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From: rfg@hound.UUCP (R.GRANTGES)
Newsgroups: net.audio
Subject: Re: Speaker Fusing - While we're on the subject ...
Message-ID: <826@hound.UUCP>
Date: Fri, 11-Jan-85 13:56:10 EST
Article-I.D.: hound.826
Posted: Fri Jan 11 13:56:10 1985
Date-Received: Sat, 12-Jan-85 07:12:56 EST
References: <271@olivej.UUCP>, <10834@watmath.UUCP>
Organization: AT&T Bell Labs, Holmdel NJ
Lines: 63

[!]
Oh boy! Here we go again on a subject that ought to produce a
megabyte of traffic at least. That is because speaker fusing is
both art and science and is controversial to boot.

Some say - don't fuse at all because it introduces series resistance 
into your speaker lead (bad) and one that some claim is nonlinear (worse).
Besides, they point out, by the time any self-respecting fuse will have
acted, your speakers will have 'gone west.'

Others, concerned about their investment, which is often as much or more
than they have in all their equipment put together, say - Consider the
possible sources of trouble. Many things could go wrong and fusing can
help on some of them. For example, you could have a large sustained
overload at a low frequency (say a ground connection comes off and you 
get 60 hertz hum at high level). This will eventually cook your woofers
even with all the air they are blowing around. In this case you fuse
for the maximum rated power of the woofer or (if its less) the amp 
(use the relation:
power = I**2 R where R is resistance of the speaker (assume equal to the
rated impedance (like 8 ohms)) so that I = squareroot of P/R). The
result is usually a large current (e.g.4 amps is 128 watts in 8 ohms).
Fuses with this sort of capacity generally have pretty low values of
resistance and can be neglected if one takes care that the fuse clip
connection itself does not introduce worse resistance. I never heard of
"Monster Fuses", bet that will be the next big thing in Audio.


Others, noting that this may help woofers but will blow hell out of ye
average tweeter, may wish to fuse for the long term power rating of
their tweeters, if they can find out what it is. When they do find out,
they are generally depressed to discover it is only a few watts and so
abandon this line.(Biamping can help here).

Transients can blow out the spiders, surrounds, cones ,or what have you
of any speaker regardless of fuses - if a big enuf transient can be
generated. Speaker design is important also. For instance, horn loaded
speakers can be pretty rugged. I know of a case where an Altec 800C
(extremely sensitive, normal room volume from 100 milliwatts or less)
was hit with a 100 volt step function (where millivolts were usually
found in the preamp) due to an exploring screwdriver. This through a
35 watt tube amp in an apartment produced an explosion so loud the
neighbors called the fire department. Person in bedroom fell out of bed.
Building started to be evacuated. No damage to speaker.

Lastly, there are other approaches. Best I know of  David Hafler's
design in the late model amps under the Dyna name. I forget what it was called,
but it was a circuit that you could set to some specified power level less
than the amps maximum. If a signal exceeded the preset power level, the amps
gain would be reduced to bring the power back in line. The time required to do
this varied inversely with the magnitude of the overload. Hence, musical
transients could use the amps full power, but real overloads get nipped in
the bud. I use a Dyna Stereo 416 to drive Altec A-7's for sound reinforcement
in outdoor theaters. This circuit makes it possible, since the tweeters
blow at 15 watts or so and I keep hitting the speakers with 200 watt
peaks (no fuses).

 A final word: People with big amplifiers shouldn't live in glass houses.


-- 

"It's the thought, if any, that counts."  Dick Grantges  hound!rfg