AUSTRALIAN MONITOR AM1600 Manuale operativo - Pagina 14
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14 Operation
Each channel of your amplifier has a nominal
balanced input impedance of 18k Ohms (@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
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:
1.
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.
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 going to the
amplifier are connected to earth at one end only.
Signal Ground Lifting
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.
One of the most effective ways to reduce earth loop
or electromagnetically induced hum is to disconnect
input signal ground at the input connector of your
amplifier. This effectively breaks the earth loop path
or open circuits the input ground path so no
electromagnetically induced currents can flow
through the amplifier.
NOTE: If the input signal ground is lifted 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 reconnect 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 within the
amplifier.
This should only be done 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.