If you have a Sony amplifier (or receiver, as the more featured ones are called) and it's jumping into PROTECTOR mode every time you crank up the volume to a level higher than silent, check if the speaker wires at the back aren't touching things. I had a few extra ones connected that I used for an experiment and never disconnected. Removed the wires, gone was the trouble.

This was a Sony STR-DA50ES, but I suppose other models will have a similar feature.