Author Topic: TL071 distortion - bad amplifier design?  (Read 6189 times)

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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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Online Zero999

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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.
 

Online 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 »
 

Online Zero999

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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.


 

Online 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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Online Zero999

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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 »
 

Offline 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 »
 

Online 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.

 

Online Zero999

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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
 

Online 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?
 

Online 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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