Author Topic: Guitar preamp op amp tl072 input noise  (Read 34935 times)

0 Members and 2 Guests are viewing this topic.

Offline AleTopic starter

  • Contributor
  • Posts: 48
  • Country: it
Re: Guitar preamp op amp tl072 input noise
« Reply #25 on: November 03, 2017, 11:24:47 am »
I made the changes you suggested and build the circuit on a breadboard. It turn out that if I connect the input shield on GND no sound are produced. If I connect the input shield on VGND I hear the sound of the guitar, but with lots of noise. Why is that?

thanks alot guys
 

Online Zero999

  • Super Contributor
  • ***
  • Posts: 19345
  • Country: gb
  • 0999
Re: Guitar preamp op amp tl072 input noise
« Reply #26 on: November 03, 2017, 11:28:13 am »
I made the changes you suggested and build the circuit on a breadboard. It turn out that if I connect the input shield on GND no sound are produced. If I connect the input shield on VGND I hear the sound of the guitar, but with lots of noise. Why is that?

thanks alot guys
Yes, that's much better. It was my fault for saying "remove R3" when I meant R4. It will only give a maximum gain of 11 though, which might not be enough.  The original circuit was designed for a gain range of 1 to 100. It depends on how much gain you need.
 

Offline jm_araujo

  • Regular Contributor
  • *
  • Posts: 72
  • Country: pt
Re: Guitar preamp op amp tl072 input noise
« Reply #27 on: November 03, 2017, 11:58:46 am »
It should make no big difference, the input and output capacitors take care of any DC bias. Can you post a picture of the assembled circuit?

Where are you getting the 12V from and where are you connecting the circuit to?
 

Offline AleTopic starter

  • Contributor
  • Posts: 48
  • Country: it
Re: Guitar preamp op amp tl072 input noise
« Reply #28 on: November 03, 2017, 04:42:24 pm »
Ok guys, I do lots of tests and the result is the schema I attached to the post.

I tried to use my smartphone with 3.5" cable and I discovered that:
1-I have no noise from cables
2-No noise from breadboard
3-This circuit works damn good :)
4-Using 220uF for C6 and C7 I lose too many bass freq. 470uF are better and more balanced.
5-18V is better to power the tl072 (I'm using tl074, but it's the same) and get +9V/-9V  :-+

The problems are:
1-with the guitar I still have lot of noise and I don't understand why. Using the same cable to my soundcard (a professional one) I have no noise at all! :o I also used the faraday cage around the pickups and they are very silent in every situation I played. I also tested another guitar, same problem.
2-with the gain of 1 (the minumum for the non inverting amplifier if I'm not wrong) the sound is quite high, if I turn the RV1 it goes overdrive easily. I tried a resistor between C5 and pin3, but nothing change. I also tried to buffer the amp (gain 1 I think), same problem.

Do you have some ideas?

thanks
 

Offline Damianos

  • Frequent Contributor
  • **
  • Posts: 268
  • Country: gr
Re: Guitar preamp op amp tl072 input noise
« Reply #29 on: November 03, 2017, 09:22:45 pm »
Your circuit has some drawbacks!
See the attached one ... in the simulation gives good results; if I have the time, I will try it on a breadboard
I will try to write some notes tomorrow...

As a start, on your circuit:
- remove the C6, the output is always positive against ground
- remove C1 and C3, they are coupling any AC component from the power line to the input...
- put the C1 in series to R3 and connect it to the GND and not to VGND, so as to not amplify the offset...
...

BTW what you mean with "noise"? Is it thermal noise, is it hum, radio interference or something else?
« Last Edit: November 03, 2017, 09:25:30 pm by Damianos »
 

Offline danadak

  • Super Contributor
  • ***
  • Posts: 1875
  • Country: us
  • Reactor Operator SSN-583, Retired EE
Re: Guitar preamp op amp tl072 input noise
« Reply #30 on: November 03, 2017, 11:36:41 pm »
Tie the unconnected end of your pot to the wiper to reduce
potential coupling/pickup.

There should be a cap from + 6 to - 6 to shunt noise away from
opamps. A combination like C1 and C3 but strapped across 12V supply.

My mistake in making non polar cap out of two polars, I kept thinking
you were on "normal" split supply, lost focus....thanks Daminaos for catching
that.

Do you have a scope and a sine wave source so that you can feed the sine
into your layout and post a pic of the output. Or hand draw the signal out
you see.


Regards, Dana.

