Supreme Compressor (Engineer's Thumb) Unwanted Distortion

[merlinb]

Could it be measurement error because of multimeter impedance?

Yes, your meter pulls the voltage down, it's normal.

[matopotato]

Reporting back @Wook22: I  tried with the other LM13700 and expectedly (?) and sadly it was not the culprit. So no change on my Thumb Sucker, and just great in his Engineer's Thumb. He has a TL074 and me 2xTL072, but just can't see how this would matter. So something else is wrong in my case.
Will try the A1M pot. But I am at a loss nearing the end if my efforts. Last is to go through each component...
Or scrap for parts. Not sure if I should get another kit or just get something off the shelf.

[matopotato]

Any progress on your side, @Wook22?

[matopotato]

A lot of fuzz-like distortion is being produced when the input signal is higher, i.e. when a string is played hard.

Based on that description, my first thought is rectifier distortion. This would be possible if you had diodes D4/5 or electrolytic caps C4/5 reversed or otherwise installed incorrectly, or if the resistors on those components aren't right. That type of problem wouldn't show up in your stable voltage readings, but would be immediately obvious if you scoped or audio-probed the C5/opamp junction and saw/heard half-wave distortion.

Thanks for this @Keppy. I think you set me on to some new discoveries.
I did probe around with a 1-ch scope. It shows a clearly cut top peak when turning the Ratio to the area where the audio gets distorted in my case.
I noticed that only the top of the wave was flattened, and the bottom didn't get flattened until more or less diming the Ratio pot. (Not 100% sure the scope wave was "levelled", so the signal might have been floating a bit above "zero" which might be a reason if the top got flattened first. But I could be totally wrong as well. Of course.)
But I didn't think much of it as far as troubleshooting goes until I went back and re-read your comment.
So if one of them diodes is clipping too soon, or in a faulty/bad fashion, that could count for the flat top of the signal. At least made sense to me. In my head.
So I went over and the 4148's check out and are connected as should be along with the surrounding resistors.
But I realized I have gotten two 1uF MKT Film caps, but the schematics call for polarized ones. I trusted the kit assembler, but maybe I shouldn't have.
So I plan to replace them with electrolyte caps.
I have 1uF/50V and 1uF/100V. Does the max voltage matter? The sizes are similar so no issue there.

The build I have has a 100k + B1M pot for Release, while original and some other versions have 470k fixed for Release. I guess this does not matter (that my total R is 100k to 1100k), although a B500k might have been enough in my case.

I understand you are focusing on the section where the 4148's are located.
My own first clipping suspicion went to the red LEDs I have in my build close to the input. Like some sort of hard clipping, but very early on.
Anyway, I measured them and they do show the same value (less than expected from red LEDs, so I assume that is part of being in the circuit context), but one of them keeps fluctuating a lot. From the 0.728mV-ish to OverLoad, while the other is constant 0.728mV. So I am suspecting the dodgy one not being sound and plan to change it.
In my limited understanding it is a diode, doing clipping, so why not potentially part of my distortion problem?

So I will go ahead and try changing the dodgy red LED and the two 1uF (C5, C6 ?).

EDIT:
PS. I noticed that the transistor does not seem to do much. We even pulled it out during testing and there was no change imho to the sound, flaws and all...

[ElectricDruid]

So I went over and the 4148's check out and are connected as should be along with the surrounding resistors.
But I realized I have gotten two 1uF MKT Film caps, but the schematics call for polarized ones. I trusted the kit assembler, but maybe I shouldn't have.
So I plan to replace them with electrolyte caps.

Don't bother. You can use a non-polarised cap anywhere a polarised cap would do. The only reason polarised caps exist is that they tend to be cheaper/smaller for large uF values. The kit assembler knew this and sent equivalent parts.
Good spot, but this is not the problem.

The build I have has a 100k + B1M pot for Release, while original and some other versions have 470k fixed for Release. I guess this does not matter (that my total R is 100k to 1100k), although a B500k might have been enough in my case.

Just leave the pot a little bit less than centered and it's all the same!

