Author Topic: TL071 distortion - bad amplifier design?  (Read 6469 times)

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Offline AngraMelo

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TL071 distortion - bad amplifier design?
« on: December 09, 2018, 11:10:12 pm »
Hey guys,
I just got a couple of boards and populated them for an audio amplifier.
After completion and checking to see if everything was in the right place, I powered it up, injected a 1kHz sine wave from the generator, hooked up the speaker to the output and was greeted with a enormous distortion.

Schematic attached

probing the output of the TL071 (pin 6) shows that the distortion is coming from there.
Now, the other thing is that by powering the amp with +-40V (unregulated but stable) I can never get a output of more than 7.5Vpp
That is very weird. I thought the whole point of the +-40V rails was to have the power transistors swinging accross that voltage.

I have no clue what is going on. Any help?
 

Offline HB9EVI

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Re: TL071 distortion - bad amplifier design?
« Reply #1 on: December 09, 2018, 11:22:38 pm »
First of all, you have the opamp inputs wrong; the feedback network goes on the inverting input and the audio signal on the non-inverting.
That what looks like a VAS stage, is not really needed if you use an opamp - but you have problem; the opamp cannot deliver full swing of nearly +/-40V, since its supply rails are limited by the zeners, so the whole concept has big lacks.

My tip would be to construct a proper long tailed pair input stage, a VAS and then the emitter follower as output stage; but also in the output stage as-is are several things to correct, for example you have far to many miller capacitors, which limit you the bandwidth far over the needs.
« Last Edit: December 09, 2018, 11:26:52 pm by HB9EVI »
 

Online Mechatrommer

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Re: TL071 distortion - bad amplifier design?
« Reply #2 on: December 10, 2018, 03:09:36 am »
First of all, you have the opamp inputs wrong; the feedback network goes on the inverting input and the audio signal on the non-inverting.
its common to feed the signal in inverting manner, but the feedback path, it depends.

That what looks like a VAS stage, is not really needed if you use an opamp - but you have problem; the opamp cannot deliver full swing of nearly +/-40V, since its supply rails are limited by the zeners, so the whole concept has big lacks.
the opamp doesnt have to swing full ±40V, there's app note for voltage gain stage like this...

it seems the OP copied the design from somewhere else, he may want to post here the original diagram so somebody may check for him.

Now, the other thing is that by powering the amp with +-40V (unregulated but stable) I can never get a output of more than 7.5Vpp
That is very weird. I thought the whole point of the +-40V rails was to have the power transistors swinging accross that voltage.
the TL071 will never be supplied from ±40V it will smoke otherwise. the zeners will limit the incoming supply to maybe ±15V. full swing is achieved in the 1st or 2nd stage of the transistor amplifer, you may check the swing at TIP41/42 emitter pins or after the 100R resistors.
« Last Edit: December 10, 2018, 03:16:28 am by Mechatrommer »
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Offline oPossum

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Re: TL071 distortion - bad amplifier design?
« Reply #3 on: December 10, 2018, 03:15:26 am »
The stage following the op-amp is inverting, so the feedback to the + input of the op-amp is correct. That stage also has voltage gain, so the output can get near the rails.

I don't see anything obvious wrong with the circuit. It may be a build error. Pictures of the build and the distortion you are seeing would be helpful.
 
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Online Mechatrommer

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Re: TL071 distortion - bad amplifier design?
« Reply #4 on: December 10, 2018, 03:22:10 am »
The stage following the op-amp is inverting, so the feedback to the + input of the op-amp is correct. That stage also has voltage gain, so the output can get near the rails.
the feedback should stabilize/equalize the inverting input path. if inp goes -ve, out will goes +ve, if further voltage gain stages goes +ve, it should go to inverting pin, otherwise it can go to non inverting pin. but you might be correct. if probing the output is flat low or high, the feedback path is probably wrong, but if excessive swing, maybe too much voltage gain there or even TL071 goes phase reversal. ymmv..

edit: err.. your speaker output should be connected to the last emitter follower (current gain) stage 52ee/1943 after 0.22R. not the earlier stage tip41/42. there dont have enough current power, you load that node you'll have distortion. ymmv.
« Last Edit: December 10, 2018, 03:29:41 am by Mechatrommer »
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Offline Bassman59

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Re: TL071 distortion - bad amplifier design?
« Reply #5 on: December 10, 2018, 03:25:27 am »
The stage following the op-amp is inverting, so the feedback to the + input of the op-amp is correct.

no.
 

Offline AngraMelo

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Re: TL071 distortion - bad amplifier design?
« Reply #6 on: December 10, 2018, 03:39:12 am »
I actually bought the boards. I got them on our local "ebay" kind of website.
Both boards were assembled exactly as shown on the schematic.

it did not matter how many volts I put in, the output would never swing more than 7.5Vpp. After that it would distort until it became a nearly square wave.

By probing the output of the TL071, the wave became a whole new thing completely, it was distortion in the theory sense of the word but what I mean is that it did not resemble a sine wave anymore.

How could the 1015-1815 and/or Tip41/42 be a VAS? They are both emitter followers. I dont understand.
 

Offline AngraMelo

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Re: TL071 distortion - bad amplifier design?
« Reply #7 on: December 10, 2018, 03:54:39 am »
That is exactly what happened. I used a 4ohm speaker, the output went nuts and all anyone could hear was distortion galore.
I think that amp is based on this one here
 

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Re: TL071 distortion - bad amplifier design?
« Reply #8 on: December 10, 2018, 04:07:47 am »
sorry i thought 100R and 0.22R is disconnected, you forgot big dot in OP :D the original schematics using ±70V, you are saying you are using ±40V the circuit maybe tuned to gain from ±70V, ±40V maybe not enough to stabilize feedback. you need to decrease gain (need some expertise on that) or reduce input level. and you dont have 100R R14, R16 and R15 at 2 biasing diodes there that may screw things up significantly. ymmv.
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Offline oPossum

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Re: TL071 distortion - bad amplifier design?
« Reply #9 on: December 10, 2018, 04:48:40 am »
How could the 1015-1815 and/or Tip41/42 be a VAS? They are both emitter followers. I dont understand.

A1015 and C1815 are common emitter configuration, so they have voltage gain and invert. The two following stages are common collector (also known as emitter follower), so they have only current gain and are non-inverting.
 
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Offline oPossum

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Re: TL071 distortion - bad amplifier design?
« Reply #10 on: December 10, 2018, 04:55:14 am »
The voltage rating on the A1015 and C1815 is too low. You should have at least 100V Vceo devices for +/- 40 volt rails. The TIP41/42 must be the C versions (TIP41C, TIP42C).
 

Offline Treehouseman

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Re: TL071 distortion - bad amplifier design?
« Reply #11 on: December 10, 2018, 05:00:46 am »
The stage following the op-amp is inverting, so the feedback to the + input of the op-amp is correct.

no.
k
 

Offline oPossum

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Re: TL071 distortion - bad amplifier design?
« Reply #12 on: December 10, 2018, 05:04:00 am »
You should also have the Zobel network (10 ohm resistor + 100 nF capacitor) on the output to help with stability.
 
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Offline AngraMelo

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Re: TL071 distortion - bad amplifier design?
« Reply #13 on: December 10, 2018, 05:08:08 am »
Like this?
 

Offline oPossum

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Re: TL071 distortion - bad amplifier design?
« Reply #14 on: December 10, 2018, 05:17:52 am »
 

Offline Circlotron

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Re: TL071 distortion - bad amplifier design?
« Reply #15 on: December 10, 2018, 05:52:55 am »
By probing the output of the TL071, the wave became a whole new thing completely, it was distortion in the theory sense of the word but what I mean is that it did not resemble a sine wave anymore.
The TL071 output is inside the feedback loop so it may well look distorted. If the circuit is operating properly, the TL071 output will be whatever it needs to be to make the amplifier output "zero" distortion.
 

Offline David Hess

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Re: TL071 distortion - bad amplifier design?
« Reply #16 on: December 10, 2018, 08:55:29 am »
First of all, you have the opamp inputs wrong; the feedback network goes on the inverting input and the audio signal on the non-inverting.

The first transistor stage inverts the output so feedback does need to go from the output to the non-inverting input.

Quote
That what looks like a VAS stage, is not really needed if you use an opamp - but you have problem; the opamp cannot deliver full swing of nearly +/-40V, since its supply rails are limited by the zeners, so the whole concept has big lacks.

The first stage has voltage gain so the operational amplifier can have a lower output voltage range.



There are better but more obscure ways to design an amplifier like this but it should still work and I have done it this way in the past.  The TL071 is faster than a 741 so it might require a separate feedback network from its output (not the amplifier's output) to inverting input to maintain stability.
 

Online Zero999

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Re: TL071 distortion - bad amplifier design?
« Reply #17 on: December 10, 2018, 09:10:03 am »
The stage following the op-amp is inverting, so the feedback to the + input of the op-amp is correct.

no.
Take another look. The second stage, A1015 and C1815, is in a common emitter configuration, which is inverting and has the voltage gain required to boost the output voltage from the op-amp. The third and final stages TIP41 and TIP42 and 2SC52EE and 2SA1943 are emitter followers, which just amplify the current.

I actually bought the boards. I got them on our local "ebay" kind of website.
Both boards were assembled exactly as shown on the schematic.

it did not matter how many volts I put in, the output would never swing more than 7.5Vpp. After that it would distort until it became a nearly square wave.

By probing the output of the TL071, the wave became a whole new thing completely, it was distortion in the theory sense of the word but what I mean is that it did not resemble a sine wave anymore.

How could the 1015-1815 and/or Tip41/42 be a VAS? They are both emitter followers. I dont understand.
When the input is connected to 0V and the output open circuit, please measure the following voltages, all relative to 0V:

The +/-40V supply
Output of the circuit.
TL071: pin 2, 3, 4, 6 & 7.
« Last Edit: December 10, 2018, 10:31:51 am by Hero999 »
 

Offline HB9EVI

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Re: TL071 distortion - bad amplifier design?
« Reply #18 on: December 10, 2018, 09:37:47 am »
Thanks for the feedback; you never stop learning. I've not yet met such an amp, so it looked all wrong to me;
not even in Self on Audio I found such an amp.

good to know!
 

Offline mikerj

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Re: TL071 distortion - bad amplifier design?
« Reply #19 on: December 10, 2018, 09:50:01 am »
The stage following the op-amp is inverting, so the feedback to the + input of the op-amp is correct.

no.

Yes.  The drive stage after the op-amp is inverting, so the feedback connection to the non-inverting op-amp input is correct.

However "distortion" doesn't tell us much without an example of the output waveform.  e.g. could be the amp is oscillating, or incorrectly biased and clipping.
 

Offline t1d

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Re: TL071 distortion - bad amplifier design?
« Reply #20 on: December 10, 2018, 09:59:08 am »
I actually bought the boards. I got them on our local "ebay" kind of website.
Well, that may be a good portion of the problem. Even with the extremely knowledgeable and gracious experts here on the forum sorting out the design, the components are probably not up to specs and there may be no chasing down the problem, due to that fact. Don't get me wrong, I buy Ebay stuff, when appropriate. So, for learning the circuit, maybe what you have is a good tool. But, for your discerning ear, maybe not. Again, I am not faulting you, by any means.
 

Offline spec

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Re: TL071 distortion - bad amplifier design?
« Reply #21 on: December 10, 2018, 10:39:46 am »
Hi AngraMelo

That is a pretty standard audio power amplifier architecture and has the potential for high quality reproduction, but there are two areas that you need to know about.

[1] The drive from the TL071 to the voltage amplification stage is troublesome.

[2] The amplifier will generate severe cross-over distortion.

To fix [1] above change the two 4.7nF capacitors for 47uF. The amplifier should then work.

I am quite busy at the moment but, maybe in about ten hours time. I will post a revised circuit to fix both issues properly.

« Last Edit: December 10, 2018, 10:41:55 am by spec »
 
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Offline perieanuo

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Re: TL071 distortion - bad amplifier design?
« Reply #22 on: December 10, 2018, 10:55:06 am »
And for god's sake put limiting resistors on base on final resistors, once they saturate you're in trouble.
On input I will use in your place bigger capacitors like 10 uF


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Online Zero999

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Re: TL071 distortion - bad amplifier design?
« Reply #23 on: December 10, 2018, 11:51:24 am »
And for god's sake put limiting resistors on base on final resistors, once they saturate you're in trouble.
On input I will use in your place bigger capacitors like 10 uF


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You mean the final transistors, not resistors, i.e. the 2SC52EE and 2SA1943?

If so, how can they saturate? They're emitter followers.
 

Offline perieanuo

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Re: TL071 distortion - bad amplifier design?
« Reply #24 on: December 10, 2018, 11:56:01 am »
And for god's sake put limiting resistors on base on final resistors, once they saturate you're in trouble.
On input I will use in your place bigger capacitors like 10 uF


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You mean the final transistors, not resistors, i.e. the 2SC52EE and 2SA1943?

If so, how can they saturate? They're emitter followers.
They can.for example, from thermal considerations or if driver goes short.that's why between emiter driver and final transistor base it's good idea to put resistor.see the second schematic that does have.

Look here and try to understand in practice it can arrive this situation"The bias resistor must connect to a voltage that is at least 0.7V higher than the supply voltage for the upper emitter-follower transistor to saturate." (eg if you have voltage loss at high power on your wires inside the amplificator, yes, the wires between the final transistor and the +Vdd, I seen this in a clone philips schematic, in practice) :
https://www.electro-tech-online.com/threads/current-limiter-using-transistor-how-to-saturate-an-emitter-follower-bjt.118323/

What you are saying should never arrive, but in practice can arrive, that's the trick.
I agree final transistor will die in short-circuit load scenario and 0.22 resistor should prevent thermal derive.
For me this schematic is not the most stable choice possible, but maybe you're right ....
Anyway there are better ways to prevent signal to be overamplified, thermal and oscillation tendances.
It's too complicated to talk about all the aspects with so simple amplifier.
Pierre


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« Last Edit: December 10, 2018, 01:05:15 pm by perieanuo »
 

Online Zero999

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Re: TL071 distortion - bad amplifier design?
« Reply #25 on: December 10, 2018, 12:21:25 pm »
And for god's sake put limiting resistors on base on final resistors, once they saturate you're in trouble.
On input I will use in your place bigger capacitors like 10 uF


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You mean the final transistors, not resistors, i.e. the 2SC52EE and 2SA1943?

If so, how can they saturate? They're emitter followers.
They can.for example, from thermal considerations or if driver goes short.that's why between emiter driver and final transistor base it's good idea to put resistor.see the second schematic that does have.
Pierre


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If the driver goes short circuit, the whole thing is toast anyway, whether there's a small base resistor before the output stage or not. Suppose the TIP41 goes short. How will a 2R2 resistor in series with the 2SC52EE's base possibly offer any protection? A huge current will flow through the 2SC52EE. Hopefully the amplifier will blow up before the speaker is destroyed.

