Fused sub?

William from NOLA · Post by **William from NOLA** » Wed May 04, 2005 11:14 am

Fishy wrote:The Kenwood has a 25 amp fuse which means with the voice coils(3 ohm) in parallel you need to start worrying about the fuse blowing with about 1000 watts.

P = I^2*R = ((25)^2)*(1.5) = 937.5 watts

That fuse would do very little if the vc's were in series. You'd need to provide over 3750 watts for it to blow with that setup(R=6ohms).

I know. More geeky math.

:)

-Fishy

and don't you guys reading this think he's talking about blowing the sub

...only the fuse :)

Post by **GlassWolf** » Wed May 04, 2005 3:35 pm

a: the fuse would blow instantly with a dead short, which is the intent.
b: remember R isn't a fixed value. a speaker is a reactive load, and the impedance changes with frequency, thus power changes with frequency as well.

Fishy · Post by **Fishy** » Wed May 04, 2005 7:42 pm

I guess I should of used Z instead of R, but then that power equation would've seemed even more alien to a lot of people. Still, if Znom(1.5 ohms) is the minimum impedance shouldn't there be a frequency where 25 amps flows at 1000 watts? In other words couldn't you blow that fuse with an amplifier rated at 1000 watts at 1.5 ohms? or is Z actually a lot higher most of the time for typical subwoofer/box combos. RMS current is what blows a fuse in an AC circuit right?

Its been a while, but IIRC when we measured the impedance circle of a driver in acoustics lab we "blocked" the cone. How does a manufacturer measure the nominal impedance of a driver? free air? or blocked.

-Fishy

daykareboards · Post by **daykareboards** » Thu May 05, 2005 7:49 am

wow, all that math just confused me and gave me a headache.

Post by **GlassWolf** » Thu May 05, 2005 8:35 am

Znom is, as the name implies, nominal, or an average impedance. It's not the minimum impedance the amplifier will see, depending on the enclosure and the frequency being played, so yes as you noted, the impedance can drop below Znom, at least briefly, during use. Remember, a fuse rating is based on % overcurrent and time. the higher the % of overcurrent, the shorter the time the fuse will last before blowing, so while the fuse may be rated @ 100A, it won't instantly blow at 110A nor at 120A, but as that current increases over 100A, the length of time the fuse will last is reduced.
http://www.bcae1.com has a great section on fuses that explains the overcurrent vs time relation.
Nominal impedance is like a lot of things in the car audio world.. there's no set way to measure specs. Not everybody uses IEEE, FCC, or CEA guidelines etc. If Nominal Impedance is 4 ohms, the DCR is usually 3.6 ohms.

Mithrandir · Post by **Mithrandir** » Thu May 05, 2005 9:51 am

Kiki, you should make an faq on what all effects a drivers impedance. Like the frequency being played, box size, and all that good stuff.

Fishy · Post by **Fishy** » Thu May 05, 2005 11:17 am

I always thought Znom was the average impedance as well, until I read this:

http://www.google.com/search?hl=en&lr=& ... +Impedance

After looking at a couple graphs in my old acoustics text Z doesn't vary all that much within the frequency response of the driver except around resonance where it becomes substantially higher. Maybe "minimum" and "average" aren't actually that much different.

Bottom line I guess is that most amps aren't going to produce the kind of power for the length of time necessary to blow that stupid fuse. Its seems it would be awfully difficult to predict the actual power output into a reactive load anyways and that fuse is there, just as you said, to protect the amp.

Maybe thats why my PPI amp ran so much hotter with a pair of 10w3's as opposed to 10w5's.

-Fishy

[note]oh and in that acoustics lab I think we blocked the cone on that driver in order to figure out the inductance of the voice coil and left it free-air to measure the impedance circle. I guess if manufacturers do measure Znom they do it free-air as well. Maybe a better definition of "nominal" would simply be "best guess".

Post by **GlassWolf** » Thu May 05, 2005 11:57 am

example: I had an Orion 280GX running the two CV 12" subs in the Festiva once when I changed the speaker terminals and put gold pin connectors on the speaker leads for easy disconnection to reach the spare tire.
The terminals were vertical, and one time, one lead got tugged and touched the opposite lead and caused a dead short across the amplifier channel and blew up the amp.. fried the entire finals for that chanel.. the amp was fused at the power lead, but the power supply still held enough power to kill the amp before the power lead fuse went, so the amp sizzled. If I'd fused the subs, that fuse would have gone faster than the amp could've gone most likely.
ah the days before internal fusing for amps.. so long ago.

spkrman · Post by **spkrman** » Sat May 07, 2005 2:09 am

Impedence will normally always be higher than DCR

hot coils, frequency... blah blah :)

Fusing isn't a bad idea and for most installs it probably helps alot. Particularly if the shop sells the sub, the n00b owner messes with it and bam, shorts the wires. or perhaps the gain gets cranked... would have otherwise popped the sub.

Works out well in some situations.. and if you need to throw more power, put in bigger/short out the fuses.