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The Direct Input box takes a high impedance, unbalanced signal and converts it to a low impedance, balanced signal.

This allows the signal to be sent over long cable runs with significantly less signal loss (especially in high frequencies) due to the lowering of the impedance, and greater rejection of interference due to the benefit of common mode rejection in a balanced signal. Furthermore, the lower impedance (around 600 ohms is normal) allows an insignificant load to the input of a mixing console or preamp which is also designed to accept input from low impedance microphones.

A passive DI unit typically consists of an audio transformer used as a balun. The turns ratio is typically chosen to convert a nominal 50 kΩ signal source (such as the magnetic pickup of an electric guitar) to the 100–200 Ω expected by the input of an audio mixer. Typical turns ratios are in the range of 10:1 to 20:1.

Less commonly, a passive DI unit may consist of a resistive load, with or without capacitor coupling. Such units are best suited to outputs designed for headphones or loudspeakers.

The cheaper passive DI units are more susceptible to hum, and passive units tend to be less versatile than active. However, they require no batteries, are simpler to use, and the better units are extremely reliable when used as designed.

Some models have no settings, while others can have a ground lift switch (to avoid ground loop problems), a pad switch (to accommodate different source levels) and a filter switch for coloring the sound.

An active DI unit contains a preamplifier. Active DI units can therefore provide gain, and are inherently more complex and versatile than passive units.

Active DI units require a power source, which is normally provided by batteries or a standard AC outlet connection, and may contain the option for phantom power use.

Most active DI units provide switches to enhance their versatility. These may include gain or level adjustment, ground lift, power source selection, and mono or stereo mode. Ground lift switches often disconnect phantom power.

A pass-through connector is a second jack, sometimes simply paralleled to the input connector, that delivers the input signal unchanged, to allow the DI unit to be inserted into a signal path without interrupting it. Pass-through is also commonly referred to as a bypass. True-bypass occurs when the signal goes straight from the input jack to the output jack with no circuitry involved and no loading of the source impedance. False bypass (or simply 'bypass') occurs when the signal is routed through the device circuitry with buffering and no other intentional change to the signal. However, due to the nature of electrical designs there is almost always some slight change in the signal. The extent of change and how noticeable it may be can vary from unit to unit.

Direct boxes are typically used in instances of instruments or other devices that only contain an unbalanced 1/4" phone output which needs to be connected to an XLR input.

Multiple direct box circuits can be mounted inside one housing. These are used for multiple unbalanced outputs, such as for a bank of electronic keyboards.

Acoustic or electric instruments

DI's can be used on instruments with electronic circuitry and pick ups that do not contain an XLR balanced output. An example of this application would be an electric keyboard that needs to be connected to a mixer board, either directly or through an audio snake. Another example would be an acoustic guitar with pickups, an electric guitar or bass guitar that would be mixed through a mixing console into a main or monitor mix.

Instrument amplifiers

Some instrument amplifiers contain built-in DI units, and can be connected to a mixing console directly without needing an external direct box. This would be a typical setup for a person who wanted to run their instrument through a Public Address (PA) system while keeping the unique sound of the amplifier. Some instrument amplifiers have the ability to turn off the amplifier EQ though a pre eq/post eq switch. This can be used if a "clean" direct output from amplifier is desired.

It is common to use both a DI and a microphone on the same source. One method is to connect a guitar amplifier speaker level output to a DI and then run it to one channel of the mixing console, and run a miked guitar speaker signal into another channel of the mixing console. Another method is to connect a DI between the guitar and the amplifier. The DI signal and miked guitar speaker can then be selectively blended, with the DI providing a more immediate, present, bright, un-equalized sound, and the microphone providing a more 'live' sound, with instrument amplifier characteristics and some room ambience.


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