Field Coils for Audio Systems

[Audiophile Bill]

IME there are no wideband drivers that work well above 12-15 kHz. You get output, but compared to a high quality tweeter it's uneven and has artifacts from breakup and the limits of the mechanical crossover... i.e. transition to the whizzer or small diameter VC. This includes all of the AER, Voxativ and Feastrex drivers. It's like expecting a full range driver to do the same thing as a 15" woofer. No way that's ever going to happen!

However, many can't hear clearly in that frequency range anyways... in my testing there are only a minority of folks who are very sensitive to this. So, despite the fact the HF quality isn't great you can still get output and get that sparkle and air, and some may prefer that to a tweeter for various reasons.

My midrange driver has no whizzer but it does have a smaller diameter VC that allows for 20 kHz output, however I prefer to roll it off and use a tweeter. I think it's possible to use a 1st or 2nd order xo with good results, but in my speaker that'll be optional and I have decided to allow the user to adjust the system. You'll be able to run the mid full range at the top end or use the xo, as well as having the option to not use the tweeter at all.

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As far as FCs, bonzo got it right imo, it just depends on implementation... like everything else.

Also, you may be able to get a FC to not sag as much but if people PREFER SAG then who cares? People often prefer FC power supplies that use unregulated tube outputs just so they DO sag. People roll rectifier tubes in amps so they get the sag characteristics they prefer. Certain cheap power cables touted on here are what they are because they sag... they are basically a resistive power conditioner. Resistors can be used for AC power conditioning just like you use in a PS after rectification.

IMO the only plus to a FC power supply is it's adjustability, it gives the user something else to tweak to their liking, or to adapt TS parameters to various enclosures. The proof of this is the existence of excellent drivers using ALL types of motors. Comparisons are usually not valid because they are seldom both optimized, like trying to compare a tube vs SS amp on a speaker with a high phase angle. Tells you absolutely nothing but gives people things to speculate on, and most likely derive incorrect conclusions from.

Morning Dave / all,

Please could you try to put some thoughts / words together in general terms how a field coil driver sounds with a regulated (non sag) supply vs one that does indeed sag. Without having done these comparisons yet myself, I am curious to know how it manifests in the music.

I assume at this point (based on physics) that the non sag supply will yield a driver that sounds exceptionally fast and inner nuanced - more stat like in its presentation. Whereas one with lots of sag will sound more euphonic and cuddly?

 

[Carlos269]

Morning Dave / all,

Please could you try to put some thoughts / words together in general terms how a field coil driver sounds with a regailated (non sag) supply vs one that does indeed sag. Without having done these comparisons yet myself, I am curious to know how it manifests in the music.

I assume at this point (based on physics) that the non sag supply will yield a driver that sounds exceptionally fast and inner nuanced - more stat like in its presentation. Whereas one with lots of sag will sound more euphonic and cuddly?

Bill,
I think that you can only make that characterization if you drive the field coil with a “stiff” or well regulated power-supply and are comparing field coil motors to permanent magnet assemblies of equivalent field strength. Driving the field coil with a Tungar type supply will yield similar results to an equivalent field strength permanent magnet and even perhaps have more magnetic field flux sag than an equivalent Alnico permanent magnet.

Furthermore, the field coil’s magnetic field strength provides the magnetic flux for the electromotive force for the moving coil’s pistonic movement so adjustments in voltage are adjustments in field-strength so it is difficult to compare to the static magnetic field of the permanent magnet unless instrumented measurements are carefully implemented.

Ralph and others have years of experience in the use of field coils and how their sound compare to permanent magnets but I think that one should be very careful with drawing sweeping conclusions unless careful approach is undertaken and detailed test set-ups are provided, which would need to show an Apples-to Apples comparison.

