Allen&Heath PA28 - Manual (Page 13)

PA User Guide 13
The MONO Channel
MIC INPUT
XLR input for connecting microphone level
signals in the range -63 to +5dBu. These are balanced and
wired pin 2 hot. Use low impedance microphones (less than
600 ohms) and professional grade mic cables (2 core with
shield) for best performance. You can also use these inputs
with DI boxes and other low level sources.
48V Phantom Power
Press this switch if the
microphone needs phantom power. Some active DI boxes
may also require power. Check the manufacturers
specification regarding power requirements. If phantom power
is not required leave the switch in its up position.
WARNING Do not connect unbalanced sources or
cables to the XLR inputs when phantom power is applied.
No damage will be done to non powered balanced
microphones as long as balanced cables are used.
To avoid loud clicks always mute the channel before
plugging or unplugging phantom powered inputs.
Phantom Power is so called because the DC voltage
required to power certain microphones is sent down the same
wires as the audio signal. It is applied equally to the + (hot)
and – (cold) signal wires through closely matched 6.8kohm
resistors. The power is tapped off from both signal wires at the
microphone end where it is used to power a little booster
amplifier circuit or energise the transducer. The audio signal
does not ‘see’ the phantom power. This is because the voltage
is the same on both the signal + and – wires and cancels out
at the differential console input. This would not be the case if
an unbalanced cable was used. For this reason phantom
power must only be used with balanced sources.
LINE INPUT
TRS input for connecting line level signals in
the range -25 to +10dBu. The input is balanced but can be
used with both balanced or unbalanced sources as shown in
the cable diagrams. Plugging into the line input automatically
overrides the mic input.
Do not use Y-adaptors to combine stereo L and R
signals such as those from a CD player into one input. Use
the stereo or two mono channels for stereo sources.
Do not plug amplifier speaker outputs into the
console line inputs. Use a DI box with speaker level input if
you need to do this.
☺
Working with different sources Choose your
microphones carefully according to their application. Low
impedance balanced types are recommended in all cases.
Dynamic mics are well suited to hand held vocal or general
instrument work. Condensers can provide greater accuracy
but may not be as rugged. Boundary mics work well for
ambient pickup such as front of stage floats or conference
tables. Rifle mics are very directional and best used for distant
pickup such as across a stage. It is best to use a directional
mic pattern such as cardioid in live situations where feedback
may be a problem. Make sure phantom power is not selected
when plugging in radio mic receivers. A guitar can be plugged
in directly only if using a short cable. For long cable runs you
should use a DI box which converts it to balanced mic signal.
A popular practice is to use a microphone placed in front of the
guitar amplifier. This captures the sound heard by the
musician.
1
Balanced and unbalanced connections
An unbalanced connection uses two cable wires,
signal and ground (screen). A balanced
connection uses three wires, signal + (hot),
signal - (cold) and ground (screen). The
balanced connection has the advantage that it
rejects noise and interference that may be picked
up on long cable runs. Here is how it works: The
audio signal is sent at equal level but opposite
polarity on the + and – wires. The – signal gets
inverted and adds to the + signal at the receiving
end. However, any interference picked up by the
cable is injected equally into both the + and –
wires and therefore cancels out at the receiving
end where the interference on the - wire is
inverted.