Musical instruments usually
have an output signal that is not directly
compatible with either the microphone
input (which is designed for signals
from a low impedance, balanced microphone)
- or the line input, which is usually
of insufficient sensitivity, or not
of optimum impedance. In addition the
mixer is often a long distance
With a balanced audio connection
from a mixer microphone input - which
consists of a pair of conductors surrounded
by a braid, or screen - it is possible
to feed power in addition to the audio
signal. A DC supply (48 volts) is fed
via a pair of resistors (6K8 ohms) onto
each of the two audio carrying conductors.
This can be picked up in the microphone
(or DI Box) and used to power active
circuitry. The current returns via the
screen of the cable. It is called "Phantom"
because if you should measure across
the audio pair with a sensitive voltmeter,
nothing would be detected. This is because
both wires are at exactly equal potential
- so nothing can be measured. If you
should check between either of the wires
and the screen, however,
Because the output of the DI
Box is intended to feed the microphone
input of a mixer (which is balanced)
- it is appropriate to employ balanced
working. The advantage is that the cable
connection between DI Box and mixer
can be very long (hundreds of metres
if necessary), without picking up hum,
noise and interference from other sources.
In a big concert, or festival situation,
the distance between the stage and the
mixing position can be enormous - so
The Standard DI Box can be powered
from either an internal battery, or
Phantom power fed from the mixer. The
battery is housed in a convenient drawer,
which can be removed with a coin, or
even with finger pressure. This enables
batteries to be changed, or removed,
easily - without the use of tools. If
using Phantom power it is not necessary
to remove the battery, as the DI Box
senses and uses the higher voltage of
the two voltages presented to it. If
the Phantom supply should fail the battery
will take over instantly, if fitted.
Please note that the red LED only illuminates
if the Phantom supply is present. This
can be useful when setting up as it
can be seen from the mixing position,
assuring the sound engineer of the
Earth lift isolation to prevent
the formation of hum loops is taken
care in the circuitry of our DI Boxes
- so no switch is fitted. On some manufactures
DI Boxes you cannot use Phantom power
with the switch in the lifted (or earth
disconnected) position. It is also our
experience that anything that can be
fiddled with (or adjusted) - will be!
On several occasions the sound has been
fine at the sound check, but when the
concert begins a mystery hum appears
on one of the audio feeds due to the
spurious operation of an earth lift
switch.
