Field-coil drivers, pros and cons. Relics of the past, or still relevant?

[fpitas]

Adjustable field configuration, not only strength.

Compression drivers have relatively low excursion, so it's not a big problem. But still TD-4003 have 4" voice coil and weights almost 10 kg.

Just think how big, heavy and hot it would be with a field coil.

 

[fpitas]

The switch from AlNiCo to ceramic for magnets was driven by the Cold War and the use of cobalt in other technologies (not so hifi-friendly ones) and supply constraints that also had geopolitical underpinnings.

The ability to make more or less arbitrarily powerful electromagnets is at least possibly a "plus".
Your high field FFT NMR instruments, e.g., use superconducting electromagnets. Could audiophile drivers be too far behind?


From ASR's perspective, field coil (electrodynamic) drivers probably make no sense.
Not everyone shares the canonical ASR perspective, though.

Make it yer own Tungar Supply

Went to the "Secret Land of Walter" last weekend to have some fun with some friends. On the agenda for the weekend were some Tungar Supplie...

hifiheroin.blogspot.com

Low voltage, high current power supplies for field coil drivers using Tungar tubes.

View attachment 259979

TUNGAR TUBE S-289415-C WESTINGHOUSE MADE IN USA 2 Pcs – Audio Tubes

www.nosaudiotubes.com

Argon filled. Welcome to the Golden Age of Wireless!

I am completely down with superconducting field coils. The refrigeration machinery tucked away in the basement, I assume.

 

[Plcamp]

Does control of the field coil (Bl) allow adjustment of system q by altering qes of the driver?

 

[fpitas]

Does control of the field coil (Bl) allow adjustment of system q by altering qes of the driver?

It would, yes. A lot of things would change besides that.

An example, the SEAS W18 is offered in two flavors with different gap flux. The lower flux version has the higher Q and is perhaps better for sealed enclosures.

 

[DonH56]

I'm not a speaker designer, but when I see variable windings on voice coil in Purifi woofers, I think there are some options of winding up fully electromagnetic motor with specific qualities.
It's better to discuss with currently employed speaker designers.
Most probably everything is stuck at extremely high costs.

Here is a basic look at a field coil driver: https://www.edn.com/field-coil-speakers-obsolete-or-the-future/

The field coil is separate from the voice coil and does not move. As @Philbo King mentioned above, the field coil also served as the DC choke (power supply filter) for tube amplifiers, which meant they had to be insulated to handle several hundred (at least) volts. I, and probably many others, have seen the aftermath of the field coil insulation breaking down and turning the driver in to a charred mess, including catching the (paper) cone itself literally on fire. I would expect, but do not know, that today's field coils would operate at much lower voltage and higher current, and probably not feed the amplifier (though see below).

Multiple voice-coil windings have been used for many years; the first time I ran into them was on the Infinity IRS system's woofers that had two windings to help control the response. I used one in my first DIY subwoofer but used the second coil as a feedback source for a servo system. A lot of woofers these days have two coils so the speaker designer (that would not be me) can trade impedance and sensitivity, etc. by how the two windings are used (connected and driven).

A field coil design could offer a lot of options for parameter control, but also adds a number of additional variables that must all work together, at a significant cost in complexity and power. I have a vague memory of some papers discussing dynamically varying the magnetic field with the signal to optimize power and distortion, basically turning the DC power supply into an active audio amplifier driving the field coil, but do not know if anything made it to the commercial product phase. Another vague memory is of a paper showing field coils on either side of the voice coil to create a sort of push-pull field; again, no idea if a practical commercial example was ever created. You could use the field coil to power the amplifier, as done in the past (for tubes), thus varying the field coil current (and thus magnetic field) with amplifier output level to provide an interesting feedback loop to reduce standing power. All sorts of interesting possibilities, but whether really worthwhile, I could not say (but that they are barely a niche now says something in my mind).

Maybe I'll go fire up my old spark-gap transmitter to create a signal for my crystal radio now. The arcs from the transmitter create a very wideband signal so must be better than the current band-limited AM stations, don't you think?

Edit: I'll defer to @fpitas -- I was typing and held in place for a while and he answered (fielded? ) the questions more thoroughly than I.

 

[Plcamp]

. I have a vague memory of some papers discussing dynamically varying the magnetic field with the signal to optimize power and distortion, basically turning the DC power supply into an active audio amplifier driving the field coil, but do not know if anything made it to the commercial product phase.

Would the modulation of Bl using field coil show a difference in the Klippel measured limits of performance for a driver? I presume there would be some ability to “flatten” the parametrics vs cone displacement.

That would be interesting to see.

 

[DonH56]

Would the modulation of Bl using field coil show a difference in the Klippel measured limits of performance for a driver? I presume there would be some ability to “flatten” the parametrics vs cone displacement.

That would be interesting to see.

