Lab.gruppen PLM series Operation Manual

Also see for PLM 10000Q: Supplementary guide User manual addendum Specifications Quick start Technical data

Contents

Lab.gruppen PLM series Operation Manual Manual pdf 29 page image

Operation and Performance23PLM Series Operation Manual rev 1.1.95.3.9 DC ProtectionDC protection is implemented on each power output to prevent damage to connected loudspeakers or anyPLM components. DC present at the output will cause the PLM’s power output module breaker to blow. Inthis instance a red LED will illuminate and NEEDS SERVICE will display on the LCD.The power output channel modules are independent ofthe input voltage. Both 115 and 230 V models have ampchannel breakers. This is not a user-servicable faultcondition and the unit should be returned for repair.5.3.10 VHF ProtectionThe PLM includes protection circuits that detect Very High Frequency (VHF) content in the input signal. Thedetection is frequency-dependent, initiated from 10 kHz upwards. If VHF signals are detected above thethreshold, the output will mute for approximately 6 seconds before a further measurement is taken. Whencontinuous VHF signal stops, 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 ormusic. Any such content can therefore be considered as a fault condition. VHF protection is essential toavoid damage to HF drivers.VHF protection is dependent on a combination of output power level and frequency. Figure 5-2 shows adecreasing power threshold, from approximately 10 kHz upwards, which illustrates increasing sensitivity ofthe protection system with frequency. When continuous output power above the threshold line is detected,VHF protection becomes active.Figure 5-2: VHF Protection Frequency SensitivityThe attack time of the VHF protection circuitry also changes with frequency, becoming shorter at higherfrequencies. This is shown in Figure 5-3.