What is a DI box. When it is used?

DI Box - What is It?

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Recording direct is also known as Direct Injection or DI. The electric guitar produces an electrical signal, so it can be plugged right into the mixing console - no microphone is needed. Because the mix and guitar amp are bypassed, the sound is clean and clear, it lacks distortion and colouration or the amp.

You need a DI box because there is a frequent requirement to interface equipment that has basically non standard unbalanced outputs with the standard balanced inputs of mixers, either at line level or microphone level. An electric guitar, for example, has an unbalanced output of fairly high impedance - around 10 kilo ohms or so. The standard output socket is the 'mono' quarter-inch jack, and output voltage levels of around a volt or so (with the guitar's volume controls set to maximum) can be expected.

Plugging the guitar directly into the mic or line level input of a mixer is unsatisfactory for several reasons:

* the input impedance of the mixer will be too low for the guitar, which likes to drive impedances of 500 kilo ohms or more;
* the guitar output is unbalanced so the interference-rejecting properties of the mixer's balanced input will be lost;
* the high output impedance of the guitar renders it incapable of driving long studio tie-lines;
* and the guitarist will frequently wish to plug the instrument into an amplifier as well as the mixer, and simply using the same guitar output to feed both via a splitter lead electrically connects the amplifier to the studio equipment which causes severe interference and low-frequency hum problems.

Similar problems are encountered with other instruments such as synthesisers, electric pianos, and pickup systems for acoustic instruments.

To connect such an instrument with the mixer, a special interfacing unit known as a DI box (DI=direct injection) is therefore employed. This unit will convert the instrument's output to a low-impedance balanced signal, and also reduce its output level to the milli volt range suitable for feeding a microphone input. In addition to the input jack socket, it will also have an output jack socket so that the instrument's unprocessed signal can be passed to an amplifier as well. The low-impedance balanced output appears on a standard three-pin XLR panel-mounted plug which can now be looked upon as the output of a microphone.

An earth-lift switch is also provided which isolates the earth of the input and output jack sockets from the XLR output, to trap earth loop problems.

Passive DI box

The simplest DI boxes contain just a transformer, and are termed 'passive' because they require no power supply. The transformer in this case has a 20:1 step-down ratio, converting the fairly high output of the instrument to a lower output suitable for feeding microphone lines. Impedance is converted according to the square of the turns ratio (400:1), so a typical guitar output impedance of 15 kilo ohms will be stepped down to about 40 ohms which is comfortably low enough to drive long microphone lines. But the guitar itself likes to look into a high impedance.

The transformer isolates the instrument from phantom power on the microphone line.

This type of DI box design has the advantages of being cheap, simple, and requiring no power source - there are no internal batteries to forget to change. On the other hand, its input and output impedances are entirely dependent on the reflected impedances each side of the transformer. Unusually low microphone input impedances will give insufficiently high impedances for many guitars. Also, instruments with passive volume controls can exhibit output impedances as high as 200 kilo Ohms with the control turned down a few numbers from maximum, and this will cause too high an impedance at the output of the DI box for driving long lines. The fixed turns ratio of the transformer is not equally suited to the wide variety of instruments the DI box will encounter, although several units have additional switches which alter the transformer tapping giving different degrees of attenuation.

Active DI box

The active DI box replaces the transformer with an electronic circuit which presents a constant very high impedance to the instrument and provides a constant low- impedance output. The box is powered either by internal batteries, or preferably by the phantom power on the microphone line.

If batteries are used, the box should include an indication of battery status; a 'test switch is often included which lights an LED when the battery is good. Alternatively, an LED comes on as a warning when the voltage of the battery drops below a certain level. The make-and-break contacts of the input jack socket are often configured so that insertion of the jack plug automatically switches the unit on. One should be mindful of this because if the jack plug is left plugged into the unit overnight, for instance, this will waste battery power. Usually the current consumption of the DI box is just a few milliamps, so the battery will last for perhaps a hundred hours.

Some guitar and keyboard amplifiers offer a separate balanced output on an XLR socket labelled 'DI' or 'studio' which is intended to replace the DI box, and it is often convenient to use this instead.

DI boxes are generally small and light, and they spend much of their time on the floor being kicked around and trodden on by musicians and sound engineers. Therefore, rugged metal boxes should be used (not plastic) and any switches, LEDs, etc. should be mounted such that they are recessed or shrouded for protection. Switches should not be easily moved by trailing guitar leads and feet. The DI box can also be used for interfacing domestic hi-fi equipment such as cassette recorders and radio tuners with balanced microphone inputs.

Related: Looking for a good DI Box

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