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

[Tom Wiltshire]

On 6 Jul 2011, at 01:58, <lanterma at ece.gatech.edu> <lanterma at ece.gatech.edu> wrote:

> 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

I've been reading around this, and Rod Elliot does it again!

http://sound.westhost.com/articles/active-filters.htm#s3

The best explanation of Sallen-Key filters by far, and he explains the trick to getting constant unity gain with varying Q; take the output from the feedback tap point (the -ve op-amp input) via a buffer. The feedback voltage divider counteracts the gain and gives you the same signal level you put in. Nice!

T.