[sdiy] Interesting VCO features?

[Jeff Farr]

Cool, thanks for the input, I'm hoping this becomes a project other people
get interested in and I'll be posting all my data about it (including eagle
files and gerbers) once I feel confident about it's quality.

"BTW, is your clone going to be programmable like the original OB-1?"

Indeed. In fact, the programmability is the reason I chose the OB-1 as I
assumed that it would require alot less work than trying to fully CV another
synth. I plan on using one of TK's MIDIBoxes to be the brain, hopefully with
an onboard sequencer/arpeggiator with parameter morphing. So far things have
been going quite well, I've made alot of progress towards understanding how
this beast works and so far my initial assumptions seem to be correct, the
core analog section is separate enough from the digital circuits to be
transplanted without alot of headaches. My only concern at the moment is how
many different power rails are required: 18v, +-15v, 10v, 7v ... ouch! If
I'm lucky a bit of cleverness will cut that down, or at least keep the
different rails on different PCBs. Hopefully that, along with some star
grounding, will fix the obnoxious noise problems this synth has.

What really needs to be added to the OB1 mainly has to do with modulation
flexibility. A second LFO, better S&H along with more than one attenuated
input to each destination would open up alot of possibilities. After that
the off/-3db/on scheme has to go in favor of using some digital pots for
finer control of the levels for the VCOs and noise (noise in particular). It
has also occurred to me that there is no reason not to implement the SEM's
filter (it's actually the same circuit, minus the extra taps) and while in
2p mode that actually leaves us with another 2p filter to use if I can come
up with a separate control circuit that can be coupled together for 4p mode.
I'm still on the fence about the 3rd VCO, the sound has never felt thin to
me, certianly not in a way another osc could fix. The cross mod sounds are
interesting enough that it would be nice to have the extra osc around for
harmonics tho. Also, being able to sync VCO3 to VCO2 while leaving VCO1
independant might prove interesting. How about an arbitrary function
generator?
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