Field Coils for Audio Systems

Audiophile Bill

Well-Known Member
Mar 23, 2015
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Hi All,

Per my other speaker build thread, please find new thread to discuss all things field coil.

Where to begin? Well we see electromagnetic tech being deployed in multifarious hifi devices (I am not sure if I missed anything but feel free to add):

> Field coil modern cone drivers such as Supravox or Atellier Rullit, Gamut Magnetics Lab, Focal Grande Woofer and Utopia sub
> Field coil compression drivers such as Cogent being used by OMA in their upper models or GIP WE rebuilds
> Tonearms such as the bearing in the Primary control totl arm
> Cartridges as used in AN / Kondo IO and DaVa
> Turntable bearings such as those reported in the new Wilson Benesch flagship
> Others?

I think it is fair to say that there is a certain sense of the exotic associated with the deployment of such technology but does it have any merit in terms of sound? I tried to read the web and learn from a couple of engineers but didn’t really learn a huge amount and struggled to extract the wheat from the chaff. In terms of the purported merits of field coil as deployed for drivers, the following list of advantages seemed to make some sense:

> The t/s parameters of the driver change quite substantially according the voltage and/or current you apply to the coil. As a consequence, you have a very flexible speaker since you can quite dramatically adjust the sound. Obviously this can also be a problem. For me, it is (on paper) quite handy. For instance, you can adjust the QTS of the driver to get more / less bass or you can adjust the sensitivity to match say your chosen bass solution.

> Fixed magnets lose their field over time. Buying vintage alnico drivers is a little bit of a lottery in this regard since the t/s parameters of the driver could be very different depending on how old they are. Obviously I take that risk and buy vintage drivers. Clearly this doesn’t apply to an electromagnetic as long as it isn’t broken.

> Fixed magnets lose their field as the voicecoil heats up with extended use. Not so much a problem at home of course. Samarium Cobalt and Alnico are much better at retaining their flux under heat. Ferrite performs much worse in this aspect.

> Fixed magnets lose their field as the voicecoil heats up with extended use. Not so much a problem at home of course. Samarium Cobalt and Alnico are much better at retaining their flux under heat. Ferrite performs much worse in this aspect.

Okay all very interesting maybe, but what about the negatives? Well as far as I see it, the main issues are very large motor structures with considerable mass. Need for yet another power supply to generate the flux. Perhaps too much scope to never land on preferred settings - plays into the audiophile illness.

What about the field itself? I heard something about the voicecoil interaction with the magnetic coil requiring a significantly stiff supply? But can this lead to audio related changes or distortion? Like for like, will a similarly spec’d field coil sound different to a fixed magnet?

How about sound? Well let me say this - whilst not the holy grail for everyone, the WE field coil 555 + large horn left an indelible mark on my audio soul that I can’t deny. It just put me into a different audio world. Would that have been the same with a similarly designed alnico magnet? I heard the Azzolina Gran Sfera and particularly enjoyed a magical mid range courtesy of field coil Lowther. I love the field coil DaVa cartridges - again they have a musicality that I find seductive.

So let’s discuss all things field coil…

Best.

field-coil.jpg
 
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spiritofmusic

Well-Known Member
Jun 13, 2013
13,125
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+1 re the Azzolina Gran Sfera, imho superior to the Pnoe.
Anyone have experience of the Maxonic field coil horn-loaded spkrs?
 

morricab

Well-Known Member
Apr 25, 2014
5,780
2,078
553
Switzerland
Hi All,

Per my other speaker build thread, please find new thread to discuss all things field coil.

Where to begin? Well we see electromagnetic tech being deployed in multifarious hifi devices (I am not sure if I missed anything but feel free to add):

> Field coil modern cone drivers such as Supravox or Atellier Rullit, Gamut Magnetics Lab, Focal Grande Woofer and Utopia sub
> Field coil compression drivers such as Cogent being used by OMA in their upper models or GIP WE rebuilds
> Tonearms such as the bearing in the Primary control totl arm
> Cartridges as used in AN / Kondo IO and DaVa
> Turntable bearings such as those reported in the new Wilson Benesch flagship
> Others?

