A DC Fault Protection Circuit for Audio Amplifiers

The standard Class AB audio power amplifier allows for direct coupling of the output of the amplifier to speakers. This is very good in that no capacitors or transformers get in the way of the sound quality coming out. The speakers are connected directly (more or less!) to the amplifying devices. This has the unfortunate side effect that if an out

put device fails, this usually causes the raw DC power supply to be connected to the speakers. Most speakers burn out or are mechanically damaged by this very quickly. Protection circuits have been designed to prevent this kind of damage, and come in many flavors. They range from the simple - such as an added fuse in series with the amplifier output - to the complex, with all sorts of monitoring. The current favorite is a series relay circuit at the output of the amplifier, driven by some sort of DC detection circuit. Relays have their own set of problems, though, not the least of which is reliability. Relay contacts corrode, arc and stick over time. Worse, each time they are tested could well be the last time they operate, so there are some built in problems. Fuses are also problematical, as they must be excruciatingly well sized to offer good protection, and when that is done, the fuse resistance is modulated by the heat dissipated in the fuse near maximum power, so there are audible side effects. Some recent advances in power MOSFETs make them attractive for replacing or sidestepping relays and fuses on the output of the power amp. Power MOSFETs make good switches, changing from a totally nonconductive state to a fractional-ohm resistor in nanoseconds with the proper drive signal. Unlike bipolar transistors, there is no offset voltage or rectification associated with a MOSFET`s on-state conduction. As long as the current...

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