Your upcoming exploration is an interesting one that will yield some useful information as to the same drivers sound with a field coil motor compared to their original permanent magnet but in order for the findings to be meaningful you must set up the field coil to match the the magnetic field strength of their original permanent magnet assemblies, otherwise you will be comparing two different drivers/speakers; which is precisely the benefit of adjustable field coils, the ability to adjust the magnetic field flux and therefore the related Thiele small parameters.

Take for instance the Supravox 215-2000 EXC driver, which field coil can be powered from 6Vdc to 13Vdc, it provides a number of different drivers in one, suitable for Open Baffle application when powered at the 6 Vdc setting because it raises its Qts to 0.7 but at the expense of efficiency as it lowers the drivers efficiency from a maximum of 102dB down to 93dB; if you want to raise the field coil voltage to 13 Vdc to take advantage of the 102dB efficiency you can no longer use it in an Open Baffle application as the Qts drops.

Many parameters at play.

 

[Audiophile Bill]

Bill,
I think that you can only make that characterization if you drive the field coil with a “stiff” or well regulated power-supply and are comparing field coil motors to permanent magnet assemblies of equivalent field strength. Driving the field coil with a Tungar type supply will yield similar results to an equivalent field strength permanent magnet and even perhaps have more magnetic field flux sag than an equivalent Alnico permanent magnet.

Furthermore, the field coil’s magnetic field strength provides the magnetic flux for the electromotive force for the moving coil’s pistonic movement so adjustments in voltage are adjustments in field-strength so it is difficult to compare to the static magnetic field of the permanent magnet unless instrumented measurements are carefully implemented.

Ralph and others have years of experience in the use of field coils and how their sound compare to permanent magnets but I think that one should be very careful with drawing sweeping conclusions unless careful approach is undertaken and detailed test set-ups are provided, which would need to show an Apples-to Apples comparison.

Your upcoming exploration is an interesting one that will yield some useful information as to the same drivers sound with a field coil motor compared to their original permanent magnet but in order for the findings to be meaningful you must set up the field coil to match the the magnetic field strength of their original permanent magnet assemblies, otherwise you will be comparing two different drivers/speakers; which is precisely the benefit of adjustable field coils, the ability to adjust the magnetic field flux and therefore the related Thiele small parameters.

Take for instance the Supravox 215-2000 EXC driver, which field coil can be powered from 6Vdc to 13Vdc, it provides a number of different drivers in one, suitable for Open Baffle application when powered at the 6 Vdc setting because it raises its Qts to 0.7 but at the expense of efficiency as it lowers the drivers efficiency from a maximum of 102dB down to 93dB; if you want to raise the field coil voltage to 13 Vdc to take advantage of the 102dB efficiency you can no longer use it in an Open Baffle application as the Qts drops.

Many parameters at play.

Yes totally agree on your point about comparing similar flux between perm and field coil.

I am interested at this point in subjective experiences of regulated versus not regulated power supplies to Sam’s field coil driver at the same voltage if possible?

 

[Carlos269]

This sounds a bit Off topic to me since this sounds like a question about why we're working with class D.

But its an easy answer. First, we can't get distortion that low just for the record, but our class D Beta units are lower distortion than our OTLs. They do use feedback; the trick there is that in most amps that use feedback the simple fact is they don't use enough. This results in the feedback adding distortion of its own in the form of higher ordered harmonics which are highly audible- contributing to harshness and brightness. We've been hearing this in solid state amps for the last 60 years.

If you run enough feedback you can get around that problem! But most amps lack the phase margin and the gain bandwidth product to allow for that much feedback. But in a class D amp you exceed the phase margin on purpose- which causes it to oscillate- and the oscillation is used as the switching frequency. In this way you can have enough feedback to do the job properly; the feedback will clean up the mess it creates and even correct for phase shift.

I agree about your comment about distortion; in any amplifier that sounds organic and musical, it will be found that the lower ordered harmonics (in particular the 2nd and 3rd) will be the primary distortion product. In fact there will be enough of it that it masks the higher ordered harmonic content (which is what happens in a good tube amp). It works out in our class D amp that the non-linearities that exist in the encoding scheme and dead time circuit result in lower ordered harmonic generation. So it sounds very much like our tube amps- smooth, detailed, relaxed- no brightness or harshness.

