jbier@janus.berkeley.edu (Jeff Bier) (07/27/89)
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
ingoldsb@ctycal.COM (Terry Ingoldsby) (08/02/89)
In article <30238@ucbvax.BERKELEY.EDU>, jbier@janus.berkeley.edu (Jeff Bier) writes: > 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. > > 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 I seem to recall that tweeters are usually rated at about 1/10 the power of the woofers in most systems. You can figure it out from that. (In other words, only 10% of the energy in music occurs above 5 KHz). If you decide to fuse all the speakers (including the woofer), put a 100 ohm resistor in parallel with fuse so that if it blows the fuse the amp will still have a load. Since fuses are cheap compared to tweeters, why not start small and gradually increase the value until the fuses just stop blowing at maximum listening values. If you are really serious about this, there are a number of active circuits (ie. with semiconductors, power supplies) that are much more effective than fuses. By the way, driving an amp too hard (so that it clips) sends lots of high energy high frequency harmonics straight to the tweeters. Thus an underpowered amp (in a given application) is good at blowing even high power speaker systems. -- Terry Ingoldsby ctycal!ingoldsb@calgary.UUCP Land Information Systems or The City of Calgary ...{alberta,ubc-cs,utai}!calgary!ctycal!ingoldsb