[sdiy] Discrete OTA

[Olivier Gillet]

> The scary thing is, I believe that, after all this time, I'm still the only
> one who has worked this into an actual product.  The Intellijel VCOs use
> this scheme, and I don't believe that any other commercially available VCOs
> do.  I don't know why, as the Dixie and Rubicon have the reputation of being
> some of the most accurate and stable VCOs around, and it is really so much
> more convenient than tempco resistors (with their convoluted layouts and
> goop) or heaters.

It's in a DIY kit too:
http://mutable-instruments.net/anushri

> By the way, I came to the conclusion, after much calculation and
> experimentation, that the optimum tempco voltage is 274 mV.

The detuning as a function of temperature is a parabola. There's a
trade-off between keeping this parabola as flat as possible ; and
having its minimum at what is going to be the typical operating
temperature of the circuit so we are not very far from the sweet spot
(even if things get worse faster as we move away from it). My
experiments brought me to 280mV.

Olivier