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

[jh.]

> Me either.  A few years ago I asked a question here about what kind of
> temperature dependencies the Korg-style SCRs might have and I still
> haven't gotten or reasoned out the answer.  I'd start by guessing that
> any temperature dependencies must cancel out in the various devices,
> otherwise it wouldn't be a practical VCO

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. Switching it on
it is spot on tune to a few cents (compared to divide-down string
ensembles and organs at least). As you normally need 2 BJTs to emulate
a SCR, the 3rd BJT would be for temperature compensation of the
threshold.

Temperature stable as it is, the general "stability" may be inferior to
other
designs. It needs some "headroom" to start oscillating (That is, the fist
saw cycle after power on is larger then the normal ones), the series
resistor between the cascode FET and the composite SCR is for some reason
important, and using different BJTs makes a difference too.

I could spice it, I guess.

I did, and reporting PSpice resuluts with the usual grain of salt, I can say
the
reset times are really impressive. I don't have the numbers here, but from
memory: it was not as fast as Ian's latest saw VCO with 319 comparators,
but in a similar order of magnitude (and therfore better than many other
designs). Just think of it, in the JH-720 (and in the Mini/Maxi-Korgs) it
always runs in the highest octave (2'), with *no* means of HFT compensation.

Speaking of HFT compensation: I'm aware that it's not as crucial in linear
VCOs as in exponential ones. We don't have the Rbb error, but we still
have the reset time error, and (depending on the implementation) we also
have a latency time before a comparator or SCR or hysteretic switch
triggers.
Now how can we measure that latency time ??

JH.