During this summer among other projects I have been simulating and slowly improving old Shure Level-Loc circuitry to figure out how I could try to make it more line/Eurorack level signal friendly. Its sidechain circuit modulates the input JFET to achieve the limiting, but I think this circuit could be used as CV signal for other stuff too. Signal gating might be most useful application in addition to just VCA modulation.
With “stock” values (as in rare component values changed to common values), the release speed is VERY slow. Input signal needs to be around >800 mVpp to properly activate the envelope follower. Below is a simple example with drum-like input signal.
C3 and R9 define the release speed. R7 and R8 define the attack speed, but increasing them too much will worsen the overall performance. Some other DIY desings have switch to change the cap C3 to other values, but I think using a 1M potentiometer for R9 is more useful and allows tuning the circuit to fit different applications better. It just needs 10k or higher series resistor to prevent shorting the power rails when turned all the way down :3 Also logarithmic pot will make adjusting fast envelope speeds easier.
Lowering R9 down to 100k seems to give a nice and reasonable 500ms release:
Lowering R9 down to 10k will result in super fast release while still barely reaching 12V.
Overall very nice small circuit, should be easy to build on stripboard too. Here is the clean schematic again without any graphs:
