[sdiy] CV offset question.

[harrybissell]

Jeez, Paul  (re: $4 opamp)

<mr rogers mode on>

"can you say P-A-i-A, kids ???"   :^P

Paul's suggestion of using the positive and negative regulators is a good
on, it will prevent changes in the overall supply from affecting the offset.
In the PAiA case, there are probably 1,001 other points that need to have
similar treatment.  The simple "resistor divider" suggested by Paul Perry
might be a more expedient solution.

Engineering is usually about trading cost/complexity for performance. Sad
to say, I usually side with Mr. Schreiber in the "damn the cost I want it to work
perfectly" camp.  PAiA might be called the "anthesis" of that philosophy.
For that reason, some PAiA users report good success and others are less satisfied.
I have never heard a post to the effect of "My MOTM is erratic in operation". You
pay for the difference.

I'll split the difference and suggest the voltage regualtors a'la Paul, and just use

any common opamp you stock for the rest.  You will have stable voltages at the
pot, and the couple of extra millivolts of offset you will remove 'by ear' as is
common
with all PAiA stuff.



Paul Schreiber wrote:

> Use 2 regulators a 7805 and a 7905 to reduce the 'swing' by a factor of 3. Then,
> use a LT1013CN8 from www.digikey.com. This is a special op amp that is
> *specifically made* for very accurate scaling/summing of DC voltages. OK, it's
> $4, what's your point? :)
>
> Lastly, you can't 'add in' the voltage off the pot wiper directly, you need to
> use an op amp summer that lets you use the ratio of 2 resistors to then take the
> +-3V swing to whatever you need.
>
> Paul S.