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Thread: Eminence Flux Density Modulation

  1. #106
    Old Timer J M Fahey's Avatar
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    Hi Steve, great post.
    Much more than what I expected.
    I always stated (still do) that ear is the final judge, but any technical measurements backing (or not) what we hear (or think we hear) make for a solid foundation.
    In this particular case I am delighted (and that's an understatement) by having real world measurements backing so closely what I predicted.
    Going to your graph , I see 12 actual curves, corresponding to attenuation stages 1 to 12 below.
    They seem to be color coded on the original, grayscaled in your image, but anyway can be matched:
    full magnetic power (no attenuation by definition) must be "Memory 1", which shows the lowest Q (lower=better/louder) ; its impedance peak must be the 26 ohms one, the 40 ohms one strikes me as somewhat high.
    As you rise the attenuation (lower magnetic flux) , Q rises steadily, as I predicted, and the peak impedance rises in step, reaching an incredible maximum of 96 ohms.
    So far it matches Theory perfectly.
    At some point, although Q continues rising, the impedance peak lowers, because the speaker motor becomes too weak to generate a counter electro motive force (CEMF) or induced voltage to counter what the amplifier sends.
    Would love to see that chart re-posted in colour to be able to match curves to specs better.
    You say those are -2dB steps? Fine with me, the rise in Q from Memory1 to memory10 is incredibly smooth and regular.
    A very interesting data line is Memory 11 ; the speaker no longer has a resonance peak !!! ; the measuring software searches everywhere and finds a peak at over 5 KHz, which is certainly there (can be seen clearly on the impedance curves) but unrelated to what we are interested in.
    Thanks again for posting.
    PS: the phase curves above are also interesting, they show behaviour of a tuned system (the speaker) above and below the resonance frequency.
    It can be seen that the amount of phase shift varies a lot depending on attenuation/magnetism, confirming the Q variation.
    Nice when different pieces of data back each other.
    PS2: please tell me something about the measurement system/software you used. Is WT3 similar to LMS, used by Eminence?

  2. #107
    kg
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    Quote Originally Posted by J M Fahey View Post
    Hi kg. Interesting questions.
    As far as tube longevity, the tubes are being driven full power, they don't know whether they drive a real speaker, a power resistor or a very weak magnet speaker, I guess they get "spent up" the same with any method, that's the price you pay for full power sound after all.
    well, this kind of gets at the heart of the matter.

    IF, as you say, the tubes don't "know" what they're driving, then we should be able to substitute an electrically equivalent black box in place of the speaker load, and expect the very same response, right? at least in terms of the amplifier's output voltage/current characteristic?


    The impedance curve of a fully unmagnetized speaker must be its DC resistance plus whatever loss/parasitic inductance the voice coil has.

    snip

    Note: a magnetless speaker will be flat; a weak magnet one will not; the HF part will always be about the same but the resonant peak and the "Q" will not.
    indeed, the chart provided by mr coil backs that up.

    The voice coil will get hotter, you are right that motion cools it; in fact I already commented here that one employee of mine, probably inmersed in a Jamaican dream or floatin in a sea of beer, has on occasion mounted an unmagnetized speaker mixed with regular ones, the lower SPL was not that much noticeable (logarithmic ear response, etc.) but the cooking epoxy smell gave it away.
    hopefully the "reduced magnet strength" method manufacturers (and i say that because it's both fluxtone and FDM) take that into account and either de-rate their voice coil wattage ratings, or add ferrofluid, or some other thermal management.

    Don't think the screens will be affected, because there are two "errors" which tend to compensate each other: impedance will be lower, because the reactive component will be lower, given a weaker cone movement (the object of this attenuation); yet as you say, the voice coil might get somewhat hotter, raising the DCR; both effects act in opposite ways and will somewhat cancel or thereabouts.
    Tube plates and screens should not be much affected.
    I leave that test to you or whoever wants to do it.
    Good luck.
    i agree they are somewhat counteracting. hopefully they'll cancel each other out!

    in my experience replacing reactive speaker loads with purely resistive ones has resulted in premature output tube failure. however, this is not exactly the case here as there is still some inductive component to the load.

  3. #108
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    JM..I am happy that you found this useful.

    Just one thing though..the 96 ohms is the non attenuated reading..you got that 180 out.
    As the attenuation goes up the ohms go down...the flatter the lines are = less SPL.

    So the most attenuation = just a bump at 130 hz.

