Lab.gruppen fP2600 - Manual (Page 10)

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1. Check if the loudspeaker can take the amp’s maximum output power
(generally, twice the RMS power handling should be okay).
2. If the loudspeaker can handle the amp’s maximum output power, l
MLS at its highest position (0dB).
eave
3. If the loudspeaker is specified at a lower power handling, reduce the
amp’s output power by changing MLS to a matching lower value (-
3dB).
4. Try the setup with a musical signal. If the amp runs into thermal
protection, or into current limiting (audible as distortion, or as “clicks”
from a cone driver/woofer), reduce the amp’s output power by
changing MLS to a lower value.
If you reduce the amp’s output power by choosing lower MLS values, you only reduce the maximum
output voltage. In fact, you will get more current headroom for low impedance loads. The amp runs at
higher efficiency, and the risk for going into thermal protection is significantly lower.
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Assume that a power amplifier can deliver 1000 watts into 8 ohms, and it should be able to run also into
2 ohms. Theoretically, and according to the physical laws, it should be able to deliver 4000 watts into 2
ohms.
Very few professional power amplifiers are able to do this, as all 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. This is due to resistive losses in the power supply and the use of
current limiting to protect the output semiconductor devices from damage due to much current.
Current limiting is definitely not the best way to reduce the power at low impedances, as the limiting
can produce very nasty distortion and glitches. The reason is that the impedance curve of a loudspeaker
driver 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 may produce glitches in the signal.
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The solution is to use a “Constant Power Converter”. The nice thing with the Constant Power Converter
is that it can produce more current than it takes, and this way overcomes the losses when driving low
impedance like 2 ohms. As 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 it cannot interfere with
the requirements by the loudspeaker impedance curve.
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