If you don’t have a polarity-reversing feature on your console (or, if you don’t have a console), there are several other ways to do the same thing. If you have balanced inputs to your console, you can simply invert the balanced signal by making up an XLR polarity-inverting adapter— make up a short XLR-male to XLR-female cable and cross-wire pins 2 and 3 (the wire at pin two on one connector is connected to pin 3 on the other, and vice versa). This will invert the polarity of the signal passing through the adapter. Warning: remember to take this adapter out of the line when you aren’t trying to invert a signal!

	XLR-to-XLR polarity-inverting wiring scheme for balanced lines.

If you don’t have balanced lines, you can invert the polarity by using an isolation transformer (I use a 10KΩ/10KΩ transformer) and, again, cross-wiring the leads to the transformer windings. Wire it up with the appropriate jacks and you can patch it in just about anywhere.

	Transformer-coupled polarity-inverting wiring scheme for unbalanced lines. The primary and secondary windings of the transformer should be identical (1:1) and the nominal impedance should be high (10KΩ).

There is an even simpler way to do this, if all you want to do is listen to the A-B signal. Go to the back of the power amp driving your speakers, and hook up the leads from one speaker (or both speakers, wired in parallel, if you wish) to each of the positive (usually red) terminals of the Left and Right channels. The only voltage across those terminals will be the difference between the two channels (A-B, get it?). Variations on this scheme include building a little switching box so you don’t have to crawl around in the dust-bunnies every time you want to listen this way, or dedicating a pair of cross-wired headphones, or even cross-wiring just one stereo jack on a headphone splitter box, or even hiring an assistant engineer, for those of you who are truly lazy. Whatever, it’s cheap, easy and anyone can do it. The figure below should give you the idea.

	A-B monitoring. Note that if you switch the amp to mono you will probably hear nothing, unless the balance control is not centered. As always, when fooling around with power amps and speakers, go gently, use very modest levels until you are SURE it all is working right, and immediately retreat (i.e. turn the power off) if you see flames, smell smoke, or hear moans or other sounds of distress from the amp or speakers.

Anyway, once you have this A-B listening system in place, there is lots of stuff to listen for. To begin with, all A+B stuff is going to be nulled out, so you aren’t going to hear it (which is also to say that you have just created a “vocal eliminator”!).