So its not a circular argument- its simply applying engineering with an understanding of what distortions the ear does not care about and those that it does.

Ralph,
You can attribute that % THD figure not to Halcro, who provided impressive data but never made the claim of distortion levels that low, but to the late Roger Paul of H-CAT infamy. Just wanted to set the record straight; if only Roger Paul’s dubious claims could have actually been backed up by scientific data, he would have been an extraterrestrial.

 

[DaveC]

Isn't most audio manufacturing like this? Pretty basic mechanical assembly etc.

Not trying to excuse 5 digit drivers as obviously there is some huge margin...

The idea of making my own driver as a diy project is pretty appealing. Just buy off the shelf baskets, model the field coil magnet parts, get them made and wind it up, pick a spider and surround from someones catalog or get really crafty and make them. Wind up a voice coil on a former material you like and make, pulp some cones over a cone shape you make,

Err still sounds fun but gonna have to think a little harder before diving in

IME everything is like this. I've designed assembly processes for large scale wind turbine nacelles and hubs before I started ZenWave Audio. Manufacturing and assembly is similar for a wide variety of products.

Modern, low-cost measurement tools have made audio possible for anyone at reasonable/hobby-level costs, including making their own FC drivers.

 

[DaveC]

Morning Dave / all,

Please could you try to put some thoughts / words together in general terms how a field coil driver sounds with a regulated (non sag) supply vs one that does indeed sag. Without having done these comparisons yet myself, I am curious to know how it manifests in the music.

I assume at this point (based on physics) that the non sag supply will yield a driver that sounds exceptionally fast and inner nuanced - more stat like in its presentation. Whereas one with lots of sag will sound more euphonic and cuddly?

I've heard some Feastrex drivers in both PM and FC motors but can't really give an apples to apples comparison. I'll leave that to Ralph.

 

[Atmasphere]

Ralph, the point that I was making and the takeaway is that it doesn’t really matter where on the playback chain the non-linearities occur, whether in the DAC’s DSP design or reconstruction filter, in the amplification stage’s unregulated power-supplies, or in the speaker’s driver motor with its magnetic field flux sag; what matters is the resultant sound; and most prefer the organic nature of the SET DHT’s lower ordered harmonics (in particular the 2nd and 3rd), magnetic tape low frequencies non-linearities or transformer saturation and find that it’s what sounds most like live music.

I'm not aware that sag in the magnetic field results in lower ordered harmonics! I would expect greater IMD and slower risetimes.

While the lower orders are useful for masking the higher orders, if you can reduce the overall distortion signature, the result is greater focus, greater resolution, but otherwise the same organic (musical) presentation. Encouraging distortion, even if its lower orders, IME while euphonic, results in less resolution- it might be fun but it does not bring you closer to the musical event. In high end audio, the assignment is to get as close to the musical event as the recordings will allow. One has to be pragmatic that distortion can't be eliminated. Reducing it, as long as the resulting distortion signature is benign, is paramount to getting things to sound real.

The class D project for us was really affirming of this idea; it measures considerably lower in overall distortion compared to our OTLs but otherwise has a very similar distortion signature. If compared side by side on a speaker that both amps are happy with its really obvious from the neutrality of the class D that with our OTLs we were on the right course all along; they sound so similar they are hard to tell apart (what you hear as different is the bass presentation, since the class D acts as a near perfect voltage source, and its also a bit more focused). Its nice to have a 'solid state' amp that sounds as smooth, liquid and detailed as a good tube amp and that both are simply sounding true to the recording.

Please could you try to put some thoughts / words together in general terms how a field coil driver sounds with a regulated (non sag) supply vs one that does indeed sag. Without having done these comparisons yet myself, I am curious to know how it manifests in the music.

