inductor differences

[Paul Carmody]

when they say that "iron core" inductors distort--and therefore air-core are superior--what kind of distortion are we talking about? Is it audible--if so, where? Does it happen at all power levels, or only above a certain threshhold?

I'm asking because there seems to be a phobia about using "iron core" (OK, steel laminate, whatever) inductors in a crossover. That, if one is serious about fidelity that he should only use air-core inductors. Any truth to this, scientific or otherwise?

Oh, what what is hysteresis? Sounds like something you get from a toilet seat.

Thanks.

[dcibel]

Re: inductor differences

Hysteresis. Basically, the core not only conducts the magnetic flux, but is magnetized by it, and will tend to hold the magnetic charge it is given. For AC signal passing through an inductor, the polarity of the magnetic field within the core is constantly changing. Since the magnetic core tends to hold it's charge, it then provides an "opposing force" when the signal polarity is reversed. Looking at a sine wave on a scope, it will look skewed when a significant amount of hysteresis is present. I'm not 100%, but I think this may represent itself as intermodulation on a spectrum analysis. It will show up as side bands at least. Cores are laminated to reduce eddy currents caused by the constant changing of magnetic field.

I recall reading in I think the LDC about cored inductors. It was concluded that they were not a problem when they were the adequate size and shape. Steel laminate type such as these were found to be the best bang for the buck and indistinguishable from air core as long as the core does not become saturated.

I think the phobia of iron cores comes from cheap commercial products. I have seen inductors that are not sized properly, not laminated, and the core does not extend past the winding as the linked photo. This is not good, will distort, and will saturate at low power levels.

[pixelgarden]

Will I see significant dB gain on my woofer (1st order crossover) if the DCR from my series inductor is changed from 0.92 ohm to 0.15 ohm ? The driver is 8 ohm rated Hivi B4n. Will the change of inductors bring very noticeable change in low end? The inductor value needs to be 1.1 mH.

[donradick]

Re: inductor differences

Sheesh, I feel silly jumping in here.
ME explaining something to YOU?

But always trying to be helpful - here goes.

Hysteresis.
wikipedia is your friend.
http://en.wikipedia.org/wiki/Hysteresis

Simply put, when a voltage delta flows through a ferrite core inductor,
there is a finite response time. Most people don't seem to hear a problem
with this.
Saturation is a more audible problem, but only occurs a high power levels:
http://en.wikipedia.org/wiki/Magnetic_saturation
IIRC, saturation adds distortion.

[billfitzmaurice]

Re: inductor differences

That, if one is serious about fidelity that he should only use air-core inductors. Any truth to this, scientific or otherwise?

Nope. Insertion losses are far more of a concern than saturation, and insertion losses with large value air cores is the reason for going with solid cores. Sure, you can run into saturation problems if you use a solid core at too high a power level than you should be, but that's your fault, not the coils.

[1100xxben]

Re: inductor differences

The main concern with iron core inductors (in general, not just audio crossovers), is saturation. When you use an inductor, there is a field that builds up around the inductor, known as magnetic flux. In an air core, this field can build and build all it wants, with the limiting factor being the gauge of the wire and its electric current capacity. When you need high values of inductance, you can reduce the amount of wire needed by using an iron core. The iron increases the inductance, but the big drawback is that the iron has a fixed amount of magnetic flux that it can hold. Once you exceed that amount, the core is then saturated. When a core saturates, it no longer acts like an inductor, and now looks like a short. This is more of a concern when inductors are used in a constant power output (like an RF amplifier) and when they're used to filter a DC signal. In the filtering of a DC signal, you can saturate the core with the flux from the DC, and then it will no longer filter out any noise.

This is purely my opinion, but I would think that distortion from an iron core inductor would only be a concern at high enough levels to saturate the core, which needs to be appropriately sized in the design stage.

[edlafontaine]

Re: inductor differences

I recently had a similar question. For saturation, look at the second page of this .pdf.

http://www.bourns.com/data/global/pdfs/PM2120_series.pdf

Inductance varies with current. Music peaks or prolonged play at excessive levels would push the inductor to a different value of inductance...throwing the crossover out of whack...

[edlafontaine]

Re: inductor differences

Paul,

My interest is in finding suitable, cheaper iron core inductors for contour networks on mids, tweets and full rangers.

Typical values would be 0.30 mH and less. I'm also considering winding my own using magnet wire or transformer salvage.

[billfitzmaurice]

Re: inductor differences

I recently had a similar question. For saturation, look at the second page of this .pdf.

http://www.bourns.com/data/global/pdfs/PM2120_series.pdf

Inductance varies with current. Music peaks or prolonged play at excessive levels would push the inductor to a different value of inductance...throwing the crossover out of whack...