    I will try to find the original around here somewhere....ya I looked that was on the old puter...sorry

    WT3 = woofer tester three

    MC

  4. #109
    Old Timer J M Fahey's Avatar
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    Fine.
    I should only like to correct a small point, maybe I didn't spell it as clearly as needed:
    Originally Posted by J M Fahey
    Hi kg. Interesting questions.
    As far as tube longevity, the tubes are being driven full power, they don't know whether they drive a real speaker, a power resistor or a very weak magnet speaker, I guess they get "spent up" the same with any method, that's the price you pay for full power sound after all.
    Posted by kg
    well, this kind of gets at the heart of the matter.

    IF, as you say, the tubes don't "know" what they're driving, then we should be able to substitute an electrically equivalent black box in place of the speaker load, and expect the very same response, right? at least in terms of the amplifier's output voltage/current characteristic?
    I only refer to "load" characteristics and health/durability, not sound here.
    Your words "response" and "voltage/current" characteristics *might* be misunderstood by someone else not following this discussion, or quoting this somewhere else out of context.
    I know it's clear for us, just wanted to state this for the benefit of third parties.
    Your finding of reduced tube life on full power + attenuator setups, sounds very reasonable to me.
    Fact is, fully overdriven tube amps are very hard on screens.
    As of somewhat derating speakers when magnetically attenuated, *I* would test that and post my findings on the label or user manual, but I'm not building them, never will, so I leave that to who's directly concerned.
    Personally I'm into "live use" amplifiers, and strive for maximum sound output possible, within realistic production cost constrains.
    Bedroom rockers, may God bless them, scratch me the wrong way.
    Mind you, I provide a "3AM" switch on the back of my (few) tube amps, which lowers power output to around a still loud (by 3AM bedroom/livingroom standards) 0.1 to 0.5W , but that's provided more as an anti-divorce method than anything else.
    On my SS amps, the ones that put the bacon on the table, that's unnecessary.
    Good luck.

  5. #110
    Old Timer J M Fahey's Avatar
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    Hi Steve, thanks.
    Waiting for that full color chart, sure will make things clear.[]

  6. #111
    kg
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    Quote Originally Posted by J M Fahey View Post
    I only refer to "load" characteristics and health/durability, not sound here.

    Your words "response" and "voltage/current" characteristics *might* be misunderstood by someone else not following this discussion, or quoting this somewhere else out of context.
    I know it's clear for us, just wanted to state this for the benefit of third parties.
    Your finding of reduced tube life on full power + attenuator setups, sounds very reasonable to me.
    Fact is, fully overdriven tube amps are very hard on screens.
    As of somewhat derating speakers when magnetically attenuated, *I* would test that and post my findings on the label or user manual, but I'm not building them, never will, so I leave that to who's directly concerned.
    Personally I'm into "live use" amplifiers, and strive for maximum sound output possible, within realistic production cost constrains.
    Bedroom rockers, may God bless them, scratch me the wrong way.
    Mind you, I provide a "3AM" switch on the back of my (few) tube amps, which lowers power output to around a still loud (by 3AM bedroom/livingroom standards) 0.1 to 0.5W , but that's provided more as an anti-divorce method than anything else.
    On my SS amps, the ones that put the bacon on the table, that's unnecessary.
    Good luck.
    i agree. i'm the crazy asshole who uses a 600w tube power amp. it does have a triode/UL switch, which knocks it down a tad, but it's still a couple hundred watts.

    the best solution i've ever heard to attenuating an amp is to build a large soundproof box to put the whole thing in! or a pair of earplugs...

  7. #112
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    So KG,

    What is the tube complement in your 600 watt amp?

    Does it require 220 VAC plug?

    Does it weigh 190 lbs?

    I am guessing...12 each 6550's?

    24 ---6l6's

    40--- EL84's

    a pair of 813's
    one 3-500?

    Come on really? 600 watts rms?

  8. #113
    Supporting Member Chuck H's Avatar
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    "I've heard magic defined as "a technology you don't understand". By that aphorism, the folks in this forum are practicing wizards, able to summon AND control the lightning demon, and make charms to allow others to use the demon in certain ways." R.G.

    "...less ear-friendly but handy for jazz." Leo_Gnardo

  9. #114
    Old Timer J M Fahey's Avatar
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    I am guessing...12 each 6550's?
    Congratulations, you nailed it pretty well: 12 x KT90's.
    Too much for me , I don't find *much* justification for an over-50W amp in everyday life, but hey, to each his own.

