I'm trying to build something to change the gain of a line- or instrument-level signal very steeply using a microcontroller, to use as a guitar tremolo pedal. I've been looking at digipots but can't shake off the zipper noise associated with those.
One circuit I looked at places a vactrol between the two stages of an op amp. But I don't want to use a component with RoHS problems. So could I theoretically substitute an optoFET?
I understand how a potentiometer works, but I don't understand how the vactrol in the schematic works. (I only have high school physics from 30-odd years ago). Is it working as a current limiter?
There are a few sub-questions there, but the main one is: what other ways (besides a potentiometer) should I be exploring to vary the volume of my guitar?