I am not enough of a speaker designer to say anything other than "it's possible". The papers I (very vaguely) remember were more focused upon reducing distortion and expanding dynamic range than adjusting parameters that might influence things like frequency response. So sweeps of distortion might show improvement, followed by very rapid fall-off, akin to hard clipping in an amplifier. Now that I think back, there was a paper talking about modulating the field to yield "soft saturation" of the driver to reduce clipping and supposedly prevent hitting hard physical limits. But this is all based on memories of memories long ago...

Modern amplifier and speaker designs seem to be doing awfully well without dealing with field coils.

 

[fpitas]

Modern amplifier and speaker designs seem to be doing awfully well without dealing with field coils.

This

 

[Killingbeans]

Alnico was good because it has a magnetic permeability close to that of air. The better drivers had Alnico as a big cylinder in the center of the coil. Inductance modulation became a problem when ferrite was introduced, because the pole piece inside the coil became a hunk of high-permeability steel instead of low-permeability Alnico. The magnetic properties of ferrite did not allow it to be used as a pole piece like Alnico. So, the voice coil ended up being a big honking inductor that varies in value as it moves in and out of the gap, a major source of distortion. The solution was shorting rings. Modern drivers are tending towards magnets based on neodymium, because it offers a tremendous amount of magnetic energy storage in a small space. And, neodymium is very low permeability like Alnico.

Makes sense. And it makes my idea in post #13 that would surround the voice coil with metallic glass really, really bad

 

[fpitas]

Makes sense. And it makes my idea in post #13 that would surround the voice coil with metallic glass really, really bad

Well, getting huge flux even in wide gaps isn't as bad as it used to be. Just look how tiny the neodymium magnets are on big pro drivers, for example.

 

[egellings]

Here is a basic look at a field coil driver: https://www.edn.com/field-coil-speakers-obsolete-or-the-future/

The field coil is separate from the voice coil and does not move. As @Philbo King mentioned above, the field coil also served as the DC choke (power supply filter) for tube amplifiers, which meant they had to be insulated to handle several hundred (at least) volts. I, and probably many others, have seen the aftermath of the field coil insulation breaking down and turning the driver in to a charred mess, including catching the (paper) cone itself literally on fire. I would expect, but do not know, that today's field coils would operate at much lower voltage and higher current, and probably not feed the amplifier (though see below).

Multiple voice-coil windings have been used for many years; the first time I ran into them was on the Infinity IRS system's woofers that had two windings to help control the response. I used one in my first DIY subwoofer but used the second coil as a feedback source for a servo system. A lot of woofers these days have two coils so the speaker designer (that would not be me) can trade impedance and sensitivity, etc. by how the two windings are used (connected and driven).

A field coil design could offer a lot of options for parameter control, but also adds a number of additional variables that must all work together, at a significant cost in complexity and power. I have a vague memory of some papers discussing dynamically varying the magnetic field with the signal to optimize power and distortion, basically turning the DC power supply into an active audio amplifier driving the field coil, but do not know if anything made it to the commercial product phase. Another vague memory is of a paper showing field coils on either side of the voice coil to create a sort of push-pull field; again, no idea if a practical commercial example was ever created. You could use the field coil to power the amplifier, as done in the past (for tubes), thus varying the field coil current (and thus magnetic field) with amplifier output level to provide an interesting feedback loop to reduce standing power. All sorts of interesting possibilities, but whether really worthwhile, I could not say (but that they are barely a niche now says something in my mind).

Maybe I'll go fire up my old spark-gap transmitter to create a signal for my crystal radio now. The arcs from the transmitter create a very wideband signal so must be better than the current band-limited AM stations, don't you think?

Edit: I'll defer to @fpitas -- I was typing and held in place for a while and he answered (fielded? ) the questions more thoroughly than I.

When the field coil was used as a choke in the power supply, significant ripple current in the coil could result in an annoying background hum in the speaker. To circumvent that, a small coil was wound near the speaker's voice coil, and this was connected in series opposing with the voice coil to the amplifier output. It was called a humbucking coil for that reason. The hum picked up by the humbucking coil via transformer action would cancel the hum in the voice coil feed.

 

[mhardy6647]

Argon filled. Welcome to the Golden Age of Wireless!

Basically a tungsten light bulb -- with attitude.

I should mention that, while the frugality of consumer radio design often led to the field coil serving double duty as a P/S choke, it doesn't have to be done that way -- and more serious installations (e.g., theater audio) back in the day presumably used dedicated field coil power supplies (along the lines of the Dave Slagle P/S I referenced earlier).

 

[fpitas]

Basically a tungsten light bulb -- with attitude.

I should mention that, while the frugality of consumer radio design often led to the field coil serving double duty as a P/S choke, it doesn't have to be done that way -- and more serious installations (e.g., theater audio) back in the day presumably used dedicated field coil power supplies (along the lines of the Dave Slagle P/S I referenced earlier).

Yes, although they certainly also had some large amps (for the day).

 

[mhardy6647]

Yup.

