Electromagnetic Speakers

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kelticwizard

diyAudio Moderator Emeritus
2001-09-18 2:33 am
Connecticut, The Nutmeg State
djdan said:
What are your opinions about electromagnetic speakers about quality of the sound ?

This kind of speaker it can be used in active box .

What do you mean by an "active box"?

For some time I have been thinking of various schemes to tune a ported box instantaneously to the lowest frequency being played at the moment. This would cut down on cone excursion, as the excursion at the tuning frequency is only one fourth of what it would be with no port help.

Is this what you mean by "an active box"? Or is there some other meaning? Please explain.

Here is about the only link I know to an electromagnetic, or "field coil", speaker in the present day, but the author is still very much in the experimental stage.
http://home5.inet.tele.dk/f-hammer/

I am told that electromagnetic speakers were used in American cars back in the forties and fifties because they fit in better with the wiring systems of the time than the now-conventional permanent magnet speaker.

An American book on speakers from the fifties had a section on "field coil" speakers, but wrote of them as if they were already on the way out. That was back in the fifties.

A DJ friend in Ireland tells me that there were still some PA units using electromagnetic speakers available back in the eighties, presumably of British manufacture. Whether they were new in the eithties or were used equipment left over from an earlier period he did not mention. But they were around.

I am fascinated by the concept and if you have any links to electromagnetic speakers, please post them here.
 
Thank you for answere Kelticwizard !

Activ Box means a loudspeaker cabinet with power amplifier inside and power supply , wich means that is easy to supply
the speaker field coil , too.

Only advantage for speaker with permanent magnet is the absence of the power supply for the magnetic field coil. Electromagnetic speakers have many advantage but unfortunately the poorest and cheapest standarts win every time in our world.
(see VHS vs. VIDEO2000).

I don't understand why somebody with fully functional brain can spend 2000$ for a simple pair of speakers. In this condition why we can't make a electromagnetic speaker with a huge magnetic field (or variable) at a fraction cost ?

Permanent magnet ? No , no , no ... no ! It is time variable.

2002 95% magnetised .
2003 80% magnetised .
2005 70% magnetised .
2010 50% magnetised .
 
djdan said:
Electromagnetic speakers have many advantage but unfortunately the poorest and cheapest standarts win every time in our world.

That is exactly why field coils mostly disappeared. Fertin build FC full-ranges (there is an active thread on them here) and Lambda builds some 15" FCs for mid-bass use. Both of these are expensive since they are hand built in small numbers for a very limited market.

I wonder how one of the new Fostex banana pulp 4" or a Jordan JX92 would sound if they had field-coils (it started out by wondering if Fostex would build an alnico banana pulp 4", but if you are having fantasies why not go whole hog).

I have sold a number of old field-coils from the 30s & 40s. They often go for big dollars (compared to what i pay for them, not compared to what new ones cost) and they invariably go off to Japan.

dave
 
djdan said:
Permanent magnet ? No , no , no ... no ! It is time variable.

2002 95% magnetised .
2003 80% magnetised .
2005 70% magnetised .
2010 50% magnetised .

Nahhh... That's just you going deaf. ;)

Actually that would be wrong now I come to think of it. With conventional electric motors if you weaken the field magnets (either permanent or electromagnets) the motor goes faster. So would it follw that a weakened speaker magnet would make the speaker more sensitive, or at least lowers the impedance because it now needs a greater cone excursion to generate the same back emf as before, that stops it from drawing infinite current? (ignoring dc coil resistance)
 
advantages/disadvantages? diy?

advantages:
- you can probably build up a stronger field than with a permanent magnet
- this magnet is free from modulation by the field of the voice coil (but I have yet to see a quantitative analysis that shows that magnetic intermodulation distortion is a significant effect in typical geometries with ferrite and neodymium magnets)
- you can modulate the field strength but it's hard to see to what end

disadvantages:
- you waste a lot of energy to keep those electrons turning which a permanent magnet does for free
- the soft iron of the pole pieces is still there and it is bound to have some hysteresis, too

diy:
can a field coil speaker be made by hammering away the ferrite from a conventional speaker? this boils down to the question of whether the pole pieces are attached to each other or held together by the magnet...

Eric
 
Re: advantages/disadvantages? diy?

capslock said:
diy:
can a field coil speaker be made by hammering away the ferrite from a conventional speaker? this boils down to the question of whether the pole pieces are attached to each other or held together by the magnet...

I seem to remember it mentioned that the Fertin had a permanent magnet and this was augmented by the field coil.

mags8s.jpg


I wonder if a quick & nasty experiment would be to wind a field coil around the slug in an alnico speaker with motors like the ones in the pictures. I guess you might have to be careful with the direction of the field least you fight (and perhaps wipe out) the permanent field.

dave
 
The problem with permanent magnets is : Constant magnetic field is not always a good thing for speakers . For same cone and moving ansenbly mass we need more or less energy to get same SPL at different frequency. My idea is to modulate the magnetic field more or less to make the speaker more linear.
For example : Good subbass speakers have big magnets but the sound for middle and high is very poor.
For full range speaker the magnet is not so big.
Why can not just vary the magnetic field in relation with frequency ?

Don't get me wrong . I am not an old fashion type. I have my own company wich manufacturate pro loudpeaker cabinets for custom purpose. I have 12 years experience in this field. I just try to find the best way( Is my hobby , too !) to make loudspeakers with very good sound .

In the field coil the energy waisted is 10 - 50VA or 100V/0.1-0.5A , for 200w full-range speaker so I think that is not big deal.

Other potential advantages for field coil speaker:
- Is cheap [ only 800 - 1200 grams Copper conductor with 0.1mm diameter ] in raport with 150 oz magnet wich is ten time more expensive .
- Is light like a neodynum magnet speaker.
- Better control for over-excursion and/or abusive use.
- Easy to manufacture . 5000 turns for field coil is not a problem for a machine .
- Gold mine for people how sell tube amplifiers.
 

Bill F.

Member
2001-11-15 5:25 pm
SW MI
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.
 
Bill F. said:


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
 

Bill F.

Member
2001-11-15 5:25 pm
SW MI
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
 
There is some more info on the virtues of Nd-Magnets in this thread:
http://www.diyaudio.com/forums/showthread.php?s=&threadid=5857

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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