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Thread: DIY 802 field coil drivers may be a reality!!

  1. #11
    Senior Hostboard Member Elitopus1's Avatar
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    Re: DIY 802 field coil drivers may be a reality!!

    Quote Originally Posted by lokie View Post
    What I "want" is information... How do they sound? Have you measured them?
    The information you seek is probably better found on a different thread/forum. Not in a Altec 802 field coil thread.

    I haven't seriously listened to my pair. First impressions are they sound like they are better suited for MI or guitar speakers rather than HiFi.

  2. #12
    Senior Hostboard Member Steve Schell's Avatar
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    Re: DIY 802 field coil drivers may be a reality!!

    Quote Originally Posted by nullspace View Post
    You'll want to make sure you touch base with Steve Schell before getting too far along; he's posted here and elsewhere on fundamental differences between the Lansing 801 & Altec 802, and what exactly is required to convert the 802 to field-coil operation. Hopefully he'll see the thread and chime in.
    I will be glad to add what I can. Altec compression drivers, both large and small format, are relatively easy to convert to field coil service because the original Lansing f.c. versions were modified minimally for permanent magnet duty.

    The original versions featured an inner pole piece whose diameter was maintained down to the front (exit) plate. Later p.m. versions featured a widened skirt below the voice coil area to conform to the magnet's outer diameter. Most modern f.c. adaptations make the critical mistake of retaining the skirt, leaving inadequate cross sectional area for the field coil. This results in the oft-heard complaints of overheating field coils, low flux density and poor sound. Properly done the results are quite different.

    As to the "are field coils any good?" argument, all I can say is that I am convinced. A late 1930s Lansing Iconic two way system simply SLAYS a late 1940s Lansing Sound (JBL) permanent magnet two way system for overall naturalness of sound reproduction. As I told an temporary owner of an Iconic several years ago... "If you are certain that you want to sell it, then don't ever listen to it."

  3. #13
    Senior Hostboard Member BillWojo's Avatar
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    Re: DIY 802 field coil drivers may be a reality!!

    Steve, glad you stopped by and commented. Here is a link to a recent set of Altec small format converted field coil drivers from ebay.
    Altec Lansing 802D Field Coil | eBay
    Looks like the large flange on the phase plug was turned down, not needed anyway with the alnico magnet gone. Do you retain the zinc taper bored piece inside or do you make a whole new steel armature for the coil and taper bore it?
    I sent some dimensioned drawings of a 802 cast pot driver to Hentai, he has a great write up on the DIYaudio forum on the field coil speaker that he built from scratch. He has a really cool computer simulation program that can map out the magnetic circuit and give values for the flux density in the gap.
    I suggested that he stop by here, maybe we can convert him to an Altec fan.

    BillWojo

  4. #14
    Senior Hostboard Member BillWojo's Avatar
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    Re: DIY 802 field coil drivers may be a reality!!

    Just an update, I haven't forgot about this. Hentai and I have corresponded several times as he needed other dimensions. On Saturday he replied that he would have a FEMM file pretty soon. This is a simulation of the magnetic circuit along with calculated field strength of the gap.
    I had to send him an email this evening, I sure hope this new development that I discovered doesn't cause him undue work.
    Last week, Don had sent me an email as he knows I'm looking for a set of cast pot 808/802 drivers to convert to field coils. There is a listing on ebay for a very rough looking driver that I bookmarked, I think a bit to rough to use actually. These older drivers do not use the plastic orange colored phase plug, they are all metal.
    Anyway, earlier today I was studying the picture of the phase plug and it hit me! The center of the phase plug is not corroded at all! I grabbed my dismantled 808 and checked the phase plug with a magnet. Seems the whole center section of the phase plug is non magnetic, only the outer part is.
    I suspect that just like the center conical expansion piece that it is made of a zinc diecasting. I took a small file to it and it is soft, not hard and tough like stainless steel.
    It amazes me all the details that went into manufacturing these drivers and the high quality that they did it with.
    As soon as Hentai post his reports, I'll update this post.

    In the meantime, if anyone has an old set of 808/802 cast pot drivers, I'm looking for some. Cosmetics don't mean anything, I'll buy new diaphragms if need be, I just don't want rusty gaps, it changes the dimensions.

    BillWojo

    - - - Updated - - -

    Just an update, I just sent this to Hentai and thought that you folks would be interested in this also.

    Well I decided to look at the material a little closer to see if it was stainless steel or not. I took a small file and found that it is very soft. I'm willing to bet that it is a zinc diecasting just like the center expansion piece. Looks like the phase plug is built up of 3 pieces, the steel outer ring that mates with the alnico magnet and provides the magnetic circuit for the gap and 2 zinc diecastings.
    When you look at pictures of the phase plug you can see little metal bars that hold it all together. The center piece has 3 bars cast into it that fit into 3 slots cast into the next casting or middle piece. That middle piece has 3 bars also that press into 3 slots that are machined into the steel outer ring. After they press it all together it was sent to final machining and plating.
    Quite an assembly, those engineers were pretty smart.

    BillWojo

  5. #15
    Senior Hostboard Member LowOhms's Avatar
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    Re: DIY 802 field coil drivers may be a reality!!

    Hi Bill,

    I would like to talk to you a little about Field Coils. You may be barking up the wrong tree.

    Do you notice in post #11 in this thread, Elitopus suggesting that his initial impression was NOT favorable upon hearing a 802 field coil in his system? He says its more suitable for Musical Instrument or Guitar reproduction, NOT home hi fi.

    Has ANYONE noticed his listening report, and thought about that report ?? No one has commented, I will.