The output stage is an emitter follower so can't go into saturation. Even if the base of one of the output transistors is connected directly to +V, it will still not saturate.  The emitter voltage will rise, thus reducing the base-emitter voltage, until the emitter will sit a couple of volts below the collector.
https://en.wikipedia.org/wiki/Common_collector#Basic_circuit

There may be other reasons for including a base resistor, such as reducing the gain and making it less susceptible to oscillation, but preventing saturation is not one of them.
 

Offline perieanuo

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Re: TL071 distortion - bad amplifier design?
« Reply #26 on: December 10, 2018, 12:36:43 pm »
of course it can saturate
https://www.allaboutcircuits.com/textbook/semiconductors/chpt-4/common-collector-amplifier/
https://www.electronics-tutorials.ws/amplifier/amp_2.html
get yourself a real amplifier, put it on your basement and put it to over 80% for a while with nominal load (cut the cooler fan to do it quickly), buy a multimeter or scope and tell us the result
 

Online Zero999

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Re: TL071 distortion - bad amplifier design?
« Reply #27 on: December 10, 2018, 01:50:58 pm »
of course it can saturate
https://www.allaboutcircuits.com/textbook/semiconductors/chpt-4/common-collector-amplifier/
https://www.electronics-tutorials.ws/amplifier/amp_2.html
A common emitter amplifier yes, it can saturate, but a common collector amplifier can't saturate, unless the input voltage is taken beyond the supply voltage. The tutorial you've linked to actually explains this.

Quote
Given the voltage polarities across the base-emitter PN junction and the load resistor, we see that these must add together to equal the input voltage, in accordance with Kirchhoff’s Voltage Law. In other words, the load voltage will always be about 0.7 volts less than the input voltage for all conditions where the transistor is conducting. Cutoff occurs at input voltages below 0.7 volts, and saturation at input voltages in excess of battery (supply) voltage plus 0.7 volts.

In the circuit posted at the start of the thread, the input voltage can't exceed the supply voltage, so the output state can't saturate.

Quote
get yourself a real amplifier, put it on your basement and put it to over 80% for a while with nominal load (cut the cooler fan to do it quickly), buy a multimeter or scope and tell us the result
I seems like you're confusing clipping distortion and overheating with saturation of a BJT. If an emitter follower output stage is over-driven, it will clip, distort and possibly overheat, if the heat-sinking and power rating of the transistors is insufficient, but neither of the transistors will ever enter the saturation region. They will both be operating in the active region the entire time.

In any case, adding base resistors to  2SC52EE and 2SA1943 will do little to protect them against overheating. It may improve the stability, at the expense of a reduced voltage swing.


EDIT:
Look here and try to understand in practice it can arrive this situation"The bias resistor must connect to a voltage that is at least 0.7V higher than the supply voltage for the upper emitter-follower transistor to saturate." (eg if you have voltage loss at high power on your wires inside the amplificator, yes, the wires between the final transistor and the +Vdd, I seen this in a clone philips schematic, in practice) :
Sorry, I missed this.

The wires between the final transistor and the supply voltage would have to be a very high resistance, or carrying a large current to drop nearly 1V (more more than 0.7V is required to saturate a transistor). An additional circuit would be required to protect against the high current scenario, as without it the amplifier would be toast anyway.

I see what you mean now and agree: a base resistor would protect against a high resistance or open circuit collector connection, because the maximum base current rating would be exceeded. Are you sure the extra resistor in the Philips amplifier wasn't to improve the stability?

There will be plenty of other points in the amplifier which can be destroyed by a bad connection. For example, what will happen if the connection to one of the zener diodes is bad? The TL071 will get 40V on one of its supply rails.

It's not feasible to protect against everything that can possibly go wrong!
« Last Edit: December 10, 2018, 08:15:05 pm by Hero999 »
 

Offline Kleinstein

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Re: TL071 distortion - bad amplifier design?
« Reply #28 on: December 10, 2018, 04:19:24 pm »
The circuit is problematic: There is quite some gain inside the loop of the TL071. This requires quite some extra capacitors for the compensation to slow it down.  With those rather large capacitors the TL071 sees quite some capacitive load and it might thus still oscillate at a high frequency.

The output stage is class B and thus would need a rather fast feedback to keep cross over distortion low. However as shown the first transistor stage is rather slow.

So the circuit is more like a poor example and can not expected to give low distortion output.

Another point is that the biasing current for the C1815 and A1015 depends on the supply voltage. So voltage drops (e.g. due to small filter caps) could make it leave the reasonable working range. Too high a voltage could cause excessive current and blow the small transistors.
For understanding the circuit it might help to simulate it - however there is likely little success in improving this type of circuit.
 

Offline AngraMelo

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Re: TL071 distortion - bad amplifier design?
« Reply #29 on: December 10, 2018, 04:23:17 pm »
Could you please explain why the first transistor stage is slow?
 

Online Zero999

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Re: TL071 distortion - bad amplifier design?
« Reply #30 on: December 10, 2018, 04:33:55 pm »
The circuit is problematic: There is quite some gain inside the loop of the TL071. This requires quite some extra capacitors for the compensation to slow it down.  With those rather large capacitors the TL071 sees quite some capacitive load and it might thus still oscillate at a high frequency.

The output stage is class B and thus would need a rather fast feedback to keep cross over distortion low. However as shown the first transistor stage is rather slow.

So the circuit is more like a poor example and can not expected to give low distortion output.

Another point is that the biasing current for the C1815 and A1015 depends on the supply voltage. So voltage drops (e.g. due to small filter caps) could make it leave the reasonable working range. Too high a voltage could cause excessive current and blow the small transistors.
For understanding the circuit it might help to simulate it - however there is likely little success in improving this type of circuit.
Are you sure the final stage is class B? Have you done any calculations?

Whether the final stage is class B or AB, will depend on how much current is flowing through the 100R resistors in the TIP41 and TIP42 stage. I suspect if it is class B, it can be tweaked into class AB fairly easily.

EDIT: I see how it's class B now.
« Last Edit: December 10, 2018, 05:03:57 pm by Hero999 »
 

Offline AngraMelo

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Re: TL071 distortion - bad amplifier design?
« Reply #31 on: December 10, 2018, 04:39:06 pm »
Well, on the scope, the crossover distortion was clear. There were bumps on the side of the waves near the 0V line. I guess lowering the 220R resistor would help bring it to the "AB" side.
 

Offline AngraMelo

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Re: TL071 distortion - bad amplifier design?
« Reply #32 on: December 10, 2018, 04:57:09 pm »
Are you sure the final stage is class B? Have you done any calculations?

Whether the final stage is class B or AB, will depend on how much current is flowing through the 100R resistors in the TIP41 and TIP42 stage. I suspect if it is class B, it can be tweaked into class AB fairly easily.
[/quote]


Arent the 220R resistor also responsible for the bias of the TIPs? Or they are so low that the 1015 and 1815 have plenty of base current and what is limiting their output is just the 100R on their emitters?
 

Online Zero999

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Re: TL071 distortion - bad amplifier design?
« Reply #33 on: December 10, 2018, 05:09:13 pm »
Looking at it again, Kleinstein was right, it's definitely class B. The diodes on the bases of the TIP4x transistors hold their bases at 1.4V apart, so hardly any current flows through the 100R resistors. Adding a couple of extra diodes should increase the current flow and bias it into class AB operation.
 

Offline Kleinstein

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Re: TL071 distortion - bad amplifier design?
« Reply #34 on: December 10, 2018, 05:49:13 pm »
The TIP41/42 run kind of class AB, but the SD52xx / SA19.. are running class B. One could at least add a 3 rd diode. To make is class AB it would need something like an VBE multiplier instead of the diode, set to a little more than 4 diode drops to adust the standing current. It won't be very stable however, as the VAS current is not stable.

Changing the 220 Ohms resistors is tricky, as there correct value depends on the supply voltage - which is one of the weak points. Too much and the 1015/1815 overheat. To little and the VAS stage is also kind of working in an class B mode with more distortion.
It might get a little better with a diode in series with a smaller resistor - though this might load the OP too much. The rabbit hole just starts here.
 

Offline spec

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Re: TL071 distortion - bad amplifier design?
« Reply #35 on: December 10, 2018, 07:15:23 pm »
And for god's sake put limiting resistors on base on final resistors, once they saturate you're in trouble.
On input I will use in your place bigger capacitors like 10 uF


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The output transistors are fine. But it is quite true, that in common with many audio amplifiers, that this amplifier is not short circuit protected
 
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Offline spec

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Re: TL071 distortion - bad amplifier design?
« Reply #36 on: December 10, 2018, 07:21:34 pm »
Well, on the scope, the crossover distortion was clear. There were bumps on the side of the waves near the 0V line. I guess lowering the 220R resistor would help bring it to the "AB" side.
The bumps 'on the sides' are crossover distortion. If you can see them on the scope, it is gross distortion.
The amplifier as it stands is essentially class B, not class AB.

On the schematic that you originally posted there is a missing dot connection between the two 0.22R resistors. Is this a typo and is that connection actually made in practice?

Did you try changing the capacitors as I suggested in reply #21?
« Last Edit: December 11, 2018, 02:36:00 am by spec »
 
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Re: TL071 distortion - bad amplifier design?
« Reply #37 on: December 10, 2018, 07:54:03 pm »
It is a typo. The resistors are connected to the output.
Im trying your suggestin right now. Ill be back in a moment
 

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Re: TL071 distortion - bad amplifier design?
« Reply #38 on: December 10, 2018, 08:43:42 pm »
It is a typo. The resistors are connected to the output.
Im trying your suggestin right now. Ill be back in a moment
Thanks

Attached below is the mod for converting the amplifier from deep class B to class AB. Set Iq to 30mA to 40mA. Physically place the flat part of the BC337 case in contact with one of the driver transistor cases.
« Last Edit: December 10, 2018, 08:46:00 pm by spec »
 
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Re: TL071 distortion - bad amplifier design?
« Reply #39 on: December 10, 2018, 08:47:29 pm »
Thank you so much!!!! Im about to try it!!!

I just soldered the 2 47uf capacitors.
Should I keep them on your desing? Or I go back to the 4.7nf?
 

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Re: TL071 distortion - bad amplifier design?
« Reply #40 on: December 10, 2018, 09:10:59 pm »
New 47uF capacitors are a success. Much better sound and no more noise at lower volumes.
Now to try the BC337.

Oh man, Im so freaking happy.
 

Offline David Hess

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Re: TL071 distortion - bad amplifier design?
« Reply #41 on: December 10, 2018, 10:36:56 pm »
The circuit is problematic: There is quite some gain inside the loop of the TL071. This requires quite some extra capacitors for the compensation to slow it down.

It is not quite dire.  The extra voltage gain inside the loop is attenuated by the feedback network before reaching the non-inverting input.  Indeed, if there was no extra voltage gain, then the operational amplifier would be overcompensated.

But I agree there are better configurations which use local feedback to control the gain added by the output stage.  Below is the example I was thinking off where local feedback is applied to the *output* of the operational amplifier which becomes an input.  How weird is that?
 

Offline spec

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Re: TL071 distortion - bad amplifier design?
« Reply #42 on: December 11, 2018, 01:06:53 am »
New 47uF capacitors are a success. Much better sound and no more noise at lower volumes.
Now to try the BC337.

Oh man, Im so freaking happy.
:) Great news- it is nice when theory and practice match.
 
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Offline AngraMelo

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Re: TL071 distortion - bad amplifier design?
« Reply #43 on: December 11, 2018, 01:08:12 am »
I sent you a message inbox!
 

Offline spec

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Re: TL071 distortion - bad amplifier design?
« Reply #44 on: December 11, 2018, 01:27:32 am »
CROSSED POSTS

UPDATE #1 2018_12_11 (two resistor values changed on amplifier schematic)
UPDATE #2 2018_12_11 (BC556 transistor added to better controlled constant current)

Attached below is the amplifier circuit fully worked over, but some detailed components have been omitted, like decoupling capacitors and padding resistors, for example. You will notice that there are no 47uF capacitors, but an extra transistor has been added. This transistor has no voltage or current gain and mainly functions as a level shifter. It has two other functions which I can describe if you want to know.

You will also notice that the supply rails have been reduced to 23V and -23V.
The reason for this is that, while looking at the transistor datasheets I noticed the following:

2SC5200: 230VCE
2SA1043: 230VCE
TIP41     : 60VCE
TIP42     : 60VCE
A1015    : 50VCE
C1815    : 50VCE

The two output transistors are Toshiba beauties and, obviously have no problem with +-40V supply lines, but I haven't checked the other parameters. I presume you are using 8 Ohm speakers.
The other transistors are only average in terms audio amplifiers and they limit the supply line voltage.

But, as you say that the amp is working OK with +-40V supply lines, that means that your particular transistors probably have a high enough VCE rating, so you can continue testing at risk with +40 and -40 supply lines.
« Last Edit: December 11, 2018, 08:26:32 am by spec »
 
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Re: TL071 distortion - bad amplifier design?
« Reply #45 on: December 11, 2018, 01:38:00 am »
That solid tantulan is 2.2uf or 220uf? Did you see my message?
 

Offline spec

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Re: TL071 distortion - bad amplifier design?
« Reply #46 on: December 11, 2018, 01:42:00 am »
That solid tantulan is 2.2uf or 220uf? Did you see my message?
220uF if you want a really low solid base. 100uF would be very good and 22uF would knock off some of the very low bass. You can make the choice quite freely and you may find that 2u2F suits your room/speakers/sound source/personal preference

Of course, you can go really high-end (low distortion and frequency response down to DC ) and do away with the capacitor altogether: just replace it with a short circuit.The consequence would be a worst case DC current of 42mA through your speakers, but that would not worry your speakers in the slightest.

The overall frequency response of the amplifier would be a single unit slope defined only by the input polypropylene capacitor and the opamp input bias resistor. This would give a beautiful open, rolling base with the right speakers.

And if you changed the opamp for ...

Will look at your message now.
« Last Edit: December 11, 2018, 08:36:16 am by spec »
 
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Re: TL071 distortion - bad amplifier design?
« Reply #47 on: December 11, 2018, 05:16:27 am »
awesome! I will try different capacitors and check out the sound!
What is the op amp you were saying??? you got me curious!
 

Offline spec

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Re: TL071 distortion - bad amplifier design?
« Reply #48 on: December 11, 2018, 08:21:32 am »
awesome! I will try different capacitors and check out the sound!
What is the op amp you were saying??? you got me curious!
Will post in about 10 hours.

Note the revised schematic in reply #44

Once this amplifier is working OK what do you want to do with it?

[1] Keep as is which will be a good quality audio amplifier with a sweet smooth sound

[2] More definition, lower distortion, more articulation, more impact (opamp change mainly)

[3] Maximum fidelity for this architecture (opamp change, component changes, and physical layout changes)

By the way, I meant to say before, all resistors, except perhaps the two 0R22 resistors, must be metal film types. A future modification will possibly include changing the 0R22 resistors for 0R1 low inductance resistors.

