[sdiy] MS20 VCA - how does it work?

[Nils Pipenbrinck]

Hi Again.

I have some problems with a MS20 VCA I've built. It does not make any sound
at all. It just passes some hum and noise if I touch one of the inputs with
my fingers, but a real signal won't make it through the transistors to the
buffer-input (well, it does, but it's so weak you can hardly hear it through
the noisefloor).

The problem is, that I don't understand the circuit at all. It's built
around two matched transistors in a configuration I haven't seen yet. Could
someone please explain me how this circuit works?

Schemantic is here:

http://www.synthdiy.com/files/1/200312/KLM-307.gif

It's on the upper right corner, built around Q3 and Q4.

Thanks,
  Nils