Fuse rating and output

[jake760 10+ year member]

I went to a car audio shop the other day. I was arguing with the guy about fuses and output. He said that an amp can only put out 100 watts for every 10 amps of current it draws. I told him my Hifonics bxi1206d should be doing a clean 900 watts @ 1 ohm, and it has an 80 amp fuse. He was telling me the fuse would pop as soon as the amp put out over 800 watts. He also told me 1 ohm is not enough resistance and my amp would soon fry running at that impedance. I know that's total BS, but I was curious about the fuse question. Thoughts?

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[gunz4me2 10+ year member]

Volts X Amps X efficiency = watts! (A slight variation on Ohm's law).

The 100 watts RMS for every 10 amps worth of fusing is a "general" rule accounting for voltage drop and class ab style efficiency.

Do the math yourself.... 14.1 volts x 80 amps x 75% efficiency = 846 watts

Of course, that assumes that you can provide a full 14.1 volts of input voltage with 80 amps of current draw AND your amplifier will run 75% efficient at 1 ohm. Sadly, the case is that many budget class D amplifiers generally have efficiency in the mid 60 to low 70% range at 1 ohm.

 

[BassAddictJ 5,000+ posts]

Volts X Amps X efficiency = watts! (A slight variation on Ohm's law).
The 100 watts RMS for every 10 amps worth of fusing is a "general" rule accounting for voltage drop and class ab style efficiency.

Do the math yourself.... 14.1 volts x 80 amps x 75% efficiency = 846 watts

Of course, that assumes that you can provide a full 14.1 volts of input voltage with 80 amps of current draw AND your amplifier will run 75% efficient at 1 ohm. Sadly, the case is that many budget class D amplifiers generally have efficiency in the mid 60 to low 70% range at 1 ohm.

x2

 

[KyleBechtold 5,000+ posts]

its a little different than that...

 

[gunz4me2 10+ year member]

its a little different than that...

I know, I missed impedance rise.//content.invisioncic.com/y282845/emoticons/frown.gif.a3531fa0534503350665a1e957861287.gif ETA: And the fact that a subwoofer is the most inefficient link in the car audio chain...

 

[n2audio 5,000+ posts]

An amp can easily produce 1000w+ w/an 80A fuse.

At 1200w you'd have to be blasting test tones non stop near or even into clipping to heat the fuse up enough for it to open.

As for what the shop employee said -- it's obvious he doesn't know how fuses work. An 80A fuse is designed specifically to pass 80A continuously, they will pass 90, 100, 110, even 120A for several minutes. Even at 2x rated current it will take a few seconds to open a fuse.

Now when you consider the RMS current draw of 1000w worth of full volume music will be in the 30-40A range it is easy to see how an amp with an 80A fuse could easily produce 1000-1200w of music power.

80A on a 1000-1200w amp is definitely on the small side - it's more a question of how HF uses fuses -- if they're for protection against over-current you'd want it on the small side. if it's for protection against catastrophic failure it would probably be larger.