[sdiy] CV questions..hardwiring

[harrybissell at wowway.com]

If your CV inputs are true summing amps (inverting opamp summer) then there
will be NO interaction between them. Current flowing into a true summing amp
goes to (virtual) ground) and nothing comes back out.

The driver will see the parallel impedance (resistance) of all the loads it
drives.

If you have a 'pseudo-summing' junction... like a 100K / 100 ohm divider
(common with a lit of OTA inputs BTW)... there will be some interaction.
If its a mod source yoy might get away with it... for a pitch CV I bet you
will not.

You can add non-inverting buffers in front of all the resistors in a
pseudo-summing junction and then there will be no interaction... the very low
output inpedance holrs all the resistors at a 'near virtual' ground when no
signal is applied.

This method is useful if you are retrofitting a circuit that used passive
summing. I did this with an EFM VCO3 ... in the Moog design they had opamps
driving all the inputs... but the clone just had the passive summer. A small
but fatal CV change occurs when you switch in another CV.  In a modular you
might just re-tune... but that did not work for me.  Making a new
'non-inverting' summer would have taken way more parts than the four buffers
(one quad opamp) that I needed.

Bug... or ~feature~ ???   You decide...

H^) harry




n Wed, 22 Oct 2008 15:57:36 +0000, Dan Snazelle wrote
> oh and i forgot to mention...when i did it on the breadboard 
> (connected one CV signal to a bunch of different places, i think i 
> might have used a diode on the INPUT end to keep the other CV 
> signals that were tied to that input from going back up the wire and 
> into the other sections trying to get that same signal....
> 
> sounds confusing.
> 
> lets say i had a lfo out going to 3 places.
> 
> VCLFO OUT---------------------------diode----------vco cv in
>                         -
>                          -------------------diode-----------vclfo2 cv in
>                          -
>                           --------------------diode----------glide in
> 
> are the diodes necessary in the case where the glide and the vclfo 
> also have some other signal plugged into that same input? if i had a 
> sample and hold plugged into the same cv in input of the vco, i fear 
> it would travel up the line and back out to the vclfo cv in or glide 
> cv in. and then i would have no control over what got what for cv!!
> 
> the problem right now is it seems very unlikely that i would want to 
> add all the opamps required to do this with buffers. for instance if 
> i have 5 cv ins on average for each section, and there are at least 
> 5 sections, that would be tons of opamps or transistors.....
> 
> hope this clarifies....
> 
> i am going to go get out my electronotes and see if i can find any 
> info on this issue. _______________________________________________ 
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Harry Bissell & Nora Abdullah 4eva