VCOs (time to change the subject line (;->) )

[jh.]

> > In practice, of all the synths I've built, the JH-720 with its MS-20
style
> > thyristor core is the most temperature stable by far.
>
> Are you suggesting that the thyristor plays a part in the stability?
> That seems sort of improbable, since everybody seems to focus on the
> expo converter stability.

Ah yes, I should have mentioned that I'm strictly going V/Hz here (even
when the VCO contains an expo converter for fine tuning and vibrato).

Are linear VCOs inherently temperature stable ? Not necessarily - ask
CS-80 owners. The V/Hz method avoids just one temperature dependency
(and the most annoying one): the change of scale factor with temperature.
Trigger thresholds that depend on pn junctions, capacitors, trimpots - all
that
can still be a source of temperature drift.

> the series
> > resistor between the cascode FET and the composite SCR is for some
reason
> > important,
>
> In what way?  I omitted the cascode FET entirely since I was mainly
> trying to figure out how the SCR worked, and my circuit oscillated,
> but I didn't check the frequency tracking, which is the most obvious
> thing I could see the resistor affecting.

All right, you got me! I have not further investigated. I saw that the
cascode
and resistor made *some* difference on the breadboard (VCO got stuck
once at power on), so I decided to stay on the save side and include them
in the JH-720 as well. Maybe I should do some more research, 'cause in
the PolyModular I'll probably put the whole circuit upside down, with
all polarities changed and oscillating between -10V and -5V (this will
simplify the divider circuit big time) - and that would mean
expensive p-channel FETs for the cascode ;-(

JH.