[sdiy] CV questions..hardwiring

[Dan Snazelle]

wow

it is good to know that in a true summer, the inputs wont interact with other opamps being fed by the same signal!

let me ask...is any opamp circuit with the + input going to gnd a TRUE summing circuit?

most if not all of the circuits i am worried about are on the ASM2 board...which seems to have all "real" opamps at the input stage.

so what you bring is GOOD NEWS.

your last paragraph confused me though

" 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."

by needed...did you mean that was all you had left over on the pcb?

THANKS SO MUCH

--------------------------------------------
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> From: harrybissell at wowway.com
> To: subjectivity at hotmail.com; synth-diy at dropmix.xs4all.nl
> Subject: RE: [sdiy] CV questions..hardwiring
> Date: Wed, 22 Oct 2008 12:00:48 -0500
>
> 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. _______________________________________________
>> Synth-diy mailing list Synth-diy at dropmix.xs4all.nl
>> http://dropmix.xs4all.nl/mailman/listinfo/synth-diy
>
>
> Harry Bissell & Nora Abdullah 4eva
>