Speakers causing amplifier instability

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joel

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A friend has a strange problem that we cant figure out.

The short version is as follows.

The system comprises of highly regarded amplifier and speakers.

After playing at highish levels for a few hours a scratching sound appears on the speakers. (possible coil damage)
If left for a day everything plays fine until the system is played loud again and then after a slightly shorter period speakres show signs of damage.

The system has been set up so that amplifier clipping doesn't occur. (using test tones a safe maximum vol is found and never exceeded)
There is no audible distortion until speakers display scratching sound.

This has happened with two sets of speakers.

Amp has been checked by agent, no fault found.

After exhausting most other logical thoughts my latest one is that due to heat on amplifier and or speakers the amp becomes unstable and pumps out enough DC or whatever to damage speakers.

My last suggestion to my friend was that he should build a reactive dummy load hook a scope up to the load and simply play the amp/into the load until he sees something wrong.
Ideally he should test with speakers but no one wants to break yet another pair.

My only other thought is that nothing is wrong and that Jack just happens to be talking so loud at the end of the evening that the golden rule  of volume settings gets broken.

Any ideas

 
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