[sdiy] MS 20 FILTER

[harrybissell]

Tim Stinchcombe wrote:

> Coincidentally I am in the process of analysing/'deconstructing' the
> MS10/20/Korg35 filter - there are a few bits I hope to clear up over the
> next few weeks, and then I shall write up my findings in a small paper. I
> also intend to build three filters: a (proper) transistor version of the
> MS10/20; one of these OTA-based 'clones' (which, to my mind, are not
> Sallen-Key filters); and (if I can do it!) an OTA-based version which *is*
> Sallen-Key, but which also has the Korg quirks, like the asymmetric
> resonance (which appears to be caused by the _signal_ impacting on the
> cut-off frequency). I also intend to make the diode limiting switchable, as
> I strongly suspect that much of the quality of the sound depends on this,
> rather than the filter topology per se.
>

I look forward to your results.  I'm inclined to think its a sallen key... but
who knows ?
I've played a lot with the diode limiting options in this filter (desperatly
trying to
come up with a formula I liked, to little avail).  You might find less options
in the
diode clipping than you might think.

imho, the diode limiting has to stop the overall volume level from completely
obliterating the normal pass band response.  That er... limits you :^P

Big difference between the MS-20 (sallen-key or not) and most other filters...
The MS-20 is an oscillator that acts like a filter, most others are filters that

act like oscillators.  The MS-20 will produce subharmonics of an input signal...

most others will just stop the input and oscillate at their natural frequency.
The MS-20 sounds more like a sync'd oscillator at medium to high resonance
settings.