A few links cropped up on AH today regarding the Korg MS20 filter:
MS20study.pdf (2.25M) by Tim Stinchcombe. Tim actually spent quite a bit of time studying the Korg MS10/MS20 filters and just made this 46 page pdf available on the list today.
RMC posted
a link to his contribution - pdfs and gifs of the schematic and trace. I noticed that RMC referred to the filter as the KORG35. I never realized this is what it was called, so I did a quick search on KORG35 and found
this site which has more info, links and a DIY clone with details and images.
My favorite Korg MS20 Clone? The
Frostwave Resonator (pictured). I picked it up years ago and will never get rid of it. It just shreds for lack of a better description. It's the aural equivalent of finding that sweet spot in your ear with a good q-tip. It will shred paint off your walls. Super aggressive with a bit of resonance. Highly, highly recommended.
I should also mention the
MOTM-420 and the Doepfer
A-106 Xtreme Filter. I'm sure there are others. If you know them, feel free to post them.
Update: Make sure to check out the comments for others. The following was sent to me via email from Tim:
"I compiled a list of known clones a while back, and this has been on it for some time. Mind you I only recently discovered that a discrete version of the Korg35 is to be seen in the Korg X-911 guitar synth (which I've not checked in any detail yet), and someone from synth DIY has just pointed out that it was probably originally developed for use in the 'PS' range of polysynths, so it looks like there is yet another avenue for me to tap into!"
Udpate via Noiseconformist in the comments:
"Hi, this is just to inform you that Tim got his new site up and running.
So the MS-20 document can be found here:
http://www.timstinchcombe.co.uk/synth/MS20_study.pdf
HTH, Michael (a.k.a
Noiseconformist)"