« Last Edit: December 11, 2018, 08:43:06 am by spec »
 
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Offline Mark Hennessy

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Re: TL071 distortion - bad amplifier design?
« Reply #49 on: December 11, 2018, 01:07:26 pm »
Just as a side-note, it's worth saying that there is a lot of confusion about amplifier class. Putting aside class A and just considering B and AB, Douglas Self shows that actually, class B is the ideal mode of operation.

Class B means that each device handles exactly half (180 degrees) of the signal. When there is crossover distortion, then the devices are carrying less than 180 degrees; hence the amplifier is in class C mode, not class B.

The other case - when the bias is increased and the devices overlap to handle more of the signal - is class AB. In an otherwise blameless amplifier, a class AB amplifier will have slightly more distortion than a perfectly biased class B amplifier because of gm-doubling.

All explained in the book: https://www.amazon.co.uk/Audio-Power-Amplifier-Design-Douglas/dp/0240526139 - PDF versions of older editions are easily found on-line, but I'd always recommend buying a copy of the latest. The books are based on his series of articles published in Wireless World back in 1993.

The difficulty in a practical amplifier is holding that exact bias point for class B operation under all circumstances, which is why most manufacturers play it safe by increasing the bias to put the amp into class AB, which is better than risk leaving it on the edge and risk having it fall into class C under certain circumstances. The book talks a lot about schemes to improve the thermal tracking so that this ideal point can be better maintained - it's really quite fascinating. Obviously, all this is well below what is audible, but it's intellectually satisfying to chase and minimise real distortion mechanism where easy and cheap to do so.

 
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Re: TL071 distortion - bad amplifier design?
« Reply #50 on: December 11, 2018, 01:29:17 pm »
i thought its class AB near 0V (with the existence of 2 biasing diodes) and B when not.
https://www.electronics-tutorials.ws/amplifier/class-ab-amplifier.html

why class C came into picture? its not suppose to be in audio anyway?
http://www.circuitstoday.com/class-c-power-amplifier

this thread seems to follow the fate of the others... as usual..
if something can select, how cant it be intelligent? if something is intelligent, how cant it exist?
 

Offline Kleinstein

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Re: TL071 distortion - bad amplifier design?
« Reply #51 on: December 11, 2018, 01:32:13 pm »
There is no need to use a large, expensive polypropylene capacitor at the input. Just a more normal polyester capacitor (PEN or PET) is well good enough.
For the 220 µF a more normal electrolytic cap is likely OK too.

If DC coupling is used, it might be a good idea to adjust the offset - at least with an TL071. It may work with other lower offset OPs.
Even with the capacitor at the input one might consider adjusting the offset in a way to get the correct polarity for the 220 µF capacitor.  Over a long time electrolytic caps may not like reversed polarity, even if only a few mV.

The distinction between class B and AB audio amplifiers is normally from the bias / standing current: class B would be without a standing current and class AB would be with some bias current at low output level, but less than the normal load current. Ideally there would be a small bias, so that in the cross over region the slow onset of operation from both halves add up - so no gm-doubling. There is still the problem of staying stable at that point of operation with changing temperature - so the coupling of the VBE multiplier to the transistors can be important.

The output stage uses a darlington configuration. So it would take about 4 diodes to get class AB.
 

Offline Mark Hennessy

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Re: TL071 distortion - bad amplifier design?
« Reply #52 on: December 11, 2018, 05:49:52 pm »
why class C came into picture? its not suppose to be in audio anyway?
http://www.circuitstoday.com/class-c-power-amplifier

As your link explains, the definition of a class C power is one where the output devices are conducting for less than 180 degrees.

If you take a class B audio amplifier and turn down the quiescent current, you will get crossover distortion. This is the effect of the output devices conducting for less than 180 degrees. Hence, the amplifier has moved from class B to class C. As you rightly say, this is not good.

Note that the quiescent current does not tell you what class of amplifier you have. This is a common myth - I'm not really sure where it comes from. In short, transistors are not binary devices, so require a particular standing current to reach class B. Slightly less bias moves into class C, and slightly more will move into class AB. Lots more standing current will move the amplifier into class A (in practice you'd choose a value that fits with the power output and minimum load impedance). All that without changing the circuit; simply adjusting a variable resistor.

It's all in the book I referred to, which is a very readable yet thorough analysis.


The distinction between class B and AB audio amplifiers is normally from the bias / standing current: class B would be without a standing current

Not so; class B requires some bias to achieve 180 degree operation. No standing current means class C  :-+


There is no need to use a large, expensive polypropylene capacitor at the input. Just a more normal polyester capacitor (PEN or PET) is well good enough.

Agree completely. Coupling capacitors lead a simple life - obviously it's good to avoid electrolytics when forming a high-pass filter as they are known to cause distortion in that role, but an ordinary polyester is more than good enough. The VAS capacitor is much more critical, but easily overlooked.


 

Offline Bassman59

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Re: TL071 distortion - bad amplifier design?
« Reply #53 on: December 11, 2018, 06:53:59 pm »
The stage following the op-amp is inverting, so the feedback to the + input of the op-amp is correct.

no.

Yes.  The drive stage after the op-amp is inverting, so the feedback connection to the non-inverting op-amp input is correct.

You guys are all correct -- I missed how the drive stage was connected. I even simulated the thing.

Ya learn something new every day, if you're willing to do so.

Thanks.
 
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Offline spec

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Re: TL071 distortion - bad amplifier design?
« Reply #54 on: December 11, 2018, 09:13:11 pm »
There is no need to use a large, expensive polypropylene capacitor at the input. Just a more normal polyester capacitor (PEN or PET) is well good enough.
For the 220 µF a more normal electrolytic cap is likely OK too.
What does 'there is no need' and what does, '(PEN or PEN) is well good enough' mean?
Capacitors have a big impact on the sound of an amplifier- I had a good reason for specifying those components.
A suitable polypropylene capacitor costs 70p UK and is not excessively big.
As I have said to you before, let us see your design for an amplifier, rather than criticizing/contradicting other people's work.
 
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Offline AngraMelo

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Re: TL071 distortion - bad amplifier design?
« Reply #55 on: December 11, 2018, 09:25:25 pm »
Spec, the only polyproplyne 1uf cap I can find in my country are these types: https://proesi.com.br/capacitor-partida-cbb61-1uf-450vac-fio.html

They seem to be for mains filtering. As they are 450v would impact negatively on the ESR or other parameter?
About the 220uf Tantalum, I think it is better for me to get a bunch of 100uf and I can combine them in series/parallel and find the sound I like.
What is the voltage rating I need for it? A 6.3V or 2V would do?

The main idea for this amp is to be a good sounding amp. I know it sounds vague but Im a musician and I have plenty of high fidelity amps around. From studio monitors to specialized stuff. Im diving in the audio amplifier world as a way to learn electronics and to be able to put together a amp that sounds the way I like, not that is 0.00000001% distortion.

I dont have a need for high power amps, given that I listen to music in a small space, so when I heard you telling me to choose the capacitors to suit the sound I like I got really excited because that is precisely the way Im tackling this subject.

Yes, it will be great to learn how to make extremely low distortion amps and maybe I will pursue that in the future. But right now, what Im looking for is finding the amp configuration/components/class that best suits my ears and speakers.
 

Offline spec

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Re: TL071 distortion - bad amplifier design?
« Reply #56 on: December 11, 2018, 09:30:52 pm »
UPDATE #1  2018_12_12

As to the class of an amplifier, there is no confusion at all:
  • In class A both output elements conduct all the time. This is, by definition, the lowest distortion class of all, but at the cost of low efficiency and thus high waste heat.
  • In class A/B both output elements conduct for small signals, but move over to class B for larger signals.
  • In class B one output element conducts for the positive half of the waveform (180 deg) and the other element conducts for the negative half (180 deg) of the waveform. AFAIK there are no low distortion class B audio amplifiers. In fact there would be little point, because class lowA/B (conventional audio amps) are so simple to implement and have essentially the same efficiency as class B, without the high crossover distortion.
Class AB  amplifiers predominate, because they give good efficiency with reasonably low distortion. Typically, a class AB amplifier will have a quiescent current of 20mA to 40mA, because this gives the lowest crossover distortion. Another approach is to have a quiescent current of 100ma to 1A. This would still be a class A/B amplifier but would operate in class A for a greater part of the signal. Guitar valve amps are nearly always class hiA/B.

Incidentally you can change the crossover point on most class A/B amplifiers, by simply adjusting the quiescent current. Of course, you need to increase the heat-sinking to get rid of the extra static heat dissipation.

You can even turn a class A/B amplifier into pure class A by increasing the quiescent current to a high value. Much more heat-sinking, would of course be required. For example, you could turn the reply #44, 72W class A/B amplifier into pure class A by increasing the quiescent current to 4A375.This wold require a change of output transistors and vastly increased heat-sinking, but that does not detract from the principle.

I have often run a standard Douglas Self amplifier in all three modes: A/B hiA/B and A.

By the way, it is dead easy to make a class B amplifier with no quiescent current: just feed the bases of a complimentary emitter follower output stage with a constant current. In fact the voltage amplification stage (VAS) of most audio power amplifiers do produce a constant current. That is one of the major objectives of most VASs.

In practical terms the absolute value of quiescent current is not that critical, so long as it does not drop below around 20mA. If it were essential to accurately fix the quiescent current exactly to a particular value, it would be done, because it is not a difficult feature to achieve.

« Last Edit: December 12, 2018, 09:30:11 am by spec »
 
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Offline spec

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Re: TL071 distortion - bad amplifier design?
« Reply #57 on: December 11, 2018, 09:46:19 pm »
Spec, the only polypropylene 1uf cap I can find in my country are these types: https://proesi.com.br/capacitor-partida-cbb61-1uf-450vac-fio.html.
There are polypropylene capacitors available for small signal use. I will advise on this.

They seem to be for mains filtering. As they are 450v would impact negatively on the ESR or other parameter?
About the 220uf Tantalum, I think it is better for me to get a bunch of 100uf and I can combine them in series/parallel and find the sound I like. What is the voltage rating I need for it? A 6.3V or 2V would do?
I will change the schematic to show a 100uF Tant, if that is what you can get. 100uf will be more than adequate.
2V will be fine, as will any higher voltage. The objective anyway, is to get rid of the capacitor altogether. The tant capacitor is one of the biggest sources of distortion/coloration because it is part of the feedback network. In general capacitors are troublesome components, so it is always a good approach to get the best you can, consistent with cost/size. Of course, the best approach is to eliminate capacitors in the signal path whenever possible.



« Last Edit: December 12, 2018, 12:56:35 am by spec »
 
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Offline AngraMelo

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Re: TL071 distortion - bad amplifier design?
« Reply #58 on: December 11, 2018, 09:57:32 pm »
Awesome! Thank you again!
 

Offline spec

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Re: TL071 distortion - bad amplifier design?
« Reply #59 on: December 11, 2018, 10:24:44 pm »

The main idea for this amp is to be a good sounding amp. I know it sounds vague but Im a musician and I have plenty of high fidelity amps around. From studio monitors to specialized stuff.
nice :)

I'm diving in the audio amplifier world as a way to learn electronics and to be able to put together a amp that sounds the way I like, not that is 0.00000001% distortion.

I don't have a need for high power amps, given that I listen to music in a small space, so when I heard you telling me to choose the capacitors to suit the sound I like I got really excited because that is precisely the way I'm tackling this subject.
OK, that gives a good picture of how perceptive you are, and the audio quality standard you would be happy with.

The amplifier of reply #44, with good +-40V supply lines will produce 72W into 8 Ohms. Which is ideal for music reproduction in a small room with modern speakers, which tend to be rather inefficient. If 72W sounds a lot, it is not because music normally consists of low level base-line signals with huge peaks every so often. So while the average power at fairly loud listening levels may be 100mW, the peaks could easily reach 72W instantaneous (theory says 1kW). Don't forget that the human ear's response to sound is logarithmic so, for example, increasing the power by 10%(+1dB) is barely perceptible. 

Yes, it will be great to learn how to make extremely low distortion amps and maybe I will pursue that in the future. But right now, what Im looking for is finding the amp configuration/components/class that best suits my ears and speakers.
It is extremely persnickety and tiresome to make a top end audio amp. Much of it is mechanical: the thickness and kind of wire and solder. The order of the connections. etc. And there are capacitors splattered all over the place- just take a look at a picture of the innards of a high end amp.

So I would advise this as a theoretical study for the future.

To summarize then, I suggest that we get the amplifier of reply #44 working well so there is a reference and reliable test bed and after that stage, which will be no mean achievement, look into improvements, like changing the opamp. Of course, if anything simple crops up on the way we can incorporate that.
« Last Edit: December 12, 2018, 03:08:45 pm by spec »
 
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Re: TL071 distortion - bad amplifier design?
« Reply #60 on: December 11, 2018, 10:28:14 pm »
That is great Spec!
That is exactly what Im looking for, im ordering the capacitors and preparing a board for the new transistors. It will be awesome!
Ill let you know of the performance and measurements I get here.
 

Offline spec

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Re: TL071 distortion - bad amplifier design?
« Reply #61 on: December 11, 2018, 10:47:17 pm »
That is great Spec!
That is exactly what Im looking for, im ordering the capacitors and preparing a board for the new transistors. It will be awesome!
Ill let you know of the performance and measurements I get here.
Glad that you like that approach because, I think it will give the best progress.

By the way, did you set up the quiescent current Iq OK?
 
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Offline spec

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Re: TL071 distortion - bad amplifier design?
« Reply #62 on: December 11, 2018, 11:14:01 pm »
[I am] preparing a board for the new transistors. It will be awesome!
Can I just say this to you. The physical layout of practically all circuits has a geat impact on their performance, and with high end audio amplifiers more so.

If you look at the schematic of reply #44 you will notice that I have changed the supply line and 0V connections to indicate how they should be physically implemented. Also the reply #44 schematic is only an outline and needs a few more components: decoupling capacitors for example.

So may I suggest that you hold the board layout and I will produce a full schematic for you to work from. The schematic will look a bit weird, but don't let that put you off. When the schematic is posted we can then discuss the board layout.

Just to illustrate how important simple things are with audio amps, you can take the most stable and best sounding amp ever and just by changing a couple of 0V connections, turn it into a temperamental distorting monster.

Apart from the OV line, which is crucial, remember that a class A/B amp is inherently distorting and, as a consequence, the output transistors take huge gulps of current from the supply line for 180 degrees of the signal and no current for the next 180 degrees. And, due to various effects, this current is a rather distorted half sine wave.

Wires and PCB traces are not perfect conductors and have resistance, so the distorted half sine wave current gulps produce a corresponding distorted voltage waveform on the supply lines. It only takes a whiff of that voltage to get induced into the amp circuitry to cause high levels of distortion.
 
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Re: TL071 distortion - bad amplifier design?
« Reply #63 on: December 11, 2018, 11:17:46 pm »
I just got home so I didnt have time to change de resistor and pot. I will do that tonight.
I have no education about pcb layout so any help would be greatly appreciated!!
 

