Field-coil drivers, pros and cons. Relics of the past, or still relevant?

[Killingbeans]

I've been thinking about so called Field-Coil speaker drivers lately. Or "electromagnet drivers" as some seem to label them.

Do they have any technical benefits that make them worth the extra expense and trouble?

Nearly all info I can find online is vague mentionings of their superior "sound". Completely useless...

Can't find any measurements at all, or even just some in-depth descriptions of how they differ in electrical and/or mechanical parameters.

The only hint to an actual performance benefit I can find, is talk about a lack of "Voice Coil Modulation". I assume that's the same phenomena as Force Factor Modulation?

If so, does Field-Coil motors really have higher potential for mitigating this effect than iron-free motors or the one designed by Purifi? And would the improvement even be audible?

One downside I can think of is the parasitic resistance of the coil windings. I assume thermal management becomes more problematic.

 

[DVDdoug]

I don't really know but I believe it's a relic and there is no advantage. And of course it requires power so it's a lot less energy efficient. I think before there were Alinco magnets they used Iron and maybe they were less efficient. Iron MAY also lose some of its magnetism when exposed to the field from the voice coil.

I did read that sometime (during WWII?) they couldn't get the materials for Alinco and they switched to ceramic magnets which are bulkier, but apparently they can get the same magnetic field. Now there is Neodymium which is smaller and lighter. An advantage in portable speakers. (And it's something "new" that speaker manufacturers can brag about. )

Nearly all info I can find online is vague mentionings of their superior "sound".

A lot of "audiophiles" are fascinated by tubes & vinyl records and other outdated technology.

 

[mhardy6647]

The switch from AlNiCo to ceramic for magnets was driven by the Cold War and the use of cobalt in other technologies (not so hifi-friendly ones) and supply constraints that also had geopolitical underpinnings.

The ability to make more or less arbitrarily powerful electromagnets is at least possibly a "plus".
Your high field FFT NMR instruments, e.g., use superconducting electromagnets. Could audiophile drivers be too far behind?


From ASR's perspective, field coil (electrodynamic) drivers probably make no sense.
Not everyone shares the canonical ASR perspective, though.

Make it yer own Tungar Supply

Went to the "Secret Land of Walter" last weekend to have some fun with some friends. On the agenda for the weekend were some Tungar Supplie...

hifiheroin.blogspot.com

Low voltage, high current power supplies for field coil drivers using Tungar tubes.

 

[Berwhale]

Nearly all info I can find online is vague mentionings of their superior "sound". Completely useless...

Found this...

https://uploads-ssl.webflow.com/62b0b204f12618a9c9de06e0/62b0b204f126180065de0816_fiche_LB12EX.pdf

From here: https://www.closeracoustics.com/electro-magnet-speaker

 

[DonH56]

There are various claims that they can (or should be able to) provide greater dynamic range with lower distortion, but I imagine it depends upon the field strength. No permanent magnets means potentially less hysteresis, thermal fading, and all that jazz. I'll have to dig up my old book as I think it has some discussion of them. Interesting topic, hopefully those with more experience will chime in. My last experience was decades ago, repairing an old, old radio, and then playing around with the idea in school. I made a working model but performance was less than stellar...

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For those who want a simple hand-waving description, a "normal" speaker uses a permanent magnet with a coil of wire (voice coil) around it. As current flows through the voice coil, driven by your amplifier, it creates a magnetic field that opposes or supports the magnet's field (poles) and the voice coil moves back and forth in response. The voice coil is attached to the actual cone (exterior part) that makes all the noise.

In contrast, a field coil is a coil of wire driven by a DC source that produces a current through the coil, which in turn creates a magnetic field (an electromagnet). Now you can control the strength of the magnetic field by how much current you send through the field coil, and there is no iron (or whatever) core to saturate. But you need a power supply for the field coil, there is still a cone or diaphragm to drive to send the sound your way, and construction may be more costly since now you have an additional coil of wire "inside" the driver to create and align (plus the cost of the power supply).

 

[Philbo King]

They were used as 'chokes' (inductors) for filtering a linear power supply output. So if you have occasion to tinker with one in tube gear, be aware that all the terminals are going to be several hundred volts above ground. Zaaaap!

They are pretty much a relic, mostly used in antique radios.

 

[Berwhale]

But you need a power supply for the field coil

...and there's an example of a power supply for a field coil driver on the Polish site I linked to above....

Closer Acoustics PSU EX field coil driver power supply

Closer Acoustics PSU EX_ field coil driver power supply. Power supply for EMS drivers. Power supply for electro magnet speakers. Designed to match field coil drivers made by French EMS (Electro Magnet Speaker - Fertin)

www.closeracoustics.com

It mentions 'tension regulation'. What would this be?

 

[Berwhale]

...and Focal use them...

Electro Magnet EM - Focal

The magnetic field in the air gap reaches 1.75 Tesla (0.9 for the woofer of the previous Grande Utopia Be).

www.focal.com

 

[DonH56]

...and there's an example of a power supply for a field coil driver on the Polish site I linked to above....

