Field Coil Speakers?

[The Ballzz]

I did a search here and only found brief discussions and some references to FluxTone http://fluxtonespeakers.com and I am truly intrigued by the concept! Unfortunately the FluxTone products are outrageously priced. It really doesn't seem that kind of price is truly warranted except for possible quick recovery of R & D costs and because of the product line's uniqueness. $1200+ for a speaker is simply out of the question, but if these speakers really do what they say they will, there must be a simpler, more cost effective way to skin this cat!

From my research, it appears that many of the Rola and/or Celestion 12" variants of field coil speakers (from older Hammond/Leslie and other organ products) will accept the same cone kits as more modern G12 permanent magnet units. This leads me to wonder how difficult it would be to simply build a variable power supply for one of these speakers?

I would love to get my hands on one or a couple of these speakers for experimentation.

Any Thoughts/Comments/Suggestions?
Gene

[Cantplay]

It also looks like most alnico speakers could be fairly easily converted to field coil by replacing the magnet with a bobbin of wire.

Gauge would be the experiment.

John

[JazzGuitarGimp]

Yup, then you'd want to apply a variable-frequency, variable-duty-cycle square* wave. Frequency either wants to be above or below the audio spectrum. The field coil would be more effecient at higher frequencies, so I'd start at about 25KHz going up from there.

* Other wave shapes should be tried as well.

[Cantplay]

I thought field coil supply had to be DC?

John

[The Ballzz]

Well Guys,
From my somewhat limited understanding of how they work, it seems they are indeed/usually DC powered. From my research, they typically get powered from either one of the "nodes" of the B+ or on a separate node of their own. My thinking is that it should not be necessary to rewind or replace a field coil, but simply design a variable DC power supply to control/modify it's efficiency.

If I'm right, it could be the greatest thing since sliced bread, for me. If I'm wrong, I'd love to find out ASAP before devoting too much time, thought, energy and $$$ into the idea. I hate even the best speaker level attenuators, power scaling has it's own set of issues and post phase inverter master volumes are, well... they are what they are!

I'm kind of assuming that the power supply needs to keep a particular balance between impedance and voltage as the voltage changes.

I have spoken at length with the owner of FluxTone and he claims they have a large expense of having new, proprietary field coils made/machined for them, but after comparing the appearances of their products to the original versions, I suspect that it is not much more complicated than rewinding the coils to work with a standardized voltage range across their product line! I assume they are simply buying up every pertinent field coil speaker they can find, making some minor mods and providing a power supply for a "Grand Slam" profit on each unit.

I'd love to get my hands on two similar 12" speakers ( one functioning properly and the other not) to experiment with the functional unit and dismantle the other for research and understanding.

I'm not looking to start a business, but would love to incorporate this concept/technology into one of my Marshall 4x12's, a 4x10 and a couple of combos, for my own use. I certainly can't afford $5,000+ just to outfit one 4x12 box!
Just My $.02,
Gene

[JazzGuitarGimp]

In my post, I am referring to pulse width modulation (varying the duty cycle from 0% to 100%). This is how DC motor speed is controlled without sacrficing torque. And I posit, would be the best way to vary field coil flux density.

[Aurora]

Pulse width control will not do for field coils,- at least the frequency will have to be in the 50-100kHz+ region,- well above audible region.
There's really no magic about them - just a field coil instead of the magnet. The plus side is high efficiency, which made them popular before powerful magnets came about. A simpel variable voltage, - or better and more complicated - a current regulator, - will do . The coil s has a DC reistance, so that's really all there is to it...
Modern field coil speakers are indeed quite expensive, - there is only a limited production, as well as demand. They are fairly popular in the HiFi crowd, but rather price prohibitive, and the you need the extra power supply as well.

EDIT: In former days, the speaker coil was often used as a choke for the B+ line - saved an extra choke...

[rp]

I looked through the Fluxtone link and it's a clever modern use of an old technology, hats off to him for trying. If it sounds good and works well then it's very clever. I agree there R&D there, he offers a lot of modern speaker variations (A field coil V30 is something I'd never thought I'd see). He had to pay for all that inventory, looks well made too, but the price is extreme. Unless they sound jaw dropping fantastic it's going to be a tough sell.

