Field Coils for Audio Systems

[Atmasphere]

Actually, an Un-regulated supply is the best way to power field coils. There’s some content online about this, but the gist is that current is what energizes the magnet and therefore current is what you need to keep constant. You do not want voltage regulation at all.
I powered my two field coil drivers with constant voltage lab supplies for years, before I upgraded to a proper constant current supply. The difference was HUGE - could not believe it!
Keep in mind there is interaction between the “amp” powering the field coils and the audio amp. With a constant voltage field coil supply, it‘s “fighting” with the audio amp…

I have supplies that display the voltage and current. I run the voltage up until I see the current specified by the manufacturer of the field coil. The higher the voltage used, the less the current is an issue.... at any rate I never see the current vary, by even so much as a milliamp (my coils run about 12V; the newer ones tend to be more like 18V). At any rate, if you are using a constant current supply, the constant current source is a regulator.

 

[Zeotrope]

Do you happen to have an online reference for this? Thx

Wolf von Langa had some background theory on his website, but I can’t find it. You can email him directly. Also, Jeff Jackson is a great person to ask.

 

[Zeotrope]

I have supplies that display the voltage and current. I run the voltage up until I see the current specified by the manufacturer of the field coil. The higher the voltage used, the less the current is an issue.... at any rate I never see the current vary, by even so much as a milliamp (my coils run about 12V; the newer ones tend to be more like 18V). At any rate, if you are using a constant current supply, the constant current source is a regulator.

That’s great. I was not able to find any off the shelf constant current supplies. All the lab supplies that claim to be constant current, are actually not what we need. They will switch to a preselected voltage when a specific current is met.
The challenge with a constant voltage supply is that the voltage needs to change while music is playing. The amp signal interacts with the field coil signal and the two can “fight” each other if the voltage is kept constant.

 

[Atmasphere]

A constant current source isn't hard to make up...
It will have better performance if the power supply is voltage regulated. The voltage would thus be a bit higher than might occur otherwise if the supply were simply unregulated and running the FC.

 

[AMR / iFi audio]

Wolf von Langa had some background theory on his website, but I can’t find it. You can email him directly. Also, Jeff Jackson is a great person to ask.

What did Wolf von Langa's background theory say?

 

[Zeotrope]

I finally found Wolf’s links:

• In electromagnetism the Lorentz force is the combination of electric and magnetic force on a point charge due to electromagnetic fields.
• Maxwell describes the interaction between electromagnetic forces and mechanical momentum.
• An eddy current brake, also known as an induction brake, is a device used to slow or stop a moving object.
• An electromagnetic field (also EM field or EMF) is a field produced by accelerating electric charges.
• The Natural Way of Loudspeaker Operation, or the serious flaws of voltage drive.
• The issue whether loudspeakers should be excited by a voltage or current signal is quite well comparable to a dispute that took place over a century ago, concerning whether the production and distribution of electricity should operate on direct or alternating current.
• Electromagnetic induction is the generation of an electromotive force via a moving electrical conductor in a magnetic field.
• Ohm's acoustic law states that a musical sound is perceived by the ear as a set of a number of constituent pure harmonic tones.
• Transducer drive mechanisms

 

[tima]

I finally found Wolf’s links:

Thanks for posting these. I found this one particularly interesting:

Current-Drive - The Natural Way of Loudspeaker Operation

Current-drive improves sound reproduction of loudspeakers and reduces distortion by suppressing EMF currents. Book on loudspeaker current-drive with DIY projects.

www.current-drive.info

"The driving force (F), that sets the diaphragm in motion, is proportional to the current (i) flowing through the voice coil according to the formula

F = Bli​

where the product Bl is called force factor (B = magnetic flux density; l = wire length in the magnetic field). B is the flux density that exists when the current is zero. (The current always causes its own magnetic field, that may react with adjacent iron, but the effect is not related to this equation.) This force, then, determines the acceleration (a) of the diaphragm, that in the main operation area is given by the Newtonian law F = ma and which determines the pressure radiated.