Closer Acoustics PSU EX field coil driver power supply

Closer Acoustics PSU EX_ field coil driver power supply. Power supply for EMS drivers. Power supply for electro magnet speakers. Designed to match field coil drivers made by French EMS (Electro Magnet Speaker - Fertin)

www.closeracoustics.com

It mentions 'tension regulation'. What would this be?

My guess is "voltage", as in high-tension (voltage) lines. But I do not speak Polish (barely handle English, American style).

 

[Berwhale]

My guess is "voltage", as in high-tension (voltage) lines. But I do not speak Polish (barely handle English, American style).

It appears so, I zoomed in on the pot on the back...

 

[lc6]

Based on my subjective evaluation of dozens of speakers/setups at a recent major audio show with hundreds of exhibitors, I have to say that the Songer field-coil speakers were in the top three. The clarity and full-range coverage of the demo setup incl. model S1 speakers were unbelievable and rivaled those of a setup spec'ed at almost $900k+. A result of 6+ years of R&D, the rep said. And it shows. Have started saving money now to get to the ~$37k MSRP (with the requisite PS).

 

[Philbo King]

Huh... I stand corrected.

 

[Killingbeans]

Now you can control the strength of the magnetic field by how much current you send through the field coil, and there is no iron (or whatever) core to saturate.

But air core isn't a necessity? It's only a limitation because of how the voice-coil sits inside the field-coil?

I'm thinking if you could instead surround the voice-coil with a number of coils wound around rods of Metglas 2714A while keeping the field strength below the saturation point, you could get some pretty powerful stuff going in a tiny package?

 

[DonH56]

But air core isn't a necessity? It's only a limitation because of how the voice-coil sits inside the field-coil?

I'm thinking if you could instead surround the voice-coil with a number of coils wound around rods of Metglas 2714A while keeping the field strength below the saturation point, you could get some pretty powerful stuff going in a tiny package?

Air core avoids saturation and hysteresis of a magnetic material, plus the whole idea of a field coil is to eliminate the magnets. Technically, field coils came first, before speakers using permanent magnets were developed and magnetic materials improved. Everything old made new again.

Never thought about a hybrid design, interesting concept, no idea how the pro/con trades would go. Not my area of expertise.

 

[egellings]

I see the field coil as an unnecessary waste of electricity. In a driver, the iron around the voice coil should be magnetically saturated. Once that is achieved, then it makes no difference how that is achieved, either by a permanent magnet or by a field coil. If the driver is being 'tuned' by varying gap field intensity, then the coil would be needed, since once a size & type is selected, a permanent magnet cannot be varied in its strength. I say, design the speaker and pick a magnetic field strength required to realize that, and then get an appropriately sized permanent magnet to generate the field and save the electricity.

 

[FeddyLost]

It's better to discuss with loudspeaker designer.
In theory it offers some interesting options, but as we see, currently it's not required by "big industry" because it's boutique luxury class by definition.
You'll need good egineer for designing, you'll need a lot of qualified labour at assembly and QA for full releasing potential of EM coils, and you'll end up with extremely expensive drivers that will require good and stable power source.
And don't forget, that you'll need to solve heating issues and parameter drift if power source is not stable.
At the pinnacle of concept one will face speaker with active cooling with transformer oil.
That will be almost as cool as plasma speakers. Maybe not as hazardous though.

 

[fpitas]

Some history and tech. From a design standpoint. Alnico was good because it has a magnetic permeability close to that of air. The better drivers had Alnico as a big cylinder in the center of the coil. Inductance modulation became a problem when ferrite was introduced, because the pole piece inside the coil became a hunk of high-permeability steel instead of low-permeability Alnico. The magnetic properties of ferrite did not allow it to be used as a pole piece like Alnico. So, the voice coil ended up being a big honking inductor that varies in value as it moves in and out of the gap, a major source of distortion. The solution was shorting rings. Modern drivers are tending towards magnets based on neodymium, because it offers a tremendous amount of magnetic energy storage in a small space. And, neodymium is very low permeability like Alnico.

FWIW, I think field coils are a retro gimmick. Of course you can get a magnetic field that way, but it no longer offers engineering advantages.

 

[FeddyLost]

Of course you can get a magnetic field that way, but it no longer offers engineering advantages.

I'm not a speaker designer, but when I see variable windings on voice coil in Purifi woofers, I think there are some options of winding up fully electromagnetic motor with specific qualities.
It's better to discuss with currently employed speaker designers.
Most probably everything is stuck at extremely high costs.

 

[fpitas]

I'm not a speaker designer, but when I see variable windings on voice coil in Purifi woofers, I think there are some options of winding up fully electromagnetic motor with specific qualities.
It's better to discuss with currently employed speaker designers.
Most probably everything is stuck at extremely high costs.

Some of the best, more or less cost-no-object drivers I know, like the TAD TD-4003, use neodymium. I can't think of a single reason to need adjustable field strength.

 

[FeddyLost]

I can't think of a single reason to need adjustable field strength.

Adjustable field configuration, not only strength.

Compression drivers have relatively low excursion, so it's not a big problem. But still TD-4003 have 4" voice coil and weights almost 10 kg.