DI unit
From Wikipedia, the free encyclopedia
A DI unit or DI box is an electronic device that connects a high impedance line level signal that uses an unbalanced cable with a 1/4" phone plug to a low impedance mic level input that uses a balanced cable and XLR connector. For example: connecting an electric guitar through a DI box and then to a mixing console's mic input. It performs level matching, balancing, and either active buffering or passive impedance bridging to minimise noise, distortion, and ground loops. (They do not perform impedance matching.)
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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[edit] 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 Ω 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.
Some models have no settings, while others can have a ground lift switch (to avoid ground loop problems) and/or a pad switch (to accommodate different source levels).
[edit] 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.
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, ground lift, power source selection, and mono or stereo mode. Ground 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.
[edit] 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.
[edit] 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.
[edit] 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.
[edit] Electric guitar
A DI can be used to take a line in from an electric guitar. When dealing with electric guitars and electric guitar amplifiers, better results will often (not always) be obtained by instead using a microphone in front of the loudspeaker. This is because the tone of the guitar is often shaped by the amplifier and speaker used in the setup. A DI in the chain before the speaker or amplifier will often result in a loss of fullness or pleasant tone. Using a microphone eliminates hum from ground loops which are often troublesome when using DI units with mains-powered amplifiers. But a microphone will of course pick up background noise which a DI connection will not, and will most often be more susceptible to feedback in live situations.
If an electric guitar is to be connected to a DI and 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.
[edit] Electric bass guitar/Acoustic guitar
When dealing with electric bass or acoustic guitar, a DI is most often preferable to using a microphone on an amplifier. This is because these instruments are often valued in a mix for being clean. The signal path from the instrument should go into the DI unit and should then pass through to any sort of instrument amplifier. Often any amp used in this setup would be for monitoring purposes only, with the major component of the sound coming from the balanced end of the DI. The DI should be chosen with the specifications of the individual instrument in mind. Often the best possible tone is achieved by one stage of preamplification. Following this idea, an active instrument, which means that the instrument has a preamplifier inside of it, should utilize a passive DI unit, while a passive instrument, meaning there is no preamplifier inside, should utilize an active DI.
[edit] 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, but caution should be used with these as they often are not transformer isolated.
Better results will often (not always) be obtained by instead using a microphone in front of the loudspeaker. This is especially true of electric guitar. Using a microphone eliminates hum from ground loops which are often troublesome when using DI units with mains-powered amplifiers. But a microphone will of course pick up background noise which a DI connection will not, and will most often be more susceptible to feedback in live situations.
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.
