Back in the 1980's we built a compressor that was quite simple and fast enough to catch the first half wave. It was similar to this:
https://www.electroschematics.com/audio-compressor-agc/
except that this is missing two important features.
1. Full wave rectification level detection. We used two NPN transistors, one common emitter and the other common base to pull down the integration capacitor. You can add a 3rd transistor so that the common base is just as sensitive and the other phase. This is the same as used to detect amp DC output in speaker protection circuits.
2. FET linearization gate signal. Half the audio signal is added to the gate control voltage so that the FET resistance remains symmetrical.
I used 2N5460s but a friend was very successful using unpowered CMOS gates. You can add pots to adjust the threshold etc., but we often just made it for instant attach and a fixed limit etc. It was similar to the classic Spectrosonic 610 compressor, but using an op-amp and P-channel FET instead of discrete transistor amplifier and an N-channel FET. The schematic for the 610 is probably on the net some place but I can't find it just now.
https://www.electroschematics.com/audio-compressor-agc/
except that this is missing two important features.
1. Full wave rectification level detection. We used two NPN transistors, one common emitter and the other common base to pull down the integration capacitor. You can add a 3rd transistor so that the common base is just as sensitive and the other phase. This is the same as used to detect amp DC output in speaker protection circuits.
2. FET linearization gate signal. Half the audio signal is added to the gate control voltage so that the FET resistance remains symmetrical.
I used 2N5460s but a friend was very successful using unpowered CMOS gates. You can add pots to adjust the threshold etc., but we often just made it for instant attach and a fixed limit etc. It was similar to the classic Spectrosonic 610 compressor, but using an op-amp and P-channel FET instead of discrete transistor amplifier and an N-channel FET. The schematic for the 610 is probably on the net some place but I can't find it just now.