Sonodyne SBA 250P - Manual (Page 12)

Page 10
SMA/ SBA SERIES
INSTALLATION
HUM PROBLEMS
Most equipment is designed for minimum hum when used under ideal conditions. When connected to other
equipment, and to a safety earth in an electrically noisy environment, problems may occur.
The three ”E”s of hum and hum related noise which can plague your audio system are:
a) Electrostatic radiation,
b) Electromagnetic radiation, and
c) Earth loops
Electrostatic radiation capacitively couples to system elements, causing an interference voltage that mainly
affects higher impedance paths, such as amplifier inputs. The source is generally a nearby high voltage, such
as a mains lead or a speaker lead. The problem can usually be reduced by moving the offending lead away, or
by providing additional electrostatic shielding (i.e. an earthed conductor which forms a barrier to the field).
Electromagnetic radiation induces interference currents into system elements that mainly effect lower
impedance paths. Radio transmitters or stray magnetic fields from mains transformers are often the cause of
this problem. It is generally more difficult to eliminate this kind of interference, but again, moving the source
away or providing a magnetic shield (i.e. a steel shield) should help.
Earth loops can arise from the interfacing of the various pieces of equipment and their connections to various
safety earths.
This is by far the most common cause of hum, and it occurs when source equipment and the amplifier are
plugged into different points along the safety earth where the safety earth wiring has a current fl owing through
it. The current flowing through the wire produces a voltage drop due to the wire's resistance. This voltage
difference between the amp earth and source equipment earth appears to the amplifier's input as a signal and
is amplified as hum.
There are three things you can do to avoid earth loop problems:
∑ Ensure the mains power for the audio system is “quiet” i.e. without equipment on it such as air-
conditioning, refrigeration or lighting that may generate noise in the earth circuit.
∑ Ensure all equipment within the system shares a common ground/ safety earth point. This will reduce
the possibility of circulating earth currents, as the equipment will be referenced to the same ground
potential.
∑ Ensure that balanced signal leads connecting to the amplifier are connected to earth at one end only