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Build A Loudspeaker Protector

Don't let this happen to you!

THE SPEAKER in the above photo is well on fire and this could be the result of a relatively minor fault in an audio amplifier. As anyone who has been following our series on the Ultra-LD Mk.3 Amplifier would be well aware, big power amplifiers have big power supplies and so a relatively minor fault in one channel of a stereo amplifier can result in a large DC voltage being applied to one of your precious (and expensive) loudspeakers.

Once that happens, your speaker is kaput! If you’re lucky, the woofer’s voice coil will quickly burn out and that will be the end of it – it’s unlikely that the amplifier’s fuses will blow in time to save your loudspeaker. You will then probably have to scrap the loudspeaker although if you are lucky and if it is economic, you might be able to have it repaired.

But you might not be that fortunate and the consequences could be a whole lot worse. Now, instead of suddenly burning out, the voice coil stays intact and gets red-hot, as you would expect it to – it is dissipating many hundreds of watts. After all, voice coils are quite small and they normally operate in the very confined space of the speaker magnet’s voice coil gap.

With a large DC fault voltage across the voice coil, the speaker will either jump forward out of the gap or jump back as far as it can go. The latter is probably the worse scenario since the voice coil can then get even hotter and soon sets the speaker cone on fire.

The sequence of photos shown elsewhere in this article show how the whole speaker cone can catch fire within just a few seconds. A few seconds later and those flames were producing copious amounts of smoke. If we hadn’t been on the spot to put the fire out by laying the speaker face down on the concrete, the fire could have spread to who knows where.

If that happened in your home and you were not present to take very quick action, you could lose your home and everything in it.

This sort of thing really can and does happen! Don’t let it happen to your stereo system. This loudspeaker protector and muting circuit can prevent such disasters.

Main features

Originally designed for the Class-A Stereo Amplifier describ­ed in the June-September 2007 issues of SILICON CHIP, the 2-channel loudspeaker protector described here (in slightly modified form) is also eminently suitable for use with the new Ultra-LD Mk.3 Amplifier module. In fact, it can be used with just about any audio amplifier, either mono or stereo. It provides the following functions:

(1) it protects the loudspeakers against catastrophic failure in the amplifier, eg, if an output transistor goes short circuit or one supply fuse blows;

(2) it provides muting at switch-on and switch-off, to prevent thumps from the loudspeakers; and

(3) it provides an input for an over-temperature switch to disconnect the loudspeakers if the output stage heat-sink rises above a certain temperature. In the latter case, disconnecting the loudspeaker from a class-B amp­lifier immediately reduces the current through the output stage to the quiescent current setting. This is typically around 50-200mA, assuming that there’s no fault in the amplifier.

So for a class-B amplifier, it makes sense to use over-temperature sensing. If the heatsink to which the output transistors are attached gets too hot, disconnecting the loudspeaker immediately reduces the dissipation to just a few watts, which allows the heatsink to cool.

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