I understand you are focusing on the section where the 4148's are located.
My own first clipping suspicion went to the red LEDs I have in my build close to the input. Like some sort of hard clipping, but very early on.
Anyway, I measured them and they do show the same value (less than expected from red LEDs, so I assume that is part of being in the circuit context), but one of them keeps fluctuating a lot. From the 0.728mV-ish to OverLoad, while the other is constant 0.728mV. So I am suspecting the dodgy one not being sound and plan to change it.
In my limited understanding it is a diode, doing clipping, so why not potentially part of my distortion problem?

The red LEDs will provide clipping *but only if the input rises above their forward voltage*. For typical red LEDs that might be 1.5V to 2V or so. So while your guitar signal stays under +/-1.5V, the LEDs won't do anything at all. They're there to provide a hard limit to the size of the signal that the rest of the pedal can see, because distortion from the rest of the circuit apparently sounds worse than LED clipping!
If one of them was somehow faulty or had a much lower forward voltage, then it's possible that you could get distortion. EXCEPT that in that situation, the distortion wouldn't be affected by the controls, since it would be happening before any of the controls have a chance to take effect.
So, again, good idea, but this is not it.

PS. I noticed that the transistor does not seem to do much. We even pulled it out during testing and there was no change imho to the sound, flaws and all...

That transistor is an essential part of the current source that drives the OTA, so if it's same with it out or in, something is wrong here. That fits with what Keppy thought - that the distortion is coming from the sidechain. Check the transistor and check the pinout. This might be the problem.

[matopotato]

So I went over and the 4148's check out and are connected as should be along with the surrounding resistors.
But I realized I have gotten two 1uF MKT Film caps, but the schematics call for polarized ones. I trusted the kit assembler, but maybe I shouldn't have.
So I plan to replace them with electrolyte caps.

Don't bother. You can use a non-polarised cap anywhere a polarised cap would do. The only reason polarised caps exist is that they tend to be cheaper/smaller for large uF values. The kit assembler knew this and send equivalent parts.
Good spot, but this is not the problem.

The build I have has a 100k + B1M pot for Release, while original and some other versions have 470k fixed for Release. I guess this does not matter (that my total R is 100k to 1100k), although a B500k might have been enough in my case.

Just leave the pot a little bit less than centered and it's all the same!

I understand you are focusing on the section where the 4148's are located.
My own first clipping suspicion went to the red LEDs I have in my build close to the input. Like some sort of hard clipping, but very early on.
Anyway, I measured them and they do show the same value (less than expected from red LEDs, so I assume that is part of being in the circuit context), but one of them keeps fluctuating a lot. From the 0.728mV-ish to OverLoad, while the other is constant 0.728mV. So I am suspecting the dodgy one not being sound and plan to change it.
In my limited understanding it is a diode, doing clipping, so why not potentially part of my distortion problem?

The red LEDs will provide clipping *but only if the input rises above their forward voltage*. For typical Red LEDs that might be 1.5V to 2V or so. So while you guitar signal stays under +/-1.5V, the LEDs won't do anything at all. They're there to provide a hard limit to the size of the signal that the rest of the pedal can see, because distortion from the rest of the circuit apparently sounds worse than LED clipping!
If one of them was somehow faulty or had a much lower forward voltage, then it's possible that you could get distortion. EXCEPT that in that situation, the distortion wouldn't be affected by the controls, since it would be happening before any of the controls have a chance to take effect.
So, again, good idea, but this is not it.

PS. I noticed that the transistor does not seem to do much. We even pulled it out during testing and there was no change imho to the sound, flaws and all...

That transistor is an essential part of the current source that drives the OTA, so if it's same with it out or in, something is wrong here. That fits with what Keppy thought - that the distortion is coming from the sidechain. Check the transistor and check the pinout. This might be the problem.

Many thanks,
A touch disheartened that my ideas probably have no bearing. Will probe with scope to try and understand where the peak gets cut down.

