Australian Monitor KA800 - Manual (Page 14)

Each channel of your KA800/KA1200 KA800/KA1200 KA800/KA1200 KA800/KA1200 KA800/KA1200 amplifier has a
nominal balanced input impedance of 30kOhms
(@1kHz) and should 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 infor-
mation.
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 sys-
tem 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 cur-
rents into system elements that mainly effect lower
impedance paths. Radio transmitters or stray mag-
netic 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 equip-
ment 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 KA800/KA1200 KA800/KA1200 KA800/KA1200 KA800/KA1200 KA800/KA1200 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
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