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

[David G Dixon]

> 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.