Offline spec

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Re: TL071 distortion - bad amplifier design?
« Reply #64 on: December 11, 2018, 11:39:57 pm »
I just got home so I didn't have time to change de resistor and pot. I will do that tonight.
No sweat. By the way most small signal BJTs will be suitable for the Iq setting transistor, including BC337 family, BC546 family, BC182 family, BC107 family, etc The transistor only needs a VCE of 10V  minimum.


I have no education about pcb layout so any help would be greatly appreciated!!
This is the most important and difficult area. The best way forward is to take a look at some PC layouts that are already done for high end amplifiers and the schematic I mentioned previously should be a help.
 
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Offline AngraMelo

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Re: TL071 distortion - bad amplifier design?
« Reply #65 on: December 12, 2018, 12:07:54 am »
I will do that. It has been an incredible journey learning electronics.
Sometimes I feel that everyday is a kick in the butt given that the content is so extent.
Slowly Im getting the hang of it!
 

Offline spec

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Re: TL071 distortion - bad amplifier design?
« Reply #66 on: December 12, 2018, 12:20:04 am »
I will do that. It has been an incredible journey learning electronics.
Sometimes I feel that everyday is a kick in the butt given that the content is so extent.
Slowly Im getting the hang of it!
I think everybody feels like that as a newbee- I certainly did. One of the most important things is getting familiar with the basics: essential math, Ohms law, and Kirchhoff's two laws are the foundation. Then you need good teachers, and good books and videos. But most of all, you need a logical mind and to be able to make mistakes and learn from them.

The trouble is, when you are young there are so many distractions: booze, music, girls, motor bikes, cars...
 
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Re: TL071 distortion - bad amplifier design?
« Reply #67 on: December 12, 2018, 12:27:09 am »
I mentioned the importance of the physical layout of audio amplifiers (or any amplifiers for that matter), so attached is the schematic for a simple class A/B audio amplifier I did some years ago. Notice the connections of the power line, 0V line and the decoupling. That is the sort of schematic I will do for the reply#44 audio power amplifier.

Just as important is the layout of the PSU, so I will include a PSU schematic too.

 
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Re: TL071 distortion - bad amplifier design?
« Reply #68 on: December 12, 2018, 12:54:50 am »
Spec, I soldered the resistor and pot correctly nothing I do brings the current between +rail and 5200 Collector above 0ma
 

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Re: TL071 distortion - bad amplifier design?
« Reply #69 on: December 12, 2018, 12:56:52 am »
The proposed modifications by spec are good: adding a VBE multiplier and a constant current source to make the bias point less dependant on supply voltage variations and only using one common emitter amplifier.

The value of the 220μF capacitor can easily be reduced without affecting the sound quality. The input capacitor is 1μF and the input impedance is 22k, which will dominate the lower cut-off frequency, long before the 1k and 220μF. See the calculations below:

FC = 1/(2πCR)

C = 1μF and R = 22k
FC = 1/(2π*1×10-6*22×103) = 7.2Hz

C = 220μF and R = 1k
FC = 1/(2π*220×10-6*1×103) = 0.72Hz

The 220μF capacitor could be reduced to 22μF. I think the value was chosen due to a calculation error, involving the decimal place, which is very easily done!

And does it have to be solid tantalum? I don't see why aluminium won't do.

To solve the biasing issue, just connect the 22μF capacitor to -15V.


I doubt the input capacitor is that critical, just don't use a ceramic capacitor: anything else should be fine. There are many myths surrounding capacitors and audio some of which are fact, others are fiction, The main issue is ceramic capacitors which can be microphonic and introduce distortion, but all film types such as: polypropylene, polyester and and Polyethylene terephthalate (PET) are good.

Another possibility is to connect two aluminium or tantalum electrolytic capacitors back-to-back, forming a non-polarised capacitor, with half the value of the two capacitors.



For the 1μF capacitor, use two 2.2μF capacitors.

I thought the board was already done? Spec's modifications are good, but will be quite tricky to perform on an existing board. If the board can be redone then great!

Spec, I soldered the resistor and pot correctly nothing I do brings the current between +rail and 5200 Collector above 0ma
What's the voltage across each of the 100R resistors, when the input is shorted to 0V?

Did you implement all of spec's changes or only some of them?

You need to add the BC546 and other 1k resistor as well as the 1k pot for this to work.
« Last Edit: December 12, 2018, 01:03:05 am by Hero999 »
 
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Re: TL071 distortion - bad amplifier design?
« Reply #70 on: December 12, 2018, 01:01:52 am »
Spec, I soldered the resistor and pot correctly nothing I do brings the current between +rail and 5200 Collector above 0ma
Very strange. I will go back and check the circuit values.
 

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Re: TL071 distortion - bad amplifier design?
« Reply #71 on: December 12, 2018, 01:13:51 am »
Hero999,
Great tips!
About the mod with the pot and 1k resistor:
Im implementing the changes as they came on the thread. Im now following the attached schemaitc
 

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Re: TL071 distortion - bad amplifier design?
« Reply #72 on: December 12, 2018, 01:16:23 am »
CROSSED POST

The circuit and values are OK.

Are you using the new design with the PNP transistor in the emitter of the top PNP VAS transistor? Whichever way there should be 10mA flowing out of the PNP VAS collector.

The voltages on the VBE multiplier transistor (BC337) should be:
Collector 1V2
Emitter  -1V2
Base      -0V6

What are you getting?
 
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Re: TL071 distortion - bad amplifier design?
« Reply #73 on: December 12, 2018, 01:22:07 am »
About the mod with the pot and 1k resistor:
Im implementing the changes as they came on the thread. I'm now following the attached schematic

That may be the problem. You need to embody the latest circuit mods as shown on the reply #44 post.
 
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Re: TL071 distortion - bad amplifier design?
« Reply #74 on: December 12, 2018, 01:23:00 am »
Spec, I did only the following modifications:
- Changed the 2 4.7nF for 2 47uF capacitors
- Added a BC337 between the colletors of both VAS transistors
- added a 1k resistor between 1015 collector and BC337 base
- added a 1k pot between 1815 collector and BC337 base
- added a 100uf cap accross the BC337 collector and emitter

I believe that is all that is included in the schematic I posted on reply#71, am I mistaken?
Do I need to short the input to measure the current?

Hero999
The board with the new transistors is not yet made. Spec will make a layout for me and Ill etch it here.
Im bodging the components I described above on the existing board.
 

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Re: TL071 distortion - bad amplifier design?
« Reply #75 on: December 12, 2018, 01:29:59 am »
I see! So you are talking about this circuit (attached)
Ill make all the changes.
What should be the voltage ratings of the 100uf caps?
 

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Re: TL071 distortion - bad amplifier design?
« Reply #76 on: December 12, 2018, 01:36:02 am »
UPDATE #1  2018_12_12 (please see reply #79)

Spec, I did only the following modifications:
- Changed the 2 4.7nF for 2 47uF capacitors
- Added a BC337 between the collectors of both VAS transistors
- added a 1k resistor between 1015 collector and BC337 base
- added a 1k pot between 1815 collector and BC337 base
- added a 100uf cap across the BC337 collector and emitter
I believe that is all that is included in the schematic I posted on reply#71, am I mistaken?
No, not mistaken. That is correct but, I now think that you need to do all reply #44 mods- I am not able to determine the VAS current of the original design. Perhaps you could measure the VAS current of the original design.

Do I need to short the input to measure the current?
No, but it would be a good idea. :)
« Last Edit: December 12, 2018, 06:24:12 am by spec »
 
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Re: TL071 distortion - bad amplifier design?
« Reply #77 on: December 12, 2018, 01:41:01 am »
I see! So you are talking about this circuit (attached)
Ill make all the changes.
What should be the voltage ratings of the 100uf caps?
The ones on the 15V zener diodes should be rated to at least 20V, to allow for the fact that the zener diode voltage might be higher than 15V.

The one on the BC556 can be a much lower voltage rating, say 6.3V, because the voltage across the BC556 is a couple of volts at most.

Of course higher voltage rating capacitors can be used, so 25V, 35V etc. is fine for all 100μF capacitors in the circuit.
 
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Offline spec

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Re: TL071 distortion - bad amplifier design?
« Reply #78 on: December 12, 2018, 01:41:22 am »
I see! So you are talking about this circuit (attached)
Ill make all the changes.
What should be the voltage ratings of the 100uf caps?
Yes, that is the correct circuit. The second added transistor can be most small signal PBJTs.
The 100uF cap only needs to be 4V7 or more. The 100uF capacitor only has 1V2 across it in normal operation of the amp.

Apologies for causing you extra work.
« Last Edit: December 12, 2018, 01:47:43 am by spec »
 
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Re: TL071 distortion - bad amplifier design?
« Reply #79 on: December 12, 2018, 06:16:39 am »
I see! So you are talking about this circuit (attached)
Ill make all the changes.
This is an update on the subject of the original schematic VAS current (now that I have slept on it).

When I proposed the BC337 VBE multiplier for use with the original VAS, I had assumed a VAS current of 5mA to 10mA, because that sort of current is normal for a VAS and I thought I had also calculated the VAS current to be around 10mA. But that is obviously not the case because, after just doing an analysis, I find that the VAS current is in fact indeterminate, but a low value in any case. This would explain why the VBE multiplier did not work with the original VAS.

So just to confirm, I would advise to get your amplifier to the reply #44 schematic build state and I will now leave that schematic as is and not update it further so that it acts as a reference point.
« Last Edit: December 12, 2018, 08:25:31 am by spec »
 
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Re: TL071 distortion - bad amplifier design?
« Reply #80 on: December 12, 2018, 09:01:45 am »
Hey Spec,
I did not have the BC556 on hand, I just ordered some. As soon as they get here Ill make the changes
 

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Re: TL071 distortion - bad amplifier design?
« Reply #81 on: December 12, 2018, 09:37:19 am »
Hey Spec,
I did not have the BC556 on hand, I just ordered some. As soon as they get here Ill make the changes
Well done.

Practically any small signal PNJT will do for the top of the VAS.

Practically any small signal PNJT with a VCE of 45V or over will do for the level-shifter.
 
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Offline Mark Hennessy

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Re: TL071 distortion - bad amplifier design?
« Reply #82 on: December 12, 2018, 08:35:39 pm »
As the PDF is still open on my computer...

Earlier the importance of PCB layout was mentioned. I appreciate that this is for much later, but the attached screengrab from the book illustrates just one cause of distortion caused by PCB layout - so-called "Distortion number 7: NFB takeoff point distortion" by Self.

Quoting the numbers given in the text, the distortion rose from 0.00097% to 0.0027% when making this mistake. We're only talking about a few mm of difference in terms of where the connection was made.

Obviously this won't be audible, but as I said in an earlier post, it's satisfying to avoid traps like this when the fix is free and easy, and doesn't cause any other harm.

Another example: using the ubiquitous LM3886, it's possible to make a pretty decent amp with 1kHz distortion in the 0.003% region. My lash-up - detailed here: https://www.markhennessy.co.uk/microamp/ - achieves this, despite the crude construction (I used an LM4780, which was a dual LM3886). A colleague designed a PCB to employ a pair of LM3886s, but the distortion was huge - from memory one channel was around 1% and the other was more like 0.1%. He had a hard job believing that the PCB layout could be the cause, despite the differences between channels, which was a pretty major clue, but agreed to humour me and revise the PCB. He incorporated most of my suggestions, but not all. Result: much improved distortion, but still not 0.003% and still different per channel. After some track cutting and bodge wires to implement the last of the changes I suggested we did get it very close to my version in the end.

One day I'll write all this up on my website, along with photos and measurements. It can be really quite dramatic - and is solid engineering that is very definitely not in the realm of audiophoolery. Capacitor choice can fall into that category, however. Check out DIY Audio if you want to see folk who believe that properly implemented passive components sound radically different - and if you want to know more about capacitors specifically, seek out Cyril Batemans landmark work on this.

All that said, I'll repeat my earlier recommendation - you won't find a better book on the subject than Self's. If you understand the basics of how transistor circuit work, then do yourself a favour and give it a go. Even the old PDFs that can be found on-line are great - later editions expand on the subject, but don't contradict what was originally written, and that's because it was very thoroughly researched right from the start.
 

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Re: TL071 distortion - bad amplifier design?
« Reply #83 on: December 13, 2018, 01:31:55 am »
All that said, I'll repeat my earlier recommendation - you won't find a better book on the subject than Self's.

Bob Cordell's book might be better but they are both excellent.
 

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Re: TL071 distortion - bad amplifier design?
« Reply #84 on: December 13, 2018, 04:31:09 pm »
All that said, I'll repeat my earlier recommendation - you won't find a better book on the subject than Self's.

Bob Cordell's book might be better but they are both excellent.

I have Bob's book, and agree that it is pretty good. Personally, I think Self's is better; it goes deeper and is more rigorous. And yet manages to be more readable - to me, at least. Individual taste will obviously be a large factor.

Of course, Self has had a lot of criticism over the years because of his strong anti-subjectivist stance. Not surprising, I guess. Rational audio engineers will always have an uphill battle when threads like this exist: https://www.diyaudio.com/forums/chip-amps/40676-try-potentiometers.html  :palm:
 

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Re: TL071 distortion - bad amplifier design?
« Reply #85 on: December 13, 2018, 04:56:19 pm »
Hey Mark,

Keep in mind that Im a noobie at all this (electronics).
So the amplifier's resistors type would have no impact on the sound?

I cant talk at all about the impact a very specific part of the design of an amplifier, what I can do is this:
As a musician that has been for 20 years listening to a very specific recording, one can get extremely familiar with the particulars of how it sounds and be able to perceive very small nuances. How scientifically small? I dont know. It was initially a song that I really liked nowadays I use it just to test speakers and amps. But I sometimes can hear the difference from the same model of amp/speaker if they are in the same room and position.
Look, most engineers would laugh (as they have done to me) at you if you say that if you wanted to buy a guitar amp, and had all the money in the world, a tube one would be brought home.
They know it has more THD than solid state but the sound at the end is what matters. I get that it must be frustrating to listen to the enormous amount of bullshit that surrounds this subject but is the example you gave on the link measurable? Correct me if Im wrong but if you wanted to see if a pot at the beginning of the chain affected the frequency response wouldnt that be easy to measure?

 

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Re: TL071 distortion - bad amplifier design?
« Reply #86 on: December 13, 2018, 06:36:03 pm »
The resistor types can have some effect, but usually only very little and the loudspeaker will have a much larger effect as it's resistance is made of copper.  So at the very high end (not this circuit) things like self heating of some resistors (e.g. the gain setting ones) can have a little effect and cause distortion. Another possible effect could be noise from some resistors that seen an sizable DC bias. However a good circuit often avoids this problem by not having a DC bias at noise critical resistors. Without a bias the noise of a carbon resistor is not higher than an expensive metal foil resistor. So there is quite some audio-foolery going on.

