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Old 11th October 2002, 09:17 PM   #1
djdan is offline djdan  Romania
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Default Electromagnetic Speakers

What are your opinions about electromagnetic speakers about quality of the sound ?

This kind of speaker it can be used in active box . Another interesting potential advantage is the posibility to modulate the magnetic field to improve some parameters .

Regards !
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Old 13th October 2002, 05:57 AM   #2
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Default Re: Electromagnetic Speakers

Quote:
Originally posted by djdan
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.
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Old 13th October 2002, 02:48 PM   #3
djdan is offline djdan  Romania
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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 .
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Old 13th October 2002, 05:26 PM   #4
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Quote:
Originally posted by djdan
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
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Old 14th October 2002, 03:59 AM   #5
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Quote:
Originally posted by djdan
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)
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Old 14th October 2002, 02:20 PM   #6
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Just thought I would throw this link in. It seems the electromagnetic loudspeaker at one time was considered the best loudspeaker available.

http://www.audioheritage.org/html/pr...mco/iconic.htm
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Old 16th October 2002, 08:21 AM   #7
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Default 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
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Old 16th October 2002, 08:43 AM   #8
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Default Re: advantages/disadvantages? diy?

Quote:
Originally posted by capslock
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.

Click the image to open in full size.

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
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Old 16th October 2002, 12:38 PM   #9
djdan is offline djdan  Romania
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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.
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Old 16th October 2002, 01:34 PM   #10
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Quote:
Originally posted by djdan

Why can not just vary the magnetic field in relation with frequency ?
Music happens to have many frequencies simultaneously. Which one do you optimize for?

I think the potentially biggest advantage is lower intermodulation distortion. Is there any quantitative analysis to show that we are not setting out to hung ghosts once more?

Eric
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