« Last Edit: November 03, 2017, 11:39:39 pm by danadak »
Love Cypress PSOC, ATTiny, Bit Slice, OpAmps, Oscilloscopes, and Analog Gurus like Pease, Miller, Widlar, Dobkin, obsessed with being an engineer
 

Offline Audioguru

  • Super Contributor
  • ***
  • Posts: 1507
  • Country: ca
Re: Guitar preamp op amp tl072 input noise
« Reply #31 on: November 04, 2017, 12:24:32 am »
We are still waiting for a description of the "noise". Is it hum, hiss, buzz interference from AC lighting or something else?
 

Offline AleTopic starter

  • Contributor
  • Posts: 48
  • Country: it
Re: Guitar preamp op amp tl072 input noise
« Reply #32 on: November 04, 2017, 08:50:59 am »
Hello again,
  I implemented the new schema here, it appears a little bit better, but not perfect. Still no noise from smartphone, but lot of hiss and buzz from guitar.

Anyway I noted this, when I turn the guitar tone pot down the noise redure alot, almost disappear. Of course it turn down the high frequencies too. I have the same effect adding a 0.1uF ceramic between the jack input and gnd. Turning the gain up I also increase the noise. Could it be a problem bound of hi-freq? how to rid off of that?

thanks
 

Offline danadak

  • Super Contributor
  • ***
  • Posts: 1875
  • Country: us
  • Reactor Operator SSN-583, Retired EE
Re: Guitar preamp op amp tl072 input noise
« Reply #33 on: November 04, 2017, 10:00:48 am »
Does the guitar have its own preamp ? Referring to the
tone control. is that just a passive pot and cap, or another
preamp ?

Is your TL072 grounds tied to amplifier ground ? Single point
ground on your board.

Try taking your design, with a set of headphones and 2 9V batteries,
outside away from indoor EMI sources, like TV, Microwave, Wireless
phones, laptop, PC.... and see how that performs.

I get the impression that guitar pickup is creating so little output that
you have to run the TL072 at very high G and that amps up the noise
quite a lot.

One other issue, R6 creates a very hiz pickup point, you might try 10K
and see how that affects performance. 10K probably not a significant
load to pickup. What is pickup, magnetic or piezo. The design of an amp
quite different between the two. If its piezo you need hiz, 10K not would not
be a good idea. Piezo wants more like 10M Zload, otherwise the signal is so
low level that causes much higher G to be needed which aggravates noise
performance. In this case a typical approach is to use a JFET design phys-
ically close to the pickup. Or the TL072 close to pickup.

Consider locating the design right at the pickup to get rid of cabling (read
antenna) coupling effects.

If you can't some comments on cabling - http://www.liutaiomottola.com/PrevPubs/Piezo/CoaxTransducer.htm

Post a picture of your construction technique.


Regards, Dana.
« Last Edit: November 04, 2017, 10:29:27 am by danadak »
Love Cypress PSOC, ATTiny, Bit Slice, OpAmps, Oscilloscopes, and Analog Gurus like Pease, Miller, Widlar, Dobkin, obsessed with being an engineer
 
The following users thanked this post: sevenTech

Offline danadak

  • Super Contributor
  • ***
  • Posts: 1875
  • Country: us
  • Reactor Operator SSN-583, Retired EE
Re: Guitar preamp op amp tl072 input noise
« Reply #34 on: November 04, 2017, 10:45:30 am »
One other issue, CM range. From the datasheet -


Quote
8.3.1 Design Requirements
• VCC must be within valid range per Recommended Operating Conditions. This example uses a value of 12 V
for VCC.
• Input voltage must be within the recommended common-mode range, as shown in Recommended Operating
Conditions. The valid common-mode range is 4 V to 12 V ( VCC– + 4 V to VCC+.
• Output is limited by output range, which is typically 1.5 V to 10.5 V, or VCC– + 1.5 V to VCC+ – 1.5 V.

So your bias point should be 8V, not 6V, to get maximum range w/o distortion.

This may or may not be an issue depending on what the pickup max output levels
are. In fact you could create a distortion control by purposely offsetting bias point
with a pot. For another day.


Regards, Dana.
« Last Edit: November 04, 2017, 10:51:28 am by danadak »
Love Cypress PSOC, ATTiny, Bit Slice, OpAmps, Oscilloscopes, and Analog Gurus like Pease, Miller, Widlar, Dobkin, obsessed with being an engineer
 

Offline AleTopic starter

  • Contributor
  • Posts: 48
  • Country: it
Re: Guitar preamp op amp tl072 input noise
« Reply #35 on: November 04, 2017, 11:32:37 am »
Sorry I didn't mention this before, I'm using an electric guitar with magnetic passive pickups (no preamp in between). I'm testing with headphones and not with a guitar amplifier. With 10K on R6 it produce a little less noise. (Also thanks for the lutherie link, it's very interesting for me).

