VCO's for homebuilt synths
mr808 at teleport.com
Fri Aug 11 02:40:48 CEST 1995
<a MOST excellent dissertation on VCOs deleted>
The CEM 3340 is a very poor choice - it's unavailable, and you're best
off keeping any you can dig up for spare parts to keep your polysynths alive.
One VCO design which no one has discussed is the Charge Pump Oscillator
from the Rhodes (Arp) Chroma. It is very simple and would appear to meet
all your requirements. As the Chroma SM put it, the Charge Pump design
is often found in instrumentation, but rarely in synths. The only catch
is, I don't know how available the charge pump chip (I can't remember the
number off the top off my head, maybe 4121 or 4151 - I'll have to look it
up). If it's used in instrumentation I would guess it's fairly
available. Apparently the charge pump is there to assure that the
sawtooth retraces w/o slew at high frequencies. The outputs are sawtooth
(w/ SYNC input!) which is also converted into a pulse. Waveshaping into
triangle should not be a problem.
If you're interested in experimenting with it, I'll try to dig up more
info on the charge pump chip...
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