[sdiy] high quality expo VCA

[Scott Bernardi]

Doing a linear and exponential response VCA is pretty easy for an OTA based VCA - don't know whether you'd call that "high quality" though. Here's an example at http://home.attbi.com/~sbernardi/elec/lockbox/lockbox_vca1.gif.
Basically it's a standard exponential converter, and the linear input is the reference current.  R47 could be driven directly  (positive voltages only). R47 is the exponential input.
You could use an LM13700 or CA3280 with linearization diodes rather than a CA3080 for improved S/N.

Seb Francis wrote:

> Fernando de Izuzquiza wrote:
>
> > But still looking for a good VCA with expo/lin switchable response...
>
> > Has anyone designed an expo circuit with the SSM2164? We have Mike Irwin LIN design, and the chip is
> > originally expo so... it would be nice to switch between the two responses.
>
> You could switch in and out a logarithmic amplifier between the CV and the VCA, I think that should allow expo/lin response with a SSM2018/2164/etc.
>
> I've got a circuit in a book which uses a matched NPN-pair, 2 opamps, 4 resistors, a cap and a diode to do this.  I can describe the circuit in more detail if you want.  I believe you can also get single chips which do this.
>
> Seb