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Old 22nd Aug 2019, 2:05 pm   #8
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Join Date: Apr 2007
Location: Hampshire, UK.
Posts: 1,514
Default Re: Bang & olufsen 3300 type 1604 fuses

The crowbar circuit is necessary for loudspeaker protection. This was the second generation of high powered DC coupled amplifier from B&O (the original Beomaster 2000 being the first), before that they had all had big DC blocking capacitors between the output transistors and the loudspeaker load (Beomaster 3000, 3000 mk2, Beolab 5000 etc).

There are various ways to go about this. The British approach more often than not was just to let 'em burn, if an output transistor fails (quite likely in British gear...) it will more than likely take the woofer of the loudspeaker connected to it with it. It is possible to put a fuse in series, but its non-linear resistance offends the purists. The Japanese approach is to use an electronically controlled relay between the amplifier and the loudspeakers which works well when new, but since it spends most of its life switching on and off at essentially zero current the contacts tend to corrode and give erratic results after some years of use - this is always my first port of call when servicing any quality Japanese amplifier. The crowbar system gives effective protection without putting anything unnecessary in the signal path so it is satisfying at a technical level, however it is strictly a 'one shot' device which makes it a nuisance from a customer relations point of view.

The bigger Beomaster 4400 model had a relay, but it was in the DC power feed to the output stages rather than in the direct signal path. This appeared to be a good compromise; the amplifier's bias current helped to keep the contacts preened and its internal PSRR and NFB loop effectively counteracted any effect it may have of marginally increasing the supply impedance.

Because the 2200 chassis (as used in the Beocenter 3300) is modular one can remove the output modules if they are suspected of firing the crowbar. If the fuses then hold, they can be plugged back in one at a time to see which is faulty. To be fair though, its normally mouldy electrolytics on the power supply regulator panel that cause it to misfire.
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