[sdiy] the summing node

[TheMysticSatin]

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