[sdiy] high quality expo VCA

[Grant Richter]

> 
> 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.