Eminence Flux Density Modulation

[kg]

jukka: certainly can be done.

as i said earlier in this thread, you can just take another strong magnet and buck their fields. it WILL negatively affect the flux in the gap. the degree to how much it will be reduced depends on how strong your bucking magnet's field strength is.

as jm said the BEST technique will buck the field within the magnetic circuit itself (ie magnet/pole piece/gap/backplate), but an externally applied one will still have an effect.

[balijukka]

To affect magnetic flux , in both cases, speaker and transformer, you should wind the auxiliary coil you suggest around the center "leg"; whether it's the speaker pole piece or the transformer core.
In the speaker case, it would not be an "ad-on" at all.

If you think of it, the back plate and center leg are just a conduit for the field. The magnet itself is a ring, opposing poles at the sides. If you wind the coil exactly around the magnet,the magnet will be your field conductor with the rest of the circuit closing it. What you do is just create the opposing field right at the source, in the magnet itself. The coil will undoubtedly end up being big, you'll have to fit the whole magnet inside it. And the magnets of different speakers being different size, you'd have to make a special coil for every speaker, but that's just a question of molding different bobbins.
What really worries me, does this kind of imposed field have any lasting effects on the original speaker magnet.

jukka

[J M Fahey]

For discussion's sake, suppose the speaker points skyward, let's say North points up *in the ring magnet*.
Let's also accept that magnetic lines of force go from North to South.
If the magnet is "alone" in free space, you all know how lines travel in different paths, "the best they can" , because space or air are terrible to conduct line forces, and the magnetic field takes the shape we all see in Physics books.
It might somewhat be compared to a quite fat dougnut.
Now to the magnet *in a magnetic circuit* which is something very different.
Both poleplates help close the circuit, let's say they force the lines to travel horizontally.
The final part of the magnetic circuit is the polepiece.
Let's follow the path of a lineforce.
By definition, either on a free magnet or inside a speaker, lines leave the top (North) magnet face perpendicular to it, "pointing up", travel inwards towards the gap thanks to the top plate, then travel *downwards* or "pointing down" through the polepiece, then horizontally "outwards" thanks to the bottom plate until they reach the bottom/South pole.
They complete the circuit "pointing up" again inside the magnet.
Now let's analyze what happens if you surround the whole enchilada with an external coil, through which you pass some DC current.
It will become an electromagnet.
It will produce a magnetic field inside it, in the space surrounded by its wire; as was suggested in the post above. The polarity of that field will be the same in any point inside that coil.
We can easily see, that if, say, we create an electromagnet with the South pole on top, to substract from and weaken the magnet, that "Top-South" field increases the field through the polepiece at the same time .
Total effect: nil. (zero)
If we invert polarity we are doing the contrary: reinforcing the magnet and weakening the polepiece: same result. It balances out to zero.
That's why I called the speaker magnetic system a "closed circuit", isolated from external forces.
If you analyze a ring Alnico (P12N) it's the same.
If you analyze a center Alnico system (P12R, P12Q) it ends up being the same, only now the North Pole is on the polepiece top, and the passive (induced) magnet which closes the circuit is outside.
Now, if we wind a coil around the polepiece but inside the ring magnet, as I suggested, the situation is absolutely different: if we pass current so it creates an North top field, this will substract from the South top field present in the polepiece, and those same "new" lineforces will complete their circuit through the ring magnet, in an opposite direction and weakening it.
I think I've demonstrated that an internal coil wound around a polepiece can be an effective magnetic attenuator; while one wound outside the whole magnetic circuit will not.
I recognize it *might* have a (very) little effect because of iron nonlinearities and such, but you'd need a monster electromagnet to make that happen.
In fact, the huge magnetic pulse in a magnetizer does exactly that
After all, speakers are magnetized "already built", and the , say, "North up" pulse goes through the magnet *and* the polepiece with the same polarity and at the same time.
As I said, that pulse must be around 5x what's needed to saturate the magnet *and* iron; that's why I talked earlier of my 3KW and 17KW magnetizers.
Compare this to the 15W Fluxtone uses to get a very loud speaker; obviously by winding around the polepiece but inside the magnetic "cup".
Excuse me for being so long winded, I understand this is not as intuitive as Electrical circuitry, too tired to draw now but if somebody finds it necessary I will gladly add some diagrams to let the magnetic paths be seen with clarity.
NOTE: exactly the same happens with a conventional EI transformer : the "useful" coils are wound around the center leg but inside the outer arms.
If you wind around the whole transformer, core and all, you have no influence on the "normal* coils.
In fact, if you wrap a copper foil around the center leg, you are shorting it, the transformer becomes useless; but if you wrap that same copper foil around the whole core (and the internal winding, of course) you not only do not short it, but you also lower interference to the outside world.
Good luck.