I assume at this point (based on physics) that the non sag supply will yield a driver that sounds exceptionally fast and inner nuanced - more stat like in its presentation. Whereas one with lots of sag will sound more euphonic and cuddly?

In a nutshell if the power supply is regulated but otherwise the same voltage and current (so driver parameters are unchanged for the most part...) then you hear a greater sense of speed and realism from the regulated supply, where an unregulated supply is not that different from Alnico in its sound- IMO making one wonder why go through the additional expense??

Field coils cost a lot more $$$ to produce according to every manufacturer I've talked to that works with them. The industry moved away from field coils in the 1950s for precisely this reason!

You can attribute that % THD figure not to Halcro, who provided impressive data but never made the claim of distortion levels that low, but to the late Roger Paul of H-CAT infamy. Just wanted to set the record straight; if only Roger Paul’s dubious claims could have actually been backed up by scientific data, he would have been an extraterrestrial.

Agreed.

I confronted him once on audiogon about his outlandish claims; he had a 'circuit' that could correct for 'Doppler Effect in amplifiers' and worked at a 'microscopic' level... Assuming it worked, somehow it produced a correction output but he was unable to produce a metric to show how it was affecting distortion or bandwidth or anything?? Seriously? I asked him why he didn't use the circuit itself as the measurement tool to know when it was 'doing its thing' and he didn't respond. Occam's Razor has a way of sorting out things like this pretty quickly- there's a very good reason its the oldest principle in science

 

[Audiophile Bill]

I'm not aware that sag in the magnetic field results in lower ordered harmonics! I would expect greater IMD and slower risetimes.

While the lower orders are useful for masking the higher orders, if you can reduce the overall distortion signature, the result is greater focus, greater resolution, but otherwise the same organic (musical) presentation. Encouraging distortion, even if its lower orders, IME while euphonic, results in less resolution- it might be fun but it does not bring you closer to the musical event. In high end audio, the assignment is to get as close to the musical event as the recordings will allow. One has to be pragmatic that distortion can't be eliminated. Reducing it, as long as the resulting distortion signature is benign, is paramount to getting things to sound real.

The class D project for us was really affirming of this idea; it measures considerably lower in overall distortion compared to our OTLs but otherwise has a very similar distortion signature. If compared side by side on a speaker that both amps are happy with its really obvious from the neutrality of the class D that with our OTLs we were on the right course all along; they sound so similar they are hard to tell apart (what you hear as different is the bass presentation, since the class D acts as a near perfect voltage source, and its also a bit more focused). Its nice to have a 'solid state' amp that sounds as smooth, liquid and detailed as a good tube amp and that both are simply sounding true to the recording.

In a nutshell if the power supply is regulated but otherwise the same voltage and current (so driver parameters are unchanged for the most part...) then you hear a greater sense of speed and realism from the regulated supply, where an unregulated supply is not that different from Alnico in its sound- IMO making one wonder why go through the additional expense??

Field coils cost a lot more $$$ to produce according to every manufacturer I've talked to that works with them. The industry moved away from field coils in the 1950s for precisely this reason!

Agreed.

I confronted him once on audiogon about his outlandish claims; he had a 'circuit' that could correct for 'Doppler Effect in amplifiers' and worked at a 'microscopic' level... Assuming it worked, somehow it produced a correction output but he was unable to produce a metric to show how it was affecting distortion or bandwidth or anything?? Seriously? I asked him why he didn't use the circuit itself as the measurement tool to know when it was 'doing its thing' and he didn't respond. Occam's Razor has a way of sorting out things like this pretty quickly- there's a very good reason its the oldest principle in science

Thank you Ralph. Can I then ask whether regulation should be achieved with diodes, zener diodes, of tube regulators, or you don’t think it matters? From my basic reading, regulation like anything else comes in gradations of precision so how precisely should we endeavour to strive for?
Can you recommend any off the shelf adjustable regulated power supplies for 9-17volts up to 5 amps?