    About 25 or so years ago, I had a BIG RCA horn system, with double 15 inch " Ubangie " Bass Bins, and the RCA 90 by 40 horns, and RCA field coil drivers. I have, in previous years, personally owned about 13 pieces of the RCA MI-1428B and 1443 field coil drivers.

    I am writing to suggest to you and others, that JUST because a driver operates as a field coil, does NOT mean its any better than a permanent magnet later version of the same driver.

    IMHO, there is an INHERENT problem with all field coils, no one discusses. The two windings in the driver, in close proximity to each other " buck " each other. I don't think ANY ONE has ever openly expressed this, but I am now. Certainly Elitopus' post #11 herein, should be considered, along with my comments.

    Frankly, I am happy with the re-magged ( GPA ) 802Ds I run. IMHO, I tend to always consider the system's wiring and amplification, as the weakest areas where we need to gain performance improvements .

    As George Gershwin once wrote and I say to you all, " It Ain't Necessarily So".


    Regards and best wishes,

    Jeff Medwin

  6. #16
    Senior Hostboard Member mah's Avatar
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    Re: DIY 802 field coil drivers may be a reality!!

    NOT ALL field-coil drivers have a humbucking winding.

    The humbucking winding allowed a lesser quality(cheaper) DC supply to be used.

    See here: Valve Radio and Audio Repair Handbook - CHAS MILLER - Google Books


    Dynamic Speaker and Power Supply


    Where a quiet, preferably well regulated, DC supply is used the humbucking winding is not required.

    It all depends on the intended application.
    Opinion is only as valid as its verifiable supporting evidence.

  7. #17
    Senior Hostboard Member BillWojo's Avatar
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    Re: DIY 802 field coil drivers may be a reality!!

    I have an antique Atwater Kent field coil speaker with a bad cone and surround that I picked up at an estate sale for cheap. Stand alone unit in a cool metal case with a cast iron stand, table top style. I'm sure this was also used as a power supply choke as well. Any of these were not particularly known for high fidelity.
    The WE and early Lansing FC drivers with the dedicated DC power supply's are a different story and were used for high fidelity. With the advent of alnico magnets it was just much cheaper to use permanent magnets than to use a coil and a dedicated DC power supply.
    I have yet to read a review of any well designed FC driver, either new or vintage that said it was inferior to it's permanent magnet sibling. Usually it's rated as being a nice step up in fidelity.
    This whole exercise is to provide a path for fellow Altec fans to be able to build a set and experiment without breaking the bank.
    For experimenting I have several PowerDesigns DC power supply's to use. If I become a field coil convert, I'll build a dedicated supply.

    Marshall, thanks for the links, they were very interesting. I found it interesting how they tried to get rid of the hum with an additional coil on the voice coil. Since the FC in those early designs were used as chokes in the B+ power supply, there was still a bit of AC ripple flowing in the FC. That additional coil was used to try and cancel out the hum induced by the FC. I doubt it was 100% effective and added mass to the field coil. I can see why most designers ran head first to alnico magnets when they became available.

    BillWojo

  8. #18
    Senior Hostboard Member mah's Avatar
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    Re: DIY 802 field coil drivers may be a reality!!

    The humbucker coil when designed to be an integral part of a particular system for more critical applications, e.g. motion picture theatre, sound stage, recording studio and premium home placback could be very effective. Of course for more money a better, dedicated DC supply could be used for either type of coil arrangement.

    Microphones also used this technology.

    Less critical applications were paging, phonograms, portable MI amps.

    I believe modern premium FC drivers can be built that are the equal of PM types - they all use the same basic magnetic theory. And, hey, you can dial up the FC Gauss if that floats your boat.

    Jeff, you said humbucking winding proximity in a FC driver is a problem. Could you elaborate?


    P.S. Seth Lover: Seth Lover - Wikipedia
    Opinion is only as valid as its verifiable supporting evidence.

  9. #19
    Senior Hostboard Member LowOhms's Avatar
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    Re: DIY 802 field coil drivers may be a reality!!

    Hello mah,

    You write " Jeff, you said humbucking winding proximity in a FC driver is a problem. Could you elaborate?"

    Actually, that is not what I said, I was never really discussing an added hum bucking winding, what so ever.

    You expounded on such a winding, and that is not what I meant at all. Allow me mah, to explain it in simple terms.

    There are two windings, a voice coil winding and a field coil winding, in close proximity to each other, so, they BUCK each other.

    Very simple, its like two cars going through an intersection, one south to north, the other east to west, and they T-Bone each other. That... is the bucking !!


    We all hear it, and some of us will actually recognize what we are hearing !! Maybe that is why Elitopus' first impression was this : a field coil 802 was " for guitar or musical instrument playback, not hi fi. " I laughed my butt off when he wrote that, but I respect him for hearing and reporting it as he did. NO ONE on this Forum commented on that report. That is hilarious to me also. Recall the original " two taps for one with a tap dancer"... using Western Electric field coil gear.

    'I personally chased the field coil mystique 25 years ago, if I thought they were the thing to own, I would not have sold every one of the 13 field coil RCA drivers I had. Live and learn.

    Pretty simple, easy to hear for some of us, and no real intellect or technical articles to justify oneself, are required !!

    I hope people understand it now. Sell that stuff abroad. Replace it with the better designed ALTEC or GPA !!!


    Jeff Medwin Low Ohms

  10. #20
    Senior Hostboard Member Elitopus1's Avatar
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    Re: DIY 802 field coil drivers may be a reality!!

    This thread is getting pretty "bucking" interesting. Before I go back under my bridge, I want to mention this. There was some confusion with lokie trying to threadjack and turn this into a Jensen thread. I did say I have a pair of Jensen A12 woofers. I do not own any, or have any experience with Altec FC drivers. Sorry for the confusion

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