[Steve Conner]

I agree with JM. Back in the days of CRT TV sets, you got magnetically shielded speakers for use near them. What they had was not an actual shield, but another magnet stuck to the back of the pole piece, creating an equal and opposite field. And yet these speakers still worked.

[J M Fahey]

Yes. They were labelled "shielded" (an easier idea to grasp for the general public); in fact they were "neutralized"
The problem with any ring magnet, specially ceramics, is that an important part of the total flux "travels" outside, that's what you see when the speaker grabs a screwdriver or erases a tape or diskette.
In a CRT TV it wreaks havoc with the image. (My own TV set by my workbench often has disgusting green or magenta splotches because I forget and carelessly walk too close with a speaker in my hand).
The external, inversely magnetized magnet which is glued to the external face of the backplate is smaller and just strong enough to cancel the *external* field; it has no influence on the internal one.
Besides that, there is one special case where an external magnet helps the internal one: in some car audio woofers, an extra magnet is glued outside but with same polarity as the main one; its field forms what could be called a "belt" around the system and forces that "loss" external field back inside, or, more precisely, doesn't let it get out.
Even more important, the Car speaker looks even more impressive, "justifying" the "Bombastic/Nuclear/Assassin/Bonecrusher/etc." label they show.
The most efficient systems are those which use the magnet inside a (truly) shielding pot, as most Alnicos and all fieldcoils (including Fluxtones)
Never had a Neodymium one in my hands, but I think they also use a magnetic slug inside a pot.
Ceramics are not powerful enough to be used there.

[balijukka]

Crap, there goes that idea
Thank you for a very detailed explanation.
I should have remembered the rule, If it would work, someone would have done it already.

jukka

[J M Fahey]

I should have remembered the rule, If it would work, someone would have done it already.

Don't be so sure about that, thanks God new things are being invented every day, be them groundbreaking or small advances in the art, all them useful.
Most of them are obvious .... *after* they are mentioned.
See Fluxtones: field coil speakers are known from the 30's on.
They were practically abandoned in the 50's.
They were reborn in the 2000's but with a twist: not to provide maximum efficiency but to get exactly the opposite, in a controlled way.
Go figure !!
PS: are you actually in Bali ? Tell us something about that. Music, electronics, our stuff.

[balijukka]

PS: are you actually in Bali ?

I'm working on a project in Malaysia at the moment. Family is in Bali.
And I'm going home for a 2 weeks holiday next saturday.http://music-electronics-forum.com/i...n_bigsmile.gif

jukka

[Diablo]

Revivng an old thread on the Fluxtone speaker concept, all in the name of science. I've been experimenting with three field coil speakers. Two of them are Jensen F12N with their original cones in good condition from the 1950s. The other speaker is a 12" Rola, and all the speakers were used in Hammond organs. The field coils measure 680-700 ohms DC at room temperature. I reconed the Rola using Weber parts - 1.75" voice coil - 100 Watt - 8 ohm, flat spider, Grey Wolf cone and large H-screen dustcap. The recone required some minor trimming on the cone to make it fit. I measured the VC gap as around .040-.045". I'm using a 100 volt DC variable power supply for the field. I find that all three speakers sound good with a guitar amp when I have the field coil voltage high. However, the range of volume attenuation is not as large as I would like - certainly not the 25dB achieved by Fluxtone. My DC voltage ranges from 7.5-100V on the field. The bigger issue is that the speakers do have a response shift as the voltage is reduced. I find that there is a lot more buzzy distortion on the low notes, and a lot of compression everywhere. So I'm very curious as to why I have these artifacts and I don't hear this with the Fluxtone videos. My Rola reconed speaker really sounds excellent with the high field voltage, and if I could retain that quality at lower SPL I would be in heaven. Any advice from Mr. Coil or JM Fahey and others is greatly appreciated.

[Alan0354]

I don't know the first thing about speakers, I listened to video demos and both red and blue ones sounded weaker when cranked down. It's like they lost something when cranked down. I just feel I can do better even with my THD Hotplate. But of cause, I have to listen to it in front of me to really tell. Together with the high price, I eliminated them when I was in market of speakers a few months ago. I ended up with two WGS speakers and never look back. The main reason I went to WGS on the first place was because they are the only one that offer refund. I even contacted Eminence, they absolutely do not give refund.

I like to hear what are your impression from using the speaker.