LD PA 400, User Manual

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Front Panel
Air Intake
Channel A Air Intake
Channel B
Power Switch
Volume Control
Channel A Volume Control
Channel B
Power LED
Statusindicator
“SIGNAL” Statusindicator
“CLIP”
Pegelsteller
Kanal A
Air intake ports left and right
The fresh air required for cooling is drawn in here. Nothing may be permitted to block the influx of air. If
the power amplifier is operated in an extremely dusty environment, the protective grilles may become
clogged. As a precaution, therefore, they should be cleaned from time to time using a vacuum cleaner and a
dry brush.
Power Switch
To turn the amplifier on, press the power switch. This activates an electronic circuit that gradually powers
up the power amplifier (soft start) and thus prevents an excessively high current surge from tripping a
circuit breaker. In addition, power-on noises are suppressed. The fans turn at minimum speed.
Status LEDs
A few seconds after power-on, the two green LEDs light up to indicate that the power amplifier is
operational. If the other LEDs blink on momentarily, this is not a fault, but may occur for technical reasons
due to non-stabilized electrical circuits during power-on.
Volume Controls Channels A & B
The input sensitivity of all power amplifiers is 0 dBV or 1 V respectively. Maximum output level is attained
with this input voltage. In order to attain optimum system dynamics, the volume controls should be turned
up all the way. This can, however, result in increased noise levels when certain signal sources are
connected, especially if these sources, e.g., mixing boards, deliver output voltages far exceeding those
required for maximum modulation. In this case it is more advantageous to turn back the volume controls so
that the maximum desired volume is still attained. The volume controls are detent potentiometers with 41
detents.
“SIGNAL” Status Indicator
Starting already at an output voltage of 20 dBu, which is equivalent to an output of 1.5 mW into 4 ohms, the
blue indicator LED shows that there is a signal on the loudspeaker outputs. If you still do not hear any
sound from the speaker cabinets, check the speaker cables, because the illuminated status LED clearly
indicates that a signal is detected at the output stage.
“CLIP” Status Indicator
Starting at approx. 3 dB overload, the blue indicator LED goes off and the red “CLIP” LED lights up. Then,
however, the limiter is also activated, which prevents the voltage from developing into a square wave. High
amplitude square waves can damage both the power amplifier and the connected loudspeakers. This circuit
is intended to prevent this. Thus the built-in limiter is consciously not designed to fulfill a dynamics
reduction function. If the CLIP LED blinks on briefly, this is acceptable, since the human ear does not
register such peaks as distortion. Under no circumstances, however, should the LED remain on, this would
mean that the power amplifier had reached its performance limits, in this case turn down the volume
controls.