Field Coils for Audio Systems
- Thread starter Audiophile Bill
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From a physics perspective , the magnetic field is the same. So, it could be the sag we have discussed or it could be subtle differences in the geometry of the field and where and how the force is applied if everything else is not exactly the same as the permanent magnet setup.
as I´ve understood, there are different qualities of neodynium
often mentioned advantage of using neodynium is less weight
however my TAD TD-4003s still weigh a substantial 9,9kg
often mentioned advantage of using neodynium is less weight
however my TAD TD-4003s still weigh a substantial 9,9kg
Alnico for the Lowther, stamm and Vox. Neo for the AER.
I have not compared Field Coil against Alnico, but have compared Alnico directly against ferrite.
I had Alon V mk2's in 1996, and moved on in 1998 to the Alon Circe, which was essentially the same speaker but with Alnico bass and tweeter. There was a big difference in performance. There was greater purity and harmonic richness across the frequency spectrum.
I had Alon V mk2's in 1996, and moved on in 1998 to the Alon Circe, which was essentially the same speaker but with Alnico bass and tweeter. There was a big difference in performance. There was greater purity and harmonic richness across the frequency spectrum.
interesting headline!
My Line Magnetic Audio LM-755 speakers are field coils. Line Magnetic is offering the LM-755 (Western Electric driver Design) as Allnico, Solid-state powered Fieldcoil and Tube (300b) powered Fieldcoil. Unfortunately very rare speakers, so I had not chance so far to compare.
A couple of years I listened to music with a full Audio Note TT3 turntable, incl the I.O. Limited cartridge, which was field coils as well
(6 x single tonearm wires instead of 4)
During that time, I also had some discussions with Wolf von Langa and others (during a Munich Audio fair).
The obvious positive arguments about field coils were the magnetic field power as such and the position of the field.
But I learned two other arguments, one is, that the weight of a magnet is much higher than a second coil, which has a huge effect on resonances of the chassis body (even higher effect for a cartridge assembly ), the second argument was the improved / free airflow of the chassis membrane, as the back movement of the membrane does not have to work against the magnet body surface.
In this regard many chassis manufacturers are working on the improvement of the airflow and do create special magnet designs, so it seems, that this is an important criteria to be included in the field coil debate as well.
My Line Magnetic Audio LM-755 speakers are field coils. Line Magnetic is offering the LM-755 (Western Electric driver Design) as Allnico, Solid-state powered Fieldcoil and Tube (300b) powered Fieldcoil. Unfortunately very rare speakers, so I had not chance so far to compare.
A couple of years I listened to music with a full Audio Note TT3 turntable, incl the I.O. Limited cartridge, which was field coils as well
(6 x single tonearm wires instead of 4)
During that time, I also had some discussions with Wolf von Langa and others (during a Munich Audio fair).
The obvious positive arguments about field coils were the magnetic field power as such and the position of the field.
But I learned two other arguments, one is, that the weight of a magnet is much higher than a second coil, which has a huge effect on resonances of the chassis body (even higher effect for a cartridge assembly ), the second argument was the improved / free airflow of the chassis membrane, as the back movement of the membrane does not have to work against the magnet body surface.
In this regard many chassis manufacturers are working on the improvement of the airflow and do create special magnet designs, so it seems, that this is an important criteria to be included in the field coil debate as well.
I guess the use of Nd magnets would also improve the point about airflow behind the driver as they are MUCH smaller than Ferrites or Alnico. Interestingly, the shape of the magnet in my Supravox drivers (Alnico) are taller and thinner than you see with Ferrite and they have shaped the field coil version exactly the same way (length and diameter look from photos identical).
Thanks for joining the discussion, Shakti. Interesting speakers you have there.
Just wanted to get some additional clarification on a couple of points:
“the magnetic field power and the position of that field”
> The flux density of a speaker magnet is ultimately limited by the raw materials at our disposal with permendur allowing for the greatest flux density in a field coil motor - in theory you can hit the maximum 2.4T of that material without super cooling. I think one can theoretically extract even higher flux by supercooling the permendur pole but even that is a little bit extreme lol. That said both neodymium and alnico can also reach the theoretical maximum of 2.4T.
So my question - what did you mean specifically when you say “field power?”
In terms of position of the field, yes that makes a lot of sense although I don’t see any reason why one can’t modify the position of a fixed magnet field by altering the magnet physical structure.
One thing we didn’t discuss is Maxwell rating of a magnet that I understand is effectively the flux density measured over a surface area - or simply put high Maxwell rating depending the area over which the magnet provides its force. I know that the size of the field in the field coil motors I have are double that as used in Lowther PM4 for instance. In theory this would allow much greater driver excursion assuming the suspension would handle it.
“Weight of the magnet is much higher than a 2nd coil which has huge effect on resonances…”
> I am not sure what you mean? You mean that a field coil motor tends to be lighter than an associated magnet? Are you saying lighter or heavier is better?
On the whole, the field coil motors I have seen in production are very big and heavy devices. Mine are 15kg each.
“Inproved airflow”
> I am not following how airflow is improved? Please could you explain a little more?
