Iron vs. air core inductors - distortion? - diyAudio
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Old 26th July 2007, 05:41 AM   #1
owdi is offline owdi  United States
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Default Iron vs. air core inductors - distortion?

Previous threads:

air core inductor vs. iron core inductor
Air core vs Iron core

I'm coming close to completing the design of my speakers, and want to replace my 19 awg air core inductors with something better. This is the BFC + lowpass inductor for my midwoofer. Crossover frequency is 2000hz.

14 awg air core, Erse EQ
1.50mh
.278 ohm

14 awg iron core, Erse Super-Q
1.50mh
.084 ohm

The DCR on the iron core looks very tempting. Not sure about how much power this coil will take, but the 16 awg version is rated for 500 watts. It will never see transient peaks higher than 50 watts.

So what is the disadvantage of going with the iron core, assuming I will never come close to saturating the core? It is rated to operate between 0 - 3000hz.. what happens above 3000hz? Is there an increase in odd, or even order harmonic distortion? Exactly how much more distortion? 1%? .1% .00001%???



Dan
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Old 26th July 2007, 12:08 PM   #2
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Hysteresis might also be an important factor in ferrous cored inductors.

It typically has an effect near the zero-crossing because the magnetized regions are more "reluctant" to change polarity entirely, compared to just growing or shrinking in response to an existing magnetizing force.

Although I'm not sure exactly how the underlying mechanisms would work with audio, in terms of choosing a solid core it's probably a case of "pick your poison":

-Solid iron or steel with a very high soft-saturation limit, giving a bit of tube-ish harmonic and IM distortion.

-Or ferrite, possibly creating small granulated jumps in the magnetic field as the signal varies, thus producing high-order harmonics and background hash.

In both cases an air gap would smooth things out and reduce the distortion, but at the cost of increased size and higher resistance. Personally, I prefer active crossovers as they avoid these sorts of issues in the first place. The amplifier's damping factor is already bad enough when taking into account that the actual air vibrations produced by a speaker are at the end of a very long list of inductive and capacitive reactances. Adding more inductors and capacitors corrupts the amplifier's feedback loop with delayed reflections.

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Old 26th July 2007, 01:53 PM   #3
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1.5mH isn't that large, so I'd never consider anything but air core. As for the couple tenths of an ohm, it's just another small term that has to be considered in the design. Inductors are always such a compromise that I long ago moved to an active crossover. Building amps and low level crossovers is, IMHO, far more fun than fooling with expensive, huge and imperfect inductors and expensive and huge capacitors, and sourcing just the right non-inductive power resistors.
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Old 26th July 2007, 05:06 PM   #4
soongsc is online now soongsc  Taiwan
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foil air core
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Old 27th July 2007, 06:41 AM   #5
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a series xover will give you smaller inductors (and larger Capacitors), hence lower resistance...
but I'm with Conrad, I don't think .2 ohms will make an appreciable difference, tolerances in drivers and boxes aren't that rigorous.
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Old 5th August 2007, 07:17 AM   #6
owdi is offline owdi  United States
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CeramicMan - that is the most convincing argument for air core inductors that I have heard to date.

I've decided to go with the 14 awg air core. I haven't found specific info on distortion of iron core inductors, so I'm playing it safe.

Dan
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Old 5th August 2007, 06:05 PM   #7
peter_m is offline peter_m  Canada
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If you feel adventurous, you can try to make your own: http://www.dpg.devry.edu/~akim/new/inductor.htm

If you wanted to go lower and use 12gage wire, this calculator indicates that the coil weight would go from 1 pound for 14g to about 1.7 pounds for 12g but with a DCR of about 0.14 ohm!

There are also other calculators online. I would suggest testing a few and comparing their output to decide which is best.


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Old 5th August 2007, 06:37 PM   #8
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As long as the iron isn't saturating, the distortion of the speaker motor will mask the distorion in the coil. Just my observations...

I'll toss in another aircore link
http://geek.scorpiorising.ca/calculator.html
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Old 5th August 2007, 07:05 PM   #9
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"It is rated to operate between 0 - 3000hz.. what happens above 3000hz? Is there an increase in odd, or even order harmonic distortion? Exactly how much more distortion? 1%? .1% .00001%??? "

Without doing the math - it is hot and I'm tired - the coil isn't doing anything much at 3Khz+ in a normal corossover - except blocking.
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Old 5th August 2007, 08:20 PM   #10
owdi is offline owdi  United States
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Quote:
Originally posted by peter_m
If you feel adventurous, you can try to make your own: http://www.dpg.devry.edu/~akim/new/inductor.htm

If you wanted to go lower and use 12gage wire, this calculator indicates that the coil weight would go from 1 pound for 14g to about 1.7 pounds for 12g but with a DCR of about 0.14 ohm!

Peter
I thought about doing that, but I don't think I can match the quality of the windings in an Erse inductor, which means I end up with higher DCR.

Quote:
Originally posted by cliff
Without doing the math - it is hot and I'm tired - the coil isn't doing anything much at 3Khz+ in a normal corossover - except blocking.
Since I am using only a series inductor in the woofer filter, and a 2khz crossover point, there will be significant output above 3khz.

Dan
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