User guide

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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.
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 temperature will rise. If the causing
conditions continue, the Protection indicator(s) will illuminate indicating that the amplifier is about to go
into thermal shutdown.
After five seconds the amplifier will go into thermal protection by muting the input signal. After some
time the amplifier will 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. Thermal
protection starts when the heatsink reaches a temperature above 90
0
C.
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If a signal above 12kHz is detected at the amplifier outputs for more than five seconds at full output
power, the VHF protection mutes the input signal. (This is indicated on the front panel labeled Protect).
After five seconds the outputs will un-mute and return to normal operation, unless the output signal has
remained unchanged, in which case the VHF protection will re-initiate.
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All Lab.gruppen amplifiers are completely short circuit protected. The protection circuit permits very
high peak currents, but still holds the output devices within the safe operation area. If a short circuit is
maintained, the channel affected will eventually go into thermal protect cycle until the short circuit has
been removed.
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There are two types of DC protection:
• Fuses on the supply branches of each channel.
• A DC crowbar protection that shorts the output if more than 10 volts DC are being detected on the
outputs.
Either of these protections comes into effect once DC voltage is being detected, independently for both
channels.
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