fuses

is there a formula ? i thought that the sizing of an AMP's output somewhat needs to be leveled to the capabilities of the Speaker you want to drive. Also from my Polk SDAII Speaker manual it says that you should never use a bigger fuse than the original --
in my case the system holds a 5ohm, 1amp, 3AG Fast-Blo

not sure if that helps..
cheers
mikeo

I’ll answer since it is concerning you, but only after I say, once again, that IMHO fusing speaker leads is unnecessary.

Mikeo,
I don't think we are talking amp fuses here. I would never change an amp’s main fuse to protect speakers. They're designed to intercept spikes coming to the amp in the AC. They are almost always fast blows.

dholmes,
Yes, if your Polk is an 8-ohm, 250w speaker, a 2-amp fuse is right (actual “answer is 1.98 amp, but rounding off is fine). If it were a 4-ohm, you’d want a 4-amp fuse.

Side Note 1: To produce the same power the current demand rises as the speaker impedance falls. This is why you often read in the Club about the need for “high-current” amps. Many Polk’s are 4-ohm and 4-ohms demand more from amps.

Ceruleance,
Absolutely. Start with your speaker manufacturer, but some will be reluctant to give guidance (liability and all that). Your situation calls for a 1.63-amp fuse. You’ll find 1.5-amp available, maybe there’s a 1.6, but I doubt it.

You use I= sq rt (P) / R to calculate your needs. Here:
“I” is the maximum fuse value in amperes (amps);
“P” is the speaker’s maximum power handling capacity in Watts; and
“R” is the nominal speaker impedance in ohms (usually 4 or 8).
Take the square root of “P”, then divide that by “R” and you have your answer.

Once you know the amperage you need, then you have to pick a fuse speed. Fast blows can offer too much protection, i.e., they could blow on a transient your speakers are well capable of handling. Slo-blows may offer too little. Normal blow is the best choice. Transients are more common in Classical music and HT (decoded sources) than they are in RnR.

RS used to, and may still, sell in-line fuse holders, but their wire was pretty lightweight. Any good electronics store should carry holders to which you can solder wire of your choice. Install the fuse in each speaker’s positive lead. Also, attach the fuse holder to the speaker (tape or “blu-tac” should do). You just don’t want it dangling where it could touch something, or someone or a pet can touch it, and go to ground.

Side Note 2: A speaker’s impedance is dynamic, i.e., it changes. Speaker impedance varies due to a few factors, but one is its drivers’ coils’ temperature. As you drive a speaker hard, its coil warms. As the coil warms, its impedance drops. As the impedance drops the current demand rises (to sustain the same power level). If this continues unchecked clipping becomes possible and damage to the amp is more likely as well. In any case, the fuse is there to keep things from getting extreme.

Side Note 3: Before proceeding, consider that you are adding three more connection to each speaker run you fuse. The potential for signal loss is more real than the threat of an “overpowered” amp doing damage to your speaker(s).

Thanks Tour2ma. Great read. I've been thinking about putting an in-line on my 4 watt RMS/ 10 max. speakers. Especially since I'll be driving them with a 200w/ch amp soon.