VCO hacks, tweaks

[terry michaels]

-------------Forwarded Message-----------------

From:   terry michaels, 104065,2340
To:     INTERNET:WeAreAs1 at aol.com, INTERNET:WeAreAs1 at aol.com
        
Date:   11/2/98 10:57 PM

RE:     Re:  VCO hacks, tweaks

Message text written by INTERNET:WeAreAs1 at aol.com
>
In a message dated 11/2/98 7:45:17 PM, 104065.2340 at compuserve.com wrote:

<<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.>>

The National LF411 is also an excellent choice for such an upgrade.  Most
(but
not all) Minimoogs have a small PC board tucked in behind the Contour
Generator board that is referred to as the "octave buffer board".  It has
nothing more than three non-inverting/unity-gain 741's which are placed
into
the path of the current coming from the three octave selector switches. 
These
opamps help prevent the octave switches from being loaded down by the VCO's
CV
inputs. (hey, they hadn't yet developed cheap hi-impedance opamps back in
1971... the TLO81 was but a glimmer in some engineer's eye...)

Anyway, you can greatly improve the stability and tuning accuracy of any
Minimoog by simply replacing the three octave buffer 741's with LF411's. 
The
411's low offset really improves the operation and accuracy of the octave
switches.  If your Minimoog doesn't already have an octave buffer board in
it,
it's definitely a good thing to put one in, and very easy to do. (it's
detailed in the service manual)

Another excellent upgrade for Minimoogs (those with the older VCO's) is to
replace those horrible wirewound trimpots with modern multi-turn trimmers. 
I
usually replace the seven trimmers on the VCO board and the filter scale
and
offset trimmers.  After doing the mod, you'll have to remove the back panel
whenever you calibrate, but you wouldn't believe how much easier it is to
get
the sucker in *perfect* tune!  (BTW, this also helps on the "new" VCO
boards)

Michael Bacich
<


Hi Michael:

I knew about the 'octave buffer board'.  Changing op-amps here will help
some, but a few microvolts of offset voltage drift is not significant in a
1 volt/octave signal line.  It is significant when you're feeding the expo
transistor, as a few microvolts drift has 50 times greater effect, because
the control voltage has been reduced to 18 millivolts/octave.  The point I
was trying to get across is this is an area where offset drift makes a big
difference.

Changing out the trim pots is definately a good idea, I've done that also. 
I also found the "range" pots have a far greater range of adjustment than
needed, making adjustment very touchy. I put a larger value resistor in
series with the pot, and paralleled that with a fixed resistor to make up
the difference, the result is a much finer adjustment.

Terry