 

[Atmasphere]

Thank you Ralph. Can I then ask whether regulation should be achieved with diodes, zener diodes, of tube regulators, or you don’t think it matters? From my basic reading, regulation like anything else comes in gradations of precision so how precisely should we endeavour to strive for?
Can you recommend any off the shelf adjustable regulated power supplies for 9-17volts up to 5 amps?

With any regulator you want it to have wide frequency response (IOW output impedance is linear across the audio band). That's pretty hard to do with tubes at the voltages and currents field coils use. You can use Tungar rectifiers, but as soon as you regulate it really makes no difference what rectifier you used unless that rectifier is making noise. So I think I can recommend almost any off the shelf adjustable regulated power supply for 9-17volts up to 5 amps

There's a nice used Lambda on ebay right now that should suit nicely. But you'll need two for stereo.
Lambda supply
Kepco is another good brand. I think if you dig in you'll find 100s of units that will do the job.

 

[Audiophile Bill]

With any regulator you want it to have wide frequency response (IOW output impedance is linear across the audio band). That's pretty hard to do with tubes at the voltages and currents field coils use. You can use Tungar rectifiers, but as soon as you regulate it really makes no difference what rectifier you used unless that rectifier is making noise. So I think I can recommend almost any off the shelf adjustable regulated power supply for 9-17volts up to 5 amps

There's a nice used Lambda on ebay right now that should suit nicely. But you'll need two for stereo.
Lambda supply
Kepco is another good brand. I think if you dig in you'll find 100s of units that will do the job.

Thanks Ralph.

 

[dcathro]

Thanks Ralph.

Hey Bill have you received your motors yet?

How long do you think before you will be trying them out?

Cheers

David

 

[Audiophile Bill]

Hey Bill have you received your motors yet?

How long do you think before you will be trying them out?

Cheers

David

The postal system from Russia is very slow so probably will take a while still unfortunately. As soon as I have them I will be sure to keep folks posted.

 

[morricab]

I'm not aware that sag in the magnetic field results in lower ordered harmonics! I would expect greater IMD and slower risetimes.

While the lower orders are useful for masking the higher orders, if you can reduce the overall distortion signature, the result is greater focus, greater resolution, but otherwise the same organic (musical) presentation. Encouraging distortion, even if its lower orders, IME while euphonic, results in less resolution- it might be fun but it does not bring you closer to the musical event. In high end audio, the assignment is to get as close to the musical event as the recordings will allow. One has to be pragmatic that distortion can't be eliminated. Reducing it, as long as the resulting distortion signature is benign, is paramount to getting things to sound real.

The class D project for us was really affirming of this idea; it measures considerably lower in overall distortion compared to our OTLs but otherwise has a very similar distortion signature. If compared side by side on a speaker that both amps are happy with its really obvious from the neutrality of the class D that with our OTLs we were on the right course all along; they sound so similar they are hard to tell apart (what you hear as different is the bass presentation, since the class D acts as a near perfect voltage source, and its also a bit more focused). Its nice to have a 'solid state' amp that sounds as smooth, liquid and detailed as a good tube amp and that both are simply sounding true to the recording.

In a nutshell if the power supply is regulated but otherwise the same voltage and current (so driver parameters are unchanged for the most part...) then you hear a greater sense of speed and realism from the regulated supply, where an unregulated supply is not that different from Alnico in its sound- IMO making one wonder why go through the additional expense??

Field coils cost a lot more $$$ to produce according to every manufacturer I've talked to that works with them. The industry moved away from field coils in the 1950s for precisely this reason!

Agreed.

I confronted him once on audiogon about his outlandish claims; he had a 'circuit' that could correct for 'Doppler Effect in amplifiers' and worked at a 'microscopic' level... Assuming it worked, somehow it produced a correction output but he was unable to produce a metric to show how it was affecting distortion or bandwidth or anything?? Seriously? I asked him why he didn't use the circuit itself as the measurement tool to know when it was 'doing its thing' and he didn't respond. Occam's Razor has a way of sorting out things like this pretty quickly- there's a very good reason its the oldest principle in science

Strange because Supravox charges the same for Alnico and field coil...

