One of our inSync readers did an excellent follow up on the difficulties with using fuses for speaker protection (see inSync 10/19/99). This also contains some great practical info about the dynamics of music reproduction in general.
Most people don’t use fuses. A slow-blow fuse, which is less likely to fail on a transient that doesn’t cause any permanent damage, may be too slow to save a tweeter. A fast-blow will generally degrade too quickly, failing prematurely on musical signals. Several studies I’ve read have found the peak to average power ratio to be around 17 dB on recorded media, or about 50:1. An amplifier cruising along at 10 watts needs to be able to put out 500 watts to avoid clipping on most program material. Live music is much worse! It’s hard to get a fuse to do the job with such an extreme range of power. Sometimes a slow blow and fast blow fuse are used in series, but that adds a lot of resistance in the line, which muddies up the sound by badly messing up the amp’s damping factor.
To control the speaker, an amplifier (most amplifiers, anyway) needs a very low output impedance. Added resistance reduces the amplifier’s ability to control the speakers, which most often translates to muddy bass. A fuse adds resistance which is almost always much higher than that of the wire or amplifier’s output stage.
In the past I’ve used a breaker based on a bimetallic switch. This is a switch that uses two different metals. When it gets hot, the metals bend at different rates and separate. Less resistance than a fuse, but it’s very hard to find these beasts and they still won’t help when (a) somebody drops a mike or (b) the sound is overdriving the amp so the tweeters fry due to all the high harmonics generated during amplifier clipping.
Most speakers will handle a momentary overload. Most modern amps will shut down if they have a severe problem (e.g., the output stage shorts and applies 100 volts across the output terminals) so fuses aren’t going to help much beyond that. If you’re in a club, cranked to the max ‘cuz that’s what the bar owner wants, then go ahead and put a fuse (I’d use a slow blow rated at some 30 – 50% of rated power) in line. At that kind of a gig, sound probably doesn’t matter that much anyway!
Editor’s note: As alluded to in the 10/19 inSync, fuses on speaker lines are typically more trouble than they are worth.