Phantom Power Explained
By Bruce Bartlett

   By Guest Contributor   Categories: Audio EquipmentRecording

Most condenser microphones need phantom power to operate their internal circuitry. Let’s explain what phantom power is and how to use it.

DEFINITION

Phantom power is electrical power that is sent to a condenser mic through its mic cable. The cable must have XLR connectors to pass phantom power.

Each of these devices can provide phantom powering:

■ A mic preamp with phantom power built in
■ A mixing console with phantom power built in
■ A standalone phantom power supply

In each device above, a female XLR mic connector supplies the phantom power to operate the mic. The microphone receives power from, and sends audio to, the device along the same cable. It’s called “phantom” because the power does not need a separate wire; it’s “hiding” in the mic signal wires inside the mic cable.   

Technically, phantom power is a positive voltage (12–48 V DC) on XLR pins 2 and 3 with respect to pin 1. The cable shield is connected to the phantom-power ground.

Figure D.1 shows a microphone plugged into a standalone phantom power supply.

After you plug a condenser mic into a mixer, turn on phantom power in the mixer. The phantom on/off switch in many consoles is labeled “P48” to mean “phantom power 48 volts.”

USING A STANDALONE SUPPLY

What if your mixer or mic preamp does not have phantom power? You can buy a phantom power supply ( Figure D.2 ) from a dealer or online.

Connect the supply in series with the mic cable. The supply has XLR input and output connectors, one pair per channel. As shown in Figure D.1 , connect a mic cable between your microphone and the supply’s input connector. Plug another mic cable between the supply’s output connector and your mixer or preamp mic input.

Some supplies are AC powered, some are battery powered, and some are both. Some can power a single microphone; others can power several at once.

CAUTIONS FOR USE

Don’t plug a mic into an input with phantom already switched on, or you’ll hear a loud pop. If you have no choice (as during a live concert), mute the mic channel when you plug the mic in.

Make sure your phantom voltage is adequate for your microphones. Check the mic’s datasheet under the “Powering” spec. Some mics start to distort or lose level if the phantom voltage drops much below 48 volts.

DC BIAS

Some microphones or mic capsules work on “DC bias” rather than phantom power. A separate wire supplies a voltage to the mic capsule. This system is used in the cable between a mic capsule and an XLR connector in lavalier mics, choir mics, and clipon instrument mics. The mic capsule itself runs off DC bias, while the XLR connector houses a circuit that runs off phantom power. That circuit converts phantom power to DC bias for the mic capsule.

 

The above is an excerpt from Bruce’s book, Practical Recording Techniques, 6e.

Bruce Bartlett is a microphone engineer, audio journalist, and recording engineer. A member of the Audio Engineering Society and Syn Aud Con, he holds a degree in physics and several patents on microphone design. He is also a musician and runs a 16-track digital studio specializing in live recording.He has written over 600 articles on audio topics for such magazines as Modern Recording, db, Recording, EQ, Mix, Recording Engineer/Producer, Radio World, Pro Audio Review, Audio, and the Journal of the Audio Engineering Society. He has also written a number of books including Stereo Microphone Techniques, Recording Music on Location, and the best-selling Practial Recording Techniques, all published by Focal Press.  The new sixth edition of Practical Recording Techniques has just published!  The above post is an excerpt from Chapter 4 – Equipping Your Studio.

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