PAG E 9
O P E R AT I O N
H U M P R O B L E M S
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 connection 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 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:
1. 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.
2. 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.
3. Ensure that balanced signal leads connecting to the amplifier
are connected to earth at one end only.
S I G N A L G R O U N D - L I F T S W I T C H
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 IS Series 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.
DO NOT CONNECT PIN-1 DIRECTLY TO THE DRAIN
CONNECTION.
You will defeat the amplifiers internal grounding scheme and possibly
cause instability in 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.
I S S E R I E S I N S TA L L AT I O N & O P E R AT I O N M A N UA L