Lab.gruppen fP 3400 - Manual (Page 11)

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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”. This converter is connected in between the power
supply and the linear power amplifier. The nice thing with the Constant Power Converter is that it can
produce more current than it takes from the power supply, and this way overcomes the losses in the
power supply 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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The amplifier is two rack units high (2U) and will fit into a standard EIA 19” rack. Amplifiers may be
stacked directly on top of each other; there is no need for spacing in between units. If it is the intention
to fill a rack with amplifiers, we recommend to start racking from the bottom of the rack. It is also
recommended that rear supports are used for amplifiers mounted in the middle of the rack, especially if
used as part of a portable system.
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The amplifier uses a forced air cooling system to maintain a low and even operating temperature. All
fan-cooled Lab.gruppen amplifiers have front to rear airflow. There are several reasons for this, one
being that there is usually cooler air outside the rack than inside, and therefore the amplifiers can run at
higher continuous power levels without thermal problems. Never try to reverse the airflow, as the
Intercooler® needs a pressure chamber between the fans and heat sink, and this only works in one
direction of the airflow.
Should a heat sink get too hot, its sensing circuitry will mute the hot channel. If the power supply
overheats, another sensing circuitry will mute all output channels, until it cools down to a safe operating
temperature.
Make sure that there is an adequate air supply in front of the amplifier, and that the rear of the amplifier
has sufficient space to allow the exhaust to escape. If the amplifier is rack-mounted, do not use covers
or doors on the front or rear of the rack.
For installations with a central cooling system, as usually found in fixed installations with a dedicated
rack room, it may be necessary to calculate the maximum heat emission. Refer to power consumption
on page
11 .
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