SoundCraft MPM 20/2 - Manual (Page 10)

10
Polarity (Phase)
You will probably be familiar with the concept of polarity in electrical
signals and this is of particular importance to balanced audio signals.
Just as a balanced signal is highly effective at cancelling out unwanted
interference, so two microphones picking up the same signal can cancel
out, or cause serious degradation of the signal if one of the cables has
the +ve and -ve wires reversed. This phase reversal can be a real prob-
lem when microphones are close together and you should therefore al-
ways take care to connect pins correctly when wiring audio cables.
Grounding and Shielding
For optimum performance use balanced connections where possible
and ensure that all signals are referenced to a solid, noise-free earthing
point and that all signal cables have their screens connected to ground.
In some unusual circumstances, to avoid earth or ground ‘loops’ ensure
that all cable screens and other signal earths are connected to ground
only at their source and not at both ends.
If the use of unbalanced connections is unavoidable, you can minimise
noise by following these wiring guidelines:
•
On INPUTS, unbalance at the source and use a twin screened
cable as though it were balanced.
•
On OUTPUTS, connect the signal to the +ve output pin, and the
ground of the output device to -ve. If a twin screened cable is
used, connect the screen only at the mixer end.
•
Avoid running audio cables or placing audio equipment close to
thyristor
dimmer units or power cables.
•
Noise immunity is improved significantly by the use of low imped-
ance sources, such as good quality professional microphones or
the outputs from most modern audio equipment. Avoid cheaper
high impedance microphones, which may suffer from interference
over long cable runs, even with well-made cables.
Grounding and shielding is still seen as a black art, and the suggestions
above are only guidelines. If your system still hums, an earth/ground
loop is the most likely cause. Two examples of how an earth loop can
occur are shown below.
Warning!
Under NO circumstances must the AC power mains earth be disconnected
from the mains lead.