[sdiy] Digital pots as the gain element in a filter

[Neil Johnson]

Hi Tom,

> What kind of filter design is that? I've have guessed 12dB SVF, but it's
> such a tangle I couldn't work it out. And why does he have 3 Q controls? (I
> suppose the filter type will tell me that)

They're both Sallen-Key 2nd order.  Each filter has three resistors -
one sets the gain of the op-amp, the other two set the cutoff
frequency and Q:

http://en.wikipedia.org/wiki/Sallen_Key_filter

The bandpass mode is done by feeding the output of the LPF into the HPF.

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