Similar with capacitors - usually no need to absolutely have PP caps in an audio circuit. The cheaper polyster caps add some small phase shift - but this is more like equivalent to having the speaker moved a fraction of a millimeter.  It a common misunderstanding to think that dielectric absorption would cause distortion - it's a linear effect and only gives a minor effect on phase and amplitude, usually no harmonics.

It may be a little overreacting, but if one sees specs for PP caps in an amplifier, expect over-specified parts.  :popcorn:
 
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Offline Mark Hennessy

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Re: TL071 distortion - bad amplifier design?
« Reply #87 on: December 13, 2018, 07:54:33 pm »
Hey Mark,

Keep in mind that Im a noobie at all this (electronics).
So the amplifier's resistors type would have no impact on the sound?

I cant talk at all about the impact a very specific part of the design of an amplifier, what I can do is this:
As a musician that has been for 20 years listening to a very specific recording, one can get extremely familiar with the particulars of how it sounds and be able to perceive very small nuances. How scientifically small? I dont know. It was initially a song that I really liked nowadays I use it just to test speakers and amps. But I sometimes can hear the difference from the same model of amp/speaker if they are in the same room and position.
Look, most engineers would laugh (as they have done to me) at you if you say that if you wanted to buy a guitar amp, and had all the money in the world, a tube one would be brought home.
They know it has more THD than solid state but the sound at the end is what matters. I get that it must be frustrating to listen to the enormous amount of bullshit that surrounds this subject but is the example you gave on the link measurable? Correct me if Im wrong but if you wanted to see if a pot at the beginning of the chain affected the frequency response wouldnt that be easy to measure?



Broadly, there are two schools of thought in amplifier design:

1. Design an amplifier that has medium to high amounts of harmonic distortion, but play around until the circuit produces a particular spread of harmonics that is subjectively "nice".

2. Design an amplifier that has low to very low amounts of harmonic distortion, and then you don't have to worry about how the distortion sounds because it's utterly inaudible.

Perhaps unexpectedly, option 2 is much easier.

Option 1 is for guitar amplifier designers. And similar things used in the creative process - for example, a recording engineer might favour a particular vintage valve/tube microphone preamp because of what the distortion adds to female vocals. Or perhaps part or all of a mix might get "bounced" via a 1/4" tape machine to add some "analogue warmth". And I have no problem with any of that - it's all part of the performance.

However, when it comes to hi-fi, some might favour valve amps, or perhaps analogue replay media like vinyl, because of the subjective change in sound quality that these impose on the signal. OK, that's a personal choice, but my view is that I prefer to hear what the recording engineer wanted, without adding my own flavour of "niceness" to it.

In terms of how much distortion is too much for a home hi-fi, that's a difficult question to answer because not all distortion is the same. There are many different distortion mechanisms in an amplifier, and they result in different characters. But any well-designed solid-state amplifier will have less distortion than the loudspeakers, and probably less than the microphones too. So it's difficult to know what you've reached the "good enough" point. Better than 0.1% is probably a good rule of thumb - that means that the harmonics will be less than 60dB below the level of the fundamental, and if you research how the human hearing system works, you'll see why that seems like a reasonable number. Also, it's spectacularly easy to come in at well below that target. I mentioned 0.003% from an LM3886 earlier - that's 90dB of separation.

This is a good presentation from a guy from Audio Precision who (informally) demonstrates the audibility of distortion. It's very interesting.



I said earlier that there are folk who believe that properly implemented passive components sound radically different. They do not. The key part of that sentence is "properly implemented". Douglas Self has written about this in his book "Small Signal Design". Improperly implemented passive components might result in higher than ideal levels of harmonic distortion, but whether that's genuinely audible in strict AB or ABX testing (with tightly matched signal levels) is up for debate. Unfortunately, people don't just listen with their ears  :-+

If you're new to all this, the best thing you can do is immerse yourself in books from Self and other rational authors. My biggest regret is paying far too much attention to all the BS when I was very much younger. I became a teenager in the '80s, when the subjectivist movement was gathering serious momentum and every hi-fi magazine that I enthusiastically absorbed was spouting utter nonsense about cables and suchlike. As an engineer, I'm lucky to have some out the other side of all that - many people get trapped there for ever.
« Last Edit: December 16, 2018, 03:37:56 pm by Mark Hennessy »
 
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Re: TL071 distortion - bad amplifier design?
« Reply #88 on: December 14, 2018, 11:53:21 pm »
I've done a quick and dirty simulation of this circuit. The original version wouldn't work because the 220R resistors didn't give the transistors in the second stage enough voltage to turn on, so I increased them to 1k. I used a VBE multiplier, similar to that proposed by spec. In real life a potential divider would need to be used to adjust the operating point.

[edit]I was wrong. It works when proper transistor models are used, rather than the generic ones. I think it's because the beta of the internal, default models was too low.[/edit]

I've used generic models for the components, so the simulatio isn't very accurate, but it gives the general idea of how this is supposed to work.
« Last Edit: December 16, 2018, 04:48:13 pm by Hero999 »
 

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Re: TL071 distortion - bad amplifier design?
« Reply #89 on: December 15, 2018, 03:17:57 am »
The resistor types can have some effect, but usually only very little and the loudspeaker will have a much larger effect as it's resistance is made of copper.  So at the very high end (not this circuit) things like self heating of some resistors (e.g. the gain setting ones) can have a little effect and cause distortion. Another possible effect could be noise from some resistors that seen an sizable DC bias. However a good circuit often avoids this problem by not having a DC bias at noise critical resistors. Without a bias the noise of a carbon resistor is not higher than an expensive metal foil resistor. So there is quite some audio-foolery going on.

Similar with capacitors - usually no need to absolutely have PP caps in an audio circuit. The cheaper polyster caps add some small phase shift - but this is more like equivalent to having the speaker moved a fraction of a millimeter.  It a common misunderstanding to think that dielectric absorption would cause distortion - it's a linear effect and only gives a minor effect on phase and amplitude, usually no harmonics.

It may be a little overreacting, but if one sees specs for PP caps in an amplifier, expect over-specified parts.  :popcorn:
I notice that you were absent when the OP asked why his amplifier wasn't working. If you are such an expert, why didn't you point out the problem, which was glaringly obvious even from a cursory inspection of the circuit.  And now that has been fixed you are, once again, on the band-wagon passing judgement from on high and, once again, in a negative and contradictory fashion.

You are making some pretty sweeping statements there, and with what purpose. What does 'there is no need' mean
And are you the arbiter of what is needed and what is not needed? You keep saying things are expensive like metal film resistors- this is not true, not in the order of things. All my low wattage resistors are metal film, and that is the complete range- they did not cost that much.

About what you need and what you don't need. Well that all depends on your own personal preferences and perception. There are no hard and fast rules, like you are implying. One person may be happy with the sound of a TV and another may only be happy with the sound of the very best high end system. Also the sound of a TV may be acceptable for listening to the news, but not serious listening to music.  And there are no morals about this, the important thing is to get a system that you like the sound of. And, in your case, if you are happy with carbon resistors and aluminum and polyester capacitors in the audio signal path  good for you, but don't keep lecturing and forcing your view on others? By the way, what is your audio equipment. Can you describe it so we know what your level is. If you like music and have never listened to a high end system you are in for a shock, of course you may have cloth ears in which case it would be wasted.

The other thing is that you need to match your system components. If you have cheap low-definition speakers, a cheap low-definition amplifier will be compatible and will produce a cheap low-definition sound. But if you have high-end speakers that same amplifier will sound awful. I have already established that the OP is a musician, has good sound perception, and has high-end speakers and other equipment.

About distortion and fidelity.  The human sensory system is not fully understood and is strange. For example second harmonic distortion can sound quite pleasant while just a sniff of third harmonic distortion will sound awful. And the distortion produced by amplifiers does sound  bad. The odd thing is that speakers have a relatively high level of distortion, even the best, but they sound OK, and even the human ear distorts.

And finally, if you are so worried about costs, the answer is to buy a ready-made amplifier from ebay for around £10UK or, failing that, buy one of the audio power amplifier chips for about £2.50.
« Last Edit: December 15, 2018, 05:49:51 am by spec »
 
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Re: TL071 distortion - bad amplifier design?
« Reply #90 on: December 15, 2018, 04:54:32 am »
As a musician that has been for 20 years listening to a very specific recording, one can get extremely familiar with the particulars of how it sounds and be able to perceive very small nuances. How scientifically small? I dont know. It was initially a song that I really liked nowadays I use it just to test speakers and amps. But I sometimes can hear the difference from the same model of amp/speaker if they are in the same room and position.
This is the crux of the matter- what the system sounds like. That is the final arbiter.

Look, most engineers would laugh (as they have done to me) at you if you say that if you wanted to buy a guitar amp, and had all the money in the world, a tube one would be brought home.
They know it has more THD than solid state but the sound at the end is what matters. I get that it must be frustrating to listen to the enormous amount of bullshit that surrounds this subject but is the example you gave on the link measurable? Correct me if Im wrong but if you wanted to see if a pot at the beginning of the chain affected the frequency response wouldn't that be easy to measure?
Hmm, sounds like you have been talking to the wrong engineers, it that is what they actually were. :)
Any engineer worth his salt would evaluate the customer's requirement and try to understand it. Besides which, it is well known that guitar amplifiers and guitar speakers color the sound, some amplifiers in one way and another in another way. For example Fender amps produce a clean sonorous sound, while Marshal amps give a thicker bluesy sound. So guitar amplifiers are not like high end audio amplifiers which, in the main, aim to reproduce sound without any distortion. If you play a solid electric guitar through a normal audio amplifier the sound is thin, truncated, and lifeless.

There is an enormous mountain of BS about sound reproduction. but it is on both sides. On one side there is the "audiofool" brigade who like the pun in the accolade and use it on every occasion. Very often the closest they come to quality sound reproduction is a sound bar on their TV. Then there is the other end of the spectrum, the true audiofools, who talk about such things as rewiring the mains supply to their house with extra thick wire directly connected to their consumer unit, or fitting gold plated mains sockets to their house.

You mention about measuring the quality of an audio amplifier and ask if it can be measured. The straight answer is no. To illustrate this, some amplifiers, especially valve, sound fantastic, but show high levels of distortion, when measured. On the other hand, other amplifiers, nearly always transistor, have distortion figures that are so low that they literally cannot be measured, but only sound mediocre. In general, though, an amplifier with a measurably low distortion will sound better.

There is a coming together of the musician and engineer though. Through painstaking testing it has been found that certain techniques and components sound better than others and Douglas Self's book, describes these techniques and throughout his book Self develops, what he calls a 'blameless'' amplifier, which embodies these techniques. This book is for engineers though and not intended for musicians or the general public.

Just a word about my own experience in this field. I have been designing, building, and auditioning audio preamps, power amps, and speakers for years and, being technical, I dismissed the audiofoolery of the day. For example, I said categorically that turntables made no difference- they just went round, that amplifiers had so low distortion that they were like a piece of wire with gain and made no difference, that speaker cable made no difference. All of these were demonstrated to be wrong. I cringe remembering the time I told a guitarist that the wood of a Strat guitar made no difference. So you have to be very careful to separate armchair theory with actual listening.

And finally, any audio engineer who does not take note of the views of the listener, especially a musician, is a genuine audiofool.
« Last Edit: December 15, 2018, 05:54:13 am by spec »
 
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Offline AngraMelo

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Re: TL071 distortion - bad amplifier design?
« Reply #91 on: December 15, 2018, 06:14:39 am »
Spec,
I had BATTLES with the father of a long time ex girlfriend. He was a badass engineer that was mainly into cameras, digital image and security system. The guy was a local pop star and traveled the country with big security corporations developing new ways to see a dude in the dark dressed in black 200m away from climbimg your backyard wall and charging the least possible for it so they could compete with China.
The day I told him that most good guitar amps take tubes, he FLIPPED and had a crazy argument with me.
I told him this. If you find a way to charge the price of a solid state amp and deliver a tube sound and make a good profit, I'll sell everything I own and invest every penny in your company. I said thad we would retire in 10 years if he accomplished that. I told him that I would even pay for a prototype! No need to say that it never got out of paper and we quickly started avoiding that subject. I still remembers his face when I said That, the whole.family was there and silence was all one could hear.
But for you to have a laugh... after years playing tube amps, having intense pain from carrying that huge thing around and spending way more than reasonable, the last 5 years or so of my career was playing a small solid state. It is literally called Jazz Amp. A single 12' speaker with a kickass sound that was carried back and forth on the subway. Nowadays it is a major decoration piece in my living room.


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Re: TL071 distortion - bad amplifier design?
« Reply #92 on: December 15, 2018, 07:36:27 am »
The day I told him that most good guitar amps take tubes, he FLIPPED and had a crazy argument with me... I still remembers his face when I said That, the whole.family was there and silence was all one could hear.
maybe he had no problem with tube or solid state. his issue i guess that you play guitar >:D

But for you to have a laugh... after years playing tube amps, having intense pain from carrying that huge thing around and spending way more than reasonable, the last 5 years or so of my career was playing a small solid state. It is literally called Jazz Amp. A single 12' speaker with a kickass sound that was carried back and forth on the subway. Nowadays it is a major decoration piece in my living room.
so have you give a thought what you have gained throughout all those years? other than temporal joyness. your ex-g's dad gave his life to give peace to the community... cheers. ;)

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Re: TL071 distortion - bad amplifier design?
« Reply #93 on: December 15, 2018, 02:51:08 pm »
Sigh.

Why does nearly every thread involving audio turn in to an argument about audiophoolery?

Who are the right and wrong engineers to ask? Just because the person you asked about something, violently disagreed with you, it doesn't mean their view is invalid. The advantages of forums such as this, over asking a friend, relative colleague etc. is, one can ask a lot of engineers and gather a consensus on a matter, not just take one person's word for it. Also remember this is a technical discussion and no one should feel offended, if they're contradicted.

There's a lot of the usual nonsense here about using passives and capacitors to avoid noise and distortion and speaker cables. There's nothing special about polypropylene, over polyester capacitors. I bet no one will be able to tell the difference, if subject to a blind test. Metal film resistors are less noisy, than carbon film, but it's wrong to say the latter shall be used everywhere in the audio amplifier, where there are plenty of parts of the circuit where it won't make any difference, i.e. the resistors used to bias the zeners diodes. It's amusing when a circuit mandates high end capacitors and metal film resistors, then uses carbon potentiometers, especially with a DC bias.:-DD In this case, the circuit has non-polarised capacitor, (220µF) carrying an AC signal, with no DC bias, which will distort the signal more than any polyester capacitor. :palm: Polarised capacitors act as poor diodes, (the electrolytic capacitor is derived from the electrolytic rectifier) and need a DC bias to avoid rectifying, thus distorting the signal. As far as resistors are concerned: I'd be more concerned about the type of resistor used for the 0R22 on the output stage, which could cause problems with oscillation if it's very inductive i,.e. wire wound, on a ferrous former.