Anyway, as you suggested I tested the circuit with 2 9v batteries in series and... no noise at all! Neither at maximum gain!  :-+ :-+ :-+ And it sounds great!

After that I tested 4 or 5 power supply and the best is one of an old a laptop, but still not noiseless.

Is it possible to eliminate the noise from the power supply or should I use always batteries? I'm just using a powersupply with multiple isolated output I thought it was a good choise.

ps, I'm posting the last schematic if anyone else from the forum needs it.
 

Offline Richard Crowley

  • Super Contributor
  • ***
  • Posts: 4317
  • Country: us
  • KJ7YLK
Re: Guitar preamp op amp tl072 input noise
« Reply #36 on: November 04, 2017, 11:46:52 am »
We sometimes use that switching node of the input connector (pin 2 of J1) to short or mute the input to avoid picking up noise when nothing is plugged in.  So, instead of connecting pin 2 over to pin 1 (ignoring the switching function), we would connect pin 2 to pin 3 (ground).  So that the input is shorted to ground when nothing is plugged in.
 

Online Zero999

  • Super Contributor
  • ***
  • Posts: 19345
  • Country: gb
  • 0999
Re: Guitar preamp op amp tl072 input noise
« Reply #37 on: November 04, 2017, 06:48:14 pm »
Try using the potentiometer as an attenuator before the op-amp, rather than varying the gain.

Note that this circuit is no good for op-amps with a bipolar input, such as the NE5532, because the bias currents pass through the potentiometer and will generate a scratching sound, when it's adjusted, but those sorts of op-amps are noisy in high input impedance designs.

« Last Edit: November 05, 2017, 03:51:09 pm by Hero999 »
 

Offline Damianos

  • Frequent Contributor
  • **
  • Posts: 268
  • Country: gr
Re: Guitar preamp op amp tl072 input noise
« Reply #38 on: November 04, 2017, 07:58:51 pm »
Sorry I didn't mention this before, I'm using an electric guitar with magnetic passive pickups (no preamp in between). I'm testing with headphones and not with a guitar amplifier. With 10K on R6 it produce a little less noise. (Also thanks for the lutherie link, it's very interesting for me).

Anyway, as you suggested I tested the circuit with 2 9v batteries in series and... no noise at all! Neither at maximum gain!  :-+ :-+ :-+ And it sounds great!

After that I tested 4 or 5 power supply and the best is one of an old a laptop, but still not noiseless.

Is it possible to eliminate the noise from the power supply or should I use always batteries? I'm just using a powersupply with multiple isolated output I thought it was a good choise.

ps, I'm posting the last schematic if anyone else from the forum needs it.

Some notes:
- The C3 and C6 do almost nothing for the (low impedance) ripple of the power supply; increase the impedance as I show on my schematic.
- The R1 and R2 can have ten times higher values ...
- The U1B, as it is used, does nothing useful! It supplies a few pA by loading the divider by ... a few pA!
- The C5/R6 values do not agree with a guitar pick up characteristics and an audio amplifier...
- The RV1, as it is connected, moves the operating point of the amplifier!
- It is generally a bad idea to change the gain of an amplifier, it changes also other characteristics of it.
- The value of C7 is very high unnecessarily.
- The value of R4 is low and consumes power from the output of the amplifier.

A little about the circuit I proposed:
- Instead of one amplifier with "huge" gain, the amplification is performed in two stages.
- The R8 and C7 constitute an additional filter for the sensitive input.
- The R2 and R5 set the bias to about the middle of the operating voltage.
- The C2 and C5 are blocking the DC, so as the DC voltage amplification is equal to one. They also affect the amplification at lower frequencies.
- The C10 is reducing possible radio interference and has to be adjusted according to the characteristics of the guitar's pick-up.
- The C8 and C9 are defining the upper frequency limit.

