On the breadboard: Envelope follower

I’m breadboarding an envelope follower based on the ARP 2600 schematics. Funnily enough, what I have available to feed it notes for testing is… a Behringer 2600.

For instance:

The top trace (yellow) is the output of the B2600 AR envelope generator. It’s used to control the B2600 VCA, with a triangle wave input, and the result of that is the middle (blue) trace. That’s the signal I’m feeding into the breadboarded envelope follower, and the output of that is the bottom (magenta) trace.

Looks pretty good to me.

I can get it to look less good, though. If I cut the AR attack and decay times, so the envelope is essentially a rectangular box, the EF can’t quite keep up:

The EF’s low pass filter cutoff frequency is around 60 Hz, so the transient response time is something like 17 ms or so. And if the input signal is down around 60 Hz too, you can just start to see some ripple happening:

With the frequency down around 50 Hz the ripple becomes more obvious:

But that’s EFs for you; you can’t have your cake and eat it too. To make the response time faster, you’d have to increase the filter cutoff frequency, but that would make the ripple worse. Lowering it would improve the ripple but worsen the response time.

A couple more screen shots, this time with the ADSR envelope generator. With moderate time values (and high enough signal frequency) the EF reproduces the envelope quite well:

But again, with fast decay and release times, it can only do its best…

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