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(6) Mode Switches
For normal stereophonic mode of operation, the
STEREO/BRIDGED input switch should be set to STEREO.
Otherwise, the amplifi er will operate in the bridged
mono input mode by which you will amplify only the
left channel through both outputs of the amplifi er. The
BALANCED/UNBALANCED selector switches should be set
to accommodate the type of input connectors that you
are using. The upper connectors (RCA-jacks) are used for
unbalanced input signals and the lower connectors (XLR-
jacks) are used for balanced input signals.
DESCRIPTION OF UNIT: FRONT PANEL
(7) On/Off Switch
The On/Off Switch controls AC power to the power
transformer and circuits of the GFA-555se. Whenever the
GFA-555se is energized the Power LED (8) will glow red. To
turn the GFA-555se on, press the button up. It will remain
in the “engaged” position while the unit is energized. To
turn the GFA-555se off, press the button down and it will
move to the “disengaged” position.
(8) Power LED
This LED will glow whenever the On/Off Switch (7) is
turned on and the GFA-555se is energized. The Power
LED indicates that there is AC voltage being fed to the
amplifi er, but it does not signify that all the amplifi er’s
circuits are in operation. If, for example, the Thermal
Protection LED (9) glows, that amplifi er will not produce
sound even though the Power LED may still glow. When
the GFA-555se is turned off, the Power LED will take some
time to fade out completely. It may take up to 30 seconds
to fully extinguish.
Additionally, the internal power transformer is provided
with a thermostat which will interrupt power into the
transformer if its temperature exceeds 125°C. This high
temperature will seldom, if ever, be encountered unless
the amplifi er is subjected to abnormal conditions, such
as operation into loads of less than 3 ohms at very high
listening levels, etc. If the AC Fuse (4) has not failed, the
Power LED (8) is out, and the Thermal Protection LED (9)
does not glow, this would indicate that the thermostat
within the transformer has opened.
Once the temperature within the transformer decreases
to a normal level, the thermostat will reset itself
automatically and normal operation will resume. If you
are to avoid tripping the thermostat in the transformer
continually, you must reduce the sound level demands,
correct the load impedance of the loudspeakers, or both.
(9) Thermal Protection LED
The GFA-555se is provided with a thermal protection
circuit which will shut down the amplifi er if its heatsink
temperature reaches 75°C. The Thermal Protection LED
will light whenever the thermal protection circuit has been
triggered and the amplifi er is inoperative. The thermal
protection circuitry will typically be triggered by very
high power demands into impedances much lower than
the amplifi er is capable of driving at those levels. If any
amplifi er channel’s output through the loudspeaker(s)
ceases abruptly, and its Thermal Protection LED glows,
you will know that its heatsink temperature has become
unacceptably high and the circuitry is protecting the
amplifi cation devices. Please note that the Power LED (8)
will remain on and the amplifi er will still be energized.
Once the temperature of the heatsink(s) drops to a safe
operating level, that amplifi er will automatically resume
operation.
(10) Distortion Alert LEDs
The Distortion Alert circuit is a unique ADCOM distortion
detection system which reads all forms of non-linear
distortion such as THD, IM, slew-induced, “clipping”, etc.
The Distortion Alert LEDs will light when distortion reaches
approximately 1% regardless of impedance, voltage/
current phase angle or the reactance of the loudspeakers
which the amplifi er is driving. Sometimes, when the
amplifi er is in use, the LEDs may occasionally fl icker
during high volume listening, particularly if you are driving
low impedances. This fl ickering is no cause for concern.
The LEDs are simply warning you that the amplifi er is
approaching its maximum power output into the particular
loudspeakers you are using. If, however, the Distortion
Alert LEDs glow brightly or are illuminated most of the
time during playback, you are over driving the amplifi er
and should turn down your volume control to reduce
the listening level, otherwise it may cause the Thermal
Protection to be activated or, in extreme cases, damage
your loudspeakers.