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Old 26th December 2013, 07:43 AM   #1
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Default Flux modulation techniques

Old thread here: Flux modulation

So we know there are a few ways to reduce flux modulation. Shorting ring around the base of the pole piece, copper right inside the coil gap, copper on the top and/or bottom of the pole piece extension, etc.

http://www.diy-audio.narod.ru/litr/F...lImpedance.pdf

I recently (re-)discovered Eighteen Sound's brilliant AIC technology:

http://www.eighteensound.com/staticC...minimizing.pdf

There is a coil wired in parallel, wound in reverse (I assume), fixed on the pole piece inside the coil gap. The induced flux from the fixed coil counteracts the induced flux from the voice coil. It seems perfect... Is it?

One of the benefits of AIC is that sensitivity increases slightly from a higher BL as the forces from the coils work sympathetically. However, the further away the voice coil gets from rest position, the less help the fixed coil can give. The other issue is that flux modulation in Le(i) and BL(i) vary based on excursion due to poorer coupling.

How do we fix this? Split the fixed coil into two sections a la SplitGap/XBL^2, and even better, make the voice coil gap with that technology as well. See here: https://lh6.googleusercontent.com/-Q...h986-no/13+-+1

Anyway, this is all just conjecture without seeing real data from something like the 12NDA520. It does seem like the perfect way to eliminate flux modulation, though it wouldn't hurt to throw in a shorting ring around the base just in case.

By the way, someone tell me again why we don't use electrically conductive voice coil formers? Wouldn't that be just as good as copper on the pole piece?

Last edited by 454Casull; 26th December 2013 at 07:46 AM.
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Old 26th December 2013, 07:49 AM   #2
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Ah, I guess for a pro driver the split voice coil gap would reduce the sensitivity a little bit.
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Old 26th December 2013, 08:36 AM   #3
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Thought this thread was going to be about time travel.
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Old 26th December 2013, 12:34 PM   #4
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Quote:
Originally Posted by 454Casull View Post
Anyway, this is all just conjecture without seeing real data from something like the 12NDA520. It does seem like the perfect way to eliminate flux modulation, though it wouldn't hurt to throw in a shorting ring around the base just in case.