I think it is fair to say that there is a certain sense of the exotic associated with the deployment of such technology but does it have any merit in terms of sound? I tried to read the web and learn from a couple of engineers but didn’t really learn a huge amount and struggled to extract the wheat from the chaff. In terms of the purported merits of field coil as deployed for drivers, the following list of advantages seemed to make some sense:

> The t/s parameters of the driver change quite substantially according the voltage and/or current you apply to the coil. As a consequence, you have a very flexible speaker since you can quite dramatically adjust the sound. Obviously this can also be a problem. For me, it is (on paper) quite handy. For instance, you can adjust the QTS of the driver to get more / less bass or you can adjust the sensitivity to match say your chosen bass solution.

> Fixed magnets lose their field over time. Buying vintage alnico drivers is a little bit of a lottery in this regard since the t/s parameters of the driver could be very different depending on how old they are. Obviously I take that risk and buy vintage drivers. Clearly this doesn’t apply to an electromagnetic as long as it isn’t broken.

> Fixed magnets lose their field as the voicecoil heats up with extended use. Not so much a problem at home of course. Samarium Cobalt and Alnico are much better at retaining their flux under heat. Ferrite performs much worse in this aspect.

> Fixed magnets lose their field as the voicecoil heats up with extended use. Not so much a problem at home of course. Samarium Cobalt and Alnico are much better at retaining their flux under heat. Ferrite performs much worse in this aspect.

Okay all very interesting maybe, but what about the negatives? Well as far as I see it, the main issues are very large motor structures with considerable mass. Need for yet another power supply to generate the flux. Perhaps too much scope to never land on preferred settings - plays into the audiophile illness.

What about the field itself? I heard something about the voicecoil interaction with the magnetic coil requiring a significantly stiff supply? But can this lead to audio related changes or distortion? Like for like, will a similarly spec’d field coil sound different to a fixed magnet?

How about sound? Well let me say this - whilst not the holy grail for everyone, the WE field coil 555 + large horn left an indelible mark on my audio soul that I can’t deny. It just put me into a different audio world. Would that have been the same with a similarly designed alnico magnet? I heard the Azzolina Gran Sfera and particularly enjoyed a magical mid range courtesy of field coil Lowther. I love the field coil DaVa cartridges - again they have a musicality that I find seductive.

So let’s discuss all things field coil…

Best.
Classic Audio Reproductions also has field coil drivers, based on ALTEC design I think.
 

morricab

Well-Known Member
Apr 25, 2014
5,780
2,078
553
Switzerland
Hi All,

Per my other speaker build thread, please find new thread to discuss all things field coil.

Where to begin? Well we see electromagnetic tech being deployed in multifarious hifi devices (I am not sure if I missed anything but feel free to add):

> Field coil modern cone drivers such as Supravox or Atellier Rullit, Gamut Magnetics Lab, Focal Grande Woofer and Utopia sub
> Field coil compression drivers such as Cogent being used by OMA in their upper models or GIP WE rebuilds
> Tonearms such as the bearing in the Primary control totl arm
> Cartridges as used in AN / Kondo IO and DaVa
> Turntable bearings such as those reported in the new Wilson Benesch flagship
> Others?

I think it is fair to say that there is a certain sense of the exotic associated with the deployment of such technology but does it have any merit in terms of sound? I tried to read the web and learn from a couple of engineers but didn’t really learn a huge amount and struggled to extract the wheat from the chaff. In terms of the purported merits of field coil as deployed for drivers, the following list of advantages seemed to make some sense:

> The t/s parameters of the driver change quite substantially according the voltage and/or current you apply to the coil. As a consequence, you have a very flexible speaker since you can quite dramatically adjust the sound. Obviously this can also be a problem. For me, it is (on paper) quite handy. For instance, you can adjust the QTS of the driver to get more / less bass or you can adjust the sensitivity to match say your chosen bass solution.

