|Repaired Filter is working!|
The filter design is a classic in the analog synth world, maybe not as famous as the Moog Ladder or Korg's MS20 incarnations, but it's definitely in thetop 40 anyway. Sound demo is here.
There have been a lot of commercially sold version of this filter: TipTop has one for Euro; Elby has a few incarnations for sale. (Elby took over CGS's site and sales....) The awesome and insanely productive Osamu Hoshuyama has one w/ variations on his site. It's an unusual design, having LP/BP/HP inputs (not outputs--inputs!) and an odd buzzy harsh sound.
Back in about 2005 I tied my own CGS build--one of my first attempts at audio DIwhY. But the filter soon broke--no audio--so I junked it and forgot about it. When I recently found it at the bottom of the junk box I asked around, no one in my Meetup synth nerd group wanted to take a crack at fixing it, so I decided to repair the damn thing myself.
My repair skills have improved somewhat I guess because it was really easy to find the issues. I scoped the BP audio all the way to the output, but there, right by the wire to the output jack, an important trace had lifted, probably during the initial build, and failed. There was enough of the trace left to solder a new wire in place. Now HP and BP work! I found another broken trace near the Low pass input. Easy repaired with a "kludge" or "bodge" wire:
|You fix PC's by rebooting. You fix audio with kludge wires.|
Happy to say it's working again. To my ears Ken's CGS design has the same nasal sound the Synthacon at the local Synthesizer Museum has. OK last thing to do is record it! Update! Demo done, soundcloud file is here. Have fun!