Protection Features; Clip Limiter - Lab.gruppen LAB 2002 User Manual

Figure 13. Front indicators
5. Indicators
The two bottom green "ON" LEDs indicate that the
output circuits are receiving the correct rail voltage.
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 rest of the green LEDs forms a bar for output
levels from -20dB to -5dB.

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
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 cooling fans go into high speed. If the
conditions 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
seconds the amplifier
®
Intercooler and the cycle will begin again.
Thermal protection starts when the Intercooler
reaches a temperature above 90
switch (2). Some
will cool down the
®
0
C
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.
•
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.
VHF protection
When a signal of more than 12kHz, 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 (4)
labeled VHF). After five seconds the cycle starts
again.
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.
9