An envelope follower/detector/extractor. It takes audio in and outputs an approximate volume evelope. This alternate design does amplificaton in the first stage and buffering in the last, which may be useful in some cases.
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 amplifies the incoming signal so that in the end, the envelope matches the actual signal peak voltage. The second set of opamps are used to make a precision full-bridge rectifier, which is fed into a lowpass filter. This is buffered with a final opamp configured as a buffer/voltage follower. Finally, the output is run through a current and voltage limiting resistor-diode network.
