[sdiy] Help with simple single supply VCA

[Tom Wiltshire]

I know that it's not really recommended, but I'd have thought you can get away with using the resistor as a simple voltage-to-current convertor for the VCA bias input. I've managed ok doing this for 9V single supply VCF applications (envelope wahs) and there's a "Shaped Fuzz" in Penfold's "More Advanced Electronic Music Projects" book (Babani BP174) that uses a single supply VCA like yours with an envelope follower. The envelope output is fed via a 18K resistor direct to the bias input.

Where the offset at the bias input was wrong, I've had success pulling it to a different voltage using a preset resistor across the rails, with a high value resistor (220K? 470K?) to the bias input. The current from this lets you drag the zero point to where you want. It's dirty, but it works.

HTH,
Tom


On 14 Oct 2011, at 13:52, Ben Barwise wrote:

> 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
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