L-Acoustics LA48A User Manual
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Contents
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L-ACOUSTICS LA48a Manual V1.0 27/01/2003 20short circuit condition is maintained, the affected channel will eventually go into thermal protect cycleuntil the short circuit has been removed.8.5 AUTOMATIC FUSE SAVER, AFS LIMITERAFS is an abbreviation for Automatic Fuse Saver. The main reason for its presence is to avoid blowingthe mains breaker if a bench test is made at full continuous power. The AFS circuit continuouslymonitors the mains current and if the average mains current is too high for a 16 ampere slow blow fuse(as it can be during some bench test procedures) the AFS limiter will limit the mains current to a safelevel. If the AFS limiter is activated, this will be indicated by the AFS LED on the front panel.The AFS circuit is a current limiter that limits the mains power current draw to 20 A (230 V) or 40 A(115V). AFS circuitry has the same characteristics as the mains breaker so it will allow for high peakcurrents (that do not significantly raise the long term average power) while limiting the current if theamplifier draws a high continuous current (this is normally responsible for blowing the mains breaker).If the AFS circuit is limiting the mains power current or if the regulated power supply goes out ofstabilization, it will be indicated by the AFS LED on the front panel. Note: it is possible for the powersupply will go out of stabilization at low mains voltage in combination with low load impedance duringthe start-up sequence. Since 1997, when this feature was introduced, not a single user has reportedthat AFS has been activated during music performance, even in high power demanding situations.During bench test procedures, if the average current draw exceeds what the mains breaker can takethe output power is reduced by 2 dB. Effectively, this is the same as an LA48a being reduced to thepower of an LA24a (but again, not a single user has reported that the AFS has been activated). Inreality this is much better than the mains breaker blowing and the amplifier being turned off - thelistener can hardly hear this since only the headroom will be reduced and if the peaks are around 120to 130 dB, 2 dB is not noticeable.8.6 AC MAINS VOLTAGE PROTECTIONIf the AC mains voltage is outside the allowed operational voltage (over or under voltage), the powersupply will automatically shut down. Once the mains voltage is above the minimum start voltage andbelow its maximum operating voltage the amplifier will restart. It takes a few seconds for the amplifierto self-check (this is known as the soft or slow start sequence), the gain will slowly and gradually beincreased back to the previously set level and the fans will blow at high speed before going into idle. Thetwo green îONî LEDs indicate when the power supply is operating properly.8.7 THE ìACî LEDThe green AC LED indicates that there is proper mains power present at the incoming mains powerconductors. Electrically, the AC indicator is located in front of the power switch and soft-start circuit,so it lights up even if the power switch is set to off.The LA48a has a power relay included in its soft start circuitry that switches the mains instead of thepower switch on the front panel. This method is more reliable and takes less space than theconventional method of employing huge power switch. However, some governmental safetystandards require that there is an indicator showing that there is power to the circuits if the power isrelay-switched.8.8 DC PROTECTIONThere are two types of DC protection: 1) Fuses on the supply branches of each channel; 2) DC crowbarprotection that shorts the output if more than 10 volts DC are detected on the outputs. Both thesecircuits are activated if a DC level is detected on either channel. |
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