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Old 16th October 2002, 03:08 PM   #11
djdan is offline djdan  Romania
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An amplifier can't amplify two ore more frequency in the same time. The music is pure sineway. You can read more about that subject on the Rod Elliot articles ( I don't remember what article ).
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Old 16th October 2002, 03:13 PM   #12
Bill F. is offline Bill F.  United States
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Right now, I'm working on a new bass driver design that will use a field coil in the prototype stage so I can test out a range of Qes values without building a herd of test mules.

While there are several good reasons to consider field coils, I would submit that weight and ease/cost of manufacture are not among them.

Field coils are no panacea. They are limited by the same thing that bottlenecks most permanent-magnet motors--the magnetic properties of the return circuit.

In my bass motor, I'm using relatively cheap low-carbon structural steel, which can conduct flux densities of about 1.7 Tesla. That's part of the reason I'm using a massive return circuit with a cross-sectional area of 20 square inches. Soft iron scores better than steel at 2.2 T. The champion material is probably Permendure (50/50 cobalt/iron) at 2.4 T. (That's what the flagship Fostex drivers use, but the cost is extremely prohibitive). Exotics like sintered neodymium may be up there too, I don' t know.

Though I am not prepared to personally support or dispute the idea, there are those who argue that magnetic hystersis causes audible distortion. Hysteresis is inherent in all ferromagnetic materials to varying degrees, directly related to remnance. Iron and steel have relatively wide hystersis loops, while magnetically soft materials, like Permalloy, MuMetal, etc., have narrow ones. (I don't know about Permendure and the other exotics.)

The common driver recipe (weakish magnet, steel return circuit, high-power voice coil) allows the most hysteresis. The VC field from a heavy transient can reverse the magnetic domains in the return circuit, and even in portions of the magnet. The total field will sag until the magnet has turned all domains back around.

The common way to fight hystersis is to use a very powerful magnetic circuit with a low-power voice coil (=high efficiency). In this case, VC transients will reverse fewer domains and cause less field sag.

I've been wondering about the potential merits of a field coil motor using a soft magnetic circuit. Though total circuit permeability wouldn't be as high as with some other materials, it seems to me that hystersis effects could be dramatically reduced and, of course, demagnetization would be eliminated.
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Old 18th October 2002, 03:55 PM   #13
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Quote:
Originally posted by Bill F.


1)
Iron and steel have relatively wide hystersis loops, while magnetically soft materials, like Permalloy, MuMetal, etc., have narrow ones. (I don't know about Permendure and the other exotics.)

2)
The common driver recipe (weakish magnet, steel return circuit, high-power voice coil) allows the most hysteresis. The VC field from a heavy transient can reverse the magnetic domains in the return circuit, and even in portions of the magnet. The total field will sag until the magnet has turned all domains back around.

1)
Are you sure speaker pole plates are not made from soft iron, i.e. just regular iron without much carbon and not alloyed with chromium or other metals? After all, I understand most electro-magnets have a soft iron core.

2)
I have seen it stated here and elsewhere that the domain switching in ferrites causes (Barkhausen) noise. Also, some claim Neodymium is immune from this. Neither have I seen quantitative analyses of the Barkhausen effekt nor do I currently understand why Nd would be immune. Do you have any further information?

Eric
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Old 18th October 2002, 05:00 PM   #14
Bill F. is offline Bill F.  United States
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Though I can't give you a complete list of who uses steel and who doesn't, I know that Eminence, P Audio, etc., typically use low-carbon steel magnetic circuits. They may have optional/higher-end iron components for custom builds, but I believe they largely employ steel--economics, I imagine. Sorry, I haven't done any real research on other mainstream manufacturers' alloys.

I have no info on the Barkhausen behavior of Nd, etc.--sorry. If you find it, please post.

If there is indeed a difference, I would guess it has to do with the size or interaction of the magnetic domains. Example: if the domains are smaller (or interact less), their individual switching effects would be less evident. (Just guessing )

Bill
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Old 28th October 2002, 03:46 PM   #15
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There is some more info on the virtues of Nd-Magnets in this thread:
Barkhausen noise in drivers and inductors

Found an old book published around 1990 in the library (G. Schwamkrug, Lautsprecher - Dichtung und Wahrheit).

According to the chapter on the magnetic system, the problem isn't so much the modulation of the magnet itself by the VC, but more so modulation of the magnetization of the iron system.

One way to avoid this is to run the iron in saturation (I have to look up the figure again but I believe it was on the order of 1.3 T). If the iron is well in saturation, some modulation of the external field by the VC will not have any effect on the magnetization of the iron. Moreover, the iron has no chance to pass the magnetization on into the magnet.

Apparently, it is absolutely impossible to saturate iron with a ferrite magnet because the flux density of ferrite is too low. A properly designed system with an Alnico or Nd-Magnet, on the other hand, will do that automatically.

The same chapter claims that a properly designed copper ring will both decrease inductance as well as reduce distortion by up to 20 dB by keeping the VC field from entering into the iron. I guess I will have to look up conductivities typical iron and steel materials...

An improperly designed copper ring is said to make disto worse...

Eric
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Old 29th October 2002, 08:11 AM   #16
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Had another look at the article, saturation flux should be 1.5T.
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Old 29th October 2002, 01:36 PM   #17
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Default short circuit rings

Grabbed a CRC handbook. Pure iron has roughly 6x the resistance of copper, which made me wonder originally why a short circuit copper ring would be effective at all.
Carbon steel as roughly 12x the resistance of copper, other steels are significantly worse.
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Old 29th October 2002, 01:45 PM   #18
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Quote:
Originally posted by djdan
An amplifier can't amplify two ore more frequency in the same time. The music is pure sineway. You can read more about that subject on the Rod Elliot articles ( I don't remember what article ).
Yes it can. You are probably mixing up the time and frequency domains.
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Old 29th October 2002, 05:37 PM   #19
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Default RCA Fieldcoil

A vintage 1935 RCA field coil 12" (a pr went off to Japan). They have a 5 piece chamois surround.

dave
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Old 29th October 2002, 05:52 PM   #20
Bill F. is offline Bill F.  United States
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Default Purty!

I suppose it's in your auctions?

Where do you find these gems, Dave?!


Bill
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