Amp on indicator (on speaker)

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Hi all,
My son was recently asked to use his two mobile discos in a pub/club. He was running 2x150 and 4x250 watt amps, one of them into 4 ohms, the rest 8.
Sometime during the night he noticed the 4 ohm amp had died, but actually no one had noticed that that amp was not working.
On opening it up, I saw that the protection circuit had fried, literally melted a resistor plus a BD140, I intend beefing it all up a bit.
Now to the question, is there any way I can have a small indicator, say a LED and a circuit that will indicate that an amp is not working, something one could connect across the speaker that would show its on. This would at least show him that the protection has kicked in and he needs to do something about it.
Any bright ideas?
DieterD
 
How about tapping off of the output and feeding the signal through a rectifier and filter, then a limiter, then a digital buffer to drive an LED? This will sense any AC output (i.e. audio output) and, if present, cause the LED to be on. If you lose and audio output, the LED will go out. Or vice versa. While you would get a failure indication when there is no audio output during normal breaks in music (if that ever occurs with disco), it should not be a problem - or one could add a delay.
 
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