If you just want to tinker with this why not call someone like Weber and ask them, I bet they get a few calls a week about field coils. Bet they can adapt something as cantplay says or re-cone something easy enough. They might even have coils otherwise you can have Heyboer make you the coil and they are totally reasonable w/ one offs. How much could it all be, $150-$200?

Personally I would sound out Weber as to what they can/can't do, what they prefer to work with for recones, and patiently start looking on ebay for something cheap, blown and/or non-vintage restoration sought after, and send it to them for a rebuild. Then if you can workout the VMT thing you might have something.

Wonder if as you vary the voltage and flux if the tone changes and there's also some sort of (active?) crossover network involved?

[The Ballzz]

I looked through the Fluxtone link and it's a clever modern use of an old technology, hats off to him for trying. If it sounds good and works well then it's very clever. I agree there R&D there, he offers a lot of modern speaker variations (A field coil V30 is something I'd never thought I'd see). He had to pay for all that inventory, looks well made too, but the price is extreme. Unless they sound jaw dropping fantastic it's going to be a tough sell.

If you just want to tinker with this why not call someone like Weber and ask them, I bet they get a few calls a week about field coils. Bet they can adapt something as cantplay says or re-cone something easy enough. They might even have coils otherwise you can have Heyboer make you the coil and they are totally reasonable w/ one offs. How much could it all be, $150-$200?

Personally I would sound out Weber as to what they can/can't do, what they prefer to work with for recones, and patiently start looking on ebay for something cheap, blown and/or non-vintage restoration sought after, and send it to them for a rebuild. Then if you can workout the VMT thing you might have something.

Wonder if as you vary the voltage and flux if the tone changes and there's also some sort of (active?) crossover network involved?

Your thoughts and take on this are quite similar to mine! Ebay seems to be nearly devoid of the Rola/Celestion units, but I have a couple friends who restore and rent Hammond organs and they are keeping their eyes open for me. The Rola's would be the ticket for the 4x12 and 4x10, but Jensens may be more appropriate for the couple/few Fender-ish combos that I own and have slated for building.

One of my biggest concerns is to make sure I don't infringe on FluxTone's patents/ideas. Not just from a legal standpoint, but also from a moral perspective. I have no desire to steal/cheat anyone out of their hard work and effort!

I'm thinking that R & D on my part will likely put the cost of one unit, at or even above FluxTone's prices but that subsequent units could come in at the $200-$500 price range for each, when all is said and done.

It does appear that in years gone by, many of these speakers have simply been tossed out with the trash!

Thanks To All For The Great & Thoughtful Replies/Comments,
Gene

[JMFahey]

1) it can be done;In fact I have, long ago.
I make my own (ceramic) Guitar speakers and some 25/30 years ago joined forces with a couple guys to make 30" speakers for DJ use.

No magnets large enough were available so we went the field coil route.

Worked like a charm, although financially were a flop, never ever recovered the initial capital

2) you don't need old Alnico speakers for that, in fact they are the worst choice.

Once you magnetize Alnico (or Ceramic) , that's it, it stays there, period, you don't need the field coil.

And they are not easy to modify, that's why they are called "hard" magnets.

3) what you need is to have the magnetic circuit : top and bottom plates, center polepiece and outer ring machined out of the softest iron you can find.

In theory unavailable "SAE 1000" meaning 0% carbon ... which nobody makes because it's very poor (mechanically).

Lowest available grade is SAE1008 and not that common either, I often have to machine my plates out of SAE1018

Once you have them, fit the proper coil inside, and apply proper DC.

You can use an adjustable SMPS for efficiency but don't worry about high frequency squarewaves hitting the field coil, because they won't, you use a filter cap (or even a more complex CLC filter to get rid of HF components.

4) FWIW old field coils used it as the supply choke, so they hit it with rectified but unfiltered DC, a very rough waveform ... and hum was coupled into the speaker , so voicecoils sometimes had an "anti hum winding", a couple turns where hum was injected out of phase to cancel the main one, go figure.

5) As of patent infringement, read Flutone's unique claims and don't infringe them, period.

