VCO hacks, tweaks

[terry michaels]

Hi Ian:

It looks like you did a very complete evaluation of my original VCO
circuit, and all of your upgrades make perfect sense.  The final result is
impressive.

I wonder if anyone has previously discussed upgrading the op-amp that sums
the control voltages going into the expo convertor.  This is an area that
can be improved by using a good op-amp.  Although the 1 volt/octave control
voltages are divided by a factor of 55 to generate an 18 mv/octave drive
for the expo transistor, op-amp offset voltage drift is not divided by 55,
and goes in to the transistor almost unattenuated, so a small offset drift
can cause large pitch changes,   I replaced the 741s in my Minimoog with
OP07A's for this reason.  This improvement can be applied to many other VCO
designs.

You didn't mention the type of timing cap you are using.  Is it a
polystyrene?  These have a small temperature coeficient, and might be the
source of the 200ppm/K drift you saw.

Centering the dVbe point is a good plan, as scale factor drift has no
effect at that point, but increases the farther you go from the center
point.

I'm glad you caught the 3300ppm/K vs. 3500ppm/K problem, I missed that one
completely.

My original design had an error that produces a high frequency, low level
oscillation in the expo convertor.  I assume you took care of this.  I
don't know if this has any effect on VCO drift, but it probably won't help
it, so it's a good idea to fix it.  If there is anyone on the list who
needs info on this, send me an e-mail.

Anyway, excellent  work!

Terry