[sdiy] Temperature Compensated Exponential ConverterUsingSSM2164

[cheater cheater]

But what you get in exchange is the possibility to have multiple
expo's which drift in the same way. Which makes it perfect for poly
synths.

D.

On Wed, Sep 2, 2009 at 1:26 AM, David G. Dixon<dixon at interchange.ubc.ca> wrote:
>> I think you need to trim it since values varies and repeatability of a
>> design needs to be achieved. Also, how do you repeatably achieve
>> 0,2678345615807 V or whatever we crank out as the optimal voltage?
>
> Yes.  What I was arguing was that the adjustment could be confined to one
> trimmer, specifically, the one which gives the tempco voltage.  The
> appropriate feedback resistance could simply be calculated and applied to
> the nearest 0.1k.  I believe that I have demonstrated successfully that the
> trimmed values are more or less insensitive to the VCA gain temperature
> coefficient, and that the optimal feedback resistance at the expected value
> of the VCA gain coefficient will still serve admirably for tempco even if
> that gain coefficient is several percent off spec, which is highly unlikely
> for 2164s.
>
> Having said that, I also know that the effectiveness of the tempco relies on
> coming within about +/- 250 microvolts of the optimum tempco voltage.  Even
> being 1 mV away from the optimum puts the first octave about 0.25% out of
> tune even at the reference temperature.  This extreme sensitivity is
> probably the "Achilles' heel" of the design.  Drift in the supply voltage,
> and even temperature sensitivity of the resistor and trimmer, could become
> serious issues.
>
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