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This protection serves well, when your amp is rated at higher power levels, and the output gets shorted out. Typically a short at the output means enormous current draw from the power supply, probably leading to the complete failure of transformer, rectifier, the power output transistors, and almost every power handling component in the circuit (the expensive parts). A typical solution is to use a fuse. Fuses work well up to a point. They usually react considerably slow, and some components may still fail, so it's not the solution I recommend, although it's always beneficial to have fuses here and there. The optimal solution Is much simpler. Contain the maximum current draw the power transistors can give out, even at worst case scenario. The systen uses two transistors, which sense the voltage drop across the 10ohm resistors and when a given amount is overstepped, they start stealing away from the base current of the output transistors, thus limiting the output power. The protection works very abruptly, so at normal operating conditions, it shouldn't affect the normal state whatsoever. At around 7.5A the protection will cut of any further current draw.
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