By Jon Chappell
A phase switch on a mixer is actually a polarity switch: it changes the positive voltage to negative and the negative voltage to positive. The reason it's called a phase switch is that that's how people use it: to eliminate phase cancellations in signals coming into the mixer.
Reversing the polarity of a signal (and the waveform produced by that signal) is the same thing as having the signal 180 degrees--the maximum--out of phase. That's why in this one situation you can use the terms "out of phase" and "reverse polarity" interchangeably, even though it's not quite correct: because it produces the same results.
To Phase or Not
Switching the phase--er, polarity--of a channel is often a good test to perform on elements in a mix, especially where a lot of mics are used, and where more than one mic is used to capture a source. For example, if your guitar or bass blends a mic with a pickup (as is often the case with a plugged-in acoustic guitars), you may want to do a quick polarity check to make sure the mic and pickup are in phase. And if not, reversing the polarity of one of those signals may improve things.
How do you know if you've improved it? If it gets louder or fuller. If the sound becomes hollower, boxier, or softer when you switch, chances are it was okay as is. You may even want to go with the "incorrect" configuration if it helps eliminate feedback, helps to balance a boomy instrument, or otherwise offers a more desirable sonic character. In the final analysis, you must use your ears to decide.
But what if your little stage mixer doesn't sport one of these handy switches? Well, you can create a simple XLR adaptor, consisting of: a male-to-female connector, where pins 2 and 3 are switched at one end. This goes inline between the mic cable and the mixer. You'll also hear this device referred to as a "polarity transposition lead," but it describes the same thing--a short inline (male to female) XLR adaptor cable with the positive and negative wires crossed at one end only. You can make your own, if you're handy with a soldering iron, or you can buy one of the many available pre-made ones online or at your favorite brick-and-mortar emporium.
Home Brewed vs. Store Bought
To make your own, you just swap the wires going to pins 2 and 3 on one end only of a male-to-female XLR cable. Figure 1 shows the schematic.
Fig. 1: In making your own polarity reverse adaptor, just switch the leads of pins 2 and 3 at one end (leave pin 1, the ground, connected).
Note that since this is a three-wire system (positive, negative, and ground), you do need to know which wire is the ground--and to leave that one alone. If you don't want to bother with disassembling a connector and breaking out the soldering gun, just buy a ready-made connector that does the job for you. I found several on the web, including models from Shure, Hosa, Whirlwind, and Pro Co, but the Hosa gets a mention here because it is the least expensive (see Fig. 2).
Fig. 2: Hosa has a no-frills adaptor (the GXX-195) for only about $8. You could buy two at that price for less than one from many other brands.
Whirlwind has one called the IMPHR ($16, street), which has the letters boldly displayed on the barrel housing, making it easy to identify when rummaging through your toolbox (see Fig. 3).
Fig. 3: Whirlwind's Phase Reverse is another way to deal with a miswired mic or cable.
I don't advocate doing this except in an emergency, but you can actually make an inline splice, which doesn't require soldering tools or even disassembling the connectors. If you cut a mic cable with scissors or a carpet knife, you can more easily strip the insulation of off the wires coming from pins 2 and 3 and reverse them than if your were working with the lugs at the connectors.
Use a volt-ohm meter to check the continuity of the pins and the corresponding wires to make sure you're making the right hook-ups. You can re-connect the cut wires by simply twisting them together and knotting or taping the cord such that there's no strain on the connections themselves. It ruins a cable, of course, and it's not the most secure way to make an electrical connection, but in an emergency it can be done (and I've done it). And here's another tip: if you do go this route, make the cut close to the connector so that you lose only a couple of inches of cable when you have time to do the connection the right way (i.e., with a soldering iron and onto the lugs of the connector). Alligator clips will also help hold the wire-to-wire connection together, but be careful not to bump them.
Jon Chappell is a guitarist and Associate Editor at Harmony Central. He has contributed numerous musical pieces to film and TV, including Northern Exposure, Walker, Texas Ranger, All My Children, and the feature film Bleeding Hearts, directed by actor-dancer Gregory Hines. He is the author of The Recording Guitarist: A Guide for Home and Studio (Hal Leonard), Essential Scales & Modes (Backbeat Books), and Build Your Own PC Recording Studio (McGraw-Hill), and has written six books in the popular For Dummies series (Wiley Publishing).