It is simply untrue that amplifiers can't be measured and characterised. This is a common audiophool myth. In reality, everything which can be heard and more, can be accurately measured and characterised. Measuring equipment is well over an order of magnitude better than the human ear at picking up distortion. The problem is it's important to measure the right thing and plenty of the traditional tests used to characterise amplifiers don't. For example, pure sinewaves, which in no way resemble music or speech, are often used to obtain THD figures. The video linked to by Mark Hennessy demonstrates this perfectly and is well worth watching.

As far as the thermionic vs solid state is debate is concerned. It's true that both amplifier topologies have different characteristics, yet paradoxically can be made to sound exactly the same: please read on, I'm not trolling, its true! It's possible to make a thermionic and a solid state amplifier with imperceptible amounts of distortion, when played below the clipping threshold, at which point they'll both be audibly transparent i.e. sound the same. The difference is a thermionic amplifier will have a nicer sounding distortion, than a solid state model, when over-driven and this effect is often desirable in a guitar amplifier, where the distortion is part of the sound and hi-fidelity is not a requirement.

Over 40 years ago a double blind test was done on three amplifier designs: one thermionic and two solid state, with golden eared audiophiles and they couldn't tell the difference. Of course both of the amplifiers were designed for minimal distortion and weren't driven into clipping, but the test was for hi-fi, not a guitar amplifier.
http://www.keith-snook.info/wireless-world-magazine/Wireless-World-1978/Valves%20versus%20Transistors%20DCD.pdf

As far as hi-fi is concerned, it's fairly easy to make an amplifier an imperceptible level of distortion. In reality speakers and room acoustics are the dominant factor, not the amplifier. It's possible that a high end hi-fi system will sound the same as a TV, if they're both connected to the same speakers and played at a low enough level to avoid clipping.

This goes back to the requirements of the original poster. Is this amplifier going to be used for hi-fi, where the distortion should be minimal or a guitar, where a certain type of distortion is required? With the appropriate modifications, there's no reason why this circuit can't give a reasonable faithful reproduction of the source, but if you want a guitar amplifier, with nice distortion, then look elsewhere.
 

Offline AngraMelo

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Re: TL071 distortion - bad amplifier design?
« Reply #94 on: December 15, 2018, 04:33:44 pm »
The day I told him that most good guitar amps take tubes, he FLIPPED and had a crazy argument with me... I still remembers his face when I said That, the whole.family was there and silence was all one could hear.
maybe he had no problem with tube or solid state. his issue i guess that you play guitar >:D

But for you to have a laugh... after years playing tube amps, having intense pain from carrying that huge thing around and spending way more than reasonable, the last 5 years or so of my career was playing a small solid state. It is literally called Jazz Amp. A single 12' speaker with a kickass sound that was carried back and forth on the subway. Nowadays it is a major decoration piece in my living room.
so have you give a thought what you have gained throughout all those years? other than temporal joyness. your ex-g's dad gave his life to give peace to the community... cheers. ;)

The point of the story is not to show that he was a bad person, bad engineer or anything like that. As described, he was a great professional and to this day I use stuff he produces in my house and it does help to keep my family safer, for that Im eternally greatful. The main point is that low level distortion is ONE way of pursuing greatness while designing an amplifier. If money was not a problem, there is a large portion of the music business that still would not buy your amp even if it had infinit gain, infinit power and 0% distortion. For years we had people to help us with our gear and we could afford such heavy amps. The budget got thinner and tube amps were no longer an option, by buying the solid state I had to make a compromise. It does not have the sound I want but it saves me from ending up in the hospital after a small tour.
The example illustrated my point of having a very good engineer that was not concerned about what I was talking about, or trying to understand where I was coming from instead, I was made fun of for no reason. I didnt know the first thing about electronics at that time (Im still a beginner but it is completely different) and what led me to tell him about tube amps was a comment he made that tubes were no longer in use.
 

Offline AngraMelo

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Re: TL071 distortion - bad amplifier design?
« Reply #95 on: December 15, 2018, 04:40:38 pm »
Sigh.

Why does nearly every thread involving audio turn in to an argument about audiophoolery?

Who are the right and wrong engineers to ask? Just because the person you asked about something, violently disagreed with you, it doesn't mean their view is invalid. The advantages of forums such as this, over asking a friend, relative colleague etc. is, one can ask a lot of engineers and gather a consensus on a matter, not just take one person's word for it. Also remember this is a technical discussion and no one should feel offended, if they're contradicted.

There's a lot of the usual nonsense here about using passives and capacitors to avoid noise and distortion and speaker cables. There's nothing special about polypropylene, over polyester capacitors. I bet no one will be able to tell the difference, if subject to a blind test. Metal film resistors are less noisy, than carbon film, but it's wrong to say the latter shall be used everywhere in the audio amplifier, where there are plenty of parts of the circuit where it won't make any difference, i.e. the resistors used to bias the zeners diodes. It's amusing when a circuit mandates high end capacitors and metal film resistors, then uses carbon potentiometers, especially with a DC bias.:-DD In this case, the circuit has non-polarised capacitor, (220µF) carrying an AC signal, with no DC bias, which will distort the signal more than any polyester capacitor. :palm: Polarised capacitors act as poor diodes, (the electrolytic capacitor is derived from the electrolytic rectifier) and need a DC bias to avoid rectifying, thus distorting the signal. As far as resistors are concerned: I'd be more concerned about the type of resistor used for the 0R22 on the output stage, which could cause problems with oscillation if it's very inductive i,.e. wire wound, on a ferrous former.

It is simply untrue that amplifiers can't be measured and characterised. This is a common audiophool myth. In reality, everything which can be heard and more, can be accurately measured and characterised. Measuring equipment is well over an order of magnitude better than the human ear at picking up distortion. The problem is it's important to measure the right thing and plenty of the traditional tests used to characterise amplifiers don't. For example, pure sinewaves, which in no way resemble music or speech, are often used to obtain THD figures. The video linked to by Mark Hennessy demonstrates this perfectly and is well worth watching.

As far as the thermionic vs solid state is debate is concerned. It's true that both amplifier topologies have different characteristics, yet paradoxically can be made to sound exactly the same: please read on, I'm not trolling, its true! It's possible to make a thermionic and a solid state amplifier with imperceptible amounts of distortion, when played below the clipping threshold, at which point they'll both be audibly transparent i.e. sound the same. The difference is a thermionic amplifier will have a nicer sounding distortion, than a solid state model, when over-driven and this effect is often desirable in a guitar amplifier, where the distortion is part of the sound and hi-fidelity is not a requirement.

Over 40 years ago a double blind test was done on three amplifier designs: one thermionic and two solid state, with golden eared audiophiles and they couldn't tell the difference. Of course both of the amplifiers were designed for minimal distortion and weren't driven into clipping, but the test was for hi-fi, not a guitar amplifier.
http://www.keith-snook.info/wireless-world-magazine/Wireless-World-1978/Valves%20versus%20Transistors%20DCD.pdf

As far as hi-fi is concerned, it's fairly easy to make an amplifier an imperceptible level of distortion. In reality speakers and room acoustics are the dominant factor, not the amplifier. It's possible that a high end hi-fi system will sound the same as a TV, if they're both connected to the same speakers and played at a low enough level to avoid clipping.

This goes back to the requirements of the original poster. Is this amplifier going to be used for hi-fi, where the distortion should be minimal or a guitar, where a certain type of distortion is required? With the appropriate modifications, there's no reason why this circuit can't give a reasonable faithful reproduction of the source, but if you want a guitar amplifier, with nice distortion, then look elsewhere.


The engineer´s point was clear, he thought that tubes in a amp made no sense. Even after I told him that the distortion they add is why they are still used. I dont know if he view was invalid or not. I simply stated a fact: if guitar players like some distortion in their sound, they will most likely look for an amp with tubes if they can afford it. The forum is a great place to learn! No wonder I cant keep myself from reading the posts!
I have nothing but gratitude for you guys!

If the whole point of using a tube amp is so you can have the characteristic sound of their distortion, how can you say that it is possible to make a solid state sound just like a tube amp?
They are made with low wattage on purpose so they will start to distort even at moderate volume. What we are looking for is exactly that light distortion it adds to the sound. If you say your solid state amp sounds just like a tube amp except when distortion starts to happen, isnt that the same as saying that your car does exact the same thing as an airplane except flying?
« Last Edit: December 15, 2018, 04:53:32 pm by AngraMelo »
 

Online Zero999

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Re: TL071 distortion - bad amplifier design?
« Reply #96 on: December 15, 2018, 06:49:36 pm »
The engineer´s point was clear, he thought that tubes in a amp made no sense. Even after I told him that the distortion they add is why they are still used. I dont know if he view was invalid or not. I simply stated a fact: if guitar players like some distortion in their sound, they will most likely look for an amp with tubes if they can afford it. The forum is a great place to learn! No wonder I cant keep myself from reading the posts!
I have nothing but gratitude for you guys!

If the whole point of using a tube amp is so you can have the characteristic sound of their distortion, how can you say that it is possible to make a solid state sound just like a tube amp?
They are made with low wattage on purpose so they will start to distort even at moderate volume. What we are looking for is exactly that light distortion it adds to the sound. If you say your solid state amp sounds just like a tube amp except when distortion starts to happen, isnt that the same as saying that your car does exact the same thing as an airplane except flying?
Yes, I do understand that the whole point of a tube amplifier is the distortion. Perhaps the person who you spoke to doesn't get this or believes the extra cost doesn't justifies it. In any case he's entitled to his point of view.

To answer your question. You're talking about a guitar amplifier, which will be driven into distortion, where as the paper I linked to is about hi-fi, which has totally different requirements to a guitar amplifier. A high amplifier should never be driven into distortion. The results of the experiment are that for hi-fi, both solid state and thermionic amplifiers give equally good sound quality, so there's no point in selecting one over the other, based on how they sound. The solid state designs are just cheaper and more reliable but people may like the way thermionic tubes look and the idea of having something old. For a guitar amplifier, a tube amp is a totally different beast to a solid state amp and there are good reasons to choose it, for the way it sounds.
 

Offline AngraMelo

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Re: TL071 distortion - bad amplifier design?
« Reply #97 on: December 15, 2018, 07:13:45 pm »
The engineer´s point was clear, he thought that tubes in a amp made no sense. Even after I told him that the distortion they add is why they are still used. I dont know if he view was invalid or not. I simply stated a fact: if guitar players like some distortion in their sound, they will most likely look for an amp with tubes if they can afford it. The forum is a great place to learn! No wonder I cant keep myself from reading the posts!
I have nothing but gratitude for you guys!

If the whole point of using a tube amp is so you can have the characteristic sound of their distortion, how can you say that it is possible to make a solid state sound just like a tube amp?
They are made with low wattage on purpose so they will start to distort even at moderate volume. What we are looking for is exactly that light distortion it adds to the sound. If you say your solid state amp sounds just like a tube amp except when distortion starts to happen, isnt that the same as saying that your car does exact the same thing as an airplane except flying?
Yes, I do understand that the whole point of a tube amplifier is the distortion. Perhaps the person who you spoke to doesn't get this or believes the extra cost doesn't justifies it. In any case he's entitled to his point of view.

To answer your question. You're talking about a guitar amplifier, which will be driven into distortion, where as the paper I linked to is about hi-fi, which has totally different requirements to a guitar amplifier. A high amplifier should never be driven into distortion. The results of the experiment are that for hi-fi, both solid state and thermionic amplifiers give equally good sound quality, so there's no point in selecting one over the other, based on how they sound. The solid state designs are just cheaper and more reliable but people may like the way thermionic tubes look and the idea of having something old. For a guitar amplifier, a tube amp is a totally different beast to a solid state amp and there are good reasons to choose it, for the way it sounds.

The article is great! Really appreciated it. I agree with that guitar amps and hi-fi are totally different things. I expressed myself badly on the last comment
 

Online Mechatrommer

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Re: TL071 distortion - bad amplifier design?
« Reply #98 on: December 16, 2018, 04:22:59 am »
The budget got thinner and tube amps were no longer an option, by buying the solid state I had to make a compromise. It does not have the sound I want but it saves me from ending up in the hospital after a small tour.
so i guess you dont need this thread. your heavily distorted amp originally should give you the better (or whitier) sound you required, no? ;D i agree with earlier commenter that good engineer should listen to their clients. so maybe another thread on how to purposely introduce distortion out of solid state to get tube's quality sound? ;)
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Offline spec

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Re: TL071 distortion - bad amplifier design?
« Reply #99 on: December 16, 2018, 08:18:41 am »
The budget got thinner and tube amps were no longer an option, by buying the solid state I had to make a compromise. It does not have the sound I want but it saves me from ending up in the hospital after a small tour.
so i guess you dont need this thread. your heavily distorted amp originally should give you the better (or whitier) sound you required, no? ;D i agree with earlier commenter that good engineer should listen to their clients. so maybe another thread on how to purposely introduce distortion out of solid state to get tube's quality sound? ;)
Not sure if you are leg pulling or not, but making transistor amps sound like valve amps and also producing distortion, has been standard practice for years in the music industry. :)

Incidentally, just in case, there is a vast range of guitar and bass speakers that have a tailored frequency response and generate a lot of distortion, of all kinds, to add character to the sound.

https://www.jensentone.com/
« Last Edit: December 16, 2018, 09:35:21 am by spec »
 

Offline spec

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Re: TL071 distortion - bad amplifier design?
« Reply #100 on: December 16, 2018, 08:29:01 am »
UPDATE #1 of 2018_12_19 (remove annotations from schematic)


AngraMelo

As promised, attached is a schematic for a slightly modified Self Simple Blameless Amplifier (SSBA).

I have added some notes to show various circuit functions, which. no doubt will be be discussed  ;D

This architecture, and versions of it, are ubiquitous.

I will say no more for the time being (social duties most of the day), except that, in my opinion, this design provides the best bang for the buck in home audio power amplifiers with an excellent balance of cost, complexity, and above, all audio quality: it will blow away the original opamp version.
« Last Edit: December 19, 2018, 11:01:24 am by spec »
 
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Offline spec

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Re: TL071 distortion - bad amplifier design?
« Reply #101 on: December 16, 2018, 09:09:11 am »
Also, as promised, here is some information on polypropylene (PP) capacitors that are suitable for use in amplifiers, pulse generators, and general electronics, as opposed to the PP capacitors widely used for power applications: snubbing, power factor correction, motor start, etc. Other manufacturers, make suitable PP capacitors but I like the specification for the Panasonic ECWFA (250V) series, although they are relatively new to the market and hence more expensive. The ECEFD (450V) and ECWFE(450V) series would be fine as well though.

