DEALING WITH GROUND LOOPS, BUZZ, AND INTERFERENCE
For optimum performance, all audio signals should be referenced to a solid, noise-free ground (earth) point,
also known as “star point” or “clean earth”.
A ground loop is created when potential differences exist between grounds at different points in the system,
and the signal has more than one path to ground. In most cases, ground loops do not result in audible
problems. Should you experience hum or buzz caused by a ground loop, check first that each piece of
equipment has its own separate path to ground. If so, operate ground lift switches on connected equipment
(i.e. DI boxes) in accordance to their respective instruction manuals.
Alternatively, disconnect the cable screen at the destination end only. This breaks the offending loop while
keeping the signal shielding down the length of cable.
WARNING:
For operator safety, do not remove the ground (earth) connection in the
power lead of the console or connected equipment.
To avoid interference pickup, keep audio cables away from mains power units and cables, lighting cables,
thyristor dimmer units, computer equipment, and mobile phones. Where this cannot be avoided, cross the
cables at right angle (90 degrees) to minimize interference.
A NOTE ON BALANCED CONNECTIONS
A differentially balanced connection has two signal wires, (hot) and signal - (cold), and a shield. The
signal source generates positive going polarity down the + wire and negative polarity down the - wire. The
destination input stage accepts the + signal on its non-inverting (+) input pin, but it inverts the - signal, adding
it to the + signal. The result is that the wanted signal is boosted. Now examine what happens when
unwanted interference (hums and noise) is induced into the cable. The noise is induced equally and with the
same polarity into both wires. At the destination input, the - wire signal gets inverted and added to the +
signal. Because the polarity is the same on both input wires, the noise cancels itself out at this point
(destination). For this interference rejection to work, it is important that the source, the cable, and the
destination input are all balanced. Balancing provides greatest advantage with low level signals such as
those produced by microphones.
An impedance balanced output provides similar interference rejection, but not as much maximum drive
capability as the differentially balanced output, typ20dBu 26dBu. It does not generate a
negative polarity signal at its - output. Instead the - wire has no signal but is held at the same impedance at
the + wire. This means that both wires pick up the noise equally, resulting in the advantage of noise
cancellation as described above when connected to a balanced input stage.
______________________________________________________________________________________
11
11
© GECKO MUSIC GROUP. All Rights Reserved.
