I found the envelope detection part of an analog audio dynamic range compression circuit here, and I'm trying to figure out the purpose of:
- The second series diode
- The 100k feedback path from the 2nd op amp
My guess after poking around on the internet is that it might be something related to preventing the opamp going into an open loop situation in the negative waveform cycles, but I'm not entirely sure. The output is identical in LTSpice in response to 1k sine input with these components removed. The only text relating to this part of the circuit on the website states
To get this signal I simply needed to create an RMS voltage of the audio. This is a changing DC voltage that represents the AC amplitude of the audio. This is implemented with a specialized version of an AC to DC converter, which is done with rectifying diodes and a filtering capacitor. I used an active half-wave rectifier and an active lossy peak detector.
Circuit elements U2, D2, and D1 make the half-wave rectifier, which only passes positive voltages of the audio. The capacitor C3 holds the most positive voltage passed, but this voltage reduces as current drains through R17 slowly. The output voltage Vrmsp is the unfiltered RMS voltage that becomes the control signal.
The resistor R17 determines the rate at which the signal approaches zero following a peak. This resistor became a potentiometer to ground to act as the ‘release’ control, eliminating the need for a separate ‘release’ circuit element.