A quick test on a breadboard:
Differences from the schematic: C4=470mmF, R8=34k8, C10=0, RV1=560k fixed, R3=100k, C8=C9=22pF.
Results:
Amplification inside the passband: ~115
Passband limits (~0.7 of the voltage on 1kHz): ~12.5Hz ... ~34kHz
Maximum output voltage, before visible clipping: ~2V RMS

Regards,
Damianos
 

Offline AleTopic starter

  • Contributor
  • Posts: 48
  • Country: it
Re: Guitar preamp op amp tl072 input noise
« Reply #39 on: November 05, 2017, 01:32:09 pm »
Thank you Damianos, I tested your schema and it works pretty well. But the last I made worked well too.
I rechecked the connections inside the guitar, cables, etc. With batteries I have a very silent preamp, but with a power supply it works really bad. Also with grounded input it produce very loud noise.

The thing is... I always tried with standard ones, also my music power supply start with a switching one (I think).
As I see for this kind of applications the best way is to have a toroidal power supply with multi isolated output. And I'm thinking to build one from scratch. Do you think it's a good idea? Which one to buy? I thought to build one with 4 1n4001 diode, some capacitors and lm7815 / lm7915 to regulate +15V/-15V.
 

Offline danadak

  • Super Contributor
  • ***
  • Posts: 1875
  • Country: us
  • Reactor Operator SSN-583, Retired EE
Re: Guitar preamp op amp tl072 input noise
« Reply #40 on: November 05, 2017, 03:49:23 pm »
Of build a simple transformer based linear power supply.





Regards, Dana.
Love Cypress PSOC, ATTiny, Bit Slice, OpAmps, Oscilloscopes, and Analog Gurus like Pease, Miller, Widlar, Dobkin, obsessed with being an engineer
 

Online Zero999

  • Super Contributor
  • ***
  • Posts: 19345
  • Country: gb
  • 0999
Re: Guitar preamp op amp tl072 input noise
« Reply #41 on: November 05, 2017, 03:52:45 pm »
Of build a simple transformer based linear power supply.





Regards, Dana.
Is a regulated power supply really necessary here? The supply rejection of the TL072 is pretty good and should be adequate, as long as sufficient capacitance is added to the rectifier.
 

Offline danadak

  • Super Contributor
  • ***
  • Posts: 1875
  • Country: us
  • Reactor Operator SSN-583, Retired EE
Re: Guitar preamp op amp tl072 input noise
« Reply #42 on: November 05, 2017, 09:41:11 pm »
Its complicated -

1) TL072 not well characterized for PSRR, no curves vs freq.
2) PSRR is not symmetrical for an OpAmp, one rail to the other.
3) Human ear dynamic range >> PSRR min of 072.
4) No short circuit or thermal protection.

But then -

5) Rejection of regulator worse than TL072, but combined approaches ear response.
6) Regulators cost money.
7) Regulators introduce transient response issues.
8) Regulators add noise.

I give up, lets just go with the batteries that explode and burn and....


Regards, Dana.
« Last Edit: November 05, 2017, 10:07:50 pm by danadak »
Love Cypress PSOC, ATTiny, Bit Slice, OpAmps, Oscilloscopes, and Analog Gurus like Pease, Miller, Widlar, Dobkin, obsessed with being an engineer
 

Online Zero999

  • Super Contributor
  • ***
  • Posts: 19345
  • Country: gb
  • 0999
Re: Guitar preamp op amp tl072 input noise
« Reply #43 on: November 05, 2017, 10:44:15 pm »
Its complicated -

1) TL072 not well characterized for PSRR, no curves vs freq.
2) PSRR is not symmetrical for an OpAmp, one rail to the other.
3) Human ear dynamic range >> PSRR min of 072.
4) No short circuit or thermal protection.

But then -

5) Rejection of regulator worse than TL072, but combined approaches ear response.
6) Regulators cost money.
7) Regulators introduce transient response issues.
8) Regulators add noise.

I give up, lets just go with the batteries that explode and burn and....


Regards, Dana.
You raise some interesting points. Another alternative to regulator(s) is an emitter follower with a RC circuit. It drops some of the supply voltage but can work quite well. I've done it before for the TDA2003, which has a very poor supply rejection, of just 36dB and was a poor choice for a mains powered amplifier. It should be more stable and better at rejecting high frequencies, than a linear regulator.

 

Offline Audioguru

  • Super Contributor
  • ***
  • Posts: 1507
  • Country: ca
Re: Guitar preamp op amp tl072 input noise
« Reply #44 on: November 06, 2017, 02:39:47 am »
An inductive magnetic guitar pickup resonates with the capacitance of the shielded cable that connects it to the amplifier. The amplifier input impedance has always been about 1M ohms in vacuum tube days so that the pickup resonance flattens the frequency response, or a higher amplifier input impedance is used to give some high frequency boost. But in this thread the load for the pickup was as low as 10k or a 100k volume control that muffles high frequencies and reduces the output level.
 

