Fuses directly on a woofer/ Slow or quick blow? - diyAudio
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Old 13th November 2012, 05:25 PM   #1
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Default Fuses directly on a woofer/ Slow or quick blow?

I am curious as to whether I should put quick blow or slow blow fuses on my woofers. It's not a matter of debate for me as to whether they work or not. Any of us who have owned systems with exciter lamps and self resetting breakers have come to value heat protection on a woofer. I would have otherwise killed some expensive equipment without them and have killed woofers because I did not have them.

My crossovers with exciter lamps got me to thinking that due to the frequencies being all over the place, it might be better to install a slow blow fuse rather than quick blow so if I'm not pushing the boundaries but a few random massive peaks do from time to time step over that threshold, I won't unnecessarily blow a fuse. Odds are I will never come close to meeting my speakers rated power and I put variable and bypassable bass limiters on all my builds.

Extra safety is practicality.
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Old 13th November 2012, 05:55 PM   #2
sreten is offline sreten  United Kingdom
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Hi,

Slow blow or PPTC's. Quick blow will blow on bass transients.

rgds, sreten.
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Old 13th November 2012, 07:44 PM   #3
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Hello sreten, thanks for reaffirming.
I am running woofers in series with a 750 RMS power rating each. I considered the poly switches but they're a hassle to find at large values. I think 250 watts was the highest to date.

Logic is a 250V 3A fuse can be predicted to blow with an audio power of over 750 watts RMS. I will start with a 250V 2A and see how those hold up under 450 watts RMS. Then of course if neither fuses blow on each cabinet and I do not smell voice coil, we're ready to rock!
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Old 13th November 2012, 08:19 PM   #4
sreten is offline sreten  United Kingdom
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Quote:
Originally Posted by TestTones View Post
Logic is a 250V 3A fuse can be predicted to blow with an audio power of over
750 watts RMS. I will start with a 250V 2A and see how those hold up under
450 watts RMS. Then of course if neither fuses blow on each cabinet and I do
not smell voice coil, we're ready to rock!
Hi,

Your logic is flawed. Fuses don't care about voltage, only current.

UK mains fuses are typically 3A, 5A and 13A. Into 8 ohm speakers
that represents total average powers of 72W, 200W and 1.3KW.

rgds, sreten.
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Old 13th November 2012, 09:23 PM   #5
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I agree slow blow – you're trying to prevent overheating, not current pulses – but fuses are mechanical. High levels of vibration can shake them apart over time, without ever approaching their limit current.

And if you've got a frequency your cone won't move at, for example RF instability or a major DC offset, you can cook the voice coil anyway, without exceeding current ratings (all right, commoner with HF drivers, but still possible with subs).

The lightbulb (which also shakes apart in the cabinet, better for them or the fuses to be mounted in the amp rack) would have told you there was current flowing when you weren't hearing anything; always an indication something's not as it should be.
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Old 13th November 2012, 09:39 PM   #6
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I'd start with an MDL 7A output fuse if you are serious about periodically holding 450WRMS into 8 ohms.
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Old 14th November 2012, 03:17 AM   #7
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I vote fast blow for woofers, and slo-blo for tweeters.

The reasons for fast-blo on woofs is that you are trying to prevent over excursion of the cone that will destroy the voice coil. This is highly likely to be what destroys a woofer, well before coil burnout from over heating. This physical over-excursion will happen within a half cycle when it hits. A half cycle is somewhere between 10mS and 50mS usually. I could be wrong, but I don't think a slo-blo will save you in this situation. Even a fast-blo may not be quick enough. A fast attack limiter circuit is a better approach. A compressor/ limiter combo can be better still.

Tweeters on the other hand usually have very little physical excursion, and much more often will blow out from over heating of the voice coil over a period of many many full waveform cycles. They can usually be drastically overdriven for a very short amount of time without burnout or physical excursion related problems, but a prolonged high level signal can heat them up and burn them out.

Last edited by Bob Richards; 14th November 2012 at 03:20 AM.
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