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

[Tom Wiltshire]

On 5 Jul 2011, at 12:09, Neil Johnson wrote:

> 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

Thanks Neil.
It's two filters, not one. No wonder I didn't get it!

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.

T.