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Old 17th April 2010, 01:21 AM   #1
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Default Crazy idea: Output tranny with constant magnetic gap?

Did anybody try a crazy idea of a constant magnetic gap in an output tranny? A strong magnet from hard drive may be used for experiments.
Less of iron, less copper, less of insulation, as the result -- smaller, but better in terms of losses and wider frequency response.
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Old 17th April 2010, 01:39 AM   #2
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TDK used to sell inductors that were pre-biased with a magnet to maximize the unidirectional flux swing. I have some small inductors in my junk box (from Electronic Goldmine?) that have little ceramic magnets for the same purpose.
The magnet would have to have a high energy product, be thin, and also non-conductive.

Last edited by wrenchone; 17th April 2010 at 01:40 AM. Reason: sp
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Old 17th April 2010, 02:03 AM   #3
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I don't see that it would have to be thin or non-conductive. The path length would be extended by the length of the magnet, and the gap would be considered as it appears on either side of the magnet, which could be an insulator if you imagined it shorting the ends of the laminations. The flux modulation in a high energy magnet would be low for the densities used in audio output cores, so eddy loss would also be low. Well I guess it would have to be thin in that you would want something other than a saturated core at a useful gap length.

Last edited by Andrew Eckhardt; 17th April 2010 at 02:05 AM.
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Old 17th April 2010, 02:15 AM   #4
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"The path length would be extended by the length of the magnet"

You know what, I'm not entirely sure of that. The magnet is not exactly permeable, so it probably wont change the inductance calculations except where it puts the core permeability at the re-positioned bias level.
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Old 17th April 2010, 02:28 AM   #5
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You'd still be increasing the flux swing by whatever core volume reduction you went for.
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Old 17th April 2010, 04:20 AM   #6
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I wouldn't look so much for a reduced core volume., instead going for extended flux swing. This will probably result in higher core losses due to said wider flux swing, but then, nothing comes for free. This technique may be useful for extending the operating range for nickel-iron cores with their lower saturation flux density.
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Old 17th April 2010, 04:32 AM   #7
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Another thought - I never really looked at the notion, but permanent magnets must have low permeability as part of their definition (think about it). The dipoles must resist reorientation in order for the material to be a permanent magnet - this means low permeability. That being the case, the bias magnet for a single ended transformer must needs be pretty thin, otherwise the primary inductance would suffer too much. This probably will limit you to a really high energy material like the neodymium/iron/boron magnets. This will be a thin, flat piece, with the field normal to the surface, meaning that there will be some eddy current losses, mostly at the lower frequencies, where the flux swing is greatest. I would try measuring the resistivity of the magnets I have around, but I think mine are all nickel plated, which tells me nothing about the volume resistivity of the material.
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Old 17th April 2010, 04:32 AM   #8
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I guess any way you want to slice it you could get almost a 2:1 increase in capacity, the sort of advantage you'd need to bring even permalloy cores into size competition with silicon steel. I don't know if anyone is using that for audio.
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Old 17th April 2010, 04:46 AM   #9
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You could probably think about the magnet as a level translator. It has essentually no permeability, for the highest energy types, being just about fully saturated. Whatever field you stuff into one end comes out the other +/- the magnet flux density. So it wouldn't change the inductance of the wound core except for the permeability shift of the static B level that remained in the core with bias current after compensation. The air gap would still be the principal limiter of core flux, so in all likelihood an easily practical thickness of NdFeBr material would be required. This company has a neat online calculator that could help predict some gap flux values without doing any calculations: Australian Magnetic Solutions - Your best source of magnetic products in Australia and the World - The attraction is quite simple!
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Old 17th April 2010, 04:48 AM   #10
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There was a thread about this very idea and related ones a few years ago. Someone tried a permanent magnet in the gap but got poor results I think. Caused to much loss of permeability by the time they got enough magnet strength. Later I suggested using a toroid core with a plated on hard magnetic film on the laminiation surfaces wound up to make the core. It could then be energized by a powerful pulse at turn on to set the magnet. This allowed a strong magnet due to the large surface area, but kept the gap width to a minimum.

DC compensated SE output transformer

Easiest way is just to put a bucking winding on the core and power it by a CCS.
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