VC ADSR

[macdonald at evenfall.com]

After a lot of struggling I have come up with a circuit for a voltage
controlled linear ADSR.  I have breadboarded it only, no module yet. 
The voltage control is done with OTAs (LM13700), the rest is based
loosely around my previous ADSR.  I borrowed a bit from Juergen Haible's
AD circuit (thanks!).  It is not a perfect circuit by any means, I had
trouble getting the sustain level to go entirely to 0V (minimum seems to
be 5 to 10% of max envelope magnitude) and the transition from the decay
to the sustain is a bit rounded, but overall it works okay.  I would
really appreciate any comments or suggestions on improving this circuit.

http://www.evenfall.com/diy/vcadsr.gif

I plan to use this with an expo converted LM13700 VCA.

-Chris MacDonald