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Al Klappenberger

Iron-core versus air-core woofer inductor intermod test.

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Guys,

I got interested in the question of why iron core woofer inductors don't sound

as good as air core inductors from another thread. I decided, since a series

inductor forms a lowpass filter, it wouldn't contribute harmonic distortion, so

maybe it's introducing intermodulation. An iron core tends to change its

inductance with the amount of current passing through it. It must do this

dynamically since the current waveform is continuously changing and the magnetic

flux must change with the instantaneous current as well. This says that the

inductance is also changing with the current changes. That sounds like a recipe

for distortion of some sore to me!

I set up a test using two sine wave signal generators, one set at 261.25 Hz and

the other at 321.25. These two signals were mixed in a dual trace oscilloscope

set to add A+B. The vertical output of the scope was fed to the input of an amp

(One channel of a Nakamichi AV-8 receiver). The amp was set to deliver about 1W

to the input of the 2.5 mHy iron core inductor of a AA network or to the input

to a 1 mHy air core inductor. The load was an 8 Ohm resistor. I took three shots

with my spectrum analyzer (attached). One of the amplifier output. One through

the iron core inductor and one through the air core inductor. The difference of

the two signals was 60 Hz. Sure enough, 60 Hz above the upper signal

(321.25+60=381.25) was a intermodulation product (teh red marker on each plot).

It was 65 dB down from the amp. 69.5 dB through the air core and only 55.1 dB

through the iron core. There is probably a measurement error of some degree due

to the insertion loss of the inductors, but the iron core definitely added some

distortion. I assume that power levels greater then 1W would look even worse but

I didn't test for that. The air core inductor actually seemed to clean it up a

bit, but I suspect that is probably measurement errors. I really don't believe

it!

Al K.

post-2934-13819253548894_thumb.gif

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Al,

Nice test. Why did you compare different values of inductors? Not sure there would be any difference, but the air core you tested was a lot smaller value than the iron core.

Bob

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Bob,

The reason I used the 1 mHy coil is becasue I shipped the last 2.4 mH I had with a set of networks. The 1 mHy is all I had laying around!

BTW.. I just ran the same test with 10W, then all the way to 26W input from the amp rather than 1W. To my surprise the results were nearly the same. I expected the distortion products to get worse. They didn't! Do you have any explanation for that? Cold it be the core comes to a "happy" level and stays there until it truly saturates? If that's true, all the distortion is at low level, right where a Khorn least neads the distortion!

Al k.

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Al,

There may be another explaination for what you are seeing but I don't know what it would be. Looking at anything at 50 or 60 db below 1 watt (or 1 mw!) might be asking for effects that defy explaination. That is more than the dynamic range of some recording media. I think finding something 20 to 30 db down might be significant and hearable but at 50 or 60 db down, don't know about that.

Bob

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Al,

Also, as a "by the way" seems I remember from my Radio Engineering Schoolin from back in the 60's, the iron core inductor will hang in there up to about 100 watts and then saturate. Can't remember what happens at saturation. Stops being a pure inductor perhaps.

Bob

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----------------

On 3/22/2004 10:16:58 PM Al Klappenberger wrote:

Bob,

I tend to agree. 60 dB down isn't very much distortion at all. Sooooo.. The mystery remains!

14.gif

Al K.
----------------

No sonic difference, mystery solved!

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John,

You might be right. It might be another case of subjectivie impression like high-dollar speaker cables. "I paid a bundle for that big coil, it's got to sound better"!

AL K.

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----------------

On 3/23/2004 7:00:36 AM Al Klappenberger wrote:

John,

You might be right. It might be another case of subjectivie impression like high-dollar speaker cables. "I paid a bundle for that big coil, it's got to sound better"!

AL K.
----------------

Perhaps, but there is no "downside" for the DIYer other than a few extra bucks. The upside is that it is impossible to saturate and they are interesting to look at.

For the manufacturer, there is no question. It's a properly sized ferrite core inductor.

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John,

I can always tell myself that I am simply testing for the wrong thing! I have not noted a difference between iron and air core myself becasue I never listened for it. I am simply taking other peoples word for it that air core sounds better. That's always a bad idea!

The manufacturer wants to cram the most inductance he can into the smallest space which is the iron (or ferrite) core. That's their bottom line!

Al K.

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Intermodulation product with 60 Hz? Don't suppose it has something to do with picking up powerline radiation do you? Difference between the results with the two inductors could easily be due to difference in proximity or orientation.

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----------------

On 3/25/2004 4:28:11 PM DeanG wrote:

What about Hysteresis?

----------------

An air core inductor does not suffer from hysteresis and will, under all power conditions short of sintering the windings into a solid "mass", exhibit linear behavior

That said, a ferrite core inductor,
properly sized
for the expected power will exhibit distortion levels so low as to be considered negligible. For a
properly sized
ferrite core inductor, the driver itself will go non-linear long before the power levels needed to saturate the core are realized.

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Dean,

I think the hystarisis effect, like any other kind of nonlinearity, would show up as intermodulation. It didn't! I think what John said above is probably true. The only measureable difference I see is "Q". The Solen coil of #14 wiree has a Q of roughly 25. The iron core coil Klipsch used in the AA was only about 15. Both Bob and I measured both coils and got the same results. Q, of course, represents losses. This is probably a size / performance trade-off.

Al K.

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Al,

Did you see the numbers on the Parts Express Iron core inductor that I posted yesterday in the Technical? I bumped it back to the top. Wanted to see what you think of those numbers.

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----------------

On 3/25/2004 7:08:32 PM Al Klappenberger wrote:

Dean,

I think the hystarisis effect, like any other kind of nonlinearity, would show up as intermodulation. It didn't! I think what John said above is probably true. The only measureable difference I see is "Q". The Solen coil of #14 wiree has a Q of roughly 25. The iron core coil Klipsch used in the AA was only about 15. Both Bob and I measured both coils and got the same results. Q, of course, represents losses. This is probably a size / performance trade-off.

Al K.

----------------

So Al you can be quoted as saying "Bigger is better"?1.gif

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Bob,

I misunderstood what that iron core coil you measure was. I thought it was the Klipsch inductor from the "AA". I see it is something else. It's Q and loss is still not quite as good as the Solen air-core coil but it isn't bad at all. I think going to an iron core like that might be necessary if you really need a lot of inductance in a small space. Air-core coils get pretty big when the inductance gets high. I just finished building the prototype for my 700 Hz extreme-slope network. It has a 3 mHy inductor I had intended to order of #16 wire and ordered #14 by mistake. I had to let it hang over the edge of the board a little to get it on there. Much more, and something would have to give!

Al K.

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Al,

I was impressed with the DCR of the Parts Express "I" core inductor. With a Q almost as good as the best air core and the ease of mounting with provided screw holes, it looks like a good alternative to the air core inductor. The DCR is 0.189 and that is with 16 awg wire. It is a nice 2 inch by 4 inch package.

Bob

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Bob,

I have had quite a bit of experience using ferrite or powered iron toroid cores for inductors in the 10 - 100 MHz range. Getting the right ferrit or mixture of material is vital and it varies with every frequency range. What you really need to do is run the Q test at a bunch of frequencies from 500 to 20 Khz. You will find that the Q is best at some specific frequency. I would like to know where the Q peaks out.

BTW: RF filters built using ferriet toroid cores DID have troulbe with intermod. A batch I built were rejected becasue of that. Toroids saturate easier than "I" cores though.

Al K.

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