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.
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. 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.
2
1
3
0
10
20
30
40
50
60
70
80
90
0
5
10
15
20
25
30
35
40
45
50
Current [A]
V
o
ltag
e [V
]
Figure 13. Current-voltage characteristic of different power
supply topologies.
1) Conventional power supply show voltage drop due to
internal resistance
2) Regulated power supply used by LAB.
3) Unregulated switch mode power supply with current
limiting.
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% !
9
