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Headphones
The PHONES output is a 3-pole ‘A’ gauge jack, wired as a stereo output as shown,
ideally for headphones of 150Ω or greater. 8Ω headphones are not recommended.
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 follow-
ing 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 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.
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