'at excessive levels'.
I went to the doctor, I told him 'my arm hurts when I do that'.
He said...:D

[RINNAV]

Re: inductor differences

Not to get off base here, but...


It would seem then [theoretically], placing a XO capacitor within the center of an air core inductor (as seen in one of the OS projects) would change the magnetic influence of the parallel plates thus changing the value of the capacitor. How much of an affect is the question--but it is likely the impedance would also be akin to constant change, dependent upon how much magnetic flux and/or hysteresis affected the values of +/- Q. I am trying to follow and learn here—all of the “tron-braniacs” let me know if I am off base here! :eek:

Jason

[johnnyrichards]

Re: inductor differences

Paul,

My interest is in finding suitable, cheaper iron core inductors for contour networks on mids, tweets and full rangers.

Typical values would be 0.30 mH and less. I'm also considering winding my own using magnet wire or transformer salvage.

Ed - I played with winding my own, and while I am not ready to give it up yet... Here is what I learned.

1. The wire is stiff, which can make for some hilarity at the kitchen table when you drop a coil you spent fifteen minutes carefully winding.

2. Bobbins are mysteriously hard to find.

3. Wrapping the wire around a screw, bolt or other odd piece of iron/steel will have the very predictable effect of raising inductance.

4. I am not ever going in the business of selling 1% matched, hand-wound coils. No way, no how. The 1-2% Erse has given me so far is good enough.

[1100xxben]

Re: inductor differences

Not to get off base here, but...


It would seem then [theoretically], placing a XO capacitor within the center of an air core inductor (as seen in one of the OS projects) would change the magnetic influence of the parallel plates thus changing the value of the capacitor. How much of an affect is the question--but it is likely the impedance would also be akin to constant change, dependent upon how much magnetic flux and/or hysteresis affected the values of +/- Q. I am trying to follow and learn here—all of the “tron-braniacs” let me know if I am off base here! :eek:

Jason

Capacitors work by storing electrical charge, and should not affected by magnetic fields generated by the inductors in a crossover. Also there shouldn't be any iron in capacitors, therefore placing a capacitor in the center of the inductor would not affect the inductance of the cap. If you place a capacitor next to a coil, there is still a significant magnetic field present, just not as strong as the middle of the coil.

[diy speaker guy]

Re: inductor differences

Ed - I played with winding my own, and while I am not ready to give it up yet... Here is what I learned.

1. The wire is stiff, which can make for some hilarity at the kitchen table when you drop a coil you spent fifteen minutes carefully winding.

2. Bobbins are mysteriously hard to find.

I'm just about to try my hand at this too. (Unwinding to custom values though, not winding from scratch, at least at first.) I'm going to try unwinding freehand but I can't imagine even trying to wind a coil without some type of winding jig, which should take care of points 1 and 2 above.

3. Wrapping the wire around a screw, bolt or other odd piece of iron/steel will have the very predictable effect of raising inductance.

I don't find this stuff very interesting so haven't researched much but I did uncover a very valuable piece of info in the 5 minutes I spent, assuming it's actually true. I read that the core material needs to be about 3% silicon (or some other material that starts with an "s") or it will saturate almost immediately. Can't remember where I even found that and I certainly can't verify it, so YMMV on that one but apparently the core material is extremely important.[/QUOTE]

[CJB67]

Re: inductor differences

Got some iron core inductors on the woofers in some 3-ways right now (300 hz X/O). If this is crappy iron cores sounding bad then I don't think I could handle "good".

I'll take the lower DCR and price over large air cores with big DCR values and big $$$$ any day.

Or larger air cores with lower DCR and even more $$$$.

I say this WRT low crossover woofer applications in mind.

Also I think the overall cost of the design is a consideration. Along with front end equipment considerations. How much did the drivers cost? What are you going to hook them up to?

Sometimes it is all about synergy.

Don't put a diamond necklace on a pig. Yeah, it's still a diamond necklace. But it's on a pig.

[Paul Carmody]

Re: inductor differences

I read that the core material needs to be about 3% silicon (or some other material that starts with an "s") or it will saturate almost immediately. Can't remember where I even found that and I certainly can't verify it, so YMMV on that one but apparently the core material is extremely important.

I'm going to guess "sushi." :D

[diy speaker guy]

Re: inductor differences

I'm going to guess "sushi." :D

Actually, considering the increasingly disturbing amounts of pollution found in our water supplies I wouldn't be surprised if you could use a raw fish as a core material.