[sdiy] CV offset question.

[Roy J. Tellason]

On Sunday 20 November 2005 07:11 am, Dave Kendall wrote:
> 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.

Change the resistor connecting the wiper of the pot to the op amp to a higher 
value.  What's used there now?

> 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.

No need for regulators,  just put a resistor at each end of the pot,  if the 
first suggestion doesn't do it for you.  Scaling them is simple,  if you use 
two resistors equal in value to the pot,  your range will be a third of what 
it was.  But try the above first...

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