Panasonic
ECWFA  (100nF to 6u8F) 250V, 450V, 630V
ECWFD  (47nF to 4u7F)  450V, 630V
ECWFE  (100nF to 4u7F) 450V, 630V

To give some idea of cost (£UK, from DigiKey UK):
ECWFA 100nF (£0.51), 1uF (£0.78), 4u7(£2.58) 6u8 (£3.66)
ECWFD 100nF (£0.42), 1uF (£0.64), 4u7 (£0.99)
ECWFE  100nF (£0.38), 1uF (£0.60), 4u7 (£0.99)

https://industrial.panasonic.com/cdbs/www-data/pdf/RDI0000/ABD0000C183.pdf
https://industrial.panasonic.com/cdbs/www-data/pdf/RDI0000/ABD0000C255.pdf
https://industrial.panasonic.com/cdbs/www-data/pdf/RDI0000/ABD0000C202.pdf
« Last Edit: December 16, 2018, 11:37:20 am by spec »
 
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Re: TL071 distortion - bad amplifier design?
« Reply #102 on: December 16, 2018, 12:15:01 pm »
spec,
The circuit you've posted is an operational amplifier! It looks perfectly decent to me. I agree will be better than the circuit posted at the start of the thread (at least it'll work!) but don't see why it should give superior performance, to an IC op-amp, with a suitable power amplifier on the output stage?

I noticed the lack of a significant bias on C2 and immediately thought that this could be an issue with distortion. There are two schools of thought on this: one, always ensure electrolytic capacitors have a significant DC bias, which is what I've been brought up on and two, just make sure the reverse voltage never exceeds a volt or so and all will be well. The reason why I've subscribed to #1 is the knowledge that an electrolytic capacitor has a parasitic diode element, in reverse parallel and even if the forward voltage of the diode isn't exceeded, causing significant current flow, it will be non-linear, hence distortion. Unfortunately reverse bias isn't accurately modelled in SPICE or shown on datasheets, presumably because capacitor manufacturers don't recommend it. In light of this decided to do some research to see whether anyone has actually done some tests to quantify this and after some digging through the depths of the Internet I found the paper linked below:
https://linearaudio.nl/sites/linearaudio.net/files/Bateman EW 12 2002 mar2003 1uF electrolytic or film.pdf

The results of the experiment are interesting. Although the distortion, due to the unbiased electrolytic capacitor is much worse, than a near-perfect film capacitor, it's nowhere need as bad as I thought. I was also wrong that the bias voltage across the capacitor should be as near the maximum rating, as possible to produce optimum distortion.

The main point which the experiment highlighted is the levels of distortion, even in the unbiased electrolytic are tiny <0.002% THD. In the circuit you posted, the distortion due to capacitors, whether aluminium electrolytic, polyester or polypropylene will be negligible. Even though C2 may introduce more distortion than any of the other capacitors, it will be tiny, compared to the rest of the circuit. The distortion due to the C2 will be much lower than the figures given in the experiment because the AC voltage across it will be tiny, it's oversized, FC = 1.2Hz and there will be the small positive bias voltage due to the current from Q3's base and R8, which will help matters, even though it's suboptimal.

Conclusion: use whatever capacitors you like in that circuit, it will make no difference to the distortion.
 

Offline Kleinstein

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Re: TL071 distortion - bad amplifier design?
« Reply #103 on: December 16, 2018, 05:19:25 pm »
The last circuit shown is a well known audio amplifier circuit.

The circuit with an OP at the input could work as well. However it is not the really easy way for high power:  One would need a power stage with additional voltage gain. It needs some attention to make the two gain parts to work together. The simple 1 pole compensation is usually no more suitable as this would need only one slow stage. Compared to many of the normal OPs the simple transistor longtail pair is also relatively fast and allows thus for good overall speed.  The use of inner loops with local feedback is kind of controversial - not everybody likes it. Well done, the concept with OP for the input and discrete power stage can work. Though the general concept may look simple, I consider the classical amplifier with discrete transistors easier.

However the TL07x are relatively high noise and might thus produce some noticeable hiss.

I also see a slight problem with an electrolytic  capacitor with no DC bias and possible slight ( 10s of mV) reverse bias from an offset.  In addition to possible distortion, there might be an issue with durability. The oxide layer will slowly get thinner and the distortion might only show up after longer use.
 

Offline Mark Hennessy

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Re: TL071 distortion - bad amplifier design?
« Reply #104 on: December 16, 2018, 05:28:52 pm »
I referred to Cyril Bateman's work earlier - it really is excellent.

I'll mentioned Douglas Self again. Search out "Small Signal Audio Design" and jump to chapter 2.

The important point is that most types of capacitor are pretty linear providing they are not being used to set a time constant (acting as a filter).

With that in mind, it is usual to over-size electrolytic capacitors in audio paths. Just picking a value based on the usual 1/(2piCR) way is not a good idea.

Best practice is to have just one capacitor that defines the -3dB of the whole amplifier, and make sure this is a film type rather than an electrolytic. Having done that, all other capacitors should be at least an order of magnitude bigger than what might otherwise appear to be needed. The aim being to minimise signal voltages across them so that they don't produce distortion. Of course, at higher frequencies the capacitors are hopefully akin to a short circuit, so no voltage is being dropped across them during the signal excursions.

Someone who is not aware of that distortion mechanism could easily assume that the capacitors are unreasonably large for no good reason.

It's easy to test for capacitor distortion if you can measure THD. You don't need a high-end audio analyser to see the distortion from an electrolytic capacitor. Self describes an experiment (page 58)  involving a 47uF cap and 1k resistor forming a high-pass filter with a -3dB point of 3.4Hz, which would appear to be pretty reasonable for audio coupling; at 20Hz, the loss is a mere 0.12dB, so you'd think that would be OK given that there is minimal voltage being lost across the capacitor, but there is enough there to cause the THD to be 0.01%. Whereas a 470uF cap is indistinguishable from the AP test system at 20Hz.

As always, one could argue about the audibility of 0.01%, especially down at 20Hz where no sub-woofer will be anything like as clean as that, but once you're aware of the problem and the way to solve it, then few good engineers would be comfortable to leave it alone. A larger capacitor is not all that expensive in the scheme of things.

Self also shows that polyester caps aren't as good as polystyrene and polypropylene, but they are still much, much better than electrolytics, and certainly better than a TL071. This presents a dilemma, but on balance I'd be happy with polyester most of the time. For a top-flight design - which the circuits in this thread are not - then it probably makes sense to fit a polypropylene. No-one will hear the difference, but it's not expensive and doesn't cause other problems, so why not? Marketing will thank you for it ;)
« Last Edit: December 16, 2018, 05:31:11 pm by Mark Hennessy »
 

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Re: TL071 distortion - bad amplifier design?
« Reply #105 on: December 16, 2018, 06:56:41 pm »
The last circuit shown is a well known audio amplifier circuit.

The circuit with an OP at the input could work as well. However it is not the really easy way for high power:  One would need a power stage with additional voltage gain. It needs some attention to make the two gain parts to work together. The simple 1 pole compensation is usually no more suitable as this would need only one slow stage. Compared to many of the normal OPs the simple transistor longtail pair is also relatively fast and allows thus for good overall speed.  The use of inner loops with local feedback is kind of controversial - not everybody likes it. Well done, the concept with OP for the input and discrete power stage can work. Though the general concept may look simple, I consider the classical amplifier with discrete transistors easier.

However the TL07x are relatively high noise and might thus produce some noticeable hiss.

I also see a slight problem with an electrolytic  capacitor with no DC bias and possible slight ( 10s of mV) reverse bias from an offset.  In addition to possible distortion, there might be an issue with durability. The oxide layer will slowly get thinner and the distortion might only show up after longer use.
There are a couple of errors in the annotations:

Constant current generator #2 should be current mirror.

The voltage to current converter, is mislabelled. A voltage to current converter would be a transconductance amplifier i.e. voltage in, current proportional to the input voltage out.  It's really a unity gain buffer or current booster circuit i.e. input voltage = output voltage, with a high input impedance and low output impedance. I'm aware that the poster knows this and has just worded it poorly.

I don't see why local feedback is such a bad idea. It will certainly be easier to stabilise. Set the output stage to a gain of 3 and you might be able to get away without using additional compensation capacitors.

The TL072 may have a relatively high input voltage noise, but it has a very low current noise and will beat op-amps with a much lower voltage noise, such as the LM4562, hands down in an application with a high input impedance. In this instance I agree, the TL072 will be more noisy than BJT input op-amps, as the impedance connected to the input will be much lower than the 22k bias resistor.


 

Offline Kleinstein

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Re: TL071 distortion - bad amplifier design?
« Reply #106 on: December 16, 2018, 08:48:49 pm »
Local feedback is a controversial topic.  Some think it is a good idea, some don't like it.  For a circuit with an OP at the input and a power stage with extra gain (like the circuit from the start), local feedback could in deed be a good idea, but it may not help very much at the upper frequency end, that is critical for stability.  It only gets really easy with an inner loop that is faster than the outer.
 

Offline David Hess

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Re: TL071 distortion - bad amplifier design?
« Reply #107 on: December 16, 2018, 09:56:36 pm »
My own experience is that local feedback makes the frequency compensation of a multistage design tractable.

My last audio amp designs used local current feedback around the voltage amplification stage to tightly control voltage gain and further local feedback around the output stage to reduce crossover distortion.  Local current feedback around the voltage gain stage has the advantage of multiplying slew rate independent of what the input differential pair can achieve without transconductance reduction which is needed to raise input stage current or lower Miller capacitance.
 
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Re: TL071 distortion - bad amplifier design?
« Reply #108 on: December 16, 2018, 10:33:24 pm »
Here's how I would do it: give the output stage a gain of 3 and a roll-off low enough to make the gain unity, before the phase shift exceeds 180o.



Kleinstein,
Yes of course, it's controversial. I didn't mean to suggest you're against local feedback, but I'm interested in why some are.
 

Offline spec

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Re: TL071 distortion - bad amplifier design?
« Reply #109 on: December 17, 2018, 04:06:05 am »
I also see a slight problem with an electrolytic  capacitor with no DC bias and possible slight ( 10s of mV) reverse bias from an offset.  In addition to possible distortion, there might be an issue with durability. The oxide layer will slowly get thinner and the distortion might only show up after longer use.
Not with solid tantalum capacitors which will take a slight reverse bias.

Incidentally, that capacitor is a PITA as it is part of the feedback network. For my money I would like to see it eliminated altogether and the amplifier servoed with an opamp. Or offset balancing used with a matched pair of transistors in one can for the input stage.
« Last Edit: December 17, 2018, 07:08:17 am by spec »
 

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Re: TL071 distortion - bad amplifier design?
« Reply #110 on: December 17, 2018, 05:01:35 am »
There are a couple of errors in the annotations:
That is a presumption
Constant current generator #2 should be current mirror.
That would be like calling the the input stage a long tailed pair- not very helpful or informative.
That arrangement of Q10 and Q11 has two functions, which if possible, will be explained. Just a clue: a current mirror is a constant current generator.
The voltage to current converter, is mislabeled.
Another presumption
A voltage to current converter would be a transconductance amplifier i.e. voltage in, current proportional to the input voltage out.  It's really a unity gain buffer or current booster circuit i.e. input voltage = output voltage, with a high input impedance and low output impedance.
Incorrect, the output transistor pairs convert an input voltage from the VAS to a current to drive the speaker (V/R).
It is difficult to chose titles for circuit functions, but Current Booster, gives totally the wrong message.
'unity gain current booster' that is a contradiction in terms.
I'm aware that the poster knows this and has just worded it poorly.
And yet another presumption.

« Last Edit: December 17, 2018, 08:04:31 am by spec »
 

Offline spec

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Re: TL071 distortion - bad amplifier design?
« Reply #111 on: December 17, 2018, 05:26:21 am »
Just a general note. I was rather hoping that we could have a positive and constructive discussion about the Self Simple Blameless Amplifier (SSBA). And it was my intention to discuss how the circuit works and what it's merits are, rather than grinding on about capacitor selection again and discussing audio power amplifier architecture and frequency stabilization etc, etc.

Also bear in mind that the SSBA is in wide use and has been designed by Douglass Self, no less, so criticism of the design by mere mortals does not make sense, neither does equating the TL071 amp with the SSBA.

By the way, as first mentioned on this thread by Mark Hennessy, read Douglass Self's book, 'Audio Power Amplifier Design Handbook', Edition 6, to get a bit more insight into audio power amplifier design- it is a very interesting read and it also covers circuit design and layout techniques that are applicable to electronics in general.
« Last Edit: December 17, 2018, 08:11:27 am by spec »
 
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Offline spec

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Re: TL071 distortion - bad amplifier design?
« Reply #112 on: December 17, 2018, 06:47:47 am »
AngraMelo

What are your plans now? I suggest at this stage that the way forward is:

[1] Correct/develop the TL071 amp and get it performing as well as possible to the latest mod standard that I posted.
[2] Build a Self Simple Blameless Amplifier (SSBA).

In regard to the actual build of the SSBA you have a few options:

[1] Knife and fork
[2] Your own PCB layout
[3] Stock PCB
[4] Stock SSBA kit
[5] Ready built SSBA

[1] By hand wiring using copper/brass strip/wire for the relevant signal paths, you can make a superb SSBA, but it would involve much work and development
[2] Like [1] above you could produce a superb board, but once again it would involve much work and development. And then you have to have the PCB manufactured.
[3] Last time I looked there were a load of SSBA (or similar) boards on the market.
[4] As [3]
[5] As [3]

I would recommend [5] from a practical point of view, but as you have said that you are in this for the experience, rather than to just have an amplifier, so perhaps [3] or [4] would suit.

By the way you have already got eminently suitable output transistors, or would you like to buy another pair of output transistors for the SSBA?
« Last Edit: December 17, 2018, 07:24:41 am by spec »
 
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Offline AngraMelo

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Re: TL071 distortion - bad amplifier design?
« Reply #113 on: December 17, 2018, 07:04:33 am »
AngraMelo

What are your plans now? I suggest at this stage that the way forward is:

[1] Correct/develop the TL071 amp and get it performing as well as possible to the latest mod standard that I posted.
[2] Build a Self Simple Blameless Amplifier (SSBA).

In regard to the actual build of the SSBA you have a few options:

[1] Knife and fork
[2] Your own PCB layout
[3] Stock PCB
[4] Stock SSBA kit
[5] Ready built SSBA

[1] By hand wiring using copper/brass strip/wire for the relevant signal paths, you can make a superb SSBA, but it would be a load of work and development
[2] Like [1] above you could produce a superb board, but once again it would involve much work and development. And then you have to have the PCB manufactured.
[3] Last time I looked there were a load of SSBA (or similar) boards on the market.
[4] As [3]
[5] As [3]

I would recommend [5] from a practical point of view, but as you have said that you are in this for the experience, rather than to just have an amplifier, so perhaps [3] or [4] would suit.

By the way you have already got eminently suitable output transistors, or would you like to buy another pair of output transistors for the SSBA?

I would like to go for option one!! As I friends says: aim on Mars to hit the Moon! hahahaha
I know it will be a pain in the butt but hey, Im here for the ride! If it proves a challenge to far away from my paygrade lets try option 3/4

Ill send an email to the Signal Transfer Company and see about the shipping fees, they appear to be a Licensed dealer of the boards, if they are not please let me know so I can look for someone who can actually sell them.
They are a bit pricy when doind the currency exchange (around double what I pay for my electrical bill per board not including shipping), so if I could etch them myself it would be awesome!

