Thanks for the quick reply! Does this way of wiring the mono out provide isolation between the L and R out and the mono out? Is there a volume drop on the mono out? If you want to draw up the schematic, that would really help make it idiot proof for me. thanks!
I drew up a quick schematic, see below. After checking my monitor controller, I saw that I actually used 12k resistors, so that’s what I put on the schematic.
To answer your questions: no, this mono summing circuit would not provide true isolation between the stereo output pair and the mono output. And, it probably creates some amount of crosstalk in the stereo outputs. However, I only use that output for a mono speaker, so since I never have stereo speakers plugged in, crosstalk isn’t a concern.
The way the switching on my controller works, the output options are mechanically switched between, with a rotary switch (the one on the right):
As you can see, “mono” here is its own, switched output, not a “mono-fy” button for any stereo set of speakers. And indeed, this mono summing circuit (or any other purely passive summing circuit) would not be an ideal way to add a “mono” switch to a pair of speakers.
As for the volume drop on the mono out, that is going to depend on the input impedance of the amp it is plugged into. In my case, that’s a powered Avantone speaker. As this Gearslutz post on mono summing says, the Avantone probably has around a 10k input impedance. So 1k would make a better summing resistor value in general, but the Avantone has variable gain, and plenty of it, so although the 12k summing resistors reduce the level much more than 1k summing resistors would, it actually balances out better with my other monitors to feed the Avantone a reduced level signal. And I can use the volume knob on the back of the Avantone to level match between the mono speaker and the stereo pair in my studio.
Anyways, here’s the schematic. It assumes you are feeding it a balanced signal! (Hence the + and - inputs for both L and R sides.) The sleeve pin of the output jack may or may not be chassis grounded - in any case, the sleeve does NOT go to audio ground (remember the “pin 1 problem”)!
