This is an envelop follower. Since I build an input module I kinda wished I had one of these so I made it a little project, fun for the whole family well, not really, but anyways..)
The idea of a voltage follower is rather straightforward. It consists of a rectifier circuit and you put the resulting waveform through a low pass filter to smoothen it out. You get a voltage on the other end which follows, more or less, the amplitude of the input waveform. The sole user control being the low-pass frequency. The main problem I experienced with this approach is the slow reaction time you typically get due to the low-pass filter.
A ton of different implementations can be found online, based on full wave rectifiers, half way rectifiers and a lot of other ideas. I ended up choosing a slightly different idea based on a peak detector, like what you’d typically find in a volume level meter. The output voltage will take on the peak voltage level of the input signal it will gradually drop according to the value of ‘R4 + RV1’ and ‘C1’ and is pushed back up whenever the input signal is higher then the output voltage. This approach makes for a fast reaction time, the higher the resistance of RV1, the longer the decay of the signal takes. This isn’t really ‘following’ the envelop of the audio, as a single short loud burst would generate a decaying signal rather then a sort of block-wave, but such ‘gated’ signals aren’t that often found in music, decaying sounds are more common. So this approach turned out better then I expected.
While a fast reaction time is rather fun and all in certain situations, other situations call for a more mellow approach. (you know, sometimes you do want that auto-wha feel) Therefor the signal goes through a variable low pass filter as well, which allows to smoothen the signal for a more classic envelop follower sound.
It took a bit of fiddling around with capacitor values to get a result I’m happy with. If you plan to build this I really encourage you to try different values for C1 and C2 and let me know which values you think are best here.
Since I had an extra unused opamp I also added an inverted output. Because, well, I could. It could, for example, be used to make a simple compressor. If you put the inverted signal, offset it by +5 volts or so and use that to drive a VCA you kinda get a simple compressor. As an afterthought I also added a voltage offset here using a simple trimmer.
I’ve added a trimmer for output scaling on the positive output as well because I still need to find out what kind of value would be best here for day to day use. I plan to keep it in my rack for a while before settling on a value. You can omit it and replace RV3 with a fixed resistor that works best for you.
Disclaimer: As always, expect some stupidity on my part. Errors are to be expected.