[sdiy] tempcos..

[René Schmitz]

Hi all,

harrybissell wrote:
> James Patchell wrote:
> 
> 
>>Somebody on this list has used negative tempo resistors...we need him to
>>speak up on this subject...:-)
>>
>>I'd hate to rat out RENE SCHMITZ but he's the one who should talk...   :^P

All right. :-P

I've looked at the datasheet and they seem to be suitable for the 
compensation circuits I have used so far. Especially those with the beta 
value of 4000, since that is what I had calculated with.
One instance can be found on my expo-tutor 
http://www.uni-bonn.de/~uzs159/expo_tutorial/index.html right at the 
end. For 3k or 2k all the values would need to be scaled along. I.e. 
divided by 4.7k/3k or 4.7k/2k respectively.

I figure this could be squeezed with some 3d art between the input 
summing opamp and the expo pair, much like the scaling option of the 
soundlabminisynth. In that case the feedback resistors of the summing 
stage would have to be replaced by a 100k resistor. (I would make part 
of that trimable.)

The other variant would be to add a 10k thermistor in series with the 
input summing resistors. But then you'd need a thermistor per input 
summing resistor. (Have a look at the VCO4069 for an application of this 
principle.) In that case one would change the feedback resistor (R60, 
R61) to a metal film 2.2k resistor (or include a trimmer as well.) 
THermal coupling could be somewhat problematic in this case.

This latter method is less precise than the first one, but I guess since 
the minisynth uses discrete transistors in its expo stage, the drift 
couldn't be totally cancelled anyway, so no great precision is required.
(And I would say that 6$ for a PTC would be a waste of money.)
You get closer to the ideal if the resistor that the NTC is in series 
with, is 11.2 times larger than the NTC, instead of 10 like it would be 
here.

Cheers,
  René

-- 
uzs159 at uni-bonn.de
http://www.uni-bonn.de/~uzs159