A DJ recently posed the following question on an industry chat board, “If it’s such a bad idea why do so many Djs do it?” I think the simplest answer to the question would be that there are a lot of things people don’t understand, and in the absence of good information they do things that shouldn’t be done. This isn’t exclusive to DJs, not even by a long shot. If it were, we wouldn’t have the Darwin Awards or those great shows on TruTV documenting poor decision making at it’s finest.
The topic that prompted him to post that question was whether or not you could/should sum the outputs of a mixer using a wye cable so that Left and Right would now be a summed Mono. While it’s a really bad idea, I’m not sure it rates up there with trying to break into a building and getting stuck in an air vent as far as entertainment value goes. I guess the producers of those TV shows are looking for something a little more exciting than potentially damaging some sound equipment.
There are lots of reasons we might want to sum stereo outputs into mono. Often subwoofers are used in a mono configuration, for lots of good reasons. Also, many DJs offer remote, satellite speakers for multi-room applications. With a single speaker in a remote room we don’t want to lose half of our musical information or have stereo imaging. We just want the sound, all of it, in there.
One might think we can just take a wye cable (2 in to 1 out) and sum the signal. Not so. Wye? (excuse me) Why, you ask? It’s a question of impedance. The outputs on our audio devices have a low impedance, typically 100 to 600 Ohms, where inputs have a much higher impedance, typically 10,000 to 20,000 Ohms. Low always drives high. As a sidebar, when we parallel inputs we decrease the impedance, just as when we parallel speakers. This is ok to do with a wye cable when splitting a single output into more than one input, as long as we keep a ratio of at least 10:1. For example, if we drive a 100 Ohm output into a 10,000 Ohm input we have a ratio of 100:1. If we drove that same 100 Ohm output into a parallel of 10,000 Ohm inputs we have a ratio of 50:1, and so on. The reason this doesn’t work in reverse is that we have two low impedance outputs that will try to drive each other. They will still drive the high impedance input, but with potentially significant loss, and potentially serious damage to each of the outputs.