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Old 4th October 2010, 06:12 PM   #1
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Default How do I calculate safety fuse ratings?

So I intend to include a fuse on my woofers which will handle 1000 watts RMS @ 8 ohms. The chart that seems to be floating around all over the place seems wrong to me though. It claims peak to be 4 times the continuous power handling rather than double.

Using 2,000 watts for my peak value, it tells me to run a 5 amp fuse.

Can anyone shed some light on this scenario?

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Old 4th October 2010, 06:39 PM   #2
AndrewT is offline AndrewT  Scotland
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Quote:
Originally Posted by ViolenceOnTheRadio View Post
So I intend to include a fuse on my woofers which will handle 1000 watts RMS @ 8 ohms. The chart that seems to be floating around all over the place seems wrong to me though. It claims peak to be 4 times the continuous power handling rather than double.

Using 2,000 watts for my peak value, it tells me to run a 5 amp fuse.
Hi,
F7A using the 4times rule and the nomograph.
That F7A fuse does not blow at 7A. It will pass 7A, almost forever, without blowing.
It will pass 14A and may take between 1second and 1minute to blow. Look up the fuse manufacturer's chart to see an approximate pass time for overload conditions.

An F7A fuse will allow ~400W to be sent continuously to your speaker, almost forever.
If you send 1600W to your speaker the F7A will blow, but we don't know exactly when.
If the fuse is warmed up by passing a continuous signal of 100W to the speaker, then the time it will survive the 1600W pulse will be shorter, than if the fuse starts from cold.

How often do you want your fuse to blow?
Who will complain if your fuses blows?
Will the output stage be damaged before the fuse blows?
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Last edited by AndrewT; 4th October 2010 at 06:44 PM.
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Old 4th October 2010, 08:12 PM   #3
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What is your setting? What are you trying to protect? Or against what?

If you want loud music at home, an amp or two will break your lease with most speakers.

If you want to keep from cooking a driver with long-term loud music in a large hall, that's different again.

If you want protection from sudden large unexpected transients or DC surges... you need to go electronic and/or be lucky.
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Old 4th October 2010, 08:40 PM   #4
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Just forget it. A fuse that can handle 1KW in line with the speaker...it will sound like driving pedal to the metal in a sportscar with the handbrake on.

An overloaded bass driver sounds horrible, long before it goes "kaputt"...you have half an hour to turn the volume down to the 10 watts a 1000 watt sub normally uses in your palace.
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Old 4th October 2010, 11:20 PM   #5
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All very good points.
I intend to drive these speakers in pro audio applications and have cooked woofers a few times in a pretty short amount of time. Sometimes you smell them and can shut down, sometimes you can't.

So if it's a matter of what will cook first, a quick blow fuse or your coil wire the answer is obvious. The fuse holders are installed so I suppose if nothing else, they will look very classy

My last cooked speaker was a car sub I wired in parallel to it's twin to achieve 2 ohms for a class D digital amp. I think it lived for about 20 minutes, 1 speaker died and the other did not. I no longer run that amp at 2 ohms for which it's rated lol. No smell, the amp just shut down.
I'm still convinced a fuse would have saved the sub because certainly too much current is going to blow that fuse well before it fries a coil.

Last edited by ViolenceOnTheRadio; 4th October 2010 at 11:35 PM.
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Old 5th October 2010, 02:06 AM   #6
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pro audio - you should have mentioned that earlier for not having us giving answers from a completely different planet!

I was on the track "my amp says 100W and my boxes´ label says 500 - why are they kaputt?" - Well, the Walmart amp has a powerplant worth of just10 Watts and when you overdrive it in bass, a hundred million% of THD fry the tweeter. Don´t know about your continent, but that´s that kind of amps that sells that kind of boxes over here.

Wouldn´t there be an electronic cop that cuts the box off before it smells like your next bank overdraft is due?

(All my hopes, best wishes and fuses are on stage with you - yours etc etc)

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Old 5th October 2010, 10:36 AM   #7
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The logic behind posting in this category was that since I'm running these drivers full frequency and without a crossover, this would be the place to ask.

But yes, these will be used in pro audio where both volume and wattage will be very high. The woofers will be producing their full spectrum which means a lot of bass, even more mids and very little highs.

So with that in mind, it seems logical to me that a fast acting fuse is going to blow before cooking a coil if my volume peaks are beyond the speakers tolerance or say you're running them in 90-100 degree weather outside for several hours.
I came across this formula.

wattage/(impedance x 4) = square this answer.
Much like the chart, it says multiply continuous wattage by 4.
In either case using 1000 watts RMS gives me an 11 amp fuse.
Rather high.
I know if I drive a 1000 watt woofer with 350 watts but use 36 dB of 80 Hz boost from a crossover, I can cook that woofer.

