SoundCraft NOTEPAD 102 User Manual page 14

14
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 problem when microphones are close together and you should
therefore always 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
impedance sources, such as good quality professional micro-
phones 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.
Soundcraft Notepad User Guide