[sdiy] V-I converter OpAmp offset

[Scott Gravenhorst]

When I build a linear VCO, I adjust offset in the
circuit with a trimmer by the following method:

I set up the new VCO to play against a known accurate
one (ie, digital synth or preadjusted analog VCO)
I listen to both simultaneously.  Then:

1) play the highest note on keyboard, adjust master tune
   for unison.
2) play the lowest note on the keyboard, adjust offset trimmer
   for unison.
3) repeat 1 & 2 until both notes play in unison.

This works for VCOs, but now I want to build a linear VCA who's
performance will closely match the VCO's, i.e., I want the current
drive characteristic in both to be the same.  Obviously, I can't 
use the method I described above.  Is it enough to ground the 
VCA's CV input and then trim the OpAmp output to zero on a DVM?  

In both cases the linear V-I converter is the opamp with 
darlington pair in the feedback loop type.  

Is it even worthwhile putting an offset control in this circuit?

If so, would the offset control be better as a panel control
as opposed to a trimmer?

This will be used for FM experiments with linear VCOs.  I will
be running the modulating signal thru the VCA and then mixing
that with the VCO's CV (AC coupled).

I would be using a V-I converter like the one found at:
http://home1.gte.net/res0658s/FatMan/VCO_pwm_tri_suboctave.html

Thanks for pointers and/or reassurance.

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-- Scott Gravenhorst | LegoManiac / Lego Trains / RIS 1.5
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