Unfortunately, using fuses to try
to protect loudspeakers just doesn't work very well. A fuse
is a thin piece of wire that opens when too much current goes
through it. Similarly, a voice coil is a thin piece of wire
that opens when too much current goes through it. Even if the
fuse were perfectly chosen, which will blow first?
If the situation were perfectly linear, then at least you'd
have a 50:50 chance. One complicating factor is that a loudspeaker
voice coil is complex. Every item in the electrical signal chain
has a different heat-up curve, none of which match that of a
fuse. Not only can the coil wire burn due to long-term overpowering,
the lead-in wires have a different heat up curve, there are
lead-in wire joints (to the voicecoil) that have their own shorter
term heatup capability curve (curve slope depends on the joint
impedance to coil impedance ratio), etc.
From a signal duration POV, a fast-blo fuse may react to short-term
signals that wouldn't hurt the speaker. A slo-blo fuse may not
react to high peaks that could damage the speaker, and it won't
catch longterm signals that are just above maximum longterm
capability of the speaker that might take a few minutes to result
in voicecoil burning. And then, if you do set a slo-blo to catch
these, then it will blow needlessly when this same signal level
doesn't last as long.
The only way to ensure that the fuse will protect is to use
a fuse so small that it will probably blow continuously. Otherwise,
you're giving yourself a false sense of security. And then there
is fuse impedance, which is what creates the heating of the
fuse that allows it to blow, lowering system sensitivity. (note:
I wouldn't mind lowering sensitivity if it worked, it would
be an acceptable trade-off, but it just doesn't work.)
In my opinion, you're better off making sure you're using an
amp with adequate headroom to avoid clipping, using a properly
set limiter, high-passing LF speakers so as not to drive them
below their cabinet tuning frequency.
If you're going to have fuses anyway, then a setup with an
indicator and spares at your fingertips seems like a good way
to implement it. But if it's in an amp rack behind stage, when
fuses blow needlessly you're still going to be unnecessarily
impairing your system performance. If your amps are at the mixing
location where the indicators can be reacted to quickly, are
you adding a bunch of unnecessary speaker cabling?
Unfortunately, I just don't think fuses accomplish as much
protection as one might hope it would.
More information about the use of speaker fuses can be found
at the Syn-Aud-Con website,
FAQ398. A new FAQ will most likely be posted based on the current
discussion (as of 5/18/99).