Tyler,
Thanks for the serial. Yes, we did build that one with the expression mod.
In dealing with 8 bit control, there are only 256 discrete steps of brightness on the LED (controlling the LDR). We’ve done some processing magic to acquire a few more steps toward the bottom of that range…which, I believe is what is causing the frying or distorted sound, as the final step flickers on and off. I don’t remember experiencing this before, but that is what I think is happening. Theoretically this distortion is at the lowest possible volume level of the pedal (without being turned all the way off). So this artifact should be rather quiet, but if there is a lot of gain added to the signal after the volume pedal then this sound will get boosted more into the audible range. One thing you can do is to change gain structure of your pedal board/amp set up. Keep the gain on the pedal only as high as necessary. Also keep the gain as low as possible after the pedal or change the location of the pedal in your signal path. If you are going straight into your audio interface, be mindful of keeping the gain low on that signal path too (be very careful when using compressors). Doing this will also help with the long turn off time you are experiencing.
If that doesn’t help or is not practical to do, then you might be able to mod the circuit hardware to help smooth out this fast switching of the LED. If you add a very small capacitor between the LED pins (on the both optocouplers) then that should delay the turn on time so that this artifact is less audible. This cap should be very small. I can’t remember off the top of my head what frequency the PWM controlling the LEDs is, so I can’t say what value of cap might work…but you can experiment by trying a variety of capacitors in the range of 100pF to around 5nF. Maybe start with the larger values and work down to see if that noticeably changes anything.
I’m going to be out of town for the next week so I probably won’t be able to respond right away, if you have any questions.
Good luck.
-Brach