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Old 13th February 2006, 06:17 AM   #1
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Join Date: Feb 2006
Location: BrisVegas, Australia
Default HP crossover help (+ resonant freq)

G'day,

So, I'm whacking some Focal 100CA (car speakers) into some boxes, since I have little use for them anymore and could use some hi-fi bookshelf speakers. The box design I don't require any help with, but curious about a few things:

1) F(res) for the driver is 90.28hz. I've used 3-4 calculators (for the ported box), and gotten the same results (which is a good thing). The new F(res) I'm getting is around 47hz. Is this the new resonant freq of the entire system (driver, box, port) or does it change the F(res) of the driver itself? Which one should I use for crossover calcs? (see below)

2) I would like to put in a 6db/oct HP filter, as well as a Zobel filter. I will most likely put a max of 20WRMS through these speakers. Is a HP crossover necessary, and if so, where should I make the crossover freq, considering the lowered F(res) of the speaker? Searching this forum tells me the Zobel is useful, but conflicting methods of calculating values.

3) Some specs: Nominal impedance is 4ohm, DC resistance is 3.3ohm. Res = 43.92ohm; Les = 8.56mH. This is a coaxial design. Can somebody provide a quick interpretation of the above figures, and some advice?

Cheers,

Adrian
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Old 13th February 2006, 09:34 AM   #2
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Join Date: Jul 2005
Location: North Derbyshire
I should bear in mind that car drivers are usually fairly poor?, and designed specifically for in-car use - they probably won't sound great as home speakers?.

I would also be careful over how much power you feed them, car amplifiers and speakers use very fictional ratings - a '100W' car speaker is more probably really rated between 10W and 25W?.

I'm presuming your home amp is suitable for feeding 4 ohms?.
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Nigel Goodwin
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Old 13th February 2006, 11:08 AM   #3
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Join Date: Feb 2006
Location: BrisVegas, Australia
Yes, the amp has enough currrent to push 4 ohms. And I've argued the car stereo wattage figures with so many people and they seem to ignore the fact that their 200W amp hasn't blown up the 5A fuse protecting it... so yes, I know the blowouts and exaggerations common in car audio.

While I agree with the general consensus that home hi-fi is a much higher fidelity level/requirement than car audio, these are excellent speakers for what they are, and it's more of a 'hate to see them go to waste' mindset than anything else.

Anyway, speakers and amp are fine. I'm curious about choosing a cap value for the Zobel, choosing a good crossover frequency, and whether cutting off at too high a frequency is going to reduce the bass output of the speaker system, ie should I use the 90.28hz or 47hz Fres as a guide for the crossover freq?

http://www.focal.tm.fr/car/download%...tek/100CA1.pdf here is some speaker info.

Cheers,
Adrian
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Old 13th February 2006, 11:39 PM   #4
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Join Date: Feb 2006
Location: BrisVegas, Australia
Ok, so can anybody recommend a decent source for finding this information? I guess there are two resonant frequencies now, so I will experiment a bit with crossovers. Nobody can guess as to why there is a difference between Res (44ohm) and DC resistance (3.3ohm)?
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