[sdiy] Good precision pot choice for VCO CV panel tuning ??

[David G. Dixon]

> An added tip for linearity, any load on the wiper is going to throw it
> off.
> If the pot is used as a voltage divider, what I do is place a high input
> impedance buffer between the wiper of the pot and the input (typically a
> CV input). This effectively removes all loading from the wiper. Do this
> especially if you're using a multi-turn pot and turns-counting dial. The
> indication on the dial will correlate nicely.


No pot will be linear (no matter how expensive) if it's output isn't
buffered before hitting a finite impedance, such as the input resistor to a
summer.

As an example, let's assume you have a 100k pot wired between +10V and -10V
feeding into a CV summer through a 100k input resistor.  Here is the voltage
at the wiper vs. pot rotation, compared with the perfectly linear case:

Pot%	Real	Linear

100%  10.00	 10.00
 90%   7.34   8.00
 80%   5.17   6.00
 70%   3.31   4.00
 60%   1.61   2.00
 50%   0.00   0.00
 40%  -1.61  -2.00
 30%  -3.31  -4.00
 20%  -5.17  -6.00
 10%  -7.34  -8.00
  0% -10.00 -10.00

This is Kirchhoff's Law in action.  The buffer presents a near-infinite
impedance to the pot wiper, and even a pretty ordinary pot will give a
pretty linear response.