The most remarkable thing here, regarding loudspeakers, is that the voltage between the ends of the wire does not appear anywhere in these equations. That is, a speaker driver obeys only and solely current, not minding what the voltage across the terminals happens to be. "

-- Esa Meriläinen
M.Sc. degree in electrical engineering from Tampere University of Technology, Finland

 

[kjartanb]

Does not the the voice coil resistance play a part in generating current in the voice coil in the first place via the amps output voltage. Can we have pure current gain amps with no voltage gain in any stage driving speakers?

 

[Scott Naylor]

https://audioxpress.com/article/current-driving-of-loudspeakers-book-review

This excerpt NATURALly addresses some questions regarding “current-drive”

 

[morricab]

Does not the the voice coil resistance play a part in generating current in the voice coil in the first place via the amps output voltage. Can we have pure current gain amps with no voltage gain in any stage driving speakers?

of course because V=iR. You can therefore replace i with V/R.

 

[AMR / iFi audio]

• The issue whether loudspeakers should be excited by a voltage or current signal is quite well comparable to a dispute that took place over a century ago, concerning whether the production and distribution of electricity should operate on direct or alternating current.

Are we referring to Tesla's free fluid electrical charges?

 

[Atmasphere]

In order to drive any loudspeaker you need power. It isn't current vs voltage- if you can't make the voltage you can't make the current.

So the conversation is really about the output impedance of the amplifier driving the speaker. If it is very low with respect to the speaker, the amplifier can act as a voltage source since the amp can make constant voltage with respect to load impedance.

If the output impedance is very high, usually a multiple of the load impedance, the amplifier is a current source meaning the amp will make constant current regardless of the impedance of the speaker. The only speakers I know of needing this sort of drive are open baffles with some very specific drivers.

If the output impedance is close to or slightly less than the impedance of the speaker, the amp is likely a power source.

To create a voltage source you either need an inherently low output impedance such as is available from certain semiconductors, and/or you need enough voltage feedback to cause the amp to behave as a voltage source (though servo action of the feedback voltage) even if its output section is relatively high impedance otherwise. This is why tube amps can behave as a voltage source. The idea behind voltage drive is plug and play- no having to adjust the speaker to match the voltage response of the amplifier as with the prior art.

To create a current source the amp will need to have current feedback rather than voltage feedback. Usually this is done by placing a small resistance in series with the output load, over which a voltage drop will occur. This voltage is then fed back into the amplifier and will cause the output impedance to increase, again according to the rules of control (servo) theory.

To create a power source you either use a tube amp with no feedback (which will approximate a power source) or run both current and voltage feedback and balance the two exactly against each other. This was the predominant method of loudspeaker drive until the late 1950s when EV and MacIntosh introduced the idea of an amplifier being a voltage source and the speakers thus 'voltage driven', that latter bit in quotes because its a term with a meaning. This idea took a good 20 years to take over the industry.

There are still loudspeakers that are power driven (Power Paradigm) as opposed to voltage driven (Voltage Paradigm) made today. The former are arguably less than 1% of all speakers made.

I think you can ignore Wolf von Langa's background links since they are exactly that, unless you really want to delve into a lot of stuff that won't seem directly related to audio. FWIW, field coils are a lot different from voice coils in that they are used to produce a magnetic field of constant level. I see their advantage as allowing a cone speaker to have similar speed to an ESL, on account of the magnetic field not sagging as current is applied to the voice coil. To do that though requires some form a regulation as discussed earlier.

 

[jw5115]

He

actually uses a field coil 18" JBL in the Hartsfield. I think it could be quite special.

The Hartsfield uses the same Woofer as the front Woofer in the T-1.5 and the T-3.4.
It is the Model 1501-8 or16 (CAL Underhung Field Coil Motor with a JBL K-145 Frame and NOS Jensen
all paper Cone 15LL

 

[