DC detect speaker protection
 
Even the best amplifiers can become faulty. One of the more common faults is destruction of the output stage and this presents a problem for speakers connected. If the output stage becomes short circuit, then the available rail voltage will be connected to the speaker directly and without intervention will quickly destroy it. The following is an example to demonstrate;

70 volt rail across a "nominal 8 ohm" speaker = (70 volts ^2) / 6 ohms = 816 watts (almost a bar heater on low setting). In the confined space of a voice coil the speaker will be destroyed way before you can reach the power button.

This simple circuit is designed to monitor the speaker output and disconnect the speakers if there is a DC voltage of 2 volts or more. The schematic is a variation of one used by a number of electronics magazines in Australia for more than 20 years.

Note the Nmos350/500, Nmos400 and the Nbip300 layouts have this protection on board. 
DC detect protection circuit schematic.pdf