Mono summing is done inside the 625CR module.
How is the summing to mono done?
It is very simple and nothing new really. I use 3 active stages in the 625CR. The mono summing is simply combining the outputs of the first stage opamps thru 330 ohm precision R's and then into the inverting inputs of the 2nd stage opamps. The DIM resistor array is just before the mono summing R's.
The L/R on-off and stepped level control are handled between stages 2 and 3.
Speaker select D is a single speaker mono output, so a completely different function than the above mentioned mono summing. For this, I take the output of both balanced line drivers and combine them thru 1k precision R's.
FWIW, I am very proud of this module. It is totally bad azz.
