This module converts an audio input to a control voltage output with the control voltage corresponding to the amplitude of the input signal known as an envelope.

This module converts an audio input to a control voltage output with the control voltage corresponding to the amplitude of the input signal known as an envelope. Most envelope follower designs use a rectifier followed by a low-pass filter, this design uses a track and hold circuit to capture the envelope quickly and accurately. Filtering of the signal can be applied by using the “Smooth” control, this is especially helpful when used with harmonically complex signals like cymbals

http://www.bartonmusicalcircuits.com/envelope/