 

[Audiophile Bill]

Was chatting to designer of the DaVa cartridge field coil power supplies. Obviously we are talking a cartridge now so possibly difference between this and speakers but the standard regulated ss supply with selenium recti that I was using was easily beaten by the EY500 dual recti big choke non regulated supply. Not even in same ball park.

 

[dcathro]

Was chatting to designer of the DaVa cartridge field coil power supplies. Obviously we are talking a cartridge now so possibly difference between this and speakers but the standard regulated ss supply with selenium recti that I was using was easily beaten by the EY500 dual recti big choke non regulated supply. Not even in same ball park.

My own experience is that the power supply matters very much and that you can hear things just like you can with the signal amplifier. I have not yet played with a non regulated Tungar, but I plan to build one and see.

 

[Carlos269]

I'm not aware that sag in the magnetic field results in lower ordered harmonics! I would expect greater IMD and slower risetimes.

I did not write, implied or believe that magnetic field flux sag in speaker driver magnetic assemblies results in lower ordered harmonics. I wrote “nonlinearities” and that includes “greater IMD and slower risetimes”.

Strange because Supravox charges the same for Alnico and field coil...

Atelier-Rullit also charges more for certain Alnico versions of their higher end drivers than the field-coil version.

My own experience is that the power supply matters very much and that you can hear things just like you can with the signal amplifier. I have not yet played with a non regulated Tungar, but I plan to build one and see.

I have procured a pair of Supravox 215-2000 EXC 8” field-coil drivers that I will be using in Open Baffles and will be powering their field-coils with a pair of regulated laboratory reference Hewlett-Packard adjustable DC power-supplies. The drivers’ moving coils will be powered by tube amplifiers; so will get my nonlinearities through moving-coil and not through the field-coil or through both.

 

[Solypsa]

Atelier-Rullit also charges more for certain Alnico versions of their higher end drivers than the field-coil version.

.

In the case of Rullit, Supravox etc do the 'less costly' FC drivers [vs AlNiCo] include PS costs? Just curious...

 

[Carlos269]

In the case of Rullit, Supravox etc do the 'less cosltly' FC drivers [vs AlNiCo] include PS costs? Jusr curious...

It’s hard to factor in the field-coil’s power-supplies costs as they can range from a few hundred dollars for Chinese off-the-shelf units to several thousand dollars for bespoke tube power-supplies.

Also keep in mind that most driver’s field coils are powered by around 12Vdc power-supplies, which are readily available off-the-shelf, while Atelier-Rullit field-coil drivers tend to run off of higher voltages, some as high as 250V, which makes them good candidates for custom tube power-supply applications.

 

[dcathro]

I have procured a pair of Supravox 215-2000 EXC 8” field-coil drivers that I will be using in Open Baffles and will be powering their field-coils with a pair of regulated laboratory reference Hewlett-Packard adjustable DC power-supplies. The drivers’ moving coils will be powered by tube amplifiers; so will get my nonlinearities through moving-coil and not through the field-coil or through both.

Very nice Carlos!

Will you be running them stand alone, or accompanying them with other drivers?

They are fantastic sounding drivers and good value for money (compared to other exotic drivers).

 

[dcathro]

it’s hard to factor in the field-coil’s power-supplies costs as they can range from a few hundred dollars for Chinese off-the-shelf units to several thousand dollars for bespoke tube power-supplies.

Also keep in mind that most driver’s field coils are powered by around 12Vdc power-supplies, which are readily available off-the-shelf, while Atelier-Rullit field-coil drivers tend to run off of higher voltages, some as high as 250V, which makes them good candidates for custom tube power-supply applications.

Some people also swear by running them off large batteries.