The Poly-9 Polyphonic Chorus Synthesiser
After I'd built the
I wanted to put together a polyphonic
machine. At the time I had no knowledge of microprocessor designs, and
not enough money for something like that either. So I put together
a design based on a String Ensamble from Practical Electronics,
cut out the string instrument filters
and added a 12dB/octave state variable filter with lowpass and
bandpass settings together with an ADSR envelope instead. The String
Ensemble was based on top-octave-divider organ technology which means
full polyphony but no real means of individual articulation of notes,
especially not with the common filter to all notes that I had.
To put it shortly, the machine failed miserably. In my eagerness to
get the machine together I didn't test the parts individually
(especially unwise since I'd changed the original design quite a bit) and once
it was all in the case nothing really worked properly. So, after a while
I tore most of the insides out, leaving only the keyboard circuits. If
you look carefully at the picture above, you can see that there are
quite a few empty holes in the front panel.
I re-built the chorus generator which then worked properly, and the
machine then stayed in pretty much the same state. This means:
...and that's about it! No filters. Whatsoever. Which means the machine
has a bit of a harsh (and largely unmodifyable) sound; nevertheless,
the fixed lowpass filters in the
constantly operating chorus generator tend to cut down on some of the
- Top-octave organ divider technology. An individual attack/release
circuit for each key gives the machine a bit of life.
- 4 octave keyboard, fixed split point after 1.5 octaves.
- Continuously variable mixture of 16", 8", 4" and 2" square waves
for the upper half, switchable mixture of the same square waves for the
lower half. Level controls to balance upper and lower keyboard halves.
- Variable speed and intensity vibrato (wow!).
- Bucket Brigade Delay Line-based (TDA 1022) chorus generator.
I've used the machine as a background string machine but that was a while
ago and lately it has never been used much. It never really was completed.
The insides of the machine are
as unfinished as the unpainted too-thin wood used for the cabinet. Mostly,
this machine is a reminder to me to test each part of a design before
going on with the next part. In fact, after several years of disuse, I
finally sold the machine to fellow DIY:er (who I don't think has done
much with it since either).
Poly-9 with covers off
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