L-Acoustics LA48A User Manual
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Contents
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L-ACOUSTICS LA48a Manual V1.0 27/01/2003 19The rest of the green LEDs form a bar display indication of output levels from -20 dB to -5 dB.The ìCLIPî LED indicates when the amplifier output is clipping or limiting. It has two different indicationstatuses:• If the clip limiter is engaged the CLIP LED has a short time constant and illuminates briefly.• If the clip limiter is not engaged the CLIP LED has an increased time constant and illuminates for alonger period.The remaining TEMP and VHF LEDs indicate if any protection circuits are activated. Protection featuresare described in the following section.8. PROTECTION FEATURESEach L-ACOUSTICS amplifier has many advanced protection features, protecting both the amplifier andthe loudspeakers connected to it should a fault condition arise. Under normal use these features areinaudible. All protection circuits are independent.8.1 CLIP LIMITERThe clip limiter prevents dangerous clipped signals from reaching the loudspeaker and causing damaging.When an amplifier is severely overdriven, the output waveform is clipped (peaks are squared off) ñreducing the crest factor. In extreme cases, the waveform can approach that of a square wave. Anamplifier is normally capable of producing far more power under these conditions than its normalundistorted rated output power.The clip limiter works by monitoring the output and comparing the distortion produced between theinput and output of the amplifier. If the distortion exceeds 1% THD for any reason (voltage or currentclipping), the limiter reduces the input signal proportionally. Once the amplifier comes out of a protectcondition, the output level has a slow rise time - the effect is like turning the gain up slowly. Note: if thesignal is distorted or clipped before it reaches the amplifier, the clip limiter cannot detect it and will notbe activated.Under normal operation, clip limiting is inaudible. The limiter can be turned On or Off by depressing orpressing the relevant rear panel clip limit switch. L-ACOUSTICS recommends leaving the clip limitersswitched ON at all times (Channel A, B rear panel clip limiter buttons depressed in the ìINî position).8.2 THERMAL PROTECTIONIf the amplifier is driven hard into a low impedance load, the cooling fans will run at high speed. If thiscondition persists, the Temperature indicator(s) will illuminate indicating that the amplifier is about to gointo thermal shutdown.After five seconds the amplifier will go into thermal protection by muting the input signal. After 15-20seconds the amplifier will have cooled down enough for the amplifier to come out of shutdown andoperate as normal. If the load conditions remain unchanged the thermal protection will be reinitiated.Thermal protection starts when the IntercoolerÆ heat sink reaches a temperature above 900 C.8.3 VHF PROTECTIONIf a signal above 12 kHz is detected at the amplifier outputs for more than five seconds at full outputpower, VHF protection mutes the input signal. This is indicated on the front panel LED indicatorlabeled VHF. After five seconds the outputs will un-mute and return to normal operation, unless theoutput signal has remained unchanged, in which case the VHF protection will re-initiate.8.4 SHORT CIRCUIT PROTECTIONAll L-ACOUSTICS amplifiers are completely short circuit-protected. Short circuit protection circuitrypermits very high peak currents, but still holds the output devices within the safe operation area. If a |
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