Incredibly simple VCA module based around the PC817 and a few resistors.
Features:
- CV attenuation knob
- AM capable
- Transistor saturation
- It's just a VCA, that's all
Not to be confused with a vactrol, the PC817 is an opto-isolator chip; a phototransistor and LED optically coupled inside a DIP chip. Unlike a vactrol, the response to CV is near instant, and it is capable of audio frequency AM. However, the phototransistor has its own caveats, namely the distortion, which adds a character to the audio input which some may find undesirable. The VCA does fully close though, meaning it goes fully silent without CV input.
For the X resistor, you'll have to try different values. For most chips, 1k works, but I had a few where 2.2k worked better. I'd recommend trying on a breadboard, or with alligator clips.
UPDATE: I also highly recommend adding these output amps, otherwise you might have issues sending the outputs into a mixer or preamp or whatever.
Any generic op amp like the TL082 or LM358 will work