Contents; Dc Protection; Vhf Protection; Short Circuit Protection - Lab.gruppen PLM Series Operation Manual

If an output channel temperature fault condition
arises, the output of the channel causing the condition
will be muted. Temperature checking will continue
thereafter at 0.5 second intervals, with the output
remaining muted until the fault condition clears.

7.3.7 DC protection

DC protection is implemented on each output to
prevent damage to connected loudspeakers or any
PLM components. DC present at the output will
cause the PLM's amp module fuse to blow. In this
instance a red LED will illuminate and the message
"Needs service" will be displayed.
This only applies to 230 V versions as no
fuse is fitted to 115 V models. This is not a
user-servicable fault condition and the unit
should be returned for repair.

7.3.8 VHF protection

The PLM includes protection circuits that detect
Very High Frequency (VHF) content in the input
signal. The detection is frequency-dependent,
initiated from 10 kHz upwards. If VHF signals are
detected above the allowed threshold, the output
will mute for approximately 6 seconds before a
further measurement is taken. Once no continuous
VHF signal is detected, the output unmutes and the
amplifier returns to normal operation.
This protection system recognizes that continuous
VHF signals at high levels do not appear in speech or
music. Any such content can therefore be considered
as a fault condition. VHF protection is essential to
avoid damage to HF drivers.
VHF protection is dependent on a combination of
output power level and frequency. Figure 7.3.8a
below shows a decreasing power threshold, from
approximately 10 kHz upwards, which illustrates
increasing sensitivity of the protection system with
frequency. When continuous output power above the
threshold line is detected, VHF protection becomes
active.
OPERATION AND PERFORMANCE 7
Figure 7.3.8a: VHF protection frequency sensitivity
The attack time of the VHF protection circuitry also
changes with frequency, becoming shorter at higher
frequencies. This is shown in figure 7.3.8b.
Figure 7.3.8b: VHF protection attack time variations
The VHF protection circuit is NOT a limiter and
does not alter the PLM's frequency response. It
is implemented solely to detect continuous VHF
content. HF content of normal music or speech
signals at peak levels will be passed in full.
Operation of the VHF protection circuits is indicated
by one (or more) of the output channel LEDs (in the
right-hand soft function buttons) showing steady red.
The adjacent fault message will show VHF FAULT. It
is also reported as a fault via the control network.

7.3.9 short circuit protection

A low impedance or short circuit at the power output
terminals is detected when the output current is high
(Current Peak Limiter is active) and, simultaneously,
the peak output voltage is below a predetermined
threshold (42 V with the PLM 10000Q for example).
When this situation occurs, the output stage is muted
to protect it from damage. Operation of the short
circuit protection system is indicated by an output
PLM Series Operation Manual 21