Australian Monitor KA1500 - Manual (Page 14)

not present a difficult load for any signal source.
Your signal source (i.e. the equipment feeding the
amplifier) should have an output impedance of
600 Ohms or lower to avoid unwanted high frequency
loss in the cabling.
Input overload occurs at +20.5dBu (8.25 volts).
See the specification section for more detailed
information.
Hum Problems
Most equipment is designed for minimum hum when
used under ideal conditions. When connected to
other equipment, and to safety earth in an electrically
noisy environment however, problems will often 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
safety earth.
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 in
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 your 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 going to the
amplifier are connected to earth at one end only.
Signal Ground-Lift Switch
When proper system hook-up has been carried out,
you may still have some hum or hum related noise.
This may be due to any of the previously mentioned
gremlins.
Your 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 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 possibily 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.
14 Operation
1.
2.
3.