inductor coils as tempcos

[Martin Czech]

Well, I did a spice example:

68k upper leg, 700 Ohm lower leg in series with 63mH, opamp ramp from 0
to +10V in a 1.0us.  1pF overall coil capacitance, 10p winding to winding.

OOuups: The expo voltage should step to only 100mV, but here we see an
overshot up to 4V! This is 4V out of an 700 Ohm Source into the tranny
base, where the emitter is more or less losely coupled to the usual expo
opamp via resistor...

I have underestimated the effect, this IS undeserable. It may not damage
the tranny, but how will it sound? A glitch into the integration cap?

It get's a lot better if the opamp is bandwidth limited to 20kHz (which is 
maybe a good idea anyway), but the overshot is still 400%.

So what sounded a nice solution (not too much labour and cheap) in the
beginning may turn out as undesireable sound artefact. I really don't
know if the current overshot will be audible in a vco, a step response
will always make some noise. It is still better then slewing...

In this repect the proposed relay solution might suffer even more...

Perhaps we should order tempcos @ Paul Schreiber. Bah, ... that
would be too easy then ... no sport ;->

m.c.