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

[Tom May]

jhaible at t-online.de (jh.) writes:

> > 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.

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.

> 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,

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.

> and using different BJTs makes a difference too.

fTom.