Field Coils for Audio Systems

MarcelNL

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Mar 15, 2021
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Ideally would like to achieve 40Hz to 16 KHz with a full range driver, but realistically expect about 55Hz to 10 KHz, and there are reports that perhaps high-frequencies will start to roll off as early as 7 KHz and low frequencies at 70Hz. I have additional divers to augment the frequency response but obviously the goal is to hear what the single field-coil drivers can do and then to determine if I can live with what they do well, knowing their limitations.
Sounds doable, I'm now between 30Hz and approx 10KHz but dropping off from 8-ish Khz and te low end is supported by a Horn, with the tweeter kicking in from approx 8Khz I added a little inductor to tame the loose top end a bit.

My question was a bit imprecise, I meant to add, in this baffle...I'm using a 7ft by 7ft baffle with front loaded horn (like Klangfilm Bionor) and find that you need quite a bit of baffle to give the wave front something to push off from or the sound is real thin.
I started in a small panel, the Breitstrahlgruppe, with two woofers and the sound is nothing like when these panels are mounted in a baffle (ended up installing these in a wall, works very well)
 
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dcathro

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Sep 16, 2016
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I posted on my system build thread but posting here in case folks here don’t go there.
Got the monster FC motors today. Stunning build quality - looking forward to firing them up soon. Got a couple of variable LPSs to get me started.

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Hi Bill,

You have been very quiet lately. I hope this is because you are too busy listening to music through these to be bothered with posting.

Looking forward to your impressions.

Best Regards

David
 

Carlos269

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Mar 21, 2012
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What output tube will you use for the SET OTL? I have seen the 6c19pi be a usual choice for this rare amp type..

My custom Single Ended OTL monoblocks will use a single 13E1 (7972/CV-6045) strapped as a Triode as the output tube. Output power is expected to be between 1 and 2 Watts.

Sounds doable, I'm now between 30Hz and approx 10KHz but dropping off from 8-ish Khz and te low end is supported by a Horn, with the tweeter kicking in from approx 8Khz I added a little inductor to tame the loose top end a bit.

My question was a bit imprecise, I meant to add, in this baffle...I'm using a 7ft by 7ft baffle with front loaded horn (like Klangfilm Bionor) and find that you need quite a bit of baffle to give the wave front something to push off from or the sound is real thin.
I started in a small panel, the Breitstrahlgruppe, with two woofers and the sound is nothing like when these panels are mounted in a baffle (ended up installing these in a wall, works very well)

With a 7ft by 7ft baffle, I can see how you were able to get the low frequency response down to 30Hz. I unfortunately no longer have enough free floor space for large baffles like those and have actually mounted the field-coil Supravox drivers in Open Baffles smaller than the JE Labs style Open Baffles that I’m using my Atelier-Rullit Silver Lab 10 full-range drivers on. I will be happy, and surprised, if I get extension down into the 40’s.

And how is it going so far? Any particular challenges on your way?

We are still at the gathering parts stage, but we have a good go-by starting schematic and know of two working amplifiers built upon the same Single Ended OTL concept. My monoblocks will be different than those two as the builder has some proven S.E.T. D.H.T. circuits and curious ideas that we want to try on this build. Excited to get the build started really soon. Based on my research, the amplifiers will require adjustable feedback, which I will have an option to adjust and disable through stepped-switches, to determine the level of feedback required for each speaker driver used with them. The big limitation with the amplifier is that it will only drive speakers with a 6 Ohm DC resistance or higher, so speaker driver compatibility and pairing will be critical. Any other challenges that you foresee, that I should be aware of?
 
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Audiophile Bill

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Hi Bill,

You have been very quiet lately. I hope this is because you are too busy listening to music through these to be bothered with posting.

Looking forward to your impressions.

Best Regards

David

Hi David,

Thanks for thinking about me!

Just been away on vacation and also got some issues at home to deal with - as such I haven’t managed to advance my install yet.

Basically I need to fit the AER frame to the new motors. I then need to build an internal “shelf” inside the rear chamber so that the 17kg motor doesn’t shear against the frame.
 
