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I'm working on a synthesizer and trying to keep the VCA simple. I'm also trying to avoid LM13700s, partly because I want this to run on one 9V batter.

So I'm trying to make this design work:

Two op-amp VCA

My CV is about 2V peak-to-peak, and I've tried it with different center points. The result is quite linear but the problem is that the output volume never goes to zero!

I'm not sure if this circuit is capable of taking volume down all the way to zero. It seems like Q1 would have to have infinite resistance, or act like an open circuit.

Any thoughts? Any mods that would help me get planned silence into this output?

-James

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  • \$\begingroup\$ You need to bias the source pin of the jfets to the negative voltage instead of the common ground. \$\endgroup\$ Mar 1 '16 at 1:53
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The JFET channel resistance is the limiting factor for (close to) zero output.

By ensuring that the JFET channel resistance is << Rin, you can get very close to zero output.

As it happens, I am using this base circuit in an ultrasonic pre-amp:

VGA circuit

It has this control voltage:

Gate control voltage

and this output response:

VGA output

I am using MMBFJ177 devices in the actual application and the circuit output goes down to a few 10s of microvolts.

The maximum gain you can get is a bit dependent on the pinch-off voltage.

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