Multiple CV-Ins

[johns at oei.com]

     Topher wrote...
     
     > I was wondering. My schems for VCO's, VCF's, etc. show one resistor  
     (usually a 100K) on each CV-in before going to the CV summing node. Is 
     there a problem with connecting the CV-ins BEFORE the 100K resistor 
     (and just using one resistor). I was planning on using banana plugs 
     and stacking in places where I wanted multiple inputs and it just 
     occured to me that this might be a problem. 
     
     (end of Topher's message)
     
     My answer:
     
     I will attempt to answer this as I think it addresses the simple V=IR 
     application in op-amps.  Somebody correct me if I'm wrong.
     
     1. You can't sum voltages simply by connecting several voltage sources 
     together.  No single node can be at more than one voltage potential 
     and that is what stacking banana plugs would do.  (Like saying a 
     bucket can have 2 quarts and 3 quarts of water at the same time).
     
     2. I think what the question refers to is a single resistor for CV 
     input going to a current summing node (not a CV summing node) in a 
     summing op-amp circuit.  This op-amp configuration sum voltages by 
     summing currents and converting the summed current to a voltage.  The 
     currents are developed by allowing a current to pass from one node (at 
     CV input voltage) to another node (at constant, usually 0V) through a 
     resistor.  You need a current for every CV, therefore you need a 
     resistor for every CV.  This is basic op-amp theory.
     
     So there is no way to sum voltages by using a single resistor instead 
     of one resistor per CV input.
     
     JJS