An envelope follower/detector/extractor. It takes audio in and outputs an approximate volume evelope.
The pot in the upper right adjusts the responsiveness. This gives a tradeoff between ripple amplitude and how responsive it is to volume changes. The lower left pot is just so you can make the test signal louder or quieter.
The first opamp just acts as a buffer/voltage follower. The second set of opamps are used to make a precision full-bridge rectifier, which is fed into a lowpass filter. The final opamp amplifies the envelope to match up with the actual waveform peak, and also serves to buffer the output. Finally, the output is run through a current and voltage limiting resistor-diode network.
