Minimoog old style VCO

[Terry Michaels]

Message text written by Rotwang the Mad Inventor
>What function is improved by changing OP?
I know 741's are not exactly hi-tech, and they are quite noisy too (my old
mixing console was a british monstrosity built entirely with 741's...) but
does it affect pitch stability/tracking? 
I do not want my Mini to be too stable... :)

(Always neat with some "uncertainty" like those Buchlas... :)
 
        Happy new year!

        /Patrik the Mad Inventor (caught in a blizzard! aaarggh!)
<

Hi Patrick:

The control voltage adder opamp reduces the control voltage from 1
volt/octave to about 18 mv/octave, which then goes to the expo conversion
transistor.  Unfortunately, in the Minimoog VCO design, offset voltage
variations in this op amp go directly into the expo convertor with almost
no attenuation, so even microvolts of offset drift will affect pitch.  This
is a critical circuit for an opamp, in my opinion.  The OP07 has a much
lower initial offset voltage and offset drift than the 741 and most other
opamps, so I recommend its use here.

I haven't compared noise specifications, so I'm not sure if there would be
a difference.

The typical VCO design today has the control voltage adder at about a gain
of 1, with a resistive divider going to the expo transistor.  This
arrangement divides down the opamp offset voltage along with the control
voltage, so opamp drift is not as much of a concern.

Terry Michaels