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Microphones 06
Phantom power 

Level of challenge Easy

 

Welcome to this tutorial on phantom power.

 

Phantom power is a method of powering audio devices where a connection to the mains power supply would be cumbersome. It performs an essential role in live performance and the recording studio.

 

Caption - Which devices require phantom power?

The complex active electronics in capacitor and back-electret microphones, and active DI (Direct Injection) boxes, require power. In the controlled environment of the studio, we use these devices because of their superior frequency response and lower distortion characteristics. However, if every microphone in a multi-mic setup required its own supply of power from the mains, this would be highly inconvenient. Phantom power is the solution.

 

Caption - Where does phantom power come from?

Devices which can supply phantom power include ..

The most common phantom power requirement is 48 volts. Where phantom power is unavailable, some mic's and DI boxes can operate at the 9 volts or less supplied by a battery, which may be installed in a compartment in the device itself. Similar voltages are produced by many portable stand alone phantom power devices. However, voltages below 48 volts usually produce a lower audio output and therefore increased distortion and a poorer signal to noise ratio.

 

Caption - How does the phantom power get to the device?

The standard way to get phantom power to a source device is to send it down the same XLR lead being used to carry the audio signal from the source device.

 

So for example, a mic pre-amp would send phantom power to a capacitor microphone and receive its audio output in return. The signals travel in 'opposite' directions, and out of phase with each other, and therefore do not interact. The positive phantom power signal travels on the Live and Return cores whilst the negative travels on the Earth.

 

Caption - Using phantom power safely

Demonstration Enabling phantom power can create a brief and loud peak signal (a bang!). Also connecting a cable to a device with phantom power already turned on can risk sending it down the wrong core. To avoid damage to your speakers or source device follow this procedure ...

  1. Ensure the gain, and audio output level if there is one, of the mic pre-amp is turned down
  2. Turn on the mic pre-amp
  3. Ensure phantom power is disabled
  4. Connect your microphone or DI box using a balanced XLR mic lead
  5. Enable the phantom power (turn it on) at the mic pre-amp
  6. Ask the performer to start or play the source signal
  7. Slowly raise the input gain until you have an average level around 0dB, or a level at which the clip LED does not light
  8. Slowly raise the output level until you reach the required level

Caption - Ribbon microphones and phantom power

Ribbon microphones not only do not require phantom power, but can be seriously damaged by it, so extreme care must be taken when using them with a mic pre-amp. Passive DI boxes and dynamic microphones do not require phantom power either, but will not be damaged by it.

 

Caption - Thanks for watching

The script for this video, with accompanying images, can be found at projectstudiohandbook.com 

 

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