[sdiy] VC Sallen-Key with a 2164

[David G Dixon]

> PS: the first stage in Olivier's circuit optionally switches 
> out the first capacitor, which is what the Oberheim XPander 
> did. This is the take weighted sums of the 4 output stages to 
> generate different responses, so just leave that cap in the 
> circuit like the other stages for what I propose.

There's a better way to do it.  We implemented it on the Polaris.