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Joe Cohen

Industry Expert
Jun 10, 2012
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I think the best way to sort it out is to have the same driver with both Alnico and field coil and set the voltage on the field coil such that it’s t/S parameters are very close to matching the Alnico driver and then putting them into an appropriate design. I only know Supravox that has the exact same driver bOth ways. Perhaps Voxativ could do something custom or you mentioned Lowther had one (never seem that)

Funny enough though the AER drivers are using Nd, which is not regarded as sounding so good compared to Alnico and yet these drivers seem to regarded as the best full range drivers.
I have heard both the alnico 5” Feastrex drivers and various iterations of their 5” field coil drivers in the same or similar cabinets. The differences are profound both between the passive and “exciter” versions and between field coils with different magnetic materials in their construction.
 

Joe Cohen

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Jun 10, 2012
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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.
Would be interested to hear a comparison of solid state and tube power supplies on the same drivers.
 
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Joe Cohen

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To be fair they are a no holds barred and absolutely sumptuous build. Milling permendur is a pita as is solid billet Armco iron like that. Unfortunately he won’t build any more because it was too much work / hassle with complexity and machining.
Permendur is definitely the way to go.
 

dcathro

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Sep 16, 2016
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dcathro

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Sep 16, 2016
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Melbourne, Australia
Hi David,

Thanks for thinking about me!

Just been away on vacation and also got some issues at home to deal with - as such I haven’t managed to advance my install yet.

Basically I need to fit the AER frame to the new motors. I then need to build an internal “shelf” inside the rear chamber so that the 17kg motor doesn’t shear against the frame.

Glad to hear that all is well.

I tend to do things in phases. Sometimes I will dive headlong into things and get lots of things done, and other times I will take my sweet time before I have a crack.

My field coils were sitting around for 10 years before I got round to implementing them o_O.

The past few days I have rebuilt my CD player (recap, ultra fast diodes and copper heatsink), and modified all my FC power supplies.

I find it interesting that you can hear everything you do to the power supply through the driver.
 

Audiophile Bill

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Mar 23, 2015
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No experience with this. What advantage does Permendur have over other materials?

Flux density is the simple answer imho. Permendur has the greatest flux density of all available materials. As a consequence, it will allow the greatest magnetic flux if that is what you want. There are some super specialist materials that have higher flux I believe but they basically aren’t available and prohibitive in cost.
 

Joe Cohen

Industry Expert
Jun 10, 2012
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No experience with this. What advantage does Permendur have over other materials?
Permendur is the most magnetically permeable material for plates, pole pieces and yokes. It is also very expensive and difficult to machine.
 

infinitely baffled

VIP/Donor
Jul 2, 2015
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+1 re the Azzolina Gran Sfera, imho superior to the Pnoe.
Anyone have experience of the Maxonic field coil horn-loaded spkrs?

+1 re the Azzolina Gran Sfera, imho superior to the Pnoe.
Anyone have experience of the Maxonic field coil horn-loaded spkrs?
Could you tell us more about your impressions of the Gran Sfera?
I can't even find a price for them
 

dcathro

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Sep 16, 2016
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Melbourne, Australia
Here is a quote from this hifi critic interview with Peter Qvortrup on field coils:

“We’ve been talking about this for 20 years,” he admits. “Quite a lot of people make field coils. But you have to do all of the drive units – tweeter and bass driver. We’ve made prototypes now with six different types of iron and there is a vast difference in their behaviour and sound. There is more to it than just getting the right gap flux. Different power supplies sound different on the same driver. All the measurement geeks will tell you there’s no difference, but we know there is. And it’s not subtle, either!”So what does he see as the main advantages of using field-coil drivers?“The sound, for a start – the difference is extraordinary. With a permanent magnet, when the voice coil moves in the air gap, the magnetic field sort of moves away – it is called hysteresis. The stiffer the magnetic field, the smaller that movement is and also the shorter the recovery time. As I understand it, when the music plays and the voice coil moves, the more movement there is, the more the field bends and the longer it takes to get back. “What it sounds like, and of course you can’t measure this, is that all the small signals are not picked up because the field is busy doing other things. In a field coil, what appears to happen, and you can see this on the hysteresis curve, is that rather than the field moving, it just draws more power from the supply. The electromagnetic field is far stiffer than with the best Alnico magnets and as a result you suddenly hear things you haven’t heard before.”
 

