I'm not really sure how I went about doing this, or even why. But I decided that I needed a functional compressor with a minimal parts count So I did. Bipolar transistors are not happy being used as compressive elements it seems, I basically formed this section by banging on ltspice until I had a functional circuit come out. There were a lot of different iterations of "what if I did this, or what if I did that" and I tried them all, one after another to get what's there.

The design aesthetic I decided upon while designing this was minimum of parts to get it to function. All of the op amps are inverting topology because it meant that I could just direct connect a 4.5v or 0v line to each non inverting input, thus saving myself the extra resistor that would have come with a non inverting topology.

When I got the circuit really close to what I thought it should be I made the decision to import and run some drum loops though the ltspice simulation. What I found was that the release of the envelope wasn't nearly fast enough and that it hadn't fully relaxed itself between beats. I had to cut the envelope capacitor from a 10uf to a 1uf.

I was concerned about how much distortion would be on the output. My simulations with sine waves had shown that it should top out around 7 percent when running near the detection threshold, but real audio seemed to show that it needed more mellowing than I had thought, hence the decision to add more dry signal to the final blend.