> Fixed magnets lose their field over time. Buying vintage alnico drivers is a little bit of a lottery in this regard since the t/s parameters of the driver could be very different depending on how old they are. Obviously I take that risk and buy vintage drivers. Clearly this doesn’t apply to an electromagnetic as long as it isn’t broken.

> Fixed magnets lose their field as the voicecoil heats up with extended use. Not so much a problem at home of course. Samarium Cobalt and Alnico are much better at retaining their flux under heat. Ferrite performs much worse in this aspect.

> Fixed magnets lose their field as the voicecoil heats up with extended use. Not so much a problem at home of course. Samarium Cobalt and Alnico are much better at retaining their flux under heat. Ferrite performs much worse in this aspect.

Okay all very interesting maybe, but what about the negatives? Well as far as I see it, the main issues are very large motor structures with considerable mass. Need for yet another power supply to generate the flux. Perhaps too much scope to never land on preferred settings - plays into the audiophile illness.

What about the field itself? I heard something about the voicecoil interaction with the magnetic coil requiring a significantly stiff supply? But can this lead to audio related changes or distortion? Like for like, will a similarly spec’d field coil sound different to a fixed magnet?

How about sound? Well let me say this - whilst not the holy grail for everyone, the WE field coil 555 + large horn left an indelible mark on my audio soul that I can’t deny. It just put me into a different audio world. Would that have been the same with a similarly designed alnico magnet? I heard the Azzolina Gran Sfera and particularly enjoyed a magical mid range courtesy of field coil Lowther. I love the field coil DaVa cartridges - again they have a musicality that I find seductive.

So let’s discuss all things field coil…

Best.
I think the best way to sort it out is to have the same driver with both Alnico and field coil and set the voltage on the field coil such that it’s t/S parameters are very close to matching the Alnico driver and then putting them into an appropriate design. I only know Supravox that has the exact same driver bOth ways. Perhaps Voxativ could do something custom or you mentioned Lowther had one (never seem that)

Funny enough though the AER drivers are using Nd, which is not regarded as sounding so good compared to Alnico and yet these drivers seem to regarded as the best full range drivers.
 

Audiophile Bill

Well-Known Member
Mar 23, 2015
3,461
2,521
540
UK
I think the best way to sort it out is to have the same driver with both Alnico and field coil and set the voltage on the field coil such that it’s t/S parameters are very close to matching the Alnico driver and then putting them into an appropriate design. I only know Supravox that has the exact same driver bOth ways. Perhaps Voxativ could do something custom or you mentioned Lowther had one (never seem that)

Funny enough though the AER drivers are using Nd, which is not regarded as sounding so good compared to Alnico and yet these drivers seem to regarded as the best full range drivers.

I should be able to do that exercise with a Lowther and Stamm and AER in due course
 

Audiophile Bill

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Mar 23, 2015
3,461
2,521
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UK

dcathro

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Sep 16, 2016
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Melbourne, Australia
A few names to add would be Feastrex, Fertin, and Wolf Von Langa.

There are a number of people here on the forum running field coil speakers including myself, @Parsons , and @Atmasphere.

I understand the main argument for the superiority of field coils is their ability to hold their magnatic field against the alternating field of the voice coil. Ferrite magnet's fields sag significantly, while Alnico is much better. Field Coils can be superior in this respect dependent on the strength of the power supply.

Cheers

David
 

Audiophile Bill

Well-Known Member
Mar 23, 2015
3,461
2,521
540
UK
A few names to add would be Feastrex, Fertin, and Wolf Von Langa.

There are a number of people here on the forum running field coil speakers including myself, @Parsons , and @Atmasphere.

