Lab.gruppen PLM 20K44 Operation Manual page 23

Trigger voltage
100
90
80
70
60
50
40
30
20
10
0
0 5 10
Frequency [kHz]
Fig 5.4: 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 5.5.
Full output voltage
1400
1200
1000
800
600
400
20
200
0
0 5 10
Frequency [kHz]
Fig 5.5: VHF Protection Attack Time Variations
The VHF protection circuit does not alter the amplifier'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 the Power channel's LED that will illuminate yellow and display VHF Warning when the
maximum -12 dB gain reduction is applied (20K44 and 12K44 only). The Power channel LED will light red for VHF Fault.
Both of these will be registered in the Lake Controller and CAFÉ status views.
5.3.11. 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. When this
situation occurs, the output stage is muted to protect it from damage.
Operation of the short circuit protection system is indicated by red indication on the affected Power channel
and a corresponding text description on the front panel, in Lake Controller and CAFÉ software programs.
Refer to section 9.1 for more information on warnings and faults.
The presence of a short circuit (or low impedance) is re–tested every six seconds, and the output remains muted
until the fault clears.
15 20 25 30
15 20 25 30
5. Operation and performance
PLM+ SERIES Operation Manual rev 3.0.1
23