Lab.gruppen PLM 20K44 Operation Manual page 26

5. Operation and performance
• If there are one or two "spurs" with 70V loudspeakers connected; if two in parallel is default, it will warn if one
is missing.
• If there is a short circuit in the load.
• If there is an open circuit (all loudspeakers missing).
• For passive 2– and 3–way loudspeakers, if one or two LF drivers are connected and working; if two in parallel
is default, it will warn if one is missing.
• For some passive 2–way designs, if one HF driver is disconnected. (However, with some loudspeakers the
HF impedance is dominated by a passive component i.e. crossover filter and the anomaly will not be detected.)
NOTE: LoadPilot is enabled only when the amplifier is On. It does not function while the amplifier is in
Standby.
5.5.3. Manual configuration
5.5.3.1. Functional description
Manual configuration can be used in special cases where proper functioning requires setting of custom
measurement frequencies, custom measurement levels, custom warning thresholds, or combinations of the three.
Manual configuration addresses the following issues:
High noise levels – To obtain accurate impedance measurements, it is necessary to drive a current sufficiently
above the noise floor. However, it is important to keep the level low in order to prevent audible artifacts from cone
travel. At very high impedances, the drive current may be insufficient. In the automatic mode, level is set around 60
mV which produces usable results for impedance up to 660 ohms.
Inductance in loudspeaker cables – Loudspeaker cable inductance at higher frequencies is very high relative to
passive resistance, and the inductance increases linearly with length. Consequently, monitoring at the 24 kHz pilot
tone frequency can be problematic with very long cables. The automatic mode will estimate cable impedance for
24 kHz and allow it to be used as long as it is less than 33% of total impedance presented by the load and as long
as the total impedance is below about 440 ohms.
DC blocking capacitors in loudspeakers – Some loudspeakers incorporate a capacitor designed to block
damaging voltages at very low frequencies. This may inhibit accurate measurements when using the 10 Hz pilot
tone. In automatic mode, LoadPilot will disable the 10 Hz tone if total impedance is above about 440 ohms.
Constant voltage loudspeakers – Loudspeakers driven via transformers in 70V/100V solutions are more likely
to exhibit wide variations in measured impedance as music and/or pilot tones are reproduced by the system. Also,
most constant voltage systems operate more than two loudspeakers in parallel. Therefore, in addition to manual
configuration of LoadPilot, additional step may be required. These include:
• Insertion of a series capacitor in each loudspeaker to block the 10 Hz LF tone – This will defeat measuring the
loudspeakers with the LF tone, but will allow the LF tone to propagate through the entire cable run.
• Insertion of EOL (end–of–line) resistors – This enables LoadPilot to detect cable faults in a spur of the system.
• Insertion of EOL (end–of–line) inductor – This is not mandatory, but it will avoid the loss of level at mid and high
frequencies due to the EOL resistor.
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PLM+ SERIES Operation Manual rev 3.0.1