[sdiy] high quality expo VCA

[Seb Francis]

>From what I can see, a logarithmic amplifier is basically the same component count as an exponential converter, so I would say it's really no more complicated to use a expo-response VCA chip to make a expo+lin response VCA.

I would imagine the quality (THD + CV feedthrough) of a VCA using something like an SSM2018T or THAT2180 is rather better than using an OTA.  And considering that these dedicated VCA chips do not cost much more than a CA3280, it rather seems to make sense to use them.

One thing to note about these VCA chips is that noise on the CV input can couple into the audio path - so use low noise opamps for the CV as well as audio signals - I imagine this is also a good idea with an OTA based VCA.

BTW, one of my many projects at the moment is building a expo/lin response VCA based on a THAT2180 .. eventually I will publish the circuit!

Seb

P.S. One should not assume that "high quality" is always good.  Sometimes distortion is a good thing, although I'm of the school of thought that if you want distortion you should be able to *choose* to add it when you need it.  CV feedthrough in a VCA is always (IMO) a bad thing!


Scott Bernardi wrote:

> 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