Speaker protection: how to fuse?

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I'm building a PA subwoofer using an Eminence Sigma Pro-18" (see other thread), and I want to protect it with a fuse. The driver handles 500w/RMS, and the DC resistance is 6.4 ohms. There is no crossover because I am bi-amping. How can I figure out what fuse rating to use, and should it be slo-blow or fast-blow?

Thanks,
Evan
 
Evan, assuming the driver will truly handle 500w RMS long term, then the fuse rating can be calculated as follows:
The current drawn at 500W into 6.4ohms is I=sq root (W/6.4)
This works out at 8.8 amps, so to be on the safe side try a 7.5 amp slow blow fuse.
 
hehe aluminium foil. I have alot of experience with aluminium foil. It's actually depend. I ahve see case when they just didn't last a week. It depend if you push your system hard. Also playing with aluminium foil is not very reccomended. You will always have a problem when it's they foil in place.

You can also use a higher value fuse but passed a point it's going to be the speaker protecting the fuse(tweeter make excellent fast acting fuse btw 🙂.
 
Evan, the best option for speaker protection is a relay. This relay protcetion has serveral advantages over fuse protection.
Disadvantages of fuses:
- Its difficult to determine the fuse rating because the speaker impedence varies with frequency and the type of enclosure.
- If an approximate fuse rating is used, it keeps blowing after a while and in many cases the speakers will die for the fuses.

Relay protection:
- Speaker is connected only when powersupply is available and this may include muting to avoid turn-on transients.
- If DC is detected it immediately disconnects the speaker from the amplifier.

Many kits are available for Speaker protection, you can try Velleman Kit K4700.

- XL.
 
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