  10. #115
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    I like it !

  11. #116
    kg
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    it's 100lbs flat, in its aluminum rack mount chassis. basically all the weight is iron! three plate trannies, one filament, two hammond 1650w outputs.

    still going on the original kt90s (well, most of them. i had a screen blow out on one and i think i dropped one once.) tried switching over to the sovtek 5881, and they were tough enough to take the voltages, but the sound was flat.

    the pisser is that the kt90s are no more. unobtanium. cannot source them, at least for any kind of money i can afford. i got them at cost years ago when i worked for a shop. i think i paid like $20 each. i kick myself for not buying enough for a lifetime supply. stupid me thought they'd be around forever.

    the limiting factor is the 750va toroidal main plate tranny. it's from a ss amp. i rewired the secondaries in series to give me 240vac. that, in turn, goes into a full wave doubler. idle plate voltage is ~630vdc. it's biased pretty cold. never heard much of a difference running them hotter. there is a silly amount of filter caps crammed in there... two sets of 4x500uF@500vdc LCRs, stacked one on the other, netting 1,000uF@1kvdc.

    since i built the amp, i've actually added some series resistance to the plate supply, which cuts down the continuous/rms output power a bit. about 50w iirc. however that headroom comes back in once the b+ charges back up. basically a big version of power supply sag that only affects the output tube screens/plates (the rest of the amp is driven through two additional separate plate trannies). the compression keeps things reasonably dynamic by letting through the transients.

    the other thing the series resistance did was reduce the ridiculous stress on everything whenever i'd power up. there's a 15A slow blow fuse in the mains that sometimes opened. i'd also trip magnetic circuit breakers in the building circuits! blew up a few 1n5407 diodes too in the doubler circuit. ended up paralleling them up to keep them in one piece. iirc the secondary of the main plate tranny was something like 5 or 6 ohms, and i added a 10r resistor. it's been a while.

    the amp still cranks! though i have to say that if i get back gigging again there will be a lighter version made... i'm too old now!

  12. #117
    rf7
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    I just read some of the patent stuff. It's funny how you can bring back an abandoned technology (Field coil speakers) to do something that would be much harder to do with the current technology.
    Last edited by rf7; 01-22-2011 at 02:29 AM. Reason: clarification

  13. #118
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    Quote Originally Posted by rf7 View Post
    I just read some of the patent stuff. It's funny how you can bring back an abandoned technology (Field coil speakers) to do something that would be much harder to do with the current technology.
    Most of this forum revolves around obsolete and the mostly abandoned technology of vacuum tubes. The abandonment of one technology for another is usually driven by the lower cost of manufacturing.

  14. #119
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    just a coil

    I still wonder would it work if you just make a movable coil, perhaps with some core parts, that you can just put on any speaker like a cap. Dial in some current from the power supply and attenuate the original speaker. If you don't need any attenuation, just leave it.
    This way you could attenuate your priceless vintage amps without distroying anything.

    jukka

  15. #120
    Old Timer J M Fahey's Avatar
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    I still wonder would it work if you just make a movable coil .... that you can just put on any speaker like a cap
    Fine as an "out of the top of my head" idea, but digging into it, the big problem is that speaker magnetic circuits are *closed* circuits, with only the Voice coil minuscule gap as an interruption.
    In a way similar to a transformer , where an "external" coil would have no effect.
    To affect magnetic flux , in both cases, speaker and transformer, you should wind the auxiliary coil you suggest around the center "leg"; whether it's the speaker pole piece or the transformer core.
    In the speaker case, it would not be an "ad-on" at all.
    If you wind something in that pole piece, just make it a Field Coil and use it as such.
    Good luck.

  16. #121
    kg
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    jukka: certainly can be done.

    as i said earlier in this thread, you can just take another strong magnet and buck their fields. it WILL negatively affect the flux in the gap. the degree to how much it will be reduced depends on how strong your bucking magnet's field strength is.

    as jm said the BEST technique will buck the field within the magnetic circuit itself (ie magnet/pole piece/gap/backplate), but an externally applied one will still have an effect.

