[sdiy] high quality expo VCA

[Seb Francis]

Grant Richter wrote:

> >
> > 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!
> >
>
> This is something the CA3280 excels at. CV rejection can be nulled to -80
> dB. Your mileage may vary.

If I'm reading the datasheet right, this is quite a lot better than the SSM2018T which has a CV feedthrough of about -60dB (for a 100mV RMS CV <1KHz).

I think I'm right in thinking that CV feedthrough is caused by change in output DC offset with gain.  On the SSM+THAT VCAs, output offset rises exponentially with gain in dB, so when running the VCA with higher gain, the CV feedthrough gets worse.

The THAT2180 datasheet has no actual CV feedthrough figure quoted (I guess because it varies with gain and CV frequency, so it's not very meaningful on its own).  However, the chart of output offset vs gain (even for the C grade) looks *much* better than the SSM2018T, so I would expect CV feedthrough to be a lot better.

So with some careful trimming, the CA3280 looks to be better than the SSM2018T (which requires no trimming).  But the overall winner .. the THAT2180 - the A and B grades have virtually no output offset even with gains up to about 10dB (and it requires no trimming).  No wonder this is the VCA of choice for many mixing consoles, compressors, etc..

Seb