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Speaker Fuses - Poor Insurance

Rick Kamlet , JBL Professional

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).