Lab.gruppen fP 3400 - Manual (Page 16)

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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 assist the setup of active loudspeaker systems, or to cut
down unwanted noise from the input signal.
In bridge 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 LED’s indicate if any protection
circuits are activated. The protections are
described further below on this page (
15 ).
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
increased time constant, and it illuminates
for a longer period.
Front panel indicators
The ”-25dB” 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 presence indicators.
The rest of the green LED’s form a bar for output levels from -20dB to -5dB.
The green bottom ”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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