Building a field coil speaker by itself is already an obsolete, common knowledge technology, and the fact that you use an SMPS to feed it, is also a generic use.

The fact that you select one specific voltage or another isn't by itself patentable matter.

[dorrisant]

I have a Hammond PA and 12" here that someone brought to me just a couple of days ago... I don't know what it is yet. Been contemplating the Fluxtone idea for a few years now.

I'm thinking a VVR controlling the coil voltage would work just fine.

Pulling up a chair for this discussion.

Tony

[Aurora]

The idea Works , no doubts about that, but I fail to see how there can be a patent on Technology that was developed more than 50 years ago. Reading through I see nothing but well known technology, spiced with the usual dose of marketing hype.
It is all very familiar from the HiFi business.
One must also expect the tone to change, as you turn down the mag current one way or another, as this changes the 'motor force', if you like....

[The Ballzz]

I have a Hammond PA and 12" here that someone brought to me just a couple of days ago... I don't know what it is yet. Been contemplating the Fluxtone idea for a few years now.

I'm thinking a VVR controlling the coil voltage would work just fine.

Pulling up a chair for this discussion.

Tony

Wow Tony,
That looks to be exactly what I'm looking for! If they actually all fire up, it would be fantastic to get some of the voltage readings, etc! If you would like to do a little collaboration on this endeavor, please feel free to PM me so that we could swap real email addresses and phone numbers. I'm fairly hot to go on this and have even joined a couple organ forums to ask some questions and "troll" for parts/components.
Thanx In Advance,
Gene

[JMFahey]

The idea Works , no doubts about that, but I fail to see how there can be a patent on Technology that was developed more than 50 years ago. Reading through I see nothing but well known technology, spiced with the usual dose of marketing hype.
It is all very familiar from the HiFi business.
One must also expect the tone to change, as you turn down the mag current one way or another, as this changes the 'motor force', if you like....

Yes, tone does change.

The actual "motor" driving your cone is B*L , L being the length of wire inside the gap and B the flux density.
The full "pushing force" comes from B*L*I , I being the current supplied by the amplifier.

In Fluxtone speakers the way to turn down sensitivity at will, without turning down I (so without turning amp volume down) and since L is constant,(you can't pull wire turns from the voice coil at will), leaves only B, the magnetic flix density to work with, so that's what they do: lower current through the field coil to turn down B , so weakening the driving motor.

You will lower sensitivity, no doubt about that, the extreme case being that with no magnetic field the speaker won't sound at all, period, but consider this: you are making the motor weaker and weaker, but load : cone weight, stiffness, air load, stay the same.

So sound is not only less loud (what is expected), it's also more sluggish and undefined.

You lose sharpness, attack, punch, along with volume.

Eminence does about the same with their variable flux speakers, although in that case I guess (haven't opened one yet) they have a gap in the magnetic circuit , probably a split polepiece, and adjust gap width by turning a screw.

It ends up being the same final effect.

One problem I see in either of them, besides sound losing attack (they acknowledge that by saying sound "gets warmer"), the voice coil still has to dissipate full amplifier driving power.

Personal opinion, combining lowered sensitivity and inviting the Musician to play louder (the idea behind these speakers) is not a good combination.

Personally, I much prefer lowering amp power with some of the high voltage attenuators, there's different names for basically the same idea.

Speaker sound and response stays the same, amp too, only "drawback" is that it overdrives easier.
Is that a drawback?

[dorrisant]

I spent several hours at a booth (suite), with the Fluxtone guys... I heard what their product could do. I have to say this... It works as they advertise. You could knock down SPLs while retaining the same basic tone and grind of the amp as it was set up. Was it perfect? No... Did it do a GREAT job? Yes! Enough to leave the impression that I had to try this when the opportunity presents itself. I have installed numerous master volumes in amps. None of them are perfect, some are better in a particular circuit. You can find one that works best.

The field coil speaker attenuation thing may not be perfect but it still remains one of the best solutions for lowering SPLs. I have the means to experiment, so why not?

Gene, I will PM you. I just gotta figure out what model of Hammond product that I have here.

Tony