Also, lets to the TL071!!

 
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Offline spec

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Re: TL071 distortion - bad amplifier design?
« Reply #114 on: December 17, 2018, 07:21:13 am »
AngraMelo

What are your plans now? I suggest at this stage that the way forward is:

[1] Correct/develop the TL071 amp and get it performing as well as possible to the latest mod standard that I posted.
[2] Build a Self Simple Blameless Amplifier (SSBA).

In regard to the actual build of the SSBA you have a few options:

[1] Knife and fork
[2] Your own PCB layout
[3] Stock PCB
[4] Stock SSBA kit
[5] Ready built SSBA

[1] By hand wiring using copper/brass strip/wire for the relevant signal paths, you can make a superb SSBA, but it would be a load of work and development
[2] Like [1] above you could produce a superb board, but once again it would involve much work and development. And then you have to have the PCB manufactured.
[3] Last time I looked there were a load of SSBA (or similar) boards on the market.
[4] As [3]
[5] As [3]

I would recommend [5] from a practical point of view, but as you have said that you are in this for the experience, rather than to just have an amplifier, so perhaps [3] or [4] would suit.

By the way you have already got eminently suitable output transistors, or would you like to buy another pair of output transistors for the SSBA?

I would like to go for option one!! As I friends says: aim on Mars to hit the Moon! hahahaha
I know it will be a pain in the butt but hey, Im here for the ride! If it proves a challenge to far away from my paygrade lets try option 3/4

Ill send an email to the Signal Transfer Company and see about the shipping fees, they appear to be a Licensed dealer of the boards, if they are not please let me know so I can look for someone who can actually sell them.
They are a bit pricy when doind the currency exchange (around double what I pay for my electrical bill per board not including shipping), so if I could etch them myself it would be awesome!

Also, lets to the TL071!!

OK
So it is:
[1] Develop TL071 amp
[2] Separately build the SSBA by knife and fork and, if that is not successful/takes too long, go for a stock PCB.

The PCBs from the Signal Transfer Company are very expensive and postage to Brazil would add significantly to the cost.
The PCBs that I was thinking of are from ebay, and many of the home electronics companies. If I get time, I will try to find some links, and perhaps other members may like to assist here too.
« Last Edit: December 17, 2018, 08:00:58 am by spec »
 

Offline Kleinstein

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Re: TL071 distortion - bad amplifier design?
« Reply #115 on: December 17, 2018, 08:24:42 am »
There is not very much more to say to the SSBA circuit, after the corrected annotations. A simple current limitation for protection might be a good idea and could be done with little effort in the standard way.

For the OP based circuit , the simple local feedback version shown by Hero999 could be a good idea for less cross over distortion.

To reduce the problem that the current in the extra voltage amplification stage depends on the supply voltage, one could add diodes in series to R7/ R8 and make those resistors a little smaller. The transistor / diode combination would than be simplified current mirrors.
There would be still some supply dependence, but less (maybe half or a third).

The more radical way would be to have one side a fixed current source and only use the OP to drive one side. Power supply suppression would still be a problem. A transistor in base configuration could help to transfer the signal as a current from ground references (at the OP) to supply referenced for Q5 or Q6.

 

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Re: TL071 distortion - bad amplifier design?
« Reply #116 on: December 17, 2018, 10:44:41 am »
Constant current generator #2 should be current mirror.
That would be like calling the the input stage a long tailed pair- not very helpful or informative.
That arrangement of Q10 and Q11 has two functions, which if possible, will be explained. Just a clue: a current mirror is a constant current generator.
A voltage to current converter would be a transconductance amplifier i.e. voltage in, current proportional to the input voltage out.  It's really a unity gain buffer or current booster circuit i.e. input voltage = output voltage, with a high input impedance and low output impedance.
Incorrect, the output transistor pairs convert an input voltage from the VAS to a current to drive the speaker (V/R).
It is difficult to chose titles for circuit functions, but Current Booster, gives totally the wrong message.
'unity gain current booster' that is a contradiction in terms.
  • The input stage is a long tailed pair.
  • Yes a current mirror can be used as a constant current source/sink, but that's not its function here, because the current isn't constant, but proportional to Q3's collector current.
  • This circuit uses voltage drive for the speaker, not current drive, hence the output stage provides current gain, not voltage to current conversion.

If I'm wrong and you're right, then why does entering in the terms I've given into a Google image search yield circuit elements, similar to those detailed in the Self Simple Blameless Amplifier schematic, yet the ones you've listed give something completely different?

Current mirror - the first hit looks remarkably how Q10 and Q11 are connected.

Constant current generator - mostly different results. A couple match, but nowhere near as much as looking for the correct term: current mirror.

Voltage to current converter - looks nothing like the output stage. Clicking on the links also reveals descriptions of circuits which generate a current, proportional to an input voltage, with a few the other way round: very confusing!

I admit, looking for current booster unity gain buffer doesn't give exactly the same circuit as the output stage, but with the exception of a couple rouge hits, it's fairly close (emitter followers). I admit I could have chosen a better term: push-pull current booster, push-pull unity gain buffer or class AB output stage give better hits, although the last one isn't quite as descriptive on its own.

I don't want to turn this into a debate about semantics, but using vaguely the correct terms makes it easier for newcomers to search for things and avoids confusion. I admit it can be difficult to choose descriptive terms for such abstract concepts.

There is not very much more to say to the SSBA circuit, after the corrected annotations. A simple current limitation for protection might be a good idea and could be done with little effort in the standard way.

For the OP based circuit , the simple local feedback version shown by Hero999 could be a good idea for less cross over distortion.

To reduce the problem that the current in the extra voltage amplification stage depends on the supply voltage, one could add diodes in series to R7/ R8 and make those resistors a little smaller. The transistor / diode combination would than be simplified current mirrors.
There would be still some supply dependence, but less (maybe half or a third).

Yes, that seems sensible.

Quote
The more radical way would be to have one side a fixed current source and only use the OP to drive one side. Power supply suppression would still be a problem. A transistor in base configuration could help to transfer the signal as a current from ground references (at the OP) to supply referenced for Q5 or Q6.
Similar to what speck proposed earlier on in the thread?
https://www.eevblog.com/forum/beginners/tl071-distortion-bad-amplifier-design/msg2030107/#msg2030107
 

Offline Kleinstein

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Re: TL071 distortion - bad amplifier design?
« Reply #117 on: December 17, 2018, 10:57:46 am »
........
Quote
The more radical way would be to have one side a fixed current source and only use the OP to drive one side. Power supply suppression would still be a problem. A transistor in base configuration could help to transfer the signal as a current from ground references (at the OP) to supply referenced for Q5 or Q6.
Similar to what speck proposed earlier on in the thread?
https://www.eevblog.com/forum/beginners/tl071-distortion-bad-amplifier-design/msg2030107/#msg2030107
Yes exactly that kind of circuit I was thinking of. Somehow forgot it was already in this thread.
 

Offline not1xor1

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Re: TL071 distortion - bad amplifier design?
« Reply #118 on: December 17, 2018, 11:22:19 am »
[...]
Similar to what speck proposed earlier on in the thread?
https://www.eevblog.com/forum/beginners/tl071-distortion-bad-amplifier-design/msg2030107/#msg2030107

who is speck:scared:
According to wikipedia you are probably hungry...  ;D
His nickname is spec, no K
 
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Online Zero999

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Re: TL071 distortion - bad amplifier design?
« Reply #119 on: December 17, 2018, 11:44:24 am »
[...]
Similar to what speck proposed earlier on in the thread?
https://www.eevblog.com/forum/beginners/tl071-distortion-bad-amplifier-design/msg2030107/#msg2030107

who is speck:scared:
According to wikipedia you are probably hungry...  ;D
His nickname is spec, no K
Sorry spec. I don't know why I added the k.:palm:

Yes, I've tried that stuff when I was in Venice. It's a bit on the salty side for my taste, but a a little on a pizza is OK.
 

Online Zero999

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Re: TL071 distortion - bad amplifier design?
« Reply #120 on: December 17, 2018, 12:44:27 pm »
........
Quote
The more radical way would be to have one side a fixed current source and only use the OP to drive one side. Power supply suppression would still be a problem. A transistor in base configuration could help to transfer the signal as a current from ground references (at the OP) to supply referenced for Q5 or Q6.
Similar to what speck proposed earlier on in the thread?
https://www.eevblog.com/forum/beginners/tl071-distortion-bad-amplifier-design/msg2030107/#msg2030107
Yes exactly that kind of circuit I was thinking of. Somehow forgot it was already in this thread.
It should work well with local feedback. I haven't simulated the original version, but my gut feeling is it will oscillate and be difficult to stabilise, as it has a huge gain, on the order of 20k. Local feedback sorts that out.
 
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Online Zero999

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Re: TL071 distortion - bad amplifier design?
« Reply #121 on: December 18, 2018, 12:11:45 pm »
AngraMelo

What are your plans now? I suggest at this stage that the way forward is:

[1] Correct/develop the TL071 amp and get it performing as well as possible to the latest mod standard that I posted.
[2] Build a Self Simple Blameless Amplifier (SSBA).

In regard to the actual build of the SSBA you have a few options:

[1] Knife and fork
[2] Your own PCB layout
[3] Stock PCB
[4] Stock SSBA kit
[5] Ready built SSBA

[1] By hand wiring using copper/brass strip/wire for the relevant signal paths, you can make a superb SSBA, but it would be a load of work and development
[2] Like [1] above you could produce a superb board, but once again it would involve much work and development. And then you have to have the PCB manufactured.
[3] Last time I looked there were a load of SSBA (or similar) boards on the market.
[4] As [3]
[5] As [3]

I would recommend [5] from a practical point of view, but as you have said that you are in this for the experience, rather than to just have an amplifier, so perhaps [3] or [4] would suit.

By the way you have already got eminently suitable output transistors, or would you like to buy another pair of output transistors for the SSBA?

I would like to go for option one!! As I friends says: aim on Mars to hit the Moon! hahahaha
I know it will be a pain in the butt but hey, Im here for the ride! If it proves a challenge to far away from my paygrade lets try option 3/4

Ill send an email to the Signal Transfer Company and see about the shipping fees, they appear to be a Licensed dealer of the boards, if they are not please let me know so I can look for someone who can actually sell them.
They are a bit pricy when doind the currency exchange (around double what I pay for my electrical bill per board not including shipping), so if I could etch them myself it would be awesome!

Also, lets to the TL071!!
Sorry for taking your thread on a tangent and allowing myself to become distracted.

The SSBA is a good design and is ideal for the beginner as it allows one to see how it works. Performance wise, I'm not convinced it'll be any better or worse than an op-amp+discreet power stage (I'm talking about a better design than the TL071 posted here of course) but that doesn't matter. The main advantage of a discrete op-amp, which is what the SSBA is, it's transparent i.e. the builder, who can probe and measure every single point in the circuit. An IC op-amp is just a black box, with no way to see what's going on inside.

Actually I'd recommend building a smaller, lower power discrete op-amp, before attempting the SSBA, simply because it'll be easier. I'm thinking of driving an 8Ω load with <1W or even a basic op-amp circuit which has no practical use, other than to demonstrate the concept.

Did you ever get the TL071 circuit working properly? If not, how about implementing the minimum number of modifications required to get it to work properly?
 

Offline Kleinstein

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Re: TL071 distortion - bad amplifier design?
« Reply #122 on: December 18, 2018, 07:47:50 pm »
........
It should work well with local feedback. I haven't simulated the original version, but my gut feeling is it will oscillate and be difficult to stabilise, as it has a huge gain, on the order of 20k. Local feedback sorts that out.

The local feedback would likely still need some compensation (e.g. a miller-capacitor at the VAS). The high DC gain is usually not a problem. The problem would be  more with to much bandwidth or too much gain in the MHz frequency range.

For such an amplifier circuit it is a good idea to start with a lower power (e.g. lower supply voltage) version, to limit the release of magic smoke. However it would not simplify the circuits shown so far. The SSBA circuit can also work with a lower voltage with only minimal changes. It is more like one might consider additional transistors (e.g. cascode for the input stage) if the supply is above 40 V, so that one does not need high voltage rated transistors there.
 

Offline David Hess

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Re: TL071 distortion - bad amplifier design?
« Reply #123 on: December 18, 2018, 10:28:15 pm »
The SSBA is a good design and is ideal for the beginner as it allows one to see how it works. Performance wise, I'm not convinced it'll be any better or worse than an op-amp+discreet power stage (I'm talking about a better design than the TL071 posted here of course) but that doesn't matter. The main advantage of a discrete op-amp, which is what the SSBA is, it's transparent i.e. the builder, who can probe and measure every single point in the circuit. An IC op-amp is just a black box, with no way to see what's going on inside.

The SSBA topology duplicates the most common operational amplifier implementation so understanding it goes a long way to understanding most operational amplifiers.

The TL071 could be used in a much better design if applied correctly.  I posted above what I would do although it violates unloading the operational amplifier's output.

 

Offline spec

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Re: TL071 distortion - bad amplifier design?
« Reply #124 on: December 19, 2018, 10:39:27 am »
after the corrected annotations.
The annotations have not been corrected. I posted the circuit for the SSBA in reply #100: that is the reference and has not changed. But what I am going to do is remove the annotations from the original schematic in reply #100 to avoid any further controversy.
« Last Edit: December 22, 2018, 07:43:32 am by spec »
 
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Offline spec

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Re: TL071 distortion - bad amplifier design?
« Reply #125 on: December 22, 2018, 07:50:11 am »
AngraMelo
The attached document, which will be progressively updated, has some notes on theory, components, and circuit functions relating to the TL071 Amplifier (TL071Amp) and Self Simple Blameless Amplifier (SSBA).

(WIP: no document attached at this time)
« Last Edit: December 22, 2018, 08:03:29 am by spec »
 

Offline spec

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Re: TL071 distortion - bad amplifier design?
« Reply #126 on: December 22, 2018, 07:52:56 am »
AngraMelo
The attached document, which will be progressively updated, has some notes on the TL071 Amplifier (TL071Amp).

(WIP: no document attached at this time)
« Last Edit: December 22, 2018, 08:18:18 am by spec »
 

Offline spec

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Re: TL071 distortion - bad amplifier design?
« Reply #127 on: December 22, 2018, 07:55:02 am »
AngraMelo
The attached document, which will be progressively updated, has some notes on the Self Simple Blameless Amplifier (SSBA).

(WIP: no document attached at this time)
« Last Edit: December 22, 2018, 08:02:52 am by spec »
 

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Re: TL071 distortion - bad amplifier design?
« Reply #128 on: December 22, 2018, 07:55:52 am »
Three times nothing attached...    :palm:
 

Offline spec

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Re: TL071 distortion - bad amplifier design?
« Reply #129 on: December 22, 2018, 08:05:42 am »
Three times nothing attached...    :palm:
Whooa- slow down. All in good time. :-DD
 


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