This loudspeaker and fan controller has been specifically designed to suit the Ultra-LD 100W per channel amplifier described in the March and May 2000 issues. Not only does it provide muting at switch-on and switch-off to prevent any thumps from the loudspeakers, it also protects the loudspeakers against catastrophic failure in the amplifier. In addition, it provides temperature control for the fan-cooled heatsink, switching the fan on if the heatsink temperature rises above 60°C.

However, while the circuit has been specifically designed to suit the above amplifier, it can be used to mute and protect the loudspeakers in other amplifiers and also provide fan switch-ing if that is required.

This is not the first loudspeaker protector we have published as we featured similar designs in April & October 1997. However, this latest design provides two methods of temperature sensing for the fan control as well as a temperature cutout for the speakers, if the heatsink rises above 80°C.

Why you need protection

By the far the biggest reason for incorporating speaker protection into any amplifier is for insurance – to save money in the case of a serious amplifier fault. For example, in the Ultra-LD amplifier, the main supply rails are ±55V DC. If one of the output transistors fails it means that more than 50V DC will be applied to the speaker’s voice coil. For a nominal 8Ω speaker the voice coil will have a DC resistance of around 6Ω and so the total power dissipation will be around 400W until the supply fuse blows.

But maybe the fuse won’t blow. Either way, the speaker is likely to be history. On the one hand, the huge DC power applied is likely to push the voice coil right out of the gap, damaging the voice coil and suspension in the process. But a worse scenario is if the on-board supply fuse doesn’t immediately blow – a strong possibility since a current of around 8.5A may not blow a 5A fuse straight away.