[sdiy] Sallen Key with input mixing to generate all shapes

Andrew Simper andy at cytomic.com
Mon Dec 22 04:35:20 CET 2014

Hi David,

Yes you're quite right, there is no need to stick the same input
signal at each input mixer, you can filter any signal you want. I hope
I haven't lost you any commercial advantage by outlining the theory of
operation, but I can assure you I developed it independently. Have you
published any schematics or other documents on it yourself? I haven't
seen anything in all my searches.

All the best,


-- cytomic -- sound music software --

On 22 December 2014 at 08:52, David G Dixon <dixon at mail.ubc.ca> wrote:
>> This very concept is the basis behind the Intellijel
>> Korgasmatron II, which (with the expander) allows one to
>> derive different responses from the same filter
>> simultaneously.  The only difference is, in that case, the
>> topology is that of the MS-20 2-pole filter.
> I wanted to clarify what I wrote before, and add that one can get different
> responses from different inputs to the Korgasmatron II.  Hence, one audio
> signal can be lowpass filtered, a second audio signal can be bandpass
> filtered, and a third can be notch filtered, all at the same time, all by
> the same filter.  I don't know of any other commercially available filter
> which is capable of this (although most filters are capable of this, I
> believe, if set up properly).  This trick has some interesting uses.  For
> example, if one audio signal is lowpass filtered, and another is highpass
> filtered, then sweeping the cutoff makes one disappear and the other appear
> at the output.  If a third audio input is bandpass filtered at significant
> Q, then this acts almost like an interpolating scanner, bit with more
> interesting sound.

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