very simple VCA circuit

Posted on Feb 5, 2014

Trimming is quite easy if you use matched NPNs for Q1 and Q2, and 1% tolerance resistors for R6 to R11. You will need a dual trace scope, a DVM and a sinewave generator. Using 10 turn trimmers is a little bit more expensive than using 1 turn but makes it easier to adjust with high accuracy. Adjust the trimmer A1 in order to have a perfectly

very simple VCA circuit
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symmetrical sine wave at the output (the best way is to superimpose the trace of the sinewave at input and the one at the output). You may have also to play slightly with A2 as well to reduce gain if you have saturation on both the positive and negative crests of the output sinewave. When used, A3 must be adjusted as follows : after completion of the previous settings, turn P1, P2 and P3 fully counter-clockwise (no signal, no CV) adjust A3 in order to have an output level (pin 7 -U1b) as close as possible to 0V. If necessary, repeat the settings from 2 to 7. If you find that a unit gain for maximum control voltage is not enough for you may increase the values of R10 and R11 (e. g. setting R10 & R11 to 47K will double the output gain). With the component values that are used, the VCA is perfectly linear for input signals that do not exceed 10Vpp. Above this level the VCA gets progressively overdriven and starts distorting. I choose not to troubleshoot this since overdriving a VCA can add some byte to its sound ! On some signals (saw, square) there may be some high frequency resonance on very steep edges, this may be eliminated by soldering a small 7pF to 10pF ceramic capacitor in parallel with R11.

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