Here's a very basic 3 transistor opto compressor (optical compressor). There is a buffer to give high impedance input, and amplifier, a transistor as a switch and a voltage divider. Has about 1Meg input impedance (more on that in a video later) and output impedance of less than 10k. That makes it more or less useful for a guitar pedal etc. Just a little experiment I did. I think it sounds fine. Skip to near the end to see the final schematic.
There is bass roll-off going into the amplifier section (.0047uf capacitor) which suits electric guitar well. For general music compression, I would put a bigger capacitor in to allow less rolloff. Hope you enjoy.
Next I am going to make another compressor, this time just solid state where the transistor as a switch itself ducks the signal. I may put more controls on it like attack decay threshold and ratio, and maybe make-up gain. Not sure how it will sound but I am going to give it a try!
Simple transistor circuits require good filtering on power supply or battery, keep that in mind if you try building this.
I tend to build basic circuits with just 4 components, resistors, capacitors, diodes and transistors.
