Supreme Compressor (Engineer's Thumb) Unwanted Distortion

[matopotato]

Any luck on your side, @Wook22?
I left your thread to keep things more clean. I breadboarded the whole thing from scratch apart from the ICs and a pot, but same problem.
And I had already swapped the LM13700 with no change, so I am a bit at a loss.
Either buy the same kit again and try out fresh, or get the Engineers thumb and add some of the mods, including the bright switch since I liked that option in the Thumb Sucker as a way of adapting to single coils vs humbuckers. But now I am not so sure what another Thumb Sucker kit would do good.
Might look around for other alternative compressor builds. Did an optical (Zirconia) which is just fine.

[Wook22]

Any luck on your side, @Wook22?

Not really!

I have fashioned an audio probe but that's not really shed any light as yet. I was also thinking of getting hold of a "normal" ET PCB and transferring parts / add new ones to see if perhaps there was an issue with the design of this one, but I have to say my enthusiasm is wavering a bit!

[matopotato]

Any luck on your side, @Wook22?

Not really!

I have fashioned an audio probe but that's not really shed any light as yet. I was also thinking of getting hold of a "normal" ET PCB and transferring parts / add new ones to see if perhaps there was an issue with the design of this one, but I have to say my enthusiasm is wavering a bit!

Yeah, I wish I was able to share some enthusiasm to get your hopes up. (Sometimes that is enough to break on through...) But I felt I reached som sort of thicker wall today.
How far did your probe give you clean sound?
I got to the Threshold/first IC and after that distorted.

[puretube]

see eh la bas ma`s latest thread ...
https://www.diystompboxes.com/smfforum/index.php?topic=129237.new#new

[Wook22]

see eh la bas ma`s latest thread ...
https://www.diystompboxes.com/smfforum/index.php?topic=129237.new#new

Thanks, I shall give that a try!

[Wook22]

How far did your probe give you clean sound?
I got to the Threshold/first IC and after that distorted.

Same for me.

Was interesting playing with the input level on my loop pedal to see what effect it had on the distortion.

[Wook22]

see eh la bas ma`s latest thread ...
https://www.diystompboxes.com/smfforum/index.php?topic=129237.new#new

Thanks, I shall give that a try!

Unfortunately it doesn't appear that either pin 8 or 9 are grounded on mine.

[matopotato]

see eh la bas ma`s latest thread ...
https://www.diystompboxes.com/smfforum/index.php?topic=129237.new#new

Thanks, I shall give that a try!

Unfortunately it doesn't appear that either pin 8 or 9 are grounded on mine.

Hmmm...  I was hoping for some magic here. Will try anyway since it is on breadboard anyway

[Wook22]

"Hoping for magic" is pretty much where I am at too!

[puretube]

see eh la bas ma`s latest thread ...
https://www.diystompboxes.com/smfforum/index.php?topic=129237.new#new

Thanks, I shall give that a try!

Unfortunately it doesn't appear that either pin 8 or 9 are grounded on mine.

oops, that`s a fourth different schematic - hadn`t seen that close-up yet ...
sorry it didn`t help - I thought I had found the culprit.

(but in that schemo, pins 7 & 10 (unused buffer inputs) are NOT grounded ...)

[Wook22]

oops, that`s a fourth different schematic - hadn`t seen that close-up yet ...
sorry it didn`t help - I thought I had found the culprit.

No worries. Thanks for the pointer anyway. Hopefully it will help on the other builds!

[matopotato]

see eh la bas ma`s latest thread ...
https://www.diystompboxes.com/smfforum/index.php?topic=129237.new#new

Thanks, I shall give that a try!

Unfortunately it doesn't appear that either pin 8 or 9 are grounded on mine.

oops, that`s a fourth different schematic - hadn`t seen that close-up yet ...
sorry it didn`t help - I thought I had found the culprit.

(but in that schemo, pins 7 & 10 (unused buffer inputs) are NOT grounded ...)

So in summary, OTA pin 8 and 9 are better off lifted away from ground. And pin7 and 10?
I also have 6 and 16 to ground. But I  assume at least 16 should go?
If lifting 6 7 8 9 10, should the be connected to eachother or left "in the air"?

[DrAlx]

As I understand it, the 1k resistor on the emitter of the PNP is what converts the control voltage from the rectifier stage into a control current for the OTA.
According to the datasheet for the OTA (if I understand it correctly) the peak transconductance can be from 6700 to 13000 (umho).    So practically a factor of 2.
So is it possible that the 1k resistor is too small a value for some OTAs but OK for others ?
i.e. it lets through too much control current for some OTAs but not others.

If i was going to play with things, I would add a 1k trimmer in series with that 1k resistor on the emitter of the PNP (so you can vary the total resistance from 1k to 2k) and see how things behave with increased resistance there.

[puretube]

pin 6 = "power ground" (V-): MUST ground.
pin 7 = "buffer input": ground.
pin 8 = "buffer output": not connected.
pin 9 = "buffer output": not connected.
pin10 = "buffer input": ground.

pins 12 & 13 connected together (shorted). Not going anywhere else.

pin14 (non-inverting input of unused half: should connect to Vref (4.5V). Not left open!!!
https://www.ti.com/lit/an/sboa204a/sboa204a.pdf?ts=1654522163497&ref_url=https%253A%252F%252Fwww.google.co.uk%252F

I`m not sure about pin 16 (Vbias of the unused half).
Since I don`t know whether there is an internal offset-voltage on this pin,
I`d rather ground it through a 27k resistor instead of directly. But that`s just a guess ...

[Wook22]

I'll see what happens if I ground 7 and 10, then have a play with the resistor value at R9.

Thanks both!

[matopotato]

As I understand it, the 1k resistor on the emitter of the PNP is what converts the control voltage from the rectifier stage into a control current for the OTA.
According to the datasheet for the OTA (if I understand it correctly) the peak transconductance can be from 6700 to 13000 (umho).    So practically a factor of 2.
So is it possible that the 1k resistor is too small a value for some OTAs but OK for others ?
i.e. it lets through too much control current for some OTAs but not others.

If i was going to play with things, I would add a 1k trimmer in series with that 1k resistor on the emitter of the PNP (so you can vary the total resistance from 1k to 2k) and see how things behave with increased resistance there.

Thanks, good idea. I was just thinking about try to measure the current. Trimming around will be similar but by ear.
Seems we are gravitating around the OTA and how to make it feel at home. Seems sensitive though or are we just the three unlucky ones among an ocean of happy compressor diy kit builders and owners perhaps...

[Wook22]

Conscious that there are now parallel threads on broadly the same topic, just thought I'd note that I have tried the suggestions listed above and so far the "magic bullet" evades me.

[matopotato]

Conscious that there are now parallel threads on broadly the same topic, just thought I'd note that I have tried the suggestions listed above and so far the "magic bullet" evades me.

Thanks for the update. I had some limited success, but hope for some magic in July. Next week travelling so no testing.