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Load monitoring


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#1 T Harrison

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Posted 04 December 2003 - 05:26 AM

Some questions for the IQ team regarding the detail of load monitoring:

1) Need more explanation of HOW the load monitoring is performed, in order to explain to a consultant and contractor.
2) How is it possible to calculate the minimum signal required to ensure that the system can accurately load monitor at all times, as the system needs some stimulus in order to monitor.
3) Need advice on how to set the high and low impedance limits. The system concerned would be a three-way active system, so will have different components on different channels.
4) It appears not to be possible to measure the impedance curve of the load, although this feature used to be available in the now discontinued SLM-8 module (curve viewer under the View menu), and is available on competitive product - the EV P series amps for instance. Is this planned to be available for the future?

We have scoured the documentation for greater explanation, but without success.

Any further information on the subject would be greatly appreciated.

Tim Harrison
Fuzion plc.

#2 Bradford Benn

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Posted 04 December 2003 - 02:28 PM

Average load impedance is calculated as a function of amplifier output voltage and current. The system requires approximately 20-30 mA of average amplifier output current for adequate supervision. This allows typically low average output power levels of less than ? watt with most loads. The maximum load impedance for reliable system performance is limited to about 50 ohms. Higher impedances can be measured but may require higher amplifier output levels for reliable operation.

Most amplifier/load systems can be configured and supervised by following these steps:
  • Configure your audio system using a known ?good? load, then enable the Load Supervision feature.
  • Provide typical program material at a level high enough to light the ?test? indicator.
  • Run the system at this level until the average impedance stabilizes. This may take seconds to minutes depending on level, duty-cycle, etc.
  • Set the nominal impedance at the measured value average. This optimizes the supervision algorithm for voltage and current levels versus the actual load. Note: a higher nominal setting will require higher output levels.
  • Set the high limit at twice average and the low limit at one-fourth nominal (just a suggested arbitrary starting point).
  • Let the system run for extended periods using any and all typical program material.
  • Adjust the high/low limits, if necessary, to account for any variance in average measured impedance.
  • Enable error reporting, if desired.
This procedure should work well for most applications. However, some applications can be a little more difficult. Some very low-level and/or low duty-cycle signals may not adequately ?test? the load. Lab and situation testing have shown output levels as small 40 dB below rated amplifier output to be enough for most low-impedance loads. Higher impedance loads such as those used in ?lightly-loaded? 70V distribution lines may require signal level near 20 dB below rated output.

The ?Nominal Load Impedance? control is used to optimize the system for the most accurate calculation of load impedance. It should be set to the expected nominal (or rated) impedance of the ?normal? load.

The high limit should be set for at least 2 times the expected nominal or actual measured load, while the low limit should be set to ? the expected nominal or actual measured load.

It is well known that the typical loudspeaker impedance is not the same for all frequencies. This variance is due to the effect of electrical properties such as the expected increase in impedance at high frequencies due to driver voice-coil inductance, or the peaks and valleys due to passive crossovers. Testing of various passive boxes has shown peaks of 100 ohms or more! Low frequency impedance variation can come from the interaction of the driver compliance with that of the box. The low frequency variations are usually wide bandwidth and may vary from 6 to 30 ohms on an 8-ohm driver. The load supervision algorithm easily averages out these anomalies in most systems. However, there may be some extreme situations for very narrow bandwidth (i.e. single-note) signals and/or very widely varying loads
-=Brad

Bradford Benn
Business Development Manager - Crown International
http://www.crownaudio.com/

#3 T Harrison

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Posted 16 December 2003 - 05:11 AM

Thanks Brad. All very useful and practical background which the contractor appreciates. The only area which remains unanswered is no. 4...
Tim

#4 Bradford Benn

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Posted 16 December 2003 - 08:16 AM

All that I can say about number 4 is that it would require doing a sweep to determine the entire curve, something which typically can not be done while the system is in use. Most people find that objectionable.
-=Brad

Bradford Benn
Business Development Manager - Crown International
http://www.crownaudio.com/