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The two input level attenuators on the front panel adjust 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.
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The yellow LEDs at the top indicate if
any
protection
circuits are activated.
These are described on page
13
.
The
Clip/limit indicator
tells when the
amplifier output is clipping or limiting. It
has two different indication statuses:
•
If the clip limiter is engaged it has a
short time constant and it illuminates
briefly.
•
If the clip limiter is not engaged it
has an increased time constant and it
illuminates for a longer period.
Front Indicators
The
”-40 dB
” LED’s illuminate if the output signal is greater than -40dB (with 0dB as reference to full
output power). These LED’s also act as signal present indicators.
The two 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, protecting 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.
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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.
Under normal operation the clip limiting is inaudible. The limiter can be turned On or Off by depressing or
pressing the relevant clip limit switch.
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 one exception, Lab.gruppen recommend leaving 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 the gain up slowly).
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