[sdiy] the summing node

[harry bissell]

Hello Mystic one...

(inline)

--- TheMysticSatin <elmystico at earthlink.net> wrote:
> hey all,
> My next project is a VCA based on one of Tom G's
> designs from when he had
> the old cookbook section on his site.  I believe
> that his design was in turn
> based on the Korg ms10's VCA.  Anyway it appealed to
> me because of its
> simplicity and low part count but as I'm going back
> over the design I'm
> noticing that the CV input doesn't go into an op
> amp.  I'd like to add
> multiple CV inputs, some with attenuators, and
> imagine I need to just add an
> op amp summer at the CV input with a 100k feedback
> res. and 100k input
> impedence res's on the inputs, but any reference to
> the op amp summer I can
> find uses the - input with the + input grounded.  So
> if I did it that way,
> things like envelopes would be inverted right? 

Yes, thats right.

 I
> figured now is the time to
> really understand the summing node... Can I input
> through the + input and
> ground the -? 

No you can't.  You "can" use the positive input, but
not to make a summer. The trick on the summer is that
the positive input is held at ground. The output (via
negative feedback) supplies enough current to the
inverting input, to exactly cancel all other currents
flowing in or out of that point through the input
resistors. The negative input is now "virtual ground".

The summing is very good... limited only by the match
of the resistors and the opamp itself (obviously it
cannot exceed its own supply voltage, output current,
etc...)

 should I use 2 opamps, both using the
> - input, the cv signals
> going into one as an inverting summer and then the
> second one as an
> inverter? 

Yes... that is one way to do it.

Sometimes if the input resistor is a high value... and
it goes to a reasonably low input impedance
(an example might be a 100K input resistor feeding a
100 ohm to ground...) you can just add more input
resistors. The summing is not perfect... but it can be
pretty close. On a VCO you would NOT be happy... but
for a VCA you probably could not hear a difference.

If you e-mail me that schematic I could advise you
on the specifics of your circuit.

H^) harry 
> 
> 


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