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 ...
D.I. (Direct Injection) BoxesIn the studio or on stage the D.I. box performs several functions ...
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.
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.
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.
This will accept the unbalanced signal from a guitar/bass pickup.
This normally carries a mic level signal to a mixer channel/mic pre-amp.
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.
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.
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.
Low and/or high pass to reduce extremes of treble and bass.
To allow an active D.I. box to receive and utilise phantom power should it be unable to use batteries.
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.
Reverses the phase of the unit.
None at present