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 really not that complicated once you get it, but up to that point it’s a bit of a mystery.
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 pf 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.
