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Just a speaker Protection I found on the web. It actually works rather interestingly.
On the right part you will find a simple delayed enabling circuit. A capacitor charges through a resistor divider chain, and after about 5s the voltage will reach the preset 2V threshold set by the LED on the negative terminal of the op amp. You can substitute it with a potentiometer, to manually set the threshold level.
On the left of the circuit is actually the intriguing part. When the amplifier is working normally, the 15k/47uF RC chain will attenuate the output of the amplifier to a point where it does not reach the triggering voltage for the protection circuit (which is around 2.1V). The RC chain determines the time constant and the lowest operating frequency the system will detect as audio. In this case it is 30Vm/8Hz. If an output transistor fails, and the output shifts to DC, the RC chain no longer attenuates it and a level as low as 2.6VDC will enable the protection and disengage the speakers. It works by rectifying the DC voltage, so that both a negative and a positive DC shift will enable the protection. The rectified voltage triggers the transistor, which starts stealing current from the RC chain of the delay circuit on the right, until it falls below the threshold level and the speaker disangages.
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