These comparisons are not easy. Altec was all Alnico, and when the price of cobalt went up Altec started doing ferrite and for those who had both they say the ferrite sucked. However, GPA Altec Alnicos were then not considered as good as original Altec Alnicos. Some consider it better than the original Altec Ferrite,
TAD came out with Neodynium in 4003 to improve on their Alnico 4001, but that worked for them, not sure this can be generalized. It was the same engineer who had made the 4001 and previous JBL drivers. Either way both my tope favorites in 4003 and AER are neodynium.
Altec was known for its woofers which were Alnico. WVL initially made FC woofers based on Altec then started making his own. He was famous for his woofers and was also a source for buying NOS altecs a few years ago. His speakers are not considered anything big not sure what the new thing is on WBF on WVL especially the small ones.
In Field Coil, WE, and then the GIP replica, were the famous holy grail drivers and FC. The Line magnetic FC driver is nothing special and is available quite cheap (relative to GIP)
Dave Slagle made a few Lowther FC and was then making Radian field coils. A friend owns his radian field coils but has not made a speaker from them yet
Altec produced field coils for the upper ranges (Altec 284) and were supposed to be brilliant and sell for over 20k if one can find them, which is never. A company takes the non FC drivers (288) and converts them to FC but those sell quite cheap and are not really supposed to have anything special over normal 288. The guy in the video below is field coiling the Altec and it sounds awesome in the video.
There are many field coil discussions on DIY and Lansing forums. As with anything it seems to come down to implementation.
TAD came out with Neodynium in 4003 to improve on their Alnico 4001, but that worked for them, not sure this can be generalized. It was the same engineer who had made the 4001 and previous JBL drivers. Either way both my tope favorites in 4003 and AER are neodynium.
Altec was known for its woofers which were Alnico. WVL initially made FC woofers based on Altec then started making his own. He was famous for his woofers and was also a source for buying NOS altecs a few years ago. His speakers are not considered anything big not sure what the new thing is on WBF on WVL especially the small ones.
In Field Coil, WE, and then the GIP replica, were the famous holy grail drivers and FC. The Line magnetic FC driver is nothing special and is available quite cheap (relative to GIP)
Dave Slagle made a few Lowther FC and was then making Radian field coils. A friend owns his radian field coils but has not made a speaker from them yet
Altec produced field coils for the upper ranges (Altec 284) and were supposed to be brilliant and sell for over 20k if one can find them, which is never. A company takes the non FC drivers (288) and converts them to FC but those sell quite cheap and are not really supposed to have anything special over normal 288. The guy in the video below is field coiling the Altec and it sounds awesome in the video.
There are many field coil discussions on DIY and Lansing forums. As with anything it seems to come down to implementation.
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In case someone hasn't read there was this review on the Slagle Lowther FC. I was planning to buy this set of drivers for 3k or so before I bought the TAD
I’ve enjoyed John’s T1.5 speakers at several shows (matched with @Atmasphere’s amps.) I would really like to hear his reproduction Hartsfields with field coil drivers.
the material of the diaphragm affects the sound more than the magnetic drive. I've tried a lot of drivers. metallic diaphragms have a little more resolution and detail, but never have the clean tone like a polyimide diaphragm. that's my personal opinion as I've heard.
Expensive field coil from 200hz to 20000hz
Expensive field coil from 200hz to 20000hz
I'm sure Bill and some others have already seen this content, but since this thread is a place to collect valuable information on field coil design:
https://www.facebook.com/ProjectRyu/ & https://projectryu.com/wp/ & https://www.diyaudio.com/forums/multi-way/212953-project-ryu-diy-field-coil-loudspeaker.html
https://www.facebook.com/ProjectRyu/ & https://projectryu.com/wp/ & https://www.diyaudio.com/forums/multi-way/212953-project-ryu-diy-field-coil-loudspeaker.html
John of Classic Audio Loudspeakers did a demo on this at T.H.E. Show years ago when it was at the St. Tropez. It was not subtle at all.
I've never seen an Altec that didn't use Alnico ( I've owned a bit of Altec stuff over the years). FWIW Alnico is the earliest PM material used. Ceramic magnets came along in the 1960s as best I can make out. KLH was an early advocate.
They sound quite nice but I prefer the horn on his Project (T-1 and T-3) speakers as it has smoother response. It is amazing though how much bass extension he's managed to get out of the Hartsfields!
When John of Classic Audio Loudspeakers was first working with FC stuff he was also working with pure beryllium diaphragms in his horns (the first breakup being about 35KHz). You hear either one and you don't want to go back. The beryllium drivers had a speed and smoothness not present in any of the drivers that didn't use it whether they were FC or not. But put the two together and you have something special.
The frequency is irrelevant. If the power supply is properly regulated it will work fine powering all the drivers, assuming that they all work at the same voltage. FWIW its the current you put through them that is important, although of course you can't get the current without the voltage.
This is incorrect. John started by reproducing the Hartsfield speakers which used JBL. His T-1 and T-3 speakers initially used TAD 1602s which employ Alnico. Later he modified JBL drivers to use FC operation. He's not messed with Altec much that I've ever seen. I've known him since 1990 if that helps.
I can also confirm that John uses JBL for his FC operation. I spoke to him about Hartsfield reproductions a few months ago.
This is the first time since I joined WBF that I have listened to a music system posted here on WBF on something other than an iPhone. I recently rescued my full-size Apple iMac out of dusty storage purgatory. I still believe that these videos do not make conceptual or technical sense, but I will concede that this video does not sound as bad on the iMac as it sounds on the iPhone.
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