Lab.gruppen LAB 4000 - Manual (Page 10)

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This is a useful feature as there are currently no
commercially available compression drivers that can
take 500 watts of power at 10 kHz!! this circuit is
inaudible under normal use, however it can be turned
off.
If the VHF protection is required to be turned off
i.e. for studio monitors, please consult your
supplier, as this is a non-user adjustment.
Short circuit protection
All LAB.GRUPPEN amplifiers are completely
short circuit protected. The protection circuit
permits very high peak currents, but still holds the
output devices within the safe operation area. If a
short circuit is maintained, the channel affected will
eventually go into thermal protection and the cycle
will start again.
Adaptive Fuse Saver, AFS tm -limiter;
The AFS TM circuit continuously monitors the mains
current. If the average mains current is too high for a
16 ampere slow blow fuse, as it can be during so
called bench tests, the AFS™-limiter will limit the
mains current to a safe level. If the
AFS-limiter is activated this will be indicated by
the AFS-LED on the front panel. The AFS limiter
will not be activated during normal use, even if the
music will be in the state of constant clip.
AC mains voltage protection
If the AC mains voltage is outside the allowed
operation voltage ( over or under voltage) , the
power supply will shut down itself. When the
mains voltage is above the minimum start voltage
and below its maximum operating voltage the
amplifier will restart again.
It takes a couple of seconds to check its circuits (this
is known as the "soft start" or "slow start" sequence),
the gain will slowly and gradually raised back to
normal, and the fans then blow at high speed before
going onto "idle". The two green ”ON” LED’s
indicate when then power supply is operating
properly.
D.C. protection
There are two types of DC protection:
Fuses on the supply branches of each channel (this is
an IEC 65 requirement).
A Crowbar bar protection that shorts the output.
Both these circuits come into effect once a DC level
of 10 volts or more is detected on either channel.
Design features
Cooling
The LAB.GRUPPEN amplifier runs very cool due
to a patented heat sink called Intercooler TM . The
output devices (bi-polar) are mounted directly on a
copper heat sink thereby avoiding thermal losses
normally found when using mica washers (the heat
sink is mounted horizontally in front of a pressure
chamber) The air flow is constant along the whole
heat sink,
thanks to a horizontal pressure chamber in front of it,
(in a conventional tunnel design the end of the heat
sink opposite to the fan invariably gets hotter than
that directly in front of the fan). When cool air hits
the geometric fins of the Intercooler TM , turbulence is
produced thereby enabling efficient cooling.
Light weight
The switch mode amplifiers has a very good power
to weight ratio, (480w/Kg) making it one of the
most powerful 2u amplifiers available.. The light
weight is achieved not only by the PSU, but the
Intercooler TM and the 9 piece anodized alloy chassis
contribute greatly, rear rack supports are unnecessary
in most applications.
Switch mode power supply (SMPS)
Switch mode power supplies are the modern
solution to the problems of size and weight.
However, early examples of SMPS used in power
amplification were not ground breaking, and
literally adopted the type of supply found in many
computers today. The LAB.GRUPPEN SMPS is
different in that it is regulated. How is this
achieved?
In contrast to other designs, that use push pull
converters we have managed to produce a SMPS
that has the same characteristics as a conventional
supply. We have avoided the pitfalls of push-pull
converters, by eliminating the need for limiting on
the secondary side of the switch transformer. This
is achieved by using sense windings, injecting a
magnetic pulse from the AC- line during a pulse
time segment which is separated from the output
charge current pulse.
Whilst the SMPS may sound complex it is actually
very straight forward with high reliability.
The net result is good bass and fast response.
Because the supply is regulated, the power
amplifier will produce the same power output even
if the AC line falls by more than 20% !
Efficiency is also assured, and in the case of the
LAB 4000 this is especially apparent ; its power
supply draws only 6 kW and yet it produces 2 x
2100 W.