In that, maybe I'm best off basing my fuse formula off the maximum output wattage of the amplifier driving the speaker?
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Old 5th October 2010, 11:22 AM   #8
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We used to (many, many years ago) fuse the + lead of speaker cables at the loudspeaker end, for speaker protection. I seem to recall using a 3.5A fast-blo fuse for protection with amps that could deliver roughly 250W RMS cleanly, and any amp below that level.

If that fuse blew too regularly, we'd recommend going to 4A but no higher. Broadly speaking that fuse would not blow with less than 300+ watts RMS (FTC Method) unless the amp went into hard clipping (with lots of power on tap, any clipping is more-or-less hard clipping, but smaller amps "act like" big amps when driven far too hard, thus the value doesn't really change downward).

There is no point in going less than 3.5A; if you do you will just blow a lot of fuses, regardless of the amplifier's clean power ability. That's from experience.

However, this method does not give any real protection to the loudspeaker as far as music signals go. I mean, the fuse will blow, but not very quickly and certainly not in a manner that insures no damage could be done. 'm referring here to your desire to limit power to the drive to some large, known value (say, 1000 watts with little or no clipping).

I don't know of any viable method to limit current to loudspeakers in a fast acting manner with a fuse. It merely stops the deaf from running the amp into clipping for an extended period of time ... minutes.

It does offer good protection against direct shorts, where it will blow quickly. Whether that is "quick enough"to prevent amplifier damage has more to do with the amp itself than the merits of the fuse method. Sometimes, it save you money; sometimes it's too little too late.

It's worth mentioning that my experience with fused speakers is pretty much limited to home use with systems we sold 35+ years ago ... I never fused a Sound Reinforcement system. Professional sound reinforcement is going to have a much larger continuous power average than a home system. So, a larger fuse does seem reasonable. However, exactly how large is definitely debatable. I would tend to try the 5 ~ 7A values first, and see if they refrain from blowing with your standard program material and at the SPL's you need in the rooms you run systems into. There is going to be just as much, if not more, grief if the fuses blow too often ("too often" in my mind is one fuse protecting one driver of all those being used, once per event. Ideally I'd want a fuse that almost never blew, but still would protect against unusual events, like direct shorts, amplifier oscillation or failure, some forms of feedback, etc).

Since you are doing Sound Reinforcement, the question of a fuse's effect on sound quality is moot ... when you create the sound, people get what you produce, and that's that. If you're OK with the sound with a fuse in-line, then that's the end of it.

If this were a serious reproduction system, where accuracy of reproduction becomes paramount, the fuse might be a problem. I can't really say for sure since I never personally used fuses in my own gear, and at the time we did use them, the concepts of cable interaction was poorly understood, if it was considered at all.

If it were me, I'd avoid the fuses completely and just rely on clipping indicators on the amplifier (if they are useful), listening (I realize this is not easy in sound reinforcement if there are multiple drivers + enclosures involved) and, in general, my familiarity with the gear I'm using.

Perhaps not the answer you're looking for, but I tend to go with buying known robust gear to begin with, getting to know the gear well so that I avoid overloading anything in the first place, and understanding that blown drivers are, over time, inevitable in that field. Build replacement costs into your quotes.

EDIT: Just had a second look at your chart after this post. Probably not coincidentally, using 250W x4 @ 8 ohms pretty much runs through 3.5A. Waddya know.

Last edited by Johnny2Bad; 5th October 2010 at 11:46 AM.
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Old 5th October 2010, 04:20 PM   #9
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My PA has serious oomph, my reinforcement amps however go into deep clipping at usable volumes according to the light even though there is no audible distortion what so ever.

I'll have to bargain on a 5-8 amp fuse and do some sound tests. Figure out what it takes to blow one in an instant rather than long term since my wattage will be nowhere near what the speakers can handle or are at least rated to handle. Aluminum cones aren't likely to blow so all concern is on voice coil.

I suppose I should also just consider exciter lamps which has become the popular method of pro audio and even some high performance home audio systems. I've witnessed their glow at wattage well below the crossover and speaker's ratings.
The relationship between preamp and power amps is such a wild card in pro audio. You get line/mic inputs with 60dB pads and suddenly you're cooking speakers receiving 1/4 their rated power.
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