My understanding of a passive summing mixer is to use series resistors to limit the voltage from multiple sources so as to not over drive the destination??? If you are combining two audio channels say from a VCR and jumper the left and right channel will this have serious reprocutions for the destination? For years in the AV business, I and many others, have been jumpering left and right channels on singal source to make a mono signal without using a summing mixer. I am being told this should never be done?? Anyone prove or disporve this?