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From: jbier@janus.berkeley.edu (Jeff Bier)
Newsgroups: rec.audio,sci.electronics
Subject: Question re: Speaker Fuse Sizing
Message-ID: <30238@ucbvax.BERKELEY.EDU>
Date: 27 Jul 89 16:55:16 GMT
Sender: usenet@ucbvax.BERKELEY.EDU
Reply-To: jbier@janus.berkeley.edu (Jeff Bier)
Organization: University of California, Berkeley
Lines: 33


In our digital signal processing lab, we are using a 
Kenwood KA-3300D amplifier with a pair of JBL ProVIII loudspeakers.

The JBL's are rated for 120 watts continuous power.

In the past few months, two tweeters on the JBL's have been
blown, apparently in the same way, but at different times.
This happened when a very high-frequency (21kHz)
sine wave test signal was accidentally run through the amp.
When I couldn't hear anything from the speakers (most of us can't
hear anywhere near 21kHz), I cranked up the amp volume.
When it reached about 1/2 of maximum setting, the tweeter
made a pathetic noise like 'zzzzzt', and died.

I want to place fuses on the speakers to prevent this from
happening again.  Given that the speakers' power rating is
probably assuming some kind of broadband (music) signal, can
anyone give me a reliable method for selecting a fuse size
small enough to protect the tweeters, and large enough so
that the speakers will still be usable at moderate power levels?

And no, there is no way to prevent high-frequency test signals
from being used.

Please reply by e-mail.

Thanks.

Jeff Bier			  jbier@janus.Berkeley.EDU
Department of Electrical Engineering and Computer Sciences
Unversity of California		        Berkeley, CA 94720
(415) 642-8884