Vactrol VCA

I wanted to have some fun today and create something easy and quick. I’ve been interested in the Makenoise’s Optomix and I decided to build something similar especially the strike input.  I don’t know how they built theirs but this is how I built mine.


The circuit is very simple, you send a big impulse to C1 which charge as fast as it can (through the diode) and It discharges itself via the “damp” control (POT1) You can have something really quick and sharp or something very longggggggg (if the damp control is 1M for instance).

Obviously if  sequenced gates are used at the input, that sequence needs to be slow enough to let the cap to discharge.

On the shortest setting which sounds like a “click”, I think the output load of the comparator is too big for it (470R) hence a really tame sound.

It is in prototype form right now but strangely enough sounds as expected ! I don’t know if I need to do more really… However, to give it a bit more use I’ll add a standard input CV input and an input mixer. I may also transform the led driver buffer into some kind of V/I converter (top corner). I’m currently using some of my remaining vactrol VTL5C/3 but I guess any kind of home-made vactrol can be used with some tweaks.

I recorded some sound if you’re interested ! (please don’t mind the noise and the odd oscillation going on, it was on breadboard with a nasty digital circuits nearby). I’m triggering the sound with a button. I’m planning to add things and make a PCB for it. Seems to be a good alternative to the regular VCA to enter the more “buchla” side of things.

So yeah, I think it was worth it to get up this morning ! (not always the case I must say…)



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4 responses to “Vactrol VCA

  1. Pavel

    Hey man, very good work! Simple and effective circuit. I have a question: What is the purpose of C4 and why is the second op-amp(after the LDR) in inverting configuration?

    • hi,

      thanks for your message.

      it is called a VCA (voltage controlled amplifier) but it works as a voltage controlled attenuator.

      the second op amp act as an attenuator. Let’s say, the resistor is the LRD is 0 0hm (unlikely but we assume). Therefore the gain at the output is 1 because of the inverting configuration with 2 10K. If the resistor increases, the signal is attenuated (10K in the loop and higher resistor at the input).

      C4 is used to bypass some high frequencies. It actually can be removed.

      The result largely depends on the vactrol used. I won’t recommend it as a general purpose VCA. The response is very slow and at high speed it’ll be totally useless (always open)

      But you can certainly find a quicker vactrol.

      Thinking about it, the circuit can be modifed to do filtering and gain control.

  2. Jamie

    Thanks for posting this schematic – I made one yesterday and it’s terrific!

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