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The yellow LED’s on the top, indicate if any
protection circuits are activated. The protections a
described further below on this page (
15
re
).
The
Clip indicator
tells if the amplifier output is
clipping or limiting. It has two different indication
statuses:
•
If the clip limiter is enabled, it has a short time
constant, and it illuminates briefly.
•
If the clip limiter is disabled, it has an in
time constant, and it illuminates for a longer
period.
creased
The green LEDs indicates –10 dB level below clip
Front Indicators
The ”-25 dB” LED’s illuminate if the output signal is greater than -25dB (with 0dB as reference to full
output power). These LED’s also act as signal present indicators.
The four bottom green ”ON” LED’s indicate that the output circuits are receiving the correct rail
voltage.
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Each Lab.gruppen amplifier has many advanced protection features. Should a fault condition arise they are
protecting both, the amplifier and the speakers connected to it. Under normal use these features are
inaudible. All protection circuits work independent from each other.
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The clip limiter is included to prevent from dangerous clipped signals reaching the speaker and damaging
it. If an amplifier is severely overdriven, its output waveform is clipped (its peaks are squared off) –
reducing the crest factor. In extreme cases the waveform can approach that of a square wave. An amplifier
is normally capable of producing far more power under these conditions than its normal undistorted rated
output power.
The limiter 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 cannot detect it, and will not be activated.
During normal operation the clip limiting is inaudible. The limiter can be turned on or off by using the
relevant clip limit switch. Note that in Bridge mode the limiters of both channels should be in the same
position, either on or off.
Some manufacturers of loudspeaker controllers do not recommend the use of clip limiters in amplifiers, as
they tend to upset the tracking of the controller’s limiters.
Apart from this single exception, Lab.gruppen
recommend to leave the clip limiters switched "on" (button
depressed). As a side-effect, once the amplifier comes out of a protect condition, the output level has a
slow rise time – the effect is like turning up the gain slowly.
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If the amplifier is driven very hard into a low impedance load, the cooling fans will run at high speed. If
the causing conditions continue, the “Temp” temperature indicator(s) will illuminate indicating that the
amplifier is about to go into thermal shutdown.
After approximately five seconds the amplifier will go into thermal protection by muting the input signal.
After another 15-20 seconds the amplifier should have cooled down enough for the amplifier to come out
of shutdown and operate as normal. If the load conditions remain unchanged, the thermal protection will
be reinitiated accordingly. Thermal protection starts, if the
Intercooler
®
heatsink reaches a temperature
above 90
°
C.
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