L-Acoustics LA48A, Руководство пользователя

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L-ACOUSTICS LA48a Manual V1.0 27/01/2003 9
MLS positions can be different for each channel and a variety of combinations can be selected as seen in
Table 2. There is no risk for amplifier damage since the LA48a is very well protected (even down to 0.3
ohms). However, if uncertain or where time does not allow, use the default position (MLS = 0 dB).
Table 2: MLS Switch Setting Applications
Channel Impedance MLS Power Comments
A 8 -2dB 820W Power
B 8 -2dB 820W reduction
A 4 0dB 2300W High peak
B 4 0dB 2300W power
A 8 0dB 1300W Power
B 8 -5dB 430W matching
A 8 -2dB 820W Power
B 4 0dB 2300W sharing
4.1.1 MLS TECHNOLOGY BACKGROUND
The Problem:
Assume that a power amplifier can deliver 1000 watts into 8 ohms and that it should also be able to
operate into 2 ohms. Theoretically, the power amplifier should deliver 4000 watts into 2 ohms but
very few professional power amplifiers are able to do this since conventional power amplifier designs
are compromises between power dissipation, cost, size etc. Most amplifiers deliver only 70% or less
of their theoretical power into 2 ohms due to resistive losses in the power supply and the use of
current limiting to protect the output semiconductor devices from damage due to too high current.
Current limiting is definitely not the best way to reduce power into low impedance loads because this
type of limiting can produce very nasty distortion and glitches. This occurs since the impedance curve
of a loudspeaker is not a straight line and therefore shows a reactive load to the amplifier. This
reactive load produces back energy that can trigger the current protection and, as a result, produce
glitches in the signal.
The Solution:
The solution is to use a “Constant Power Converter” connected between the power supply and the
linear power amplifier. The Constant Power Converter can produce more current than it takes from
the power supply and in this way overcomes the losses in the power supply when driving low
impedances (such as 2 ohms). Since power is the product of current and voltage, an increase in
current requires a reduction in voltage. This reduction in voltage also causes the dissipation in the
output devices to be lower. The current limit protection can be adjusted for a much higher current,
so that it does not interfere with the requirements dictated by the loudspeaker impedance curve.
4.2 CLIP LIMITER
The LA48a Channel A, B clip limiters prevent dangerous clipped signals from reaching the loudspeaker
and causing damage. When an amplifier is severely overdriven, the output waveform is clipped and
peaks become squared off, causing a reduction in crest factor. In extreme cases, the waveform can
approach that of a square wave. Normally an amplifier is capable of producing far more power under
these conditions than its normal, undistorted rated output power.
The LA48a clip 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. Once the amplifier comes out of a