This was one of my first filers. It’s based on the famous Korg MS 20 design. I believe it’s based on the late design by Korg. I’ve been looking into filter designs a lot lately and this one is really nice. A good description can be found on Kassutronics site: https://kassu2000.blogspot.com/2019/07/ks-20-filter.html
He based the filter on a design by René Schmitz. René did a lot of reverse engineering to find out how the filter in the Korg worked. Most of the MS20 filter design in eurorack are based of the schematics René posted. https://www.schmitzbits.de/ms20.html. There is a nice write up by Tim Stinchcombe about the korg filters. If your into some more background stories check out his website too https://www.timstinchcombe.co.uk/index.php?pge=korg
I enjoy this filter it can scream a lot. The only thing I didn’t like is the panel. I like to have had a black panel for this one but that doesn’t seem to be available. If anyone does know where to find a black panel for this please let me know. If you order this on Kassutronics site make sure to peel of the panel protector a lot of people have mailed him the text looks faded. The protector needs peeling off.
I do have a second one in my rack that is based of Kassutronics schematics and made by a good friend of mine. So there are a lot of MS 20 filters out there I think it might even be the most build filter in eurorack.
Difficulty to build: Medium, Good manual