VCO hacks, tweaks

[terry michaels]

Message text written by INTERNET: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

<

Hi Jorgen:

Dividing after the op-amp as in the ASM-1 VCO works too, this is probably
another case of several ways to accomplish the same goal, none of which are
truly perfect.  Putting the tempco resistor in the feedback path of the
op-amp gives a linear resistance change / voltage change transfer
characteristic, whereas putting the tempco resistor in as one leg of a
resistive voltage divider has a non-linear resistance change / voltage
change characteristic, although I think the error will be quite small
unless you are talking about a huge temperature range.

Terry