AW: VCO-1C

[jhaible]

> Super simple 4 tranny unijunction VCO. May be the worlds
> smallest descret 1V/OCT VCO. Works a lot better than it looks.
> 
> http://www.mindspring.com/~vco/misc/pics/vco1c_sd.gif

I normally have mixed feelings about such minimal circuits: They are
tempting in their minimalistic elegance, but when you consider the
price of one single potentiometer of decent quality, there is not much
benefit from such low cost circuits anymore.

Normally, that is. But I wouldn't write this mail just to word this caveat.
Because I have found one reason to look at minimum parts count VCOs
meanwhile: That's for building a full polyphonic "ensemble" type keyboard
without divide-down circuitry, but with a lot of VCOs instead.

I have this old Roland VK-07 collecting dust, with a one contact per note
keyboard in almost perfect shape. I will try to sell the whole organ,
but if I don't get a good price for it, it's time to "modify" it a little.
That is,
keep the enclosure and replace the electronics.

What I have in mind is a VCO bank like the Korg VC-10 has it. A simple
linear, thyristor-based VCO per key, and a resistor string to apply LFO
modulation with slightly different strength to each VCO.

These VCOs don't have to cover a wide range (two octaves would be enough),
but they should be so stable that there is no need to re-tune individual
VCOs
at all. (Tuning the whole instrument is no problem with one common CV,
so some drift is ok as long as the whole VCO bank drifts in the same
way.)

I really don't know if I will actually start such a project, but I'd like
to
start collecting information about such instruments and their VCOs.
Any VC-10 owners: How stable is this thing ? (the internal VCOs)
Any Korg PE-1000 and PE-2000 owners: Do these instruments really have
individual VCOs, or are they divide-down units ? (I don't have schemos,
and what I heard about these instruments is contradictory.)
Any other instruments with individual VCOs ??

And last, not least: How temperature stable is your UJT VCO, Tom ?
(not asking about the expo converter, which is a low end compromise
no doubt - but the mere UJT / capacitor pair ?) And how does it perform
in comparison with a thyristor solution (like Korg) ?

JH.