When you power up the amplifier it takes a couple of
seconds to check its circuits (this is known as the
"soft start" or "slow start" sequence), the fans then
blow at high speed before going onto "idle" and the
two bottom green LED’s come on to show the output
circuits are receiving the correct rail voltage.
3. Input attenuators
The two input level attenuators on the front panel,
alter the signal level for their respective amplifier
channel in all modes. They are calibrated in dB to
help setting up active loudspeaker systems or cutting
down unwanted noise from the input signal. In
bridged mode, both controls must be in the same
position, so that the speaker load will be shared
equally between the channels.
4. Gain switch
The gain switch located on rear panel is for changing
the input sensitivity of the amplifier. This can be
handy when using low or high nominal input signals
e.g. most professional mixing consoles operate at a
nominal level of +4dBu therefore use the 29dB
position (depressed switch), to give you plenty of
fader movement. On the other hand, for a disco
mixer that operates at a nominal level of 0dBu, use
the upper position setting, which has a sensitivity of
0 dB for full power in 4 ohms.
Figure 12. Front indicators
5. Indicators
The two bottom green ”ON” LEDs indicate that the
output circuits are receiving the correct rail voltage.
The
”-25 dB”
LEDs glow when the output signal is
greater than -25dB, 0dB is referenced to full output
power. These LEDs also act as signal present
indicators. The rest of the green LEDs forms a bar
for output levels from -20dB to -5dB.
The
Clip/limit indicator
tells when the amplifier
output is clipping or limiting. The two different states
can be told apart:
•
When the clip limiter is engaged it flickers
briefly.
•
When the clip limiter is not engaged it lights for
a longer period.
The remaining yellow LEDs indicate if any
protection circuits are activated. These are described
below.
Protection features
Each LAB.GRUPPEN amplifier has many advanced
protection features, that will protect both the
amplifier and the speakers connected to it, should a
fault condition arise. Under normal use these features
are inaudible. All protection circuits are independent.
Clip limiter
The clip limiter is included to prevent dangerous clip
signals reaching the speaker and damaging it. It
works by monitoring the output and comparing the
distortion produced between the input and output of
the amplifier. If the distortion exceeds 1%THD for
any reason ( voltage or current clipping), the limiter
reduces the input signal proportionally. Note that, if
the signal is distorted or clipped before it reaches the
amplifier, the clip limiter will not be activated.
Under normal operation the clip limiting is inaudible.
The limiter can be turned On or Off by depressing
the relevant clip limit switch (2). Some
manufacturers of Loudspeaker processors do not
recommend the use of clip limiters in amplifiers, as
they tend to upset the tracking of the processor’s
limiters.
Apart from this one exception,
LAB.GRUPPEN
recommend leaving the clip limiters switched "on"
(button depressed).
As a by-product, when the amplifier comes out of a
protect condition, the output level has a slow rise
time -the effect is like turning the gain up slowly.
Thermal protection
When the amplifier is driven very hard into low
impedance, the cooling fans go into high speed. If the
conditions that cause this continue, the temperature
indicator(s) will light as an indication that the
amplifier will soon thermal out.
After five seconds the amplifier will go into thermal
protect, by muting the input signal. After 15-20
seconds the amplifier will cool down the
Intercooler
®
and the cycle will begin again.
Thermal protection starts when the I
ntercooler
®
reaches a temperature above 90
0
C
VHF protection
When a signal of more than 20khz, at full output
power, is present at the output connectors for more
than five seconds, the VHF protection mutes the
input signal (this is indicated on the front panel (4)
labeled VHF). After five seconds the cycle starts
again.
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