Lab.gruppen LAB 1600 - Manual (Page 10)

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
ltage [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% !
Traditional PSUs behave erratically at certain
frequencies, during low frequency cycles (bass),
particularly in clip conditions, their output
impedance's rise thereby producing uncontrollable
speaker cone movement. Under these conditions clip
states are more audible.
LAB.GRUPPEN SMPS produces its rated power
output at all frequencies regardless of speaker
demands and AC line conditions.
Minimum Load Switch (MLS
(TM) )
Thanks to the regulated SMPS, the maximum
power available for the output stages can be
adjusted without increased heat dissipation or
efficiency loss. This allows users to match the
output power with the loudspeaker impedance.
The LED bargraph is also adjusted to give a correct
reading. Because the amount of power delivered into
a load is dictated by its impedance (at 2 ohms you
use the least voltage but use the most current), you
can cheat with the MLS switches thereby enabling
powers in excess of the normal power rating to be
produced.
Safety approvals
10