Offline AleTopic starter

  • Contributor
  • Posts: 48
  • Country: it
Re: Guitar preamp op amp tl072 input noise
« Reply #45 on: November 06, 2017, 09:52:36 am »
Yes Dana, it was exactly what I was thinking about. Anyway, this evening I will try to build this and test with and without regulations, evenually also the emitter follower. Hero999, the schema you posted is for reduucing ripple, right? is it possible to use on a transformer which gives +15V/-15V or should I use only the positive and then the voltage divider?

thanks again
 

Online Zero999

  • Super Contributor
  • ***
  • Posts: 19345
  • Country: gb
  • 0999
Re: Guitar preamp op amp tl072 input noise
« Reply #46 on: November 06, 2017, 10:00:11 am »
Yes Dana, it was exactly what I was thinking about. Anyway, this evening I will try to build this and test with and without regulations, evenually also the emitter follower. Hero999, the schema you posted is for reduucing ripple, right? is it possible to use on a transformer which gives +15V/-15V or should I use only the positive and then the voltage divider?

thanks again
If will work fine for a single supply, as is. If you want to use it for a dual supply (+/-15V) then a use a PNP transistor with the capacitor's polarity reversed, for the negative supply.
 

Offline Damianos

  • Frequent Contributor
  • **
  • Posts: 268
  • Country: gr
Re: Guitar preamp op amp tl072 input noise
« Reply #47 on: November 06, 2017, 03:18:06 pm »
Thank you Damianos, I tested your schema and it works pretty well. But the last I made worked well too.
I rechecked the connections inside the guitar, cables, etc. With batteries I have a very silent preamp, but with a power supply it works really bad. Also with grounded input it produce very loud noise.

The thing is... I always tried with standard ones, also my music power supply start with a switching one (I think).
As I see for this kind of applications the best way is to have a toroidal power supply with multi isolated output. And I'm thinking to build one from scratch. Do you think it's a good idea? Which one to buy? I thought to build one with 4 1n4001 diode, some capacitors and lm7815 / lm7915 to regulate +15V/-15V.

For this circuit, it is not needed a regulated power supply, anything between 12 V and 30 V will do the job. In addition, it is not necessary a bipolar power supply, as we have already seen.

The problems are filtering and shielding.
Put the circuit inside a metallic box and connect all the common/ground referenced parts to one point.

In the past, when I played with sensitive audio circuits, like this one, I found a simple solution (I am a "fanatic" of simplicity):
- Separate power supply for the sensitive circuit.
- An external power supply has serious advantages, for safety and interference.
- Shielding of the amplifier. A small tin box of candies will make a big difference.
- Additional filtering of both lines of the power supply. Due to the low current consumption (less than 10 mA), this can be performed by using RC circuits.

In the example that shown in the picture, increasing the values of resistors and C1, filtering and isolation of interference is improved.
 

Offline Damianos

  • Frequent Contributor
  • **
  • Posts: 268
  • Country: gr
Re: Guitar preamp op amp tl072 input noise
« Reply #48 on: November 06, 2017, 03:29:24 pm »
You raise some interesting points. Another alternative to regulator(s) is an emitter follower with a RC circuit. It drops some of the supply voltage but can work quite well. I've done it before for the TDA2003, which has a very poor supply rejection, of just 36dB and was a poor choice for a mains powered amplifier. It should be more stable and better at rejecting high frequencies, than a linear regulator.


In this circuit it is needed a resistor from the base of the transistor to ground, because if the capacitor is charged to the peak voltage then it can not do something and the ripple will pass from the collector to emitter ...
 

Offline Damianos

  • Frequent Contributor
  • **
  • Posts: 268
  • Country: gr
Re: Guitar preamp op amp tl072 input noise
« Reply #49 on: November 06, 2017, 03:36:23 pm »
An inductive magnetic guitar pickup resonates with the capacitance of the shielded cable that connects it to the amplifier. The amplifier input impedance has always been about 1M ohms in vacuum tube days so that the pickup resonance flattens the frequency response, or a higher amplifier input impedance is used to give some high frequency boost. But in this thread the load for the pickup was as low as 10k or a 100k volume control that muffles high frequencies and reduces the output level.
Additionally to the loading resistance, I think that an adjustment of the capacitance is also needed, for the correct frequency response. I am not an expert on that but I have already mentioned it!
 


Share me

Digg  Facebook  SlashDot  Delicious  Technorati  Twitter  Google  Yahoo
Smf