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Old 29th April 2013, 06:15 PM   #261
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Quote:
Originally Posted by gootee View Post
Good point. Several people have suggested that a gap should be left between the columns of magnets. I think they had other reasons, ie either to leave area for air to move or because it would enable the use of fewer magnets.

But it seems reasonable to believe that if the N and S were touching, the field would almost act like there was a short, there, and not as much would extend out to the mylar.
The configuration people are discussing isn't with the N and S poles touching, though. Instead it is with the N pole of one magnet pointing up and the S pole of the neighboring magnet pointing up. Even with the magnets touching, field lines will emerge from one pole and return to the neighboring pole (more or less).

Another good reason to separate the magnets (besides room for air to move) is that the change in the magnetic field with distance above the gap can be lower as a function of height above the magnets. Although increasing the gap reduces sensitivity, it also may reduce distortion.

Jeremy
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Old 30th April 2013, 03:26 AM   #262
gootee is offline gootee  United States
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Jeremy,

Thanks for getting involved and commenting. (But read the thread, please.)

The configuration being discussed was with the N and S touching at their edges. So yes, they were touching. Everyone here already realizes that the N and S are both facing the same way, and are the large flat faces of the magnets, which are all parallel to the mylar. And they realize that even if the edges are touching, there will still be field lines that emerge, to some relatively small and much less useful extent. But it is unhelpful to just state that field lines will still emerge, without any regard for the goals that are implicit in the thread, i.e. without noting that it would likely be a much less desirable configuration.

Since you seem to possibly be knowledgable about magnetics, we would welcome any help in learning to better-characterize the effects of the magnets' spacing and the effects of the height of the conductors above the magnets, with respect to the force vector exerted on the mylar when there is current in a conductor that is attached to the mylar.

Presumably there would be combinations that produce maximum force, and combinations that produce the best-behaved (most-orthogonal) forces, etc, etc. For example, I think that we would usually want the field lines to be parallel to the mylar, at the mylar. (But, of course, the mylar changes its distance from the magnets, as varying current amplitudes are passed through the conductors that are attached to it. So we would be looking for the "best" range of distances, and the best at-rest distance given the other variables.)

And it seems likely that the field strength, at some distance in front of the magnets' front plane (e.g. at the mylar), would increase as the columns of magnets start to separate, then reach a maximum, then start to decrease as they get even farther apart. At the same time, I'm guessing that the distance from the magnets' plane to the most-useful region where the strongest part of the field is most-parallel to the magnets' plane would also change, but maybe not in the same way or at the same rate.

It might be useful to be able to calculate (or simulate, e.g. with FEMM), in order to determine some sort of quasi-optimization, or at least just get a feel for how to guess at a satisfactory configuration, of the combination of 1) the foil width, 2) the foil's distance from the magnets' plane, and 3) the distance between the columns of magnets.

Ideally, maybe, it would be good to be able to characterize or even predict the maximum displacement of the mylar membrane to which the conductors are attached, for a given current amplitude, so that the resulting configuration would not cause the mylar to actually touch the magnets. My Calculus of Variations skills are basically non-existent so it would be a whole lot of work to get to the point where I could construct and then solve the forced-membrane equations. BUT, maybe we could come up with some very simple guideline, such as "tension the membrane while it's horizontal, until placing X grams in the center makes it sag by Y mm", if we could calculate or simulate to get the force on each conductor given the other variables.

In the end, we would also want the most-accurate reproduction of sound that we could possibly get for a reasonable cost, and a reasonably-large maximum sound pressure level from a typical rated maximum amplifier output power.

I was actively planning and preparing to design and build some planar magnetic speakers, a long while ago. But life got in the way and I had to put it aside. And in the meantime I have purchased some Magnepan MG-3.6/R speakers. But I wouldn't mind helping to figure out how to do it and would still love to be able to come up with something that seemed too good to not build. I do also still have the DC Gaussmeter that I bought, back then, which has a directional probe.

Cheers,

Tom

Last edited by gootee; 30th April 2013 at 03:30 AM.
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Old 30th April 2013, 07:59 AM   #263
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Default I had found an optimal way.

I had found an optima way, betvin air and magnet system. There must be more air, than magnets.
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Old 15th May 2013, 09:02 AM   #264
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Finished building my experimental full range planar headphone, very pleased with the outcome, used a cheap headphone as the frame. A bit fiddly on one half the left which contains resistor and 4 wires, the right one only contains 2 wires. 2 different neo layouts were used and 2 different sized neo magnets. One side is a little louder I think! My ears are not as good anymore, can only hear up to 8k. This makes it difficult to tell by listening alone, one of the drawbacks of old age. I made it about 29 ohms although the Koss Pro/4XTC measures 105 ohms. I thought about reducing the impedance to about 6 ohms ,any thoughts on the matter from you technos?
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Old 15th May 2013, 09:33 AM   #265
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Since a lot of people are using a speaker amp with their orthodynamic headphones I guess that 6 ohms should be fine especially if the sensitivity is a bit low.
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Old 15th May 2013, 03:04 PM   #266
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thank you, have started another rebuild of a cheap headphone, which sounds terrible in comparison to my first rebuild. Can't take my planar headphone off, so good, it's like listening to speakers not like headphones at all!! Very happy with them.Have taken the next headphone apart. Supposed to be the same model, but is entirely a different setup and shape, oval in fact. A new challenge for me.
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Old 16th May 2013, 02:34 AM   #267
gootee is offline gootee  United States
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Henry,

Cool!

My gut instinct would be to have both sides identical.

Also, if the side of the planar "speaker" that is away from your ear is open to the outside air, the sound will be diferent than if it's all sealed up. It might be much better with them open. But it depends on the details of the design. Something to try, though! (I guess in that case you might be able to just make your own open frame, instead of tearing open the existing headphone shell, if it happens to be the closed type.

Tom
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Old 16th May 2013, 02:07 PM   #268
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Red face PLANAR HEADPHONES.

Finished at last, a bit fiddly made a few goofs on the way, but very much worth the effort very happy with result. Don't sound like headphones, more like speakers inside your brain. Wore them for hours and hours never wore headphones for such a period of time, unbelievable really. The next 3 pics are of my new headphone rebuild, using a cheap chassis so to say!!
This is a more oval shape than my first one, and a different design and fit. I hope I don't make too many goofs this time, altho' I have made one already and broke 2 of my magnets already, clumsy aren't I
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Old 16th May 2013, 02:11 PM   #269
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Default MISSED THIS PIC

Silly me, old and senile, can only get worse!!!
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Old 16th May 2013, 02:16 PM   #270
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Default this time!!!

Hopefully.http://www.diyaudio.com/forums/attac...1&d=1368710119
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