A DI unit
, DI box
, Direct Box
or simply DI
is an electronic device that connects a high impedance line level
signal that has an unbalanced output to a low impedance mic level balanced input, usually via XLR connector
. DIs are frequently used to connect an electric guitar
or electric bass
to a mixing console
's microphone input. The DI performs level matching, balancing, and either active buffering
or passive impedance bridging
to minimise noise, distortion, and ground loops
. DIs do not perform impedance matching
DI (pronounced dee EYE) is variously claimed to stand for direct input, direct injection or direct interface. DIs are extensively used with professional and semi-professional PA systems and in sound recording studios.
Passive DI units
A passive DI unit typically consists of an audio transformer used as a balun. The turns ratio is typically chosen to match a nominal 50 kΩ signal source (such as the magnetic pickup of an electric guitar) to a 100-200 Ω 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 simple 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.
Active DI units
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 passthrough connector is a second output, sometimes simply connected 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. This is essential in many applications. Passthrough is more common on active than passive DI units. Passthrough 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 no 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 widely from unit to unit.
Direct boxes are typically used in instances of instruments or other devices that only contain an unbalanced 1/4" output which needs to be connected to an XLR
Acoustic or electric instruments
DIs 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 a 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.
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.
A DI box can be used to receive a signal from a headphone jack, such as those on personal stereo systems, laptops, MP3 players, samplers or keyboards. If the signal is to be connected to a single input then a mixing facility is required in the DI unit. If stereo is required, then either two DI units or a single stereo unit can be used.