 

[atmasphere]

Field coils have the advantage of no sag in the voice coil gap if the power supply is regulated. This results in greater acceleration and thus lower distortion with greater efficiency (not including the current of the FC itself). They can be run on as little as 12Volts (no need for the scary stuff in the post above...).

Alnico has traditionally been preferred as a magnet structure in loudspeakers, despite being weaker and more expensive than ceramics, because it can be easily focused in the VC gap and because when current is applied to the voice coil, it has the least sag of all the PMs. Recently I've noticed ceramic magnet drivers that have a ring of magnets on the magnet structure in addition to the main magnet. These are used to reduce sag in the gap, according to engineers at MISCO to whom I've spoken. MISCO is one of the few remaining US loudspeaker manufacturers (Eminence being one of the others) and they are here in town.

FWIW, the main reason EM drivers went away was not due to performance but cost, like so many other things in audio. A good field coil is tricky and expensive to wind so the cost is significant. For those where cost is less important than performance, EMs can be attractive.

Two other advantages: EMs tend to be more efficient in terms of amplifier power and by varying the field current, Thiele-Small parameters can be adjusted.

Of course a vented pole piece can be used with an EM, reducing air compression behind the dust cap and further focusing the magnetic field.

 

[fpitas]

Even allowing the field coil may have a superior magnetic field shape, the significant voice coil pre-heating can't be a good thing.

 

[egellings]

Again, there's no advantage to a field coil over a permanent magnet beyond the field strength adjustability afforded by the coil. That might be good for optimizing driver-box tuning, maybe, although the iron around the coil should be saturated. Find other ways to adjust the driver-box tuning. Iron around the voice coil should be magnetically saturated to the point where no counter-field from the voice coil can un-saturate the iron and change its B-field intensity, which would result in distortion. Both a coil and a permanent magnet can accomplish that. Just get a good magnet, save the electricity, and get it over with.

 

[atmasphere]

Again, there's no advantage to a field coil over a permanent magnet beyond the field strength adjustability afforded by the coil. That might be good for optimizing driver-box tuning, maybe, although the iron around the coil should be saturated. Find other ways to adjust the driver-box tuning. Iron around the voice coil should be magnetically saturated to the point where no counter-field from the voice coil can un-saturate the iron and change the B-field intensity, which would result in distortion. Both a coil and a permanent magnet can accomplish that. Just get a good magnet, save the electricity, and get it over with.

This statement is false as explained earlier. Field coils offer greater efficiency over PMs. This is why they are favored by tube aficionados, since vacuum tube power is expensive; you have to get the most out of them that you can.

You'd think that, as you say, the gap should be saturated so adding more won't make a difference. In practice though, the field does sag and this effect is both measurable and audible. This suggests that the gap isn't properly saturated.

Even allowing the field coil may have a superior magnetic field shape, the significant voice coil pre-heating can't be a good thing.

Field coils do run warm!

I think you'll find that most EM drivers are fairly high efficiency so the voice coils don't suffer as much thermal compression. As a result the higher temperature is less of a variable than it is for a lower efficiency driver in a PM application.

 

[egellings]

This statement is false as explained earlier. Field coils offer greater efficiency over PMs. This is why they are favored by tube aficionados, since vacuum tube power is expensive; you have to get the most out of them that you can.

You'd think that, as you say, the gap should be saturated so adding more won't make a difference. In practice though, the field does sag and this effect is both measurable and audible. This suggests that the gap isn't properly saturated.

Field coils do run warm!

I think you'll find that most EM drivers are fairly high efficiency so the voice coils don't suffer as much thermal compression. As a result the higher temperature is less of a variable than it is for a lower efficiency driver in a PM application.

What do you mean by "efficiency" of a magnetic field? Now that I think about it, the B in the gap could be changed by voice coil contributions to it even when surrounded by saturated iron, since the air in the gap cannot be saturated, even if the surrounding iron is. At least there'd be no hysteresis do deal with in air. It still makes no difference whether a coil or a permanent magnet saturates the iron, so long as the iron is never taken out of saturation by way of strong counterposing voice coil contributions.

 

[atmasphere]

What do you mean by "efficiency" of a magnetic field? Now that I think about it, the B in the gap could be changed by voice coil contributions to it even when surrounded by saturated iron, since the air in the gap cannot be saturated, even if the surrounding iron is. At least there'd be no hysteresis do deal with in air. It still makes no difference whether a coil or a permanent magnet saturates the iron, so long as the iron is never taken out of saturation by way of strong counterposing voice coil contributions.

I'm referring to the overall efficiency of the driver; I'm using 'EM' to refer to ElectroMagnetic.

What this suggests is that gap saturation isn't always happening (just using logic here). Were this not so, you'd not be seeing neodymium drivers with a ring of smaller magnets trying to focus on the VC gap. Early guitar speakers that used neodynium were known for terrible sound; when I asked an engineer about that at MISCO he told me that the magnets sagged too much when current was applied to the coil. I think most people assume that magnets don't react to current in the VC which isn't the case.