[sdiy] Polymoogs

[Debby and Gene Stopp]

Hi List,

Over the years I've managed to aquire two sets of tone generator boards from
the PAIA Stringz n' Thingz, which have been sitting in a box (along with
lots of other synth curiosities like Prophet keyboards and Moog wheels and
Omni chorus generators and .....). I frequently amuse myself by daydreaming
about what kind of monstrosity I can create with these PCB's. Sometimes I
think about cascading them to make one six-octave string machine (by
grafting a pair of old Pratt-Read keyboards together); sometimes I think
about making a three-octave thingey with stacked tone generators to chorus
them together. The stacked idea usually wins - this kind of top-octave
organ-type tone generation system is a little thin-sounding. If I pick the
stacked option, I can use dual VCO's to drive the top octave generators, so
I think 4046's in VCO mode (don't use the phase comparators) running at 1 or
2 Mhz. Too bad they're linear - pitch bending will go out of tune for
anything except unison tuning. But independent vibrato modulation might be
interesting... Then I think heck - drive the PLL's with a pair of
audio-frequency exponential VCO's, filter the whole mess, run the audio
through a VCF controlled by a common-gate driven ADSR. Kind of like an Omni
on steroids. I've thought that an ASM-1 with these tone generator boards and
some PLL's could make an interesting little poly thing.

I do more thinking than building these days. It actually helps me drift off
to sleep at night, thinking about synthesizers that I never get around to
building...

Anyway last month I discovered a couple of busted Polymoogs for sale, cheap.
I couldn't resist. I've never really thought much about how they worked
inside - I've always known that they were top-octave generator based, with a
custom waveshaper chip for each key, formant filters and a VCF, but they've
always been somewhat of a mystery. I'm much more familiar with the
microprocessor/DAC/voice machines than oddball designs like the Polymoog.
There's some info out on the web, most noteably the patent part of Don
Tillman's web site (thanks Don! Very nice.) Suddenly Polymoogs are
fascinating.

So I pick one and start to open it up. Between analysis of the guts, and a
peek at the schematics, it turns out that this thing is really quite similar
to that PAIA-guts thing I've been daydreaming about all these years! Two
1.3khz exponential VCO's with 726 current sources, driving a pair of
high-frequency PLL's, driving two stacked top-octave-based divider networks.
The tones go into the "modulator" cards, which have the custom waveshaper
chips. Each key on the keyboard goes to one modulator card, and an RC
network on each key provides velocity sensitivity. The audio gets routed to
formant filters, and the VCF. There's 2 independent pitch LFO's for vibrato,
an LFO for PWM on each note, and another LFO for the VCF. All  parameters
are controlled by some kind of primitive hard-wired preset system. What a
monster. I've never seen so many CA3080's in one place before - I love it.

First problem - bad power supply. This supply has +5, +15, and -15 volt
outputs. Each voltage is provided by a pass-transistor 723-regulated mass of
parts. The Polymoog, it turns out, is infamous for its power supply
failures, so I opted to do away with the hassle. I stripped off all parts
from the PCB except the rectifiers and power caps, and turned it into a more
modern design (LM323, LM317, LM337). So far so good. Screw the remote sense
lines.

Hooked up the new supply, gave it a shot. Next problem - some smoke. Just a
little.

Sorry for the suspense but that's where I left it. With the power off, of
course. I know this is a little OT, but I was thinking of providing a kind
of synthesizer soap opera over the next few weeks (or months) as I make
progress. I hope this is interesting stuff for you guys. Think of it as an
alternative to Survivor....

- Gene