In order for a fuse to work, it has to have resistance. A fuse of ZERO resistance would handle infinite power (P = E^2/R). Since that is certainly not what we want, then we have to accept fuse resistance as part of the circuit.
Let's say we have a 2 amp fuse. The power to blow that fuse assuming an 8 ohm speaker is P = I^2 x R = 2^2 x 8 = 4 x 8 = 32 watts. When you apply a sustain 32 watts, the fuse wire melts and the fuse blows.
Note the Square of the current, since we are dealing with power, things aren't linear. Jumping the fuse to 4 amps, jumps the power up to 128 watts. Which means the only advantage to 'Audio' fuses might be very fine increments of current. Personally, if you can't get those fine increments of current ratings, then I don't see much point.
And that happens regardless of what material the fuse is made out of. I would suggest you take one of the fuses from your speakers and measure the resistance, assuming your ohm meter can even measure that low, then ask yourself if you can live with the resistance in your speaker lines.
I've alway said it is not overpowered or underpowered amps that blow speakers; it is always the idiot running the volume control. Keep that idiot under control and you don't need fuses.
Make a fair and reasonable assessment of your amps power and an equally fair assessment of your speaker's ability to handle power, and adjust the volume control accordingly. Of course, there is no accounting for idiot friends and kids. If it's not your money invested, who cares how far the volume control goes.
But for those who want fuses, here is how to figure out which one you need -
Let's work this backwards. Let's say we have 60 watt rated 8 ohm speakers, and we want to fuse them at 50 watts for a little safety margin.
I = SqRt(p/r) = SqRt(50/8) = SqRt(6.25) = 2.5 amps
Let's try another, 80 watt 8 ohms fused at 70 watts
I = SqRt(70/8) = SqRt(8.75) = 2.958 amps (3 amp = 72 watts)
Now comes the hard part, how long does it take a fuse to blow? If you slam it with a massive amount of current, it blows instantly. But what about when the current and power creep up on it? I would suggest taking a very high powered 8 ohm resistor and putting the fuse in series with it. Then gradually crank you amp up and see if and when it blows. It may sustain near the rated current for quite some time before finally blowing.
Most fuses are meant to blow when confronted with a short circuit, so the time to make them blow is not that critical. Massive overload blows them instantly, and that is what they are guarding against.
Also, keep in mind that a fraction of a watt can blow a speaker. It's not just signal or power, it is signal or power over time. I remember a couple of instances where, for one reason or another, a very low level pure tone was being fed into a speaker. A loudness that most would consider faint. But because the signal was continuous and unrelenting, the gradual build up of heat was unrelenting. Eventually the speakers fried from the accumulated heat, even though the rated power was extremely low.
Things are never easy.
Steve/bluewizard