  17. #122
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    Quote Originally Posted by J M Fahey View Post
    To affect magnetic flux , in both cases, speaker and transformer, you should wind the auxiliary coil you suggest around the center "leg"; whether it's the speaker pole piece or the transformer core.
    In the speaker case, it would not be an "ad-on" at all.
    If you think of it, the back plate and center leg are just a conduit for the field. The magnet itself is a ring, opposing poles at the sides. If you wind the coil exactly around the magnet,the magnet will be your field conductor with the rest of the circuit closing it. What you do is just create the opposing field right at the source, in the magnet itself. The coil will undoubtedly end up being big, you'll have to fit the whole magnet inside it. And the magnets of different speakers being different size, you'd have to make a special coil for every speaker, but that's just a question of molding different bobbins.
    What really worries me, does this kind of imposed field have any lasting effects on the original speaker magnet.

    jukka

  18. #123
    Old Timer J M Fahey's Avatar
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    For discussion's sake, suppose the speaker points skyward, let's say North points up *in the ring magnet*.
    Let's also accept that magnetic lines of force go from North to South.
    If the magnet is "alone" in free space, you all know how lines travel in different paths, "the best they can" , because space or air are terrible to conduct line forces, and the magnetic field takes the shape we all see in Physics books.
    It might somewhat be compared to a quite fat dougnut.
    Now to the magnet *in a magnetic circuit* which is something very different.
    Both poleplates help close the circuit, let's say they force the lines to travel horizontally.
    The final part of the magnetic circuit is the polepiece.
    Let's follow the path of a lineforce.
    By definition, either on a free magnet or inside a speaker, lines leave the top (North) magnet face perpendicular to it, "pointing up", travel inwards towards the gap thanks to the top plate, then travel *downwards* or "pointing down" through the polepiece, then horizontally "outwards" thanks to the bottom plate until they reach the bottom/South pole.
    They complete the circuit "pointing up" again inside the magnet.
    Now let's analyze what happens if you surround the whole enchilada with an external coil, through which you pass some DC current.
    It will become an electromagnet.
    It will produce a magnetic field inside it, in the space surrounded by its wire; as was suggested in the post above. The polarity of that field will be the same in any point inside that coil.
    We can easily see, that if, say, we create an electromagnet with the South pole on top, to substract from and weaken the magnet, that "Top-South" field increases the field through the polepiece at the same time .
    Total effect: nil. (zero)

    If we invert polarity we are doing the contrary: reinforcing the magnet and weakening the polepiece: same result. It balances out to zero.
    That's why I called the speaker magnetic system a "closed circuit", isolated from external forces.
    If you analyze a ring Alnico (P12N) it's the same.
    If you analyze a center Alnico system (P12R, P12Q) it ends up being the same, only now the North Pole is on the polepiece top, and the passive (induced) magnet which closes the circuit is outside.
    Now, if we wind a coil around the polepiece but inside the ring magnet, as I suggested, the situation is absolutely different: if we pass current so it creates an North top field, this will substract from the South top field present in the polepiece, and those same "new" lineforces will complete their circuit through the ring magnet, in an opposite direction and weakening it.
    I think I've demonstrated that an internal coil wound around a polepiece can be an effective magnetic attenuator; while one wound outside the whole magnetic circuit will not.
    I recognize it *might* have a (very) little effect because of iron nonlinearities and such, but you'd need a monster electromagnet to make that happen.
    In fact, the huge magnetic pulse in a magnetizer does exactly that
    After all, speakers are magnetized "already built", and the , say, "North up" pulse goes through the magnet *and* the polepiece with the same polarity and at the same time.
    As I said, that pulse must be around 5x what's needed to saturate the magnet *and* iron; that's why I talked earlier of my 3KW and 17KW magnetizers.
    Compare this to the 15W Fluxtone uses to get a very loud speaker; obviously by winding around the polepiece but inside the magnetic "cup".
    Excuse me for being so long winded, I understand this is not as intuitive as Electrical circuitry, too tired to draw now but if somebody finds it necessary I will gladly add some diagrams to let the magnetic paths be seen with clarity.
    NOTE: exactly the same happens with a conventional EI transformer : the "useful" coils are wound around the center leg but inside the outer arms.
    If you wind around the whole transformer, core and all, you have no influence on the "normal* coils.
    In fact, if you wrap a copper foil around the center leg, you are shorting it, the transformer becomes useless; but if you wrap that same copper foil around the whole core (and the internal winding, of course) you not only do not short it, but you also lower interference to the outside world.
    Good luck.

  19. #124
    Noodle of Reality Steve Conner's Avatar
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    I agree with JM. Back in the days of CRT TV sets, you got magnetically shielded speakers for use near them. What they had was not an actual shield, but another magnet stuck to the back of the pole piece, creating an equal and opposite field. And yet these speakers still worked.
    "Enzo, I see that you replied parasitic oscillations. Is that a hypothesis? Or is that your amazing metal band I should check out?"

