[sdiy] Sallen-Key [was: Re: Digital pots as the gain element in a filter]

[lanterma at ece.gatech.edu]

On Jul 5, 2011, at 10:09 AM, Tom Wiltshire wrote:

> I've used the Sallen-Key design loads of times, for anti-alising filters and such like, but it wouldn't have been my first choice for a synth filter since the cutoff and Q are so mixed up together. Presumably he untangles that in the code somewhere.

They're untangled if you keep the resistor values the same - i.e. if you make them your variable element, but raise or lower the effective resistance of both equally. In that case you can control the Q with the amount of feedback. The Buchla LPGs and the Korg MS-20 use this approach. Although, the MS-20 isn't strictly an original Sallen-Key, because there's an added buffer; I've been calling it a "Bach" variation, as in some dude I found cited in some paper, not J.S. or any of his many relatives.

- Aaron