need a quick & dirty mixer?  this is a cascading summer, meaning that the first three inputs are output into OUT 1, but if no output cable is present they will add themselves to the inputs of the next stage and the sum of all six INs will be present at OUT 2.  what? no OUT 2 cable present either ? then all six get added to inputs 7, 8 & 9 and all nine are summed into OUT 3. likewise for the fourth stage. 

works for audio and CV. uses 4558 opamps but others could be used to optimize it for your desired specs.

mark
vco2@yahoo.com
9/2001