I understand the main argument for the superiority of field coils is their ability to hold their magnatic field against the alternating field of the voice coil. Ferrite magnet's fields sag significantly, while Alnico is much better. Field Coils can be superior in this respect dependent on the strength of the power supply.

Cheers

David

Thanks David. Hopefully the technical folks will wade in here soon. Really keen to get into the discussion about field sag and power supplies.
 

Atmasphere

[Industry Expert]
May 4, 2010
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Really keen to get into the discussion about field sag and power supplies.
Most power supplies sag less than permanent magnets IME, but if you want to really hear what field coils do a regulated supply is the way to do it. There's a sizable contingent that like the old Tungar bulbs for rectifiers, but if you are regulating then the kind of rectifier used isn't going to make any difference except that you will need more overhead in your design if Tungars are used.

I like the extra speed that field coil speakers are able to exhibit. You really don't want to go back after that. I regard them as similar to ESLs in that regard, but of course they are lot easier to drive. My amps at home only make 50 watts and I can't overload them in my space.
 

Audiophile Bill

Well-Known Member
Mar 23, 2015
3,461
2,521
540
UK
Most power supplies sag less than permanent magnets IME, but if you want to really hear what field coils do a regulated supply is the way to do it. There's a sizable contingent that like the old Tungar bulbs for rectifiers, but if you are regulating then the kind of rectifier used isn't going to make any difference except that you will need more overhead in your design if Tungars are used.

I like the extra speed that field coil speakers are able to exhibit. You really don't want to go back after that. I regard them as similar to ESLs in that regard, but of course they are lot easier to drive. My amps at home only make 50 watts and I can't overload them in my space.

Thanks Ralph. From a technical perspective, is there anything innate to the magnetic field of a field coil motor that makes it in anyway different to that of a permanent magnet?

Why do you think the field coils have such speed technically - is it something to do with handling the perturbations in the field as the voicecoil moves through it or something?
 

cjfrbw

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Apr 20, 2010
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the sound of Tao

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Having similar field coil attraction thoughts at the moment and have been doing preliminary researching on the web for the last month or so and I also struggled to pin down a deeper understanding of the essential characteristics traits that separate field coil, alnico and neodymium.

I have 8 x neo 15 inch mid woofers with horns in the one project but have a second open baffle under way that could have 4 x mid woofers and possibly folded OB subs so for me there’s the opportunity to consider to go field coil and even possibly field coil throughout the range.

I initially have mainly considered Supravox as it is within my budget ballpark but it also looks like they’ve now shelved their 16 inch field coils and 16 inch alnico (I was considering the 4 x silver 16 inch field coil) and they have now instead a 15 inch Alnico but no field coil variant of this.

The attraction of field coil for me also is being able to alter the supplied voltage to shape their sound… but then to get a qts to work for open baffle looks like possibly running the supravox then at the lower voltage in their range and so that potential for variability may not be as broadly applicable with OB (it seems).

Also part of my philosophy is to go simple where possible so going field coil in ways flows against that goal with more power supplies in the setup (and in general more complexity throughout) but then shaping the outcome through the power supplies as a secret sauce is also a tasty tungar attraction.

Somehow just going over to alnico when I’m already kitted up with neo for the mid woofers just isn’t as immediately attractive either and likely I could make even greater wins by instead focussing on bringing in 45 or type 50 amps for biamping instead. Audiophile dilemmas… nothing new there :rolleyes: :)
 

Atmasphere

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Thanks Ralph. From a technical perspective, is there anything innate to the magnetic field of a field coil motor that makes it in anyway different to that of a permanent magnet?

Why do you think the field coils have such speed technically - is it something to do with handling the perturbations in the field as the voicecoil moves through it or something?
Neodymium is the most powerful permanent magnet used for loudspeakers. It also sags the most when current is applied to the voice coil. It took driver designers a while to sort out how to get decent response from neodymium magnets as a result. At the other end of the spectrum AlNiCo is the lest powerful magnet but has two advantages- it can be better focused in the voice coil gap and has the least sag, which is why its the preferred magnet.

