[sdiy] Discrete OTA

[Tom Wiltshire]

On 7 Apr 2014, at 19:58, Olivier Gillet <ol.gillet at gmail.com> wrote:

>> 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

Very interesting. So does changing the 'tempo' voltage change the parabola, then?

Also, how do you produce such a specific voltage and provide it to a low impedance input like the 2164's control input?

Thanks,
Tom