I N S TA L L AT I O N
PAG E 8
AV 2 . 2 P / AV 2 . 4 P – O P E R AT I O N M A N UA L
Hum Problems
Most equipment is designed for minimum hum when used under ideal condi-
tions. When connected to other equipment, and to a safety earth in an electri-
cally 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 ele-
ments 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 flowing through it. The cur-
rent flowing through the wire produces a voltage drop due to the wire’s resist-
ance. 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 which 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.
Signal Ground-Lift Switch
When proper system hook-up has been made, you may still have some hum
or hum related noise. This may be due to any of the previously mentioned
gremlins.
Your AMAV amplifier has a “Signal Ground Lift” switch which disconnects the
input ground wiring from the amplifier. A substantial drop in hum and/or hum
related noise can result from the judicious use of this switch.
NOTE: If the input ground lift switch is used, you must ensure
adequate shielding of the input wiring. If the signal source equipment
does not provide adequate shielding (i.e. a definitive connection to
ground), you must disconnect the shield from the input connector’s
ground pin (Pin-1) and re-connect it to the ”drain” contact on the input
connector. This will ensure the shield on your input wiring actually
goes to the amplifier chassis and subsequently to earth.
IMPORTANT: Do not connect pin-1 directly to the drain connection.
You will defeat the amplifiers internal grounding scheme and possibly
cause instability to the amplifier.
Always ensure that your amplifier is off and the attenuators are down when
you engage this switch. This switch should only be used when the amplifier is
operated from a balanced signal source.
NOTE: Be wary of quasi-balanced outputs, these are often no more
than floating unbalanced outputs.