Joe Cohen

Industry Expert
Jun 10, 2012
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Here is a quote from this hifi critic interview with Peter Qvortrup on field coils:

“We’ve been talking about this for 20 years,” he admits. “Quite a lot of people make field coils. But you have to do all of the drive units – tweeter and bass driver. We’ve made prototypes now with six different types of iron and there is a vast difference in their behaviour and sound. There is more to it than just getting the right gap flux. Different power supplies sound different on the same driver. All the measurement geeks will tell you there’s no difference, but we know there is. And it’s not subtle, either!”So what does he see as the main advantages of using field-coil drivers?“The sound, for a start – the difference is extraordinary. With a permanent magnet, when the voice coil moves in the air gap, the magnetic field sort of moves away – it is called hysteresis. The stiffer the magnetic field, the smaller that movement is and also the shorter the recovery time. As I understand it, when the music plays and the voice coil moves, the more movement there is, the more the field bends and the longer it takes to get back. “What it sounds like, and of course you can’t measure this, is that all the small signals are not picked up because the field is busy doing other things. In a field coil, what appears to happen, and you can see this on the hysteresis curve, is that rather than the field moving, it just draws more power from the supply. The electromagnetic field is far stiffer than with the best Alnico magnets and as a result you suddenly hear things you haven’t heard before.”
This description is in line with my experience hearing the difference between the Feastrex 5” Alnico drivers and the 5” field coil drivers.
 
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Audiophile Bill

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This description is in line with my experience hearing the difference between the Feastrex 5” Alnico drivers and the 5” field coil drivers.

Hi Joe,

Please could you provide some more insights into the differences you hear with the Feastrex alnico and field coils?

best.
 
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Joe Cohen

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Jun 10, 2012
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The field coil drivers deliver each sound in a more “wrap around“ kind of way. That is, each sound exists in space as a more fully defined 3 dimensional object. The spaces between instruments are also more clearly defined so you have a more comprehensively defined acoustic panorama. The color and timbre of individual instruments that are part of a massed presentation (as in a string section) are not subsumed by the group but shine forth giving a sense of presence and immediacy. Though you can get some of this from the passive versions, it is nowhere near as striking.
 

Atmasphere

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My custom Single Ended OTL monoblocks will use a single 13E1 (7972/CV-6045) strapped as a Triode as the output tube. Output power is expected to be between 1 and 2 Watts.
Totally as a FWIW dept.: When going OTL there's really no reason to also be single-ended- most of that has to do with the output transformer of the SET. Single-ended circuits have what is mathematically called a 'quadratic non-linearity' which results in a prominent 2nd harmonic. If you go fully balanced you get a 'cubic non-linearity' instead. The latter produces a prominent 3rd (which is treated by the ear the same as the 2nd- it adds to 'richness') but at a much lower level (but can still be high enough to mask the higher ordered harmonics); as the harmonic order is increased the harmonic amplitude falls off faster (resulting in smoother, more detailed presentation). Because the ear assigns tonality to harmonics, and because it favors that tonality over actual frequency response, you can get something sounding much more neutral (and less like electronics) when you go fully balanced. The problem is getting the output transformer to behave when you do this; hence my opening statement.

Of course with an OTL you avoid this problem.
 

Carlos269

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Mar 21, 2012
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Totally as a FWIW dept.: When going OTL there's really no reason to also be single-ended- most of that has to do with the output transformer of the SET. Single-ended circuits have what is mathematically called a 'quadratic non-linearity' which results in a prominent 2nd harmonic. If you go fully balanced you get a 'cubic non-linearity' instead. The latter produces a prominent 3rd (which is treated by the ear the same as the 2nd- it adds to 'richness') but at a much lower level (but can still be high enough to mask the higher ordered harmonics); as the harmonic order is increased the harmonic amplitude falls off faster (resulting in smoother, more detailed presentation). Because the ear assigns tonality to harmonics, and because it favors that tonality over actual frequency response, you can get something sounding much more neutral (and less like electronics) when you go fully balanced. The problem is getting the output transformer to behave when you do this; hence my opening statement.

Of course with an OTL you avoid this problem.

Ralph, thank you for sharing this good to know information. If you were to only use a single 6AS7G output tube in your OTL design to get 6 Watts of fully balanced power, what would be the minimum impedance speaker-driver that it could drive? And how would that compare to using a single 6B4 output tube to get 2 watts of single ended power?
 

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