VCO hacks, tweaks

[jorgen.bergfors at idg.se]

>> 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.

>Well, I just stumbled on that one. I started with a garden-variety FET
>op amp there, but discovered by putting a finger on it that there was
>quite a bit of temperature sensitivity. The better op amp helped quite a
>bit. Maybe an opamp optimized for offset drift would be even better. I
>like to keep the slew rate up here, though, in case I want to do some
>high-frequency FM.  

Isn't this a less-than-optimal design to start with? In the ASM-1 VCO, the 
divider is after the op-amp (if I remember correctly). So an error in the 
op-amp will be divided down too.  And the divider use a tempco resistor. Are 
there other drawbacks with the ASM-1 approach? 

/Jorgen