What is stable enough?

[Grant Richter]

>=D8	Heating up the transistor (like the uA726 does) is not really=20
>solving the problem.
>
>Why not ?? Every synth I owned or built with temperature regulated=20
>chips was very, very stable:
>SH-7 (ua726 + some really nasty design tricks), a heated CA3046 DIY=20
>project, the  Mono/Poly

Probably a good question is "what is stable enough?".
Using a Hewlet-Packard frequency counter and comparing designs:

At 1KHz the Serge NTO's (using heated 3046) and Buchla 258's
(using uA726) both exhibit more short term instability (like +/- 1-2 Hz)
than a plain vanilla PAIA experimenter board with a CEM3340.

I built up a design using a thermostated CA3096 to stabilize a PNP
pair for Bernies (Electronotes) classic convertor.

By hand toggling the heater circuit to on/off I can pull down a
high quality linear supply (<0.1 ohm output impedence) by ~20 mv
which in the circuit I'm using gives ~10 hz change.

Using a 0.1uF cap in the feedback loop (instead of 0.01uF) there's
still enough servo cycling to give a short term variation greater
than when I cut the heater circuit out altogether.

Lacking an envionmental chamber I can't test stability over a 0C - 70C
range, but it seems obvious that theres a trade off between short term
stability and long term thermal stability using the heater circuits.

Also note, that according to Electronotes, actual thermal terms are not
completely cancelled by the use of Q81 type resistors. Actual thermal
dependent terms run around 3300-3500ppm but the match between
the actual circuit dependence and the actual Q81 factor is a crap shoot.

To summarize:
No thermal compensation scheme that I've heard about yet is perfect.
The monolithic nature of the CEM3340's seem to offer an advantage
for pure tone generation which is useful for analog FM and other critical
functions. In contrast the heater circuits with their tiny cyclical instabilty
seem to mimic the natural instability of acoustic sources (i.e. the voice)
in pleasing manners. Both would seem to have merit.