[sci.electronics] Question re: Speaker Fuse Sizing

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