By the way, someone tell me again why we don't use electrically conductive voice coil formers? Wouldn't that be just as good as copper on the pole piece?
Aluminium formers exist all over the place, from cheap cheap to expensive drivers. Due to the VC and the former moving in unison there is very little compensation and we wouldn't want it. Having the VC buck against it's former to which it is firmly secured to would be akin to welding the drive shaft to the chassis. < Extreme example for sure Also note that even when AL is used it is split eg it will not short the flux. Also Faraday rings et al do nothing for bass, they are only effective at much higher midrange frequencies.
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Old 27th December 2013, 12:48 AM   #5
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They do use aluminum formers because they are very conducive thermally BUT normally they are split or have a small gap so that they are NOT a shorted turn on the moving system which is not normally desirable. A shorted turn in the magnetic gap is a good thing as its counter emf prevents the lines of flux from moving (as much) when “pushed against” by the voice coil current.
While the best place to put that shirted turn is IN the same gap as the VC, that is also the most costly place to put it if you have a fixed flux as a target, Cubic volume in the gap X B field in the gap X fudge factor = cost of magnet structure.
Best,
Tom
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Old 27th December 2013, 07:11 PM   #6
bwaslo is offline bwaslo  United States
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Would use of a saturated electromagnet structure avoid the flux modulation?
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Old 27th December 2013, 08:04 PM   #7
tinitus is offline tinitus  Europe
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Originally Posted by Tom Danley View Post
... they are split or have a small gap so that they are NOT a shorted turn on the moving system which is not normally desirable. A shorted turn in the magnetic gap is a good thing as its counter emf prevents the lines of flux from moving (as much) when “pushed against” by the voice coil current ...
so underhung would be different in this respect too ?
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Old 28th December 2013, 01:31 AM   #8
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Hi Bill
I hope your doing well, I have an e-mail I never finished to you, maybe over Christmas holiday I can finish it and finally send it.
I think you and tinitus hit on some areas worth elaborating on since my first answer left many holes and “the magnet” is an interesting system which can be seen like an electric circuit.
In electricity we have Voltage and Current in a magnetic circuit we have the B or filed intensity or flux density and H the magnetizing force.
Air or Vacuum has a permeability or ability to conduct magnetic flux of 1, iron can have a permeability into the thousands meaning when presented with a given magnetizing force H, it has a flux density thousands of times higher than in air. One could say that higher permeability is something like lower resistance conductors are in an electrical circuit and is why we use iron to conduct the flux to the gap.
A coil with current passing through it in place of a magnet can accomplish the same thing as a magnet and if that coil has a magnetizing force proportional to the number of turns times the current with the magnetic path factored in. Thus a field coil with a current source is a purely constant H force (and often, the Rdc of the coil and Voltage source becomes in effect a constant current source).
The Voice coil is also a source of H, it has current and turns in the gap and so this H is on top of the constant force in the magnet. When it pushes against the magnetic flux the fly’s fall into the simple soup.
So, we have a magnetic gap, it has a thickness and surface area and like Voltage and Current, H and B, that area and thickness has what one can think of as magnetic impedance.
A magnet like ceramic or alnico has what you could think of as a magnetic impedance curve or BH product. The total load on the magnet is the leakage loss (lost flux not in the gap) and the load the gap presents. A magnet itself is ideally the right combination of magnet thickness AND area so that when connected to the gap (area and thickness) and leakage losses, the magnet is operating near or at the peak of it’s load line or “BH product”.
With magnets like ceramic and alnico, a large enough voice coil H force can shove the magnet’s operating point away from that maximum product and that modulates the field strength in the gap. Alnico magnets are rather fragile this way and so the way they made recharging them less of a problem was to use a large magnet volume than normal and deliberately increase the flux density at the pole tip to the point of saturation (where the irons permeability falls sharply and becomes more like air so far as magnetic conductivity).
Then when a powerful H force is presented by the VC, it only modulates a small portion of the saturated iron at the tip near the VC up and down relative to saturated. The magnet is left isolated from that de-magnetizing VC H by the small amount of iron always saturated. Another minor curiosity, iron doesn’t have a straight line BH curve, it has little steps in it. So, not only can one modulate the flux up and down with voice coil current, it can do it in little steps. Here too an under hung coil would have a vastly smaller issue with this stuff as only part of the air gap has conductor in it.
So if you’re picturing this in the static condition, there are a few more issues when dealing with an AC signal like with loudspeakers.
The voice coil pushes and pulls against the lines of flux, in a magnet, those lines are more or less pinned to the surface but in iron, they can move around and in fact only their unwillingness to be concentrated next to each other is when they arrange themselves fairly evenly within. On the other hand, the H or push from the VC current can move the lines around and to the degree they move, the VC doesn’t,
The iron top plate and iron center pole are electrically conductive and if one moves the fluc up and down with a VC, that moving flux produces a Voltage in the top plate and center pole. Those parts are conductive and so current flows because of that Voltage induced by transformer coupling.
That Current flowing in those parts produces it’s own magnetic field and interestingly, it is in opposition to the field that is inducing the current. That effect is also called eddy current or a voltage induced in a large conductor which because of it’s “short circuit” nature opposes the magnetic field. If you have a small Neo magnet lying around, try dropping it into a normal copper water pipe (around here most water pipes are copper) and see a jaw dropping demo of eddy current flux pinning.
Anyway, iron is lame compared to aluminum and copper twice as good as aluminum. While neither is magnetically conductive, if you have the space for some saturated iron, you can hog out the gap a little bit more and put a copper sleeve on either or both the in or outside.
At high frequencies, the lines of flux the voice coil produces follow a tighter and tighter path and react less and less with the magnet’s magnetic flux. Electrically this is the series L which rolls off the response above Rmin in the impedance curve. With a copper sleeve (shorted turn) in very close proximity to the voice coil this will oppose the inducing field just as the top plate does at lower frequencies and so can reduce the Voice coils series inductance and roll off which can extend the hf corner –or- let you use a larger VC with more turns which would have otherwise imposed a lower inductive corner.
Anyway, I am rambling and will sign off.
Best,
Tom
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Old 28th December 2013, 04:05 AM   #9
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No interest in the AIC or my take on it?
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Old 29th December 2013, 07:33 AM   #10
Hentai is offline Hentai  Romania
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they could've used colored pictures in the fem results :P and the images too small to see values.

Even if there is an improvement with a shorted VC instead of ring the complexity of manufacturing is increased by a large factor for now either you make the top of the central pole piece removable (which is not good for magnetic circuit) to fit the coil or you will have a process to wind the coil on the pole piece.

What i know and used is high field strength, tight gap and as less copper in the VC as possible. If i add a shorting ring in the gap i extended either above the central pole piece and add width or below the gap and add width as its effect depends on coupling (position and distances) and DC resistance of the ring (thickness the bigger the better).

Quote:
Originally Posted by bwaslo View Post
Would use of a saturated electromagnet structure avoid the flux modulation?
Electromagnets do behave better because you can rule out the effects of magnetic materials in presence of external magnetizing forces.
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