[sdiy] How does the JX3P VCA work?

Neil Johnson neil.johnson71 at gmail.com
Wed Jan 15 12:17:33 CET 2014

> http://urekarm.tripod.com/synth/jx_vca.pdf
> How exactly does it work?  I would have thought that increasing the CV would turn on both the pass transistor and the shunt transistor.  There's obviously something clever that I'm missing.

Two clues: one, the transistors need to be matched to some degree
("selected"), and two, that looks like a rather tiny translinear loop,
with the top transistor going CW and the bottom transistor going CCW
(or vice versa).  The passives and CVs then bias up the transistors to
operate at some movable point.

A quick LTSpice sim shows it does what it says it does, although to my
eye there appears to be some 2nd harmonic distortion.  Might sound


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