[sdiy] Help with simple single supply VCA

[Ben Barwise]

Hello,

I have been trying to design a single supply VCA but I am having lots
of problems with the VCO (that shares the same circuit) signal
bleeding through. I believe the issue has been caused because OTA's
need the envelope to go all the way to -V to turn off? the problem is
I use op-amps to make the envelope and they don't swing that low. I
fixed it by putting an LED between the negative supply and GND to
raise the OTA a bit, and it worked but there is still a bit of signal
coming through... veery slightly , but enough to notice. The problem
is that I cannot see it on my scope (any idea how I could do this?
better probes? )  but I can hear it in an amp.

The circuit runs off 9V with a vref supplied by an op-amp.

http://img600.imageshack.us/img600/7051/vcasinglesupply.jpg

These are the things I have explored

*OTA not completely closing - I grounded the control pin .... still there.
*through the traces / power supply? I put .1u on the VCO's in the circuit.
*the LED is causing the issue? the voltage drop seems to move when I
put my multimeter on it
*can the signal bleed through if I overdrive the OTA's input? its a
bit loud at the moment.
*at one point it went away when I connected the output jacks gnd to
Vref - but that would short if I plug it into anything else!

Any shared wisdom on what I could look for and try would be much appreciated!

-Ben