Sidechain: is that among the 4148s? Thought I checked those Ds Rs and Cs in that area between OPAs.
The transistor I will measure to see what it shows and if those values make sense.
Again thanks.
Might swap the LED anyway just to have a functioning one.

[ElectricDruid]

A touch disheartened that my ideas probably have no bearing. Will probe with scope to try and understand where the peak gets cut down.

Don't get disheartened. I think you might be getting close. It only takes one part to have some little problem and the thing doesn't work right. But at the same time, find one little thing, and it might all be fine.

Sidechain: is that among the 4148s? Thought I checked those Ds Rs and Cs in that area between OPAs.
The transistor I will measure to see what it shows and if those values make sense.
Again thanks.
Might swap the LED anyway just to have a functioning one.

Yes, the sidechain is the whole section U3A to U3B to Q1. It measures the volume level of the incoming signal and produces a proportional current which is then fed to the OTA as VBIAS. Keppy initially suspected the rectifier (U3A and those diodes) because problems there might well turn up as distortion in the audio. But the problem could be a bit later in that sidechain section, like in the current source (U3B and Q1). So definitely check that transistor.

[Keppy]

PS. I noticed that the transistor does not seem to do much. We even pulled it out during testing and there was no change imho to the sound, flaws and all...

The transistor is in parallel with a 1M resistor. If the transistor makes no difference, then that indicates that it is never turning on, and the OTA is unchangingly biased by the 1M resistor. The voltages you posted are wrong for a PNP transistor (base should be a few tenths below emitter, not above). I initially figured that it was because the voltages were taken without a signal to lower the base voltage, but now I'm not so sure.

Is that really a PNP transistor? If so, have you tried another?

What happens at the base of the transistor if you apply a signal at the input? You should see a drop in voltage at the base.

[matopotato]

Thanks @ElectricDruid and @Keppy.
Sorry for the confusion, it is my bad. I should open a separate thread for my issues.
This is @Wook22's thread and I had similar problems. But trying to help has glided over to hogging the thread. So I was going to lay low until I find something.
So I should post my readings but in a separate thread.

[merlinb]

OP seems to have disappeared, and it was just a wrong resistor value.

[Wook22]

Have been away for a few days.

Will have a crack at checking that resistor and see where it gets me!

Thanks for all the help so far.

[Wook22]

Alas, just checked R6 and it is indeed a 220R.

Was very hopeful it might be a simple fix!

[merlinb]

Alas, just checked R6 and it is indeed a 220R.

Not in your photo it wasn't?

[Wook22]

I wonder whether we are talking about the same resistor?

I have removed R6 from the board and it's definitely 220R according to my multimeter - I find differentiating the reds and browns quite tricky by eye so I tend to go for this method.

[matopotato]

From first picture it looks as if R2 has a cable to one of the eylets instead of the resistor. The other eyelet is empty. This is most likely covered somehow or there wouldn't be much of anything in the first place. Just curious.
Edit: Ignore, just read the early post. All is well.

[merlinb]

I have removed R6 from the board and it's definitely 220R according to my multimeter - I find differentiating the reds and browns quite tricky by eye so I tend to go for this method.

Hmm...colors sure do look like 10k to me

[Wook22]

I have removed R6 from the board and it's definitely 220R according to my multimeter - I find differentiating the reds and browns quite tricky by eye so I tend to go for this method.

Hmm...colors sure do look like 10k to me

You definitely have me doubting myself!

Just to clarify, we are talking about the resistor circled below?



If so, definitely 220R

[merlinb]

Just to clarify, we are talking about the resistor circled below?

Yes. Which makes me wonder if any of your other '10k' resistors are actually 220R... I suggest you check them

[Wook22]

I have re-checked every resistor value and they are all to spec.

Also checked that everything that should lead to ground according to the schematic does so, and no issues noted.

[matopotato]

I decided to try to reduce the level of confusion, so I took my problem to a separate thread: https://www.diystompboxes.com/smfforum/index.php?topic=129225.0
Thanks all who helped me so far. Still some ways to go though.
Will follow this closely, @Wook22 and hope you eventually find out what is wrong. The pedal is promising IMHO.