[sdiy] VCA circuits?

[tsheets at saturn5.com]

I am trying to debug the VCA on my AVR-X synth, but I just't can't quite
get it.  I imagine it is a pretty standard VCA circuit.  Does anyone know
of any resources that might explain this particular circuit?  Any good
descriptions of some basic VCAs out there?

The most basic thing I need to figure out is what the CV voltage range into
the first transistor (Q7) should be.  The DAC puts out 0-5V, but there is a 
trimming circuit before the input to the first transistor.  I am only seeing 
a few hundred millivolts of swing at the input of this transistor, and that 
does not seem right.

See page 5 here:

http://www.saturn5.com/tsheets/alex/avrx.pdf

-troy


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