Soundcraft EPM12 [12/36] Grounding and shielding

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 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.