TheEC: Phantom Power

Jul 12, 2013

A typical phantom power circuit.

BOO!  This time on the Engineer’s Corner, we’ll talk about PHANTOM POWER.   Usually not as ghoulish as one might expect, phantom power has to do with microphones.   Specifically, some microphones have active circuitry inside them.   That means they need power to operate, but it’s unwieldy to run a separate power cord and audio microphone cable.   So a phantom circuit is used to provide DC power on the same three wires (positive/hot, negative/cold, and ground) out to the microphone that the audio from the mic also uses.

A phantom circuit is one of those nifty things in electronics that looks, to the layman, like it can’t possibly work...but it does anyway.  

The key is to remember that an audio signal is really just electricity.   It’s electricity with voltage within a specific range which is quite low, usually less than half a volt.  But it’s still just electricity.

So with three wires to use, you can create two separate circuits of electricity…the audio signal and the power…using center-tapped transformers (aka “repeating coils”) to prevent “crosstalk”, which is just a fancy way of saying “undesired mixing of electrical signals.”

Phantom power tends to be 12 or 48 volts, although it’s limited to 1 to 2 milliamps for the most part.  That’s very little power and, contrary to some stories, not enough to hurt someone.  The “live mic” phenomena - that can indeed electrocute someone to death - has to do with grounding issues between a mic amp and a guitar amp, usually on stage where there can be many AC outlets for your amps and not all of them are properly grounded.  Phantom power, on the other hand, can…and often is…provided by a single AA battery.  For some microphones, it's built right into the handle of the mic!

The concept of phantom power also been around since about 1919, although phantom power wasn’t more common in microphones until the 1960’s and 70’s.   The phantom power allows active-circuitry mics, like condenser mics, to function.  But the really handy part is that the supply voltage is effectively invisible to a balanced mic like dynamic microphones.  So you can use the same wiring, and even have active phantom power, on a cable to a dynamic mic and it just doesn’t care.   Makes the setup a lot simpler for us engineers!