  20. #125
    Old Timer J M Fahey's Avatar
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    Yes. They were labelled "shielded" (an easier idea to grasp for the general public); in fact they were "neutralized"
    The problem with any ring magnet, specially ceramics, is that an important part of the total flux "travels" outside, that's what you see when the speaker grabs a screwdriver or erases a tape or diskette.
    In a CRT TV it wreaks havoc with the image. (My own TV set by my workbench often has disgusting green or magenta splotches because I forget and carelessly walk too close with a speaker in my hand).
    The external, inversely magnetized magnet which is glued to the external face of the backplate is smaller and just strong enough to cancel the *external* field; it has no influence on the internal one.
    Besides that, there is one special case where an external magnet helps the internal one: in some car audio woofers, an extra magnet is glued outside but with same polarity as the main one; its field forms what could be called a "belt" around the system and forces that "loss" external field back inside, or, more precisely, doesn't let it get out.
    Even more important, the Car speaker looks even more impressive, "justifying" the "Bombastic/Nuclear/Assassin/Bonecrusher/etc." label they show.
    The most efficient systems are those which use the magnet inside a (truly) shielding pot, as most Alnicos and all fieldcoils (including Fluxtones)
    Never had a Neodymium one in my hands, but I think they also use a magnetic slug inside a pot.
    Ceramics are not powerful enough to be used there.

  21. #126
    Member balijukka's Avatar
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    Crap, there goes that idea
    Thank you for a very detailed explanation.
    I should have remembered the rule, If it would work, someone would have done it already.

    jukka

  22. #127
    Old Timer J M Fahey's Avatar
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    I should have remembered the rule, If it would work, someone would have done it already.
    Don't be so sure about that, thanks God new things are being invented every day, be them groundbreaking or small advances in the art, all them useful.
    Most of them are obvious .... *after* they are mentioned.
    See Fluxtones: field coil speakers are known from the 30's on.
    They were practically abandoned in the 50's.
    They were reborn in the 2000's but with a twist: not to provide maximum efficiency but to get exactly the opposite, in a controlled way.
    Go figure !!
    PS: are you actually in Bali ? Tell us something about that. Music, electronics, our stuff.

  23. #128
    Member balijukka's Avatar
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    Quote Originally Posted by J M Fahey View Post
    PS: are you actually in Bali ?
    I'm working on a project in Malaysia at the moment. Family is in Bali.
    And I'm going home for a 2 weeks holiday next saturday.http://music-electronics-forum.com/i...n_bigsmile.gif

    jukka

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    Revivng an old thread on the Fluxtone speaker concept, all in the name of science. I've been experimenting with three field coil speakers. Two of them are Jensen F12N with their original cones in good condition from the 1950s. The other speaker is a 12" Rola, and all the speakers were used in Hammond organs. The field coils measure 680-700 ohms DC at room temperature. I reconed the Rola using Weber parts - 1.75" voice coil - 100 Watt - 8 ohm, flat spider, Grey Wolf cone and large H-screen dustcap. The recone required some minor trimming on the cone to make it fit. I measured the VC gap as around .040-.045". I'm using a 100 volt DC variable power supply for the field. I find that all three speakers sound good with a guitar amp when I have the field coil voltage high. However, the range of volume attenuation is not as large as I would like - certainly not the 25dB achieved by Fluxtone. My DC voltage ranges from 7.5-100V on the field. The bigger issue is that the speakers do have a response shift as the voltage is reduced. I find that there is a lot more buzzy distortion on the low notes, and a lot of compression everywhere. So I'm very curious as to why I have these artifacts and I don't hear this with the Fluxtone videos. My Rola reconed speaker really sounds excellent with the high field voltage, and if I could retain that quality at lower SPL I would be in heaven. Any advice from Mr. Coil or JM Fahey and others is greatly appreciated.

  25. #130
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    I don't know the first thing about speakers, I listened to video demos and both red and blue ones sounded weaker when cranked down. It's like they lost something when cranked down. I just feel I can do better even with my THD Hotplate. But of cause, I have to listen to it in front of me to really tell. Together with the high price, I eliminated them when I was in market of speakers a few months ago. I ended up with two WGS speakers and never look back. The main reason I went to WGS on the first place was because they are the only one that offer refund. I even contacted Eminence, they absolutely do not give refund.

    I like to hear what are your impression from using the speaker.

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