Hi,

some of you will have seen this famous powered speaker since it has been around for about 18 years. Tens of thousands must have been sold and it spawned numerous clones. The original is better made than most others with a hefty 12 inch woofer and a 45mm diaphragm compression horn. The amp module is particularly solidly built with generous heatsinking and so no need for a fan.

Inside are two, discrete component power amps of 300W and 60W rating. The 300W amp has four power rails for high efficiency of plus and minus 80 & 40 volts.

Both power amps are DC rail fused and have VI limiting to protect output devices in case of driver failure. Both amps have power limiting too, which reduces the output level after short period of full drive to prevent overheating the woofer and horn driver's voice coils.

So they thought of everything - right? Nope.

Both amps are direct coupled to their respective loads with no protection in case of large DC offsets. Both amps use one half of the same dual op-amp as input gain stages - also direct coupled to the rest of the amp.

Failure of one of the 15V rails that supply the op-amp sends both amp's outputs hard over to one of the 40V rails. A few seconds later, the woofer's voice coil and the horn diaphragm are burnt and destroyed.

In the example on my bench, it looks like an intermittent solder joint in the feed to the -15V supply did the job. The amps test just fine.

One 10uF bi-polar cap would have saved the horn diaphragm and a simple triac crow bar would have saved the woofer. A couple of dollars worth of components.



.... Phil