Last time, I showed demos where I simply clipped in different resistors to give me the amount of overdrive that I want. Now, I've started working out how to add a variable resistor (so, a potentiometer) to control the overdrive, yet do it in a way that the overdrive setting will get saved and recalled with the Polysix's patch memory (so, not a potentiometer).
I ended up re-using the approach of an Arduino-controlled digital potentiometer. I control the amount of overdrive by listening to the Polysix's "Attenuator" knob setting. Since I never use the lower half of the knob's values, I sense those settings and start turning up the overdrive. Works pretty well!
Here's a write-up on what I've assembed so far:
http://synthhacker.blogspot.com/2015/12/arduino-controlled-ota-overdrive.html
The next steps are to fix one little remaining issue and then to package it nicely for installation in the synth.
Chip