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Cores for XO Inductors

 
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speakerguru
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PostPosted: Fri Nov 14, 2014 9:27 am    Post subject: Cores for XO Inductors Reply with quote

The main limitations of magnetic core materials are non-linearity and saturation. Both are due to the magnetisation hysterisis loop that they following during magnetic field reversals caused by ac current.

They are used because they reduce the size and therefore the resistive losses in inductors. They make inductor designs more practical.

It should be noted that most crossover ferrite core inductors are only partly cored. The return path between the two ends of the cylinder is through the air! This is why crossover boards with many inductors should be built with all inductors at right angles and/or far apart to prevent unwanted coupling or transformer action. This air gap considerably reduces both non linearity and saturation that a closed core would have had. i.e don't use toroids for XO inductors.

Some so called "ferrites" are in fact iron dust. KEF inductors were at one stage all iron dust and had the best saturation performance available at that time.

Saturation occurs when the magnetisation causes the magnetic flux in the core to rise above the maximum specified level. The magnetic resistance of the core increases and the flux then has no reason to travel in the core rather than through the air. The inductance at this point falls (the core is doing nothing) and the current increases thereby causing distortion. This happens at peaks in the audio waveform. The point of saturation is reached sooner with small diameter cores as this increases the flux density in the core and with large number of turns which increases the magnetisation for a given current.

If you monitor the current through an inductor on a 'scope you can see when it saturated by the blips on top of the peaks of the waveform. You can crudely determine the saturation point by putting a steel croc clip against the open end of an inductor core and increasing the signal until the clip starts to bounce. Watch out for things getting hot though
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audiolabtower
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PostPosted: Fri Nov 14, 2014 6:40 pm    Post subject: Reply with quote

Most interesting. Many years ago (soon after they came out) I built around 15 pairs of LS3/5as as a job lot for interested people at work when we got all the design info from a friend of mine at the BBC. We followed all the exact parts, the only thing we could not do is the exact selection of the B110s, we measured the ones we bought and matched them into pairs as best we could (I set the final crossover tap with each pair sat in turn on top of my BC1s!). At the time it was relatively difficult (and expensive) to get the radio metal core transformers the Beeb used, and I always thought they had the best distortion or saturation performance for crossover inductors? I guessed the consumer orientated manufacturers did not use them because of the high cost?
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speakerguru
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PostPosted: Sat Nov 15, 2014 9:58 am    Post subject: Reply with quote

audiolabtower wrote:
... and I always thought they had the best distortion or saturation performance for crossover inductors?
Although I have not measured to compare them, I suspect that despite the laminations and air gap spacer, Radiometal was not as good as iron dust. The eddy current losses with bonded dust must be smaller than with solid laminations.
audiolabtower wrote:
I guessed the consumer orientated manufacturers did not use them because of the high cost?
Yes, E and I laminations must be more expensive materially and in labour to assemble, than a simple cylindrical open core.
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audiolabtower
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PostPosted: Sat Nov 15, 2014 2:24 pm    Post subject: Reply with quote

I've looked in a few boxes over the years, and was always impressed by the quality of Kef construction and parts (particularly Ref series) compared with some of the cheap tat used in crossovers in stuff getting "rave" reviews in the Haymarket rags, inevitably to be instantly forgotten and replaced by the next "flavour of the month".
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habrune
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PostPosted: Mon Nov 17, 2014 9:45 pm    Post subject: Reply with quote

Many years ago I compared Philips 3,5mH inductors with laminated closed cores to ferrite Intertechnik HQ58/46 3,3mH coils. The Intertecchniks showed consistendly 3.3mH across different measuring instruments, but the same 3.3mH Philipses showed huge differences in mH values measured across different instruments from 1,5 to 5.0mH!
Puzzled, I took an advanced HP L/C measurement device.
The problem appeared to be the different AC measuring currents across the different instruments. So: in the laminated cored Philipses, the current mattered for the 'right'reading, which has lead me to the conclusion that these PHS coils+cores must be very non-linear.
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audiolabtower
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PostPosted: Tue Nov 18, 2014 12:41 pm    Post subject: Reply with quote

I had never heard of radio metal in those days, apparently most transformers have silicon steel cores which is perfectly adequate but not ideal for audio, particularly e cores where the grain has different magnetic properties as the flux travels around. Radio metal is an expensive nickel steel with much superior magnetic properties and much lower hysteresis losses thus much more sensitive to small signals. I doubt very much if any other speaker manufacturer except the BBC licensed ones ever used it.
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iso
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PostPosted: Wed Nov 19, 2014 3:42 pm    Post subject: Re: Cores for XO Inductors Reply with quote

speakerguru wrote:


Some so called "ferrites" are in fact iron dust.

Martin Colloms mentions in High performance Loudspeakers that distortion figures of iron dust cored inductor is inferior to ferrite cored one at lower signal level, but iron dust core outperforms ferrite core near saturation level.

Are iron dust and ferrite cores not too exact specifications?

Best Regards

Kimmo

PS SG... great you have opened this discussion about this and some other basic ideas
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speakerguru
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PostPosted: Thu Nov 20, 2014 9:26 am    Post subject: Re: Cores for XO Inductors Reply with quote

iso wrote:
PS SG... great you have opened this discussion about this and some other basic ideas

I'm glad you're happy, but the purpose of a "sticky" post is not to open discussion, that should be done with a new post. A "sticky" is there for reference, to answer questions that seem to come up again and again as new members join the forum.
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