Today we're going to continue our series on the electrical side of sound. Last time, we tackled ground loops; their cause and a few solutions. This time around, it's phantom power. Phantom power is one of those often misunderstood aspects of sound. It's one of those things that's really not that complicated once you get it, but up to that point it's a bit of a mystery. So today we take the mystery out of phantom power.
Why Use It?
The first question we need to ask is why use phantom power at all? Strictly speaking, we don't need to, as the only reason we need phantom power is to power condenser microphones. Take all the condensers off stage and you can shut off phantom power forever. But most of us like to use the occasional condenser mic or active DI, so phantom power is necessary. Some condenser mics and active DIs will run on a battery, but if you don't have to power something from a battery, you shouldn't (you know it's going to die at the most inopportune time). It should be noted that it is only condenser mics and active DIs that require phantom power; for all other sources, it's best to turn it off if you have the option to do so on a channel by channel basis.
What is it?
Phantom power is a DC voltage—typically 48 volts— that is applied to both signaling lines of a balanced connector. In practical terms, 48 volts are applied to both pin 2 and pin 3 of an XLR. As both pins are getting the same voltage, in the same polarity, it's cancelled out at the input transformer. Audio signals are generated based on the differences between the "high" or "hot" (pin 2) and the "low" or "cold (pin 3) signals; thus if the same voltage (48) is appearing on them at the same time, it is effectively 0 volts. That is to say, there is no delta or difference between them, so to the input amplifier in the mixer, there is no signal. Since phantom power is direct current, it can coexist on the audio lines with no problem. A condenser mic is free to pull power from either pin 2 or pin 3, and the current returns to ground over the shield.
Generally speaking, it is safe to use phantom power with dynamic microphones. However, if you have the option to turn it off when it's unneeded, you should. The reason is safety. If there is ever a fault in the mic line that would short pin 1 (ground) to either pin 2 or pin 3, 48 volts of DC current would flow through the voice coil of the mic and it could be destroyed. This doesn't happen often, but why take the chance. If you can turn it off, do.
The original spec for phantom power called for delivering only 2 milliamps of current. Today, some mics require much more (the Shure KSM series for example, really wants 5-6 mA, while really high end mics want 10 mA). Most modern sound consoles will deliver this current, but be aware of the needs of your mic. If you have a real high end condenser that just doesn't sound as good as it should, check the phantom power current spec of your desk. Of course, plugging a high end Schoepps Colett into a cheap Crate mixer is sort of like giving Michael Schumacher a Yugo to drive. But I digress...
As noted, phantom power can be used to power active DIs, such as a Countryman Type 85 or Type 10, or a Radial J48. While the Countrymans will run on a 9v battery, giving them a steady supply of 48 volts will help you sleep better at night. Another cool use of phantom power is the Rat Sniffer. The Sniffer uses the 48 volts to diagnose mic cables, and indeed entire signal flows. If you're walking into a venue with a bunch of unlabeled snakes, you can plug the sniffer into an input on the snake, then switch phantom power on, one channel at a time. When the lights light up, that's your line. The Sniffer is also very handy at diagnosing cross patches, too. If you have an open pin, a polarity issue or other wiring malady, the Sniffer will let you know. Pretty nifty!
So there's a little about phantom power. As you can see, it's really not that mysterious. In fact, it's pretty straight forward.