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

David G Dixon dixon at mail.ubc.ca
Mon Dec 22 07:07:53 CET 2014

Hi Andrew,

I haven't published any schematics, because I don't have a website.
However, anyone who buys a Korgo II PCB from me gets the schematic and the
theory behind it.


> -----Original Message-----
> From: Andrew Simper [mailto:andy at cytomic.com] 
> Sent: Sunday, December 21, 2014 7:35 PM
> To: David G Dixon
> Cc: synthdiy diy
> Subject: Re: [sdiy] Sallen Key with input mixing to generate 
> all shapes
> 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,
> Andy
> -- 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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