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Thread: Should Field Coil Come Back?

  1. #1
    Senior Member Ducatista47's Avatar
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    Should Field Coil Come Back?

    I was wasting everyone's time again posting about efficiency in speaker design and amp power vs quality, in another thread.

    I got to thinking, now that anything that improves sound is fair game, at least to a small dedicated segment of audio fans (that includes us, I think), is a rethinking of field coil speakers desirable? For all I know they might have desirable properties. I'm sure any undesirable tendencies could be engineered out. Look what JBL has done with ferrite, which Greg Timbers has labeled a "lousy," I think it was, material to make speaker magnets from.

    What would be the advantages of field coil, if any, especially in an era of expensive Alnico?

    Clark in Peoria
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    RIP 2010 scott fitlin's Avatar
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    Field coild drivers are available, and here in the U.S. www.supravox.com.

    There are some that say nothing sounds better than field coils, Steve Schell, the forums resident expert on really old audio, he can tell you about their sound.

    They look to be interesting, but, they are very expensive too. I would imagine that making drivers that need their own DC power supply doesnt help keep costs down.

    I have always wanted to hear some.
    scottyj

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    I'm sure if you've got that room temperature surperconductor thing worked out it's absolutly the way to go! but other wise it's more heat to dissapate, one (at least) more power supply and one more way to introduce hum into the system. JBL use to brag on being the only on to use a 4 inch voice coil (before the first altec with a 4 inch) and say they would never use ceramic. Electrovoice use to say a lessor competitor of theirs (whom I never heard identified) used 4 inch voice coils because they lacked the sophisticated equipment needed to make the close tolerance adaptors needed to adapt the redily available 4 inch alnico of the day to work with the more desirable smaller voice coil diameters (I think they used 2 1/2 on their 30 inch, which went into the Patrician that I've heard can still be had in japan) Do you think any of them ever told the truth?

  4. #4
    Senior Member Ducatista47's Avatar
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    Quote Originally Posted by scott fitlin
    Field coild drivers are available, and here in the U.S. www.supravox.com.
    Scotty, thanks for that link. The drivers tab has a very interesting history section. I notice their drivers cost the same in Alnico or field coil. The motors seem to be quite large!

    I did spot one advantage of field coil, 101 dB 1w/1m from a woofer!

    For a moment I thought Mrs. Dorliac was Francoise Dorleac, Catherine (Dorleac) Deneuve's sister. She also died in an accident, but at age 25 in 1967. I remember it well.

    I'll keep reading, Clark
    Information is not Knowledge; Knowledge is not Wisdom
    Too many audiophiles listen with their eyes instead of their ears


  5. #5
    Senior Member Ducatista47's Avatar
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    Quote Originally Posted by Thom
    Do you think any of them ever told the truth?
    Not completely, I suspect.

    Well, I can't think of any reason why using smaller voice coils would be a superior method. Anybody can jump in here now...please...

    Clark
    Information is not Knowledge; Knowledge is not Wisdom
    Too many audiophiles listen with their eyes instead of their ears


  6. #6
    RIP 2010 scott fitlin's Avatar
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    You are welcome, I always look at Suparavox from time to time. They tell me they are supposed to sound very good.

    I thought about taking an empty pair of A7 cabinets I have, and loading them with the supravox field coil 15. What stopped me is that the A7,s need a bigger room than my living room, IMHO.

    I spoke to them here in the States, and I asked for the Alnico versions, and they told me, NO, what you really want is the field coils! Apparently, and this is where it gets very interesting, you can vary the PS output voltage, and this changes the woofers parameters, allowing you to really fine tune the speaker to your taste, in a way you cannot do with any conventional driver. I found it interesting, and possibly I will get back to actually doing it at some point. I have asked Steve Schell about FC speaker sound, and he tells me its the most lucid and smooth, and relaxed, realistic sounding speaker you could want to hear. I know IF I get out his way, I want to visit him and hear what he has.

