DI unit
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A DI unit or DI box is an electronic device designed for connecting a piece of equipment with an electronic audio output to a standard microphone or line level input. It performs both level and impedance matching to minimise both noise and distortion.
DI is variously claimed to stand for direct input, direct injection or direct interface. DI units are extensively used with professional and semi-professional PA systems and in sound recording studios.
There are two types, active DI units and passive DI units.
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Passive DI units
A passive DI unit typically consists of an audio transformer used as a balun. Typical turns ratio is about 500:1, to match a nominal 50k ohm signal source such as the magnetic pickup of an electric guitar to a 100 ohm input.
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 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.
A very simple passive DI
Active DI units
An active DI unit contains a preamplifier. Active DI units can therefore provide gain, and are inherently more complex and versatile than passive units.
Activedi.jpg
A versatile active stereo DI
with passthrough in mono mode only
Active DI units require a power source, which is normally provided by batteries, by phantom power, or both. Cheaper units offering both options may perform far better on fresh batteries than on phantom power, or vice versa, so it is important to test a prospective purchase in the mode in which it will be used.
Most active DI units provide switches to enhance their versatility. These may include gain or level adjustment, earth lift, power source selection, and mono or stereo mode. Earth lift switches often (perhaps unintentionally) 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, and tends to work better on the active units.
Unit selection
These are typically successful strategies. Others may work too. In all cases suitable cables are of course required in addition to the DI unit.
Headphone outputs
A DI box can be used to receive a signal from any headphone jack, such as those on personal stereo systems 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. The jack cannot normally be used for headphones as well.
Suitable units:
- Active.
- Passive resistive load.
Electric keyboards
For best results use the line output(s), unless the keyboard has built-in balanced outputs (some high end units only) which are essentially built-in DI units and should give the best results of all. If monitor amplifiers are also to be driven directly from the keyboard, the DI unit must have a passthrough connector. Alternatively, take a signal from the amplifier instead, see below.
Suitable units:
- Active.
- Passive balun type.
Electric guitar or bass
If an amplifier is to be connected as well, then the DI unit must have a passthrough connector. Alternatively take a signal from the amplifier, see below. For players using effects (including distortion) built in to their amplifiers, this is the only option, otherwise the contribution of these effects will be lost. If a passthrough is used, normally the DI unit is between any effects units and the amplifier for the same reason.
Suitable units:
- Active.
- Passive balun type.
Instrument amplifiers
Some high-end instrument amplifiers contain built-in DI units. Most of these work as well as or better than any external unit, as they are well matched to the signal.
Better results will often (not always) be obtained by instead using a microphone in front of the loudspeaker, and generally more reliable results. Using a microphone eliminates hum from earth loops which is often troublesome when using DI units with mains-powered amplifiers.
Suitable units:
- Active:
- From line or slave output.
- From loudspeaker output (parallel to loudspeakers).
- From effects loop (may need passthrough).
- Passive resistive load type:
- From loudspeaker output (parallel to loudspeakers, and check impedance and power handling capacity of the DI!).
- Passive balun type:
- From line or slave output.