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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