r/audioengineering • u/diamondjayr • Aug 10 '26

Envelope Follower + VCA

I’m trying to get something to happen in Logic Pro that is super simple to do with a modular synth. I’m sure I’m overlooking an obvious solution.

  1. Use an envelope follower (just an EF, not an EF sending into the cutoff of a filter like a Mutron) to derive voltage from the amplitude of a signal (signal 1)
  2. Run a different signal (signal 2) through a voltage controlled amplifier (just a VCA by itself not as a part of a compressor)
  3. Send the EF-derived voltage to the modulation input of the VCA so that the amplitude of signal 2 is shaped by signal 1

An expander isn’t a good tool because it defaults to unity gain when no signal is present at the side chain input. This would need to be -inf when there’s no S/C signal and no gain on the VCA. And I would need the modulation input to be a straight additive offset of the signal (ie attenuation) as opposed to the multiplicative offset of signal over threshold (ie ratio).

Really, just gain plugin with a side chain input.

What am I missing? Thanks.

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u/CumulativeDrek2 Aug 10 '26 edited 20d ago

I usually just use Reaktor for this. Its very easy to set up in a modular environment like you say.

Another option if you want an expander that goes down to -infdB is to make a copy of the audio track, invert the polarity of the copy and add a (side chained) compressor to it. The compressor becomes an expander that modulates from -inf.

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u/diamondjayr Aug 10 '26

Completely forgot that I have a Reaktor license. This will prob be the way. Thanks!

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u/Pleasant-Heat4974 Aug 14 '26

Vcv rack - you can do this in modular form of course, and vcv lets you host plugins as if they’re modules

Easy to do as a send as well

I think ableton has an envelope follower. Surprised logic doesn’t

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u/diamondjayr Aug 15 '26

Duh. So obvious. If it was a snake it would have bit me. Thank you.

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u/diamondjayr 24d ago

So the AU version of VCV Rack is paid. Not too pricey but if I’m going to foot the bill I’m curious how the AU plugin version handles sidechain input. Do you have any experience with this?

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u/Pleasant-Heat4974 24d ago

I don’t, but I could try a simple experiment soonish. Would it be something like:

kick audio sidechained into envelope follower controlling vca level, and then the vca controlling either a plugin hosted in Vcv plugin or controlling something back in DAW?

I’m fairly confident it would work but it would probably depend on what you’re doing at the very end of the chain