Phantom Power

Condenser microphones and active D.I. (Direct Injection) boxes need electrical power to operate. If they don’t accept batteries they can receive 48 volts of direct current sent from a suitably equipped device. This electrical signal is known as Phantom Power. Devices that can produce and output phantom power include ...

• Mixers
• Voice channels (essentially a single mixer input channel in a box)
• Video camcorders (so you can connect an external condensor microphone)
• Portable audio recoders (professional)
• Good quality audio interfaces (USB, Firewire etc)
• etc

Most mixers have a 48v phantom power switch located in the mic pre-amp stage of the channel input strips. When activated, it will send 48 volts of direct current down a mic (XLR) lead to the connected mic or D.I. box. The positive side runs on the Live and Return wires whilst the negative runs on the Earth. Phantom Power can travel down a lead to a mic whilst audio signals are travelling from the mic to the mixer without the signals inteferring with each other.

D.I. (Direct Injection) Boxes

In the studio or on stage the D.I. box performs several functions.

• It can convert an unbalanced mic or line level signal to a balanced one.
• It can help match the differing impedances (Z) of two devices to allow interconnection. For example, the high impedance output of an electric guitar can be converted to a low impedance to allow a better transfer of audio energy to the low impedance input of a mixer.
• It can provide a way for a guitar player to simultaneously connect to a guitar amplifier and a mixing desk.

Note: The lower the impedance (Z) of the signal, the lower the resistance offered by the wire it travels in.

Impedance (Z) = the resistance in an AC (Alternating Current) circuit.
Resistance = Resistance in a DC (Direct Current) circuit.

 

Types of D.I. Box

Active

Requires power usually in the form of batteries or Phantom Power sent from a mixer.

Passive

Unpowered type. Lower quality performance.

Typical D.I. Box features

Link output

Creates and sends out a copy of the input signal. This would allow the simultaneous connection of an instrument to a mixer and its amp/cabinet.

Earth Lift Switch

Used to eliminate 50Hz mains earth hum caused by so called earth loops (a situation in which two interconnected pieces of equipment, both with their own earthed power supplies, cause an audible hum to be sent down the audio return or earth wire). An earth lift switch allows the input earth to be disconnected from the output earth.

Pad switch

A switch to reduce the input sensitivity of the D.I. box and thus prevent it overloading when a strong (line level) signal is input.

Filters switches

Low and/or high pass to reduce extremes of treble and bass.

Phantom power switch

To allow an active D.I. box to receive and utilise phantom power should it be unable to use batteries.

Impedance (Z) switch

A switch which changes the impedance characteristics of the input. A high setting will be ideal for receiving the output of a guitar pickups, a low setting for the line level signals of electronic instruments.

Phase reverse switch

Reverses the phase of the unit.

Related downloads

None at present

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