Hi,
studying the evergreen https://www.electrosmash.com/mxr-dyna-comp-analysis
I was wondering if it was possible to add a simple envelope extractor that, when the input level drops below a selected threshold, forces the OTA to minimize noise.

In other words:
- when the input level starts to increase, the extractor/unipolar circuit provides less feedback to the OTA (less gain = compression);
- when the input level is low or absent, the feedback reaches its maximum (more gain = sustain), with some noise as well.
The key is to extract and define a threshold level below which the feedback is zeroed (or usefully attenuated). I'm not thinking of a noise gate but more precisely a downword expander.
I know this circuit has been around for 50 years...
Thanks all
m.p.

Is a total redesign. The sidechain comes from the OTA output. In silence the gain and output level are very small. It won't "open" until you SHOUT.

Expander sidechains are usually fed from the input, before gain reduction. Also they typically have a LOT of sidechain gain, and no gain in the main path. Here's one:

http://effectslayouts.blogspot.com/2016/07/one-knob-noise-gate.html

The SSM series (2165, 66, 67) does this.  Easier just to use one of those.