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

[Neil Johnson]

Hi Tom,

> 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!

A simpler approach is kind of what is done to add resonance to the
Buchla LPG, a mod devised by Mark Verbos I believe.  With an op-amp
and pot you vary the feedback signal.  Same effect.

Neil
-- 
http://www.njohnson.co.uk