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Old 5th October 2001, 01:03 AM   #1
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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
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Old 8th October 2001, 10:00 PM   #2
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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.
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Old 9th October 2001, 02:26 AM   #3
JBL is offline JBL  Canada
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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.
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Old 9th October 2001, 05:32 PM   #4
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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
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Old 10th October 2001, 12:20 AM   #5
JBL is offline JBL  Canada
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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 .
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Old 10th October 2001, 12:02 PM   #6
swede is offline swede  Switzerland
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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
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Old 10th October 2001, 12:16 PM   #7
hifi is offline hifi  Sweden
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but those are usualy rataed att above 100amps...?
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Old 10th October 2001, 12:56 PM   #8
swede is offline swede  Switzerland
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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
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Old 10th October 2001, 02:26 PM   #9
paulb is offline paulb  Canada
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That's a circuit breaker. It may affect the sound.
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Old 10th October 2001, 06:07 PM   #10
swede is offline swede  Switzerland
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Ok, never mind. ;=)

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

//magnus
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