Phantom Power

Condenser microphones and active D.I. (Direct Injection) boxes (explained below) require electrical power to operate. Some models can get their power from batteries or a separate power supply, but most rely on power sent from a mixer or mic pre-amp via an XLR/mic cable.

The power and audio signal, which are moving in opposite directions, do not interfere with each other. The positive side of the signal runs on the Live and Return wires whilst the negative runs on the Earth.

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 the XLR/mic cable to the connected mic or D.I. box.

Although the standard phantom power voltage is 48 volts of direct current, some mics and DI boxes can operate with less, though often with a reduction in signal to noise/quality.

Devices that can produce and output phantom power include ...

• Dedicated power supplies
• Microphone pre-amplifiers
• Mixers
• Voice channels (essentially a single mixer input channel in a box)
• Video camcorders (so you can connect an external condenser microphone)
• Portable audio recorders (professional)
• Good quality audio interfaces (USB, Firewire etc)
• etc

 

D.I. (Direct Injection) Boxes

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

• It can help match the differing output impedances (Z) and levels of two devices to allow interconnection. For example, the high impedance unbalanced instrument level output of an electric bass or guitar pickup can be converted to low impedance mic level to allow the signal to be sent to the mic input of a mixer, soundcard, audio interface or pre-amp and to be sent over a greater distance without quality loss.
• Convert an unbalanced mic or line level signal to a balanced one.
• Solve earth/ground-loop hum problems.
• 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

Passive

The passive DI box has no powered electrical circuitry and comprises only passive components, a few resistors and a transformer etc. When an electric guitar or bass is connected, the circuitry is driven solely by the signal from the pickup. In cheaper designs, this can result in the so called "loading effect" which causes a deterioration in sound quality, often noticeable by a loss of the natural tonal balance and high frequency. In addition, the response of the pickup is altered leading to a change in the playability of an instrument. This effect can be large or minimal depending on the design/quality of the DI box and the design/type of pickup, which itself may be active or passive.

Active

An active DI box contains a buffering amplifier which isolates a guitars pickup from the loading effect, effectively allowing the pickup to operate as it should. A disadvantage is that they are more expensive and require power, usually in the form of batteries or Phantom Power sent from a mixer.

Typical D.I. Box features

Instrument input

This will accept the unbalanced signal from a guitar/bass pickup.

Balanced output

This normally carries a mic level signal to a mixer channel/mic pre-amp.

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.

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