Field coils essentially have no sag at all since they are powered by a power supply and the magnetic field can be nicely focused in the voice coil gap. That is why they are fast like ESLs (which no coincidence also use a power supply for their MO).

The speaker industry didn't move away from FC operation because permanent magnets were better, it did it because PMs are cheaper, like so many other things that have happened in audio. Put another way if you want the best performance you can get with a driver in a box, a field coil version of that driver will be the best performance.

I've often wondered what a field coil magnetic planar would be like...
 

dcathro

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Sep 16, 2016
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Having similar field coil attraction thoughts at the moment and have been doing preliminary researching on the web for the last month or so and I also struggled to pin down a deeper understanding of the essential characteristics traits that separate field coil, alnico and neodymium.

I have 8 x neo 15 inch mid woofers with horns in the one project but have a second open baffle under way that could have 4 x mid woofers and possibly folded OB subs so for me there’s the opportunity to consider to go field coil and even possibly field coil throughout the range.

I initially have mainly considered Supravox as it is within my budget ballpark but it also looks like they’ve now shelved their 16 inch field coils and 16 inch alnico (I was considering the 4 x silver 16 inch field coil) and they have now instead a 15 inch Alnico but no field coil variant of this.

The attraction of field coil for me also is being able to alter the supplied voltage to shape their sound… but then to get a qts to work for open baffle looks like possibly running the supravox then at the lower voltage in their range and so that potential for variability may not be as broadly applicable with OB (it seems).

Also part of my philosophy is to go simple where possible so going field coil in ways flows against that goal with more power supplies in the setup (and in general more complexity throughout) but then shaping the outcome through the power supplies as a secret sauce is also a tasty tungar attraction.

Somehow just going over to alnico when I’m already kitted up with neo for the mid woofers just isn’t as immediately attractive either and likely I could make even greater wins by instead focussing on bringing in 45 or type 50 amps for biamping instead. Audiophile dilemmas… nothing new there :rolleyes: :)

You must be the Dragon Warrior of Audio! :)
 
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the sound of Tao

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I had read a couple of comments on diy forums that Alnico was in some ways characteristically more in nature like SET (unsure if that comment is aligned at a more stereotyped warm fleshy end of the SET spectrum although SET clearly isn’t just of one nature) or perhaps could read that even as classic 300B like.

I’ve also then seen other references that field coil is more like electrostatics in nature or even OTL or 45 valve so Ralph’s comments seem in ways to also mirror that.

I’ve mostly just heard Voxativ and OMA as field coils and this makes some connection for me but generally my field coil experience and more so alnico driver experience is limited. If any of this resonates with anyone that’d be great to get some thoughts. My aim is to just leave that as largely anecdotal either way but helps (for me at any rate) as a loose tangential frame of reference.

So thinking that with that just as conjecture then this might leave neodymium (which I have much better reference on) as somewhere else, possibly more as a hybrid or as a bold, full-bodied and fruity 805 valve. Run at 8 ohms the neos here have a great blend of nuance with also a bit of punch and so play across rock and dance music happily as well as jazz and most classical. That said I am running them with a 300b driven 805 amp with an otl like preamp… so maybe that’s coming together in some approximate middle place.

The 8 x neos however run parallel wired and with their 16ohm load the neodymiums then shift characteristic to bring even more flow and low level resolution but also punch just that little less hard than at 8 ohm load… the 16 ohm paralleled configuration more perfect for large scale orchestral forces as well as chamber music and also any jazz you’d want to play so I figure even neodymium can likely be implemented to shift its characteristic to move towards other ends of the spectrum from its firmly anchored and fluent balance of attack and decay out to a more reckless and joyous simple enthusiasm.
 
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