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The "-25 dB" LEDs glow when the output signal is
greater than -25dB, 0dB is referenced to full output
power. These LEDs also act as signal present
indicators.
The Clip/limit indicator tells when the amplifier
output is clipping or limiting. The two different states
can be told apart:
•
When the clip limiter is engaged it flickers
briefly.
Protection features
Each LAB.GRUPPEN amplifier has many advanced
protection features, that will protect both the
amplifier and the speakers connected to it, should a
fault condition arise. Under normal use these features
are inaudible. All protection circuits are independent.
Clip limiter
The clip limiter is included to prevent dangerous clip
signals reaching the speaker and damaging it. It
works by monitoring the output and comparing the
distortion produced between the input and output of
the amplifier. If the distortion exceeds 1%THD for
any reason ( voltage or current clipping), the limiter
reduces the input signal proportionally. Note that, if
the signal is distorted or clipped before it reaches the
amplifier, the clip limiter will not be activated. Under
normal operation the clip limiting is inaudible. The
limiter can be turned On or Off by depressing the
relevant clip limit switch (3). Some manufacturers of
Loudspeaker processors do not recommend the use
of clip limiters in amplifiers, as they tend to upset the
tracking of the processor's limiters. Apart from this
one exception, Lab.Gruppen recommend leaving
the clip limiters switched "on" (button depressed).
As a by-product, when the amplifier comes out of a
protect condition, the output level has a slow rise
time -the effect is like turning the gain up slowly.
Thermal protection
When the amplifier is driven very hard into low
impedance, the temperature at the heat sinks will rise.
If the condition that cause this continue the
temperature indicator(s) will light as an indication
that the amplifier will soon thermal out. After five
seconds the amplifier will go into thermal protect,by
muting the input signal. After 15-20
Design features
Safety approvals
Lab.Gruppen amplifiers are designed to meet the
IEC65, a stringent electrical safety approval from the
International Electro-technical Commission. The
IEC65 is recognized world-wide with most countries
having an equivalent.
EMC approvals
EMC stands for Electro Magnetic Compatibility. This
implies that the equipment should have low emission
•
When the clip limiter is not engaged it lights for
a longer period.
The remaining yellow LEDs indicate if any
protection circuits are activated. These are described
below.
seconds the amplifier will cool down and the cycle
will begin
again. Thermal protection starts when the heat sinks
reaches a temperature above 90
VHF protection
When a signal of more than 20khz, at full output
power, is present at the output connectors for more
than five seconds, the VHF protection mutes the
input signal (this is indicated on the front panel (3)
labeled PROTECT). After five seconds the cycle
starts again.
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.
The LAB 300 is equipped with LAB.GRUPPEN's
Adaptive limiting system (ALS
circuit continuously monitors the supply rails, and
limits the current going to the output transistors. If
TM
the ALS
is activated the input signal is muted,
and the yellow LED labeled "PROTECT" will light.
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.
of radio frequencies, directly as electromagnetic fields
in the air, and as conducted from the cables from and
to the unit. The unit should also be able to handle
electromagnetic fields, high voltage flashes, and radio
interference, coming into the unit via the air or cables.
Emission
The regulation now cover the frequency range from
150 kHz to 300 MHz. The maximum level radiated
from a unit has to be below 100 µV,
( 0.0001 V ).
Immunity
0
C.
TM
TM
). The ALS
8
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