[sdiy] Polymoog Polycom chip block diagram?

Donald Tillman don at till.com
Fri Feb 18 08:57:45 CET 2022

On Feb 17, 2022, at 9:20 PM, Michael E Caloroso via Synth-diy <synth-diy at synth-diy.org> wrote:
> They dropped the ball on the two pole VCF on the PolyCom IC in that the VCF has zero resonance capability, zero modulation (no EG, no LFO, no key tracking, no velocity), and its cutoff is varied by fixed resistors per preset.  It is merely a "brightness" filter.


Well, they wanted to implement a Moog Ladder Filter for each key.  But that involves 4 floating caps, which would mean 8 pins on the IC.  That's a lot of pins.  And it leaves no room for other functions.

So they chose to implement a 2 stage Ladder Filter with 4 pins for the caps.

And yeah, that's a problem.  It's not a full Ladder Filter, and there's no resonance, which is where the Ladder Filter shines.

> My Polymoog is one of the later ones that are more reliable.  But as I acquired more analog polyphonics and other gear, the Polymoog saw less and less use.  I almost never use it anymore with all this cool MIDI stuff.  It still works but it is now the coffee table in my living room, as the music room in my new house has limited space.

Another Polymoog coffee table?!?!  What's with this turning Polymoogs into coffee tables?

  -- Don
Donald Tillman, Palo Alto, California

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