What is the purpose of R38 in the compressor part of the Rockman Ace ?
Is it to somehow linearize the FET by feeding the integrating cap voltage across its source and drain ?
Or somehow linearize it by feeding bit of guitar signal to noninverting pin of Opamp U4, thereby changing the FET gate voltage.
(https://i.postimg.cc/cJkY9tt0/Rockman-Ace.jpg) (https://postimg.cc/t1PY7TzM)
It looks like a bug. If you have the PCB traces it would be worth checking.
For linearization you would apply the drain voltage to R37. R37 and R36 provide the divide by 2 so you get Vds/2 applied to the gate. That's the normal method of linearization.
The bug: as drawn the R38 (10M) forms a low-pass filter with C27 (560n). The cap C27 pretty much shorts out the AC signal from the drain meaning the linearization is removed.
A possible fix: There should be a 10M resistor between R38 and C27. What happens now is C27 shorts the added 10M to ground. So the Vds voltage gets divided by 2 from the 10M + 10M divider formed from R38 and the added resistor. The opamp gain is adjusted to 2 which undoes the divide by 2 from the 10M+10M divider and the output of the opamp then has an AC component equal to Vds. When applied to R37 and R38 you get the correct linearization.
That fix needs an added resistor. An identical scheme is to lift R39 and move that the resistor between C27 and R38. C27 then discharges through R39 in series with R38 which means the time constant increases from C * (10M parallel 10M) to C*(10M + 10M) ie. four times longer from 2.8sec to 11.2sec. The problem with that is it looks too long.
So there's something wrong with the linearization. There is a way to fix it but how it is in the real unit I don't know.
Alternatively it's a stuff-up and that's how the unit was built.
Go here, and you can download some very clear shots of the top and bottom of the PCB.
PCB top and bottom pics (https://reverb.com/item/25264515-sr-d-rockman-guitar-ace-headphone-amp-w-new-dunlop-ecb005-power-supply-new-caps-free-shipping)
It seems to me that part of the circuit is different and different part values are used!
EDIT:
So here's a quick trace before I go to bed ;D
- R40, R38, R39, TP1, U4A are all removed
- D5 still connects to C27
- C27 is 100nF
- R35 is 560 ohm
Next is the main change, a common linearization scheme but DC coupled,
- R37 and R36 are connected differently.
- C27 connects to R37 (14MEG)
- other side of R37 goes to the gate
- R36 (12M) goes to the drain
- other side of R36 goes to the gate