Speaker protection: how to fuse?

[Evan Andersen]

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

 

[AndrewJ]

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.

 

[JBL]

Remember that a fuse on a speaker will always blow after a time. It can get very annoying(trust me) when it's come to change it and you don't have some.

 

[Evan Andersen]

Hmm..I was not aware of this problem. Maybe I'll nix the idea, or keep an emergency wad of aluminum foil on hand! How long will a slow-blow fuse usually last in this application?

Evan

 

[JBL]

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 .

 

[swede]

Why not use a fast auto-fuse (the kind with a small button to press when you've removed the load on the speaker)? They usually last more than 1000 times, before you need to replace them.

//magnus

 

[hifi]

but those are usualy rataed att above 100amps...?

 

[swede]

Not the ones I mean.

They look like:

<img src=http://www.elfa.se/images/ct_tnail/A10672_C.JPG>

http://www.elfa.se/images/ct_tnail/A10672_C.JPG

And they are rated from 2 A and up.

//magnus

 

[paulb]

That's a circuit breaker. It may affect the sound.

 

[swede]

Ok, never mind. ;=)

I thought they'd do.
But I've seen similar fuses on older Yamaha speakers.

//magnus

 

[Xavier]

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.