[sdiy] CV offset question.

[Dave Kendall]

Hi All.

Hope this is an easy one.
In getting the pitch wheel CV from my trusty PAIAmidi2CV to produce -5, 0,
+5 instead of the 0, +5, +10 that it does, I've tried a few of the simple
offset and scaling schemos around, but all have the same problem. The offset
adjustment is a pot with wiper connected via a resistor to an opamp input,
and the pot ends are connected to ±15V. This makes setting the desired
offset almost impossible, as just breathing on the pot seems to make it
drift. Using 22-turn trimmers doesn't help, as the range covered is still
too great, and there is drift with temperature changes.

First idea was to find a way of providing smaller ± voltages to the pot
ends. The smaller the voltages, the easier it should be to set the pot. As
one common setting would be ±2 semitones, the total range would only need to
be -.08V to +.08V, so the negative offset would only need to be -.08, so the
voltages sent to the pot ends could be very small.
Would using ± voltage regulators work?
I haven't found any regulators that produce such small voltages yet (but I'm
still a newbie really......)
Has anyone cracked this problem before?
Any ideas would be welcome.

cheers,

Dave