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Old 29th December 2002, 07:09 PM   #1
Ilianh is offline Ilianh  Canada
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Question Air core vs Iron core

I am making a crossover for a set of speakers, and after seeing that air coil coils would cost twice as much as Iron core ones...

Some people tell me air coil coils sound better, but they are not able to explain why, or what makes them sound bettter, therefore I cant take this for true.

And IF theres a difference, i'd guess it would be only in higher frequensies.

Anyone here has any real info on air coils vs iron core?

and I'd need like 2 9.4 mH and 2 7mH coils, I have the tools to measure mH's and I was wondering woud it better (cheaper) if I winded them myself.
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Old 29th December 2002, 07:36 PM   #2
SY is offline SY  United States
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Air core coils have the advantage of being more difficult to saturate. They have the disadvantage of having higher DC resistances, more bulk, and higher cost for the large values you cited. If an iron core coil is kept well below its saturation current, it will perform just fine, unless superstition rules in your life. Saturation is more of an issue at low frequencies, by the way.

Judging from your question, I'd advise buying your coils pre-made, especially because there will actually be a saturation current specification. Madisound is an excellent source in the US.
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Old 29th December 2002, 08:21 PM   #3
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SY is right - inductors that use an iron or ferrite core suffer from a drop in inductance as current through them increases. I think "saturation current" is defined as when the inductance has dropped to 90% of it original value.

Air core inductors are almost impossible to saturate but require many more windings for the same inductance value so have a higher DC resistance, or they use thinker wire and hence more cost.

So, it depends what you're after. Both the values you need are quite high; the 9.4mH will probably have to iron core to keep the DC resistance reasonable but the 2.7mH could be of either type primaryily depending on the location in the crossover.

Where abouts are they used in the crossover?

Nice one,
David.
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Old 29th December 2002, 08:33 PM   #4
Ilianh is offline Ilianh  Canada
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The 7 mH one is used in the low pass second order
and the 9.4 mH is used in the bandpass for the mid

the speaker has a 8" woofer, a 5" mid/bass and a 1" tweeter
the low pass is at 200 hz, the highpass is at 2000 hz
and the Fmid is at 963 hz

actualy Im not even sure if a bandpass would be necessary
I have the feeling that a simple highpass on the mid/bass would do the job better than a bandpass,
but then maybe impedance problems would occure?
the woofer is 4 ohm, the mid is 6 ohm, and the tweeter is 8 ohm (because the woofer has the lowest sensibility, and the tweeter has the highest, i chose the drivers so i wouldnt need lpads)

I really need a spl meter to see whats the natural roll off of that driver.....

I have a multimeter with dB's and i was wondering if i could make myself a simple electret microphone and use it with the multimeter, that way i could have an idea of whats happening...

what do you guys think of all that?
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Old 29th December 2002, 08:48 PM   #5
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I miss-read your post regarding the 7mH (i took it to be 2.7mH). I would be inclined to use iron or ferrite cores for both otherwise DC resistace could be a problem. Does your XO schematic include DCR values for the inductors?

As for measurement gear, if you're going to the effort of making a mic pre-amp for your multimeter then I suggest you get a copy of Speaker Workshop from Audua (it's free!):
http://www.speakerworkshop.com

Simply connect your soundcard output via an amp to you speakers then connect your mic via a pre-amp into the line-in on your soundcard and you get lovely, lovely frequency response plots in a matter of seconds.

Nice one,
David.
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Old 2nd January 2003, 01:36 PM   #6
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Actually, disto of cores will be highest at low frequencies were their impedance is on the same order of magnitude as the driver, i.e. in their pass band! Iron cores, due to their strong hysteresis, will have disto on the order of 1% which is largely indepencent of power. Ferrites will be pretty benign at low power but distort heavily once they approach saturation.

Eric
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