    From an audio buffs point of veiw, I have never heard FC,s and really want to simply because its something I never even knew about till fairly recently and is so scarce.
    scottyj

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    The only thing that could make them any different than pm would be that you could control mag strength and therefor efficiency so you could pay more for them because you thought you could correct what the engineer didn't understand. I know, a Linn was supposed to sound best if you put it on a chessboard supported by four upside down watertumblers. I should think it should be cheaper, machines wind coils (I koow single crystle, oxygen free) and you wouldn't have to go through the magnetizing process and the hardening process but hey some people preach on TV some people sell audio. selling audio must be less harmfull. You know you could go moving magnet while your at it. I'm not serious but someone might me.With field coils today I can just envision the arguments about which polarit sounds best. Perhaps negative polarity through the early moon. And the arguments over regulation whether you should use shunt or series regulation. Whether discrete diodes or a one piece bridge. And what caps should you use to filter your supply or should it only be powered from batteries. I see it now! The state of the art would be a little mg set the motor would run from a vfd and it would either be a dc generator or a polyphase set-up
    there would definetly be two schools of thought about that. I've got to go befor I offend somone it's not my intention

  8. #8
    Senior Member Ducatista47's Avatar
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    Thom, that is one hilarious take on clueless audiophiles. You might look down at the quote in my digital signature to see how much I appreciate it.

    Obviously field coils in the wrong hands could complicate audio beyond reason, but I am suffering from increasing interest now. I started this question with idle curiosity uppermost in my mind, but Scotty's revelations have increased my curiosity manyfold. In the right hands, they sound like a diy dream come true. Tweaking the transducer/cabinet combo at the turn of a knob, good lord.

    Most of all, the promise of even sweeter sound than otherwise possible would make the development of modern field coil drivers a desirable path of inquiry. Perhaps compression drivers might benefit as much or more. Did Lansing or Altec ever build field coil horn drivers? I'm not in a financial position to pursue this now, but I am excited that other more able explorers might take this forward.

    I confess that my initial mention of cost was a red herring. I always imagined cost being greater, but with $3500 compression drivers upon us all things are possible.

    Clark
    Information is not Knowledge; Knowledge is not Wisdom
    Too many audiophiles listen with their eyes instead of their ears


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    I'm sure that yesterday's field coil drivers would benefit from today's power supplies. Back when EMs ruled, consumer gear had the field winding integral to the same power transformer that ran the rest of the system, while pro use dictated multiple remote field supplies. The integrals' field supply interacted with the amplifier directly (for both good and bad), while the remotes acted more like a PM, and saved stringing 200-400 volt field wiring to all the speakers.

    My only personal experience with EMs has just involved a couple of 12"s, a Jensen and a Rola that I'm playing with in a couple of DIY MI amps. Both amps are still works in progress, though I played them quite a bit before I ripped into them. For guitar and organ, they're clear and sweet, until you pour on the gain, then the HV sags, dragging the field down with it, yielding a compressed effect you won't get with any digital rig. The characteristics I find worthy of pursuing for MI apps would be undesireable for stereo listening though, and if it wasn't for that, the heat alone would be enough to turn me off.

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    Seems to me that if one had a machine shop, nothing major, a little skill, doing it wrong a few times will usually give you that, and here comes the tough one, a lot of time. You could play with it. Problem is their always seems to be some magic involved. You could probably water cool. If you started out with one of those JBL's, with the magnets that are apart, that are on EBAY right now, it might even be easier. I'v got a (very old ) JBL 15 where the magnet assy obviously had been slated to be a 375 before they snatched it for a 15 so with some work (less than starting from scratch) one could probably go the other way. There is so much difference in drivers, that if field coils were common, there would be pm's that people swore were better than any field coils, and field coils that people swore were better than any pm. If you actually built your own (I think I'd leave the armature and as much else as I could to off the shelf parts) and it sounded good you'd have some braggin rights.

  11. #11
    Senior Member Tom Brennan's Avatar
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    A pal has a pair of those Supravox field-coil drivers, he's used them both in Oris horns and as direct radiators on large open baffles.

    They sound very good but nothing to get rid of your Altecs or JBLs for. One wonders if their goodness is due to the field-coils or some other aspects of their design.

    Now the fella is running a mono rig with a B&C woofer and a JBL 2435 on some big old anonymous prosound radial horn. Sounds better than the Supravoxes. My opinion of course.

  12. #12
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    some of the most interesting systems just sort of died when stereo came about. The poor guys just didn't have another basement or spare room, or basement + spare room +add on to build another horn into or even infinite baffle. Plus can you imagine you've spent your life putting this system together and now all at once someone is telling you you need another one on the other side?

  13. #13
    Senior Member Steve Schell's Avatar
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    I have been addicted to field coil speakers for about a decade now and have accumulated a house full of them. I have also founded a company with a friend to manufacture field coil compression drivers and horn systems, so my comments here are not impartial in any way.

    Field coil drivers tend to have a sort of family sound that one encounters over and over. The sound is very dynamic and compelling, yet relaxed and relaxing to listen to at the same time. It is the basic type of sound that sends us all in search of high efficiency speakers to begin with, but is realized most fully with field coil drivers in my experience. I think that this reflects a more accurate job on the part of the speaker of tracing the delicate audio signal. The most popular theory to explain this is that the magnetic flux in the voice coil gap is less modulated by voice coil currents and is therefore more stable, allowing the movements of the voice coil to more perfectly mirror the input signal. A permanent magnet driver stores a finite amount of energy in its magnetic circuit from the moment it was originally charged. A field coil is usually connected to a low impedance DC power supply however, and does not share this limitation. To my knowledge no one has yet conducted a scientific study to determine whether field coil drivers really do have greater stability of gap flux; I hope to investigate this one of these days.

    Many vintage field coil drivers have been in great demand for decades now and sell for high prices due both to their scarcity and fine sound. There are a few companies currently (sorry, pun unavoidable) producing field coil drivers, and the trend is on the rise. I have also corresponded with numerous hobbyists who have taken on the challenge of converting permanent magnet drivers to field coil operation. This is quite feasable for Altec and JBL compression drivers, as their ancestors were field coil units.

    Clark, the Altec small format driver began life in 1937 as the Lansing 801, and the large format Altec began as the Lansing 284 in 1934 or 1935. This is covered in the history section of this site- check out the profiles of the Lansing Iconic and the Shearer Horn System. Disassembling and comparing, say, a 1940 Lansing 801 and a 1970 Altec 802 shows many more similarities than differences. Also, the JBL D-175 owes its origins to the Lansing 801, and the JBL 375 was a permanent magnet adaptation of the Western Electric 594A of the late 1930s.

    Tom, I have heard the Supravox drivers a couple of times. They do the usual field coil thang very well, but no driver can transcend its basic type just because it uses a field coil. A small f.c. direct radiator may sound smooth and punchy, but it will still sound like a small direct radiator. When we delve into large multi way horn systems using field coil compression drivers throughout, then things really get interesting.

    It is true that the current delivered to the field coil can be varied within limits and the speaker's characteristics tuned to an extent. I have heard that this can be effective in a guitar amp. In my approach to hi fi speaker design the desired response is achieved primarily through horn design, not enclosure tuning. For horn applications the strongest motor possible provides the highest efficiency and widest bandwidth. Saturating the pole tips to 21 kilogauss, or to 24 kilogauss if one springs for permandur pole tips, is a Good Thing.

  14. #14
    Senior Member Ducatista47's Avatar
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    Wonderful Post

    Steve, thank you so much. I feel like I did when Greg Timbers posted on the 4345 thread, and these experiences have - ahem - more similarities than differences.

    Would you care to either post some links to your current product development (I remember seeing some spectacular images previously either here or in a link) or treat us to some presentation here of what you are up to? I had a feeling that compression drivers would benefit from this technology, but in my case it was an educated hunch.

    Thank you again for posting here,

    Clark
    Information is not Knowledge; Knowledge is not Wisdom
    Too many audiophiles listen with their eyes instead of their ears


  15. #15
    Senior Member Steve Schell's Avatar
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    Clark, thank you for the very kind words.

    I'm limping a bit at the moment as my computer is away for repair and I've pressed our business laptop into service. Most of my images are inaccessible at the moment.

    I blabbed on and on about our Cogent drivers in a previous thread. There are also some pictures and a link to an old article I wrote here:

    http://www.audioheritage.org/vbullet...ghlight=cogent

    Since then we have begun production and sales of our drivers. I have been building our web site and we plan to launch it soon. We have accepted Ron Welborne's invitation to share his room at the upcoming Rocky Mountain Audio Fest in October, so we will have a Cogent system playing there. I highly recommend this show for its size, quality, and interesting mix of both high end and DIY audio.

    http://www.audiofest.net/

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