Phantom Power Explained

Phantom Power

and Microphone interconnect basics

From the "ground" up!

© 1999 by Eddie Ciletti for the July issue of EQMagazine

additional updates ©2000, 2004 and 2007

If you’re new to the audio scene, let’s start with a few basic electronic concepts then move on to Microphones and see how all of the connections are made, audio and phantom power.

Before taking the time to understand Phantom Power, let's look at the specs so you have a better understanding of why your mic or direct box might not be working.

Answers to Frequently Asked questions...

The Phantom Power spec is 48 volts dc from a standard 3-pin XLR connector.
Phantom Powered microphones have a wide operating range, from 9vdc to 48 vdc.
Some console / mixer manufacturers take advantage of the above range by not supplying the full 48-volts. They do this because it is easier and cheaper.
Computer microphones that use a mini 1/8-inch (3.5mm) phone plug do require power, but not phantom power. No simple adapter will make these mics work in a pro system.
The power supplied by the computer / sound card to the 1/8-inch (3.5mm) jack not configured to power professional microphones (or powered direct boxes).

AC/DC

What’s that you say? You don’t know your AC from your DC? Audio is considered an Alternating Current, a.k.a. "AC." (So is 120-volt "wall" power.) But electronic circuits need Direct Current (DC) to turn them on, from batteries or power supplies. Like a speaker in reverse, a dynamic mic consists of a coil of wire suspended in a magnetic field. When vibrations move the cone or "diaphragm," the energy stored in the magnet is transferred to the wires. (A Dynamic mic is passive and needs no power.)

A DEDICATED SUPPLY
The preamplifier inside Vacuum Tube microphones requires both plate and filament voltages. Power and audio are delivered via special, multi-conductor cables and non-standard connectors from a dedicated power supply. Only then does the mic-level signal appear at a standard three-pin XLR connector. Transistorized microphones require much less power and can operate from a battery, hence the idea for phantom power, a system of distributing a DC voltage through a standard mic cable. All condenser mics (except electrets) requires a fairly large, but low current DC polarizing voltage that is applied to a diaphragm — similar to a drum head, but thinner and plated with a molecularly thin conductive layer that is typically gold. The signal is not strong enough to venture into the outside world without an internal buffer / preamp (active electronics) that also requires power.

A BALANCED BREAKFAST

Compared to both consumer (-10dBV) and professional (+4dBu) Line levels, Microphones produce a signal that can be considerably lower in level, hence the need for an external preamplifier. Every precaution is taken to minimize noise. By design, this begins with using two wires for the signal — referred to as "balanced" — plus a shield. Contrast this with a passive electric guitar — that is, one with no active internal electronics (i.e., a battery is required). A guitar cable uses a single conductor plus a shield, an unbalanced signal.

PHANTOM POWER: First you see it, then you don't

The rear of a Female XLR is shown in Figure One with a Red wire on Pin-2 and a Black wire on Pin-3. Pin-1 is called "ground" and the reference to terra firma implies that the metal body of the mic will ultimately connect to the "earth" and is therefore safe to touch even if you are barefoot in a pool of water (the Green wire). A good ground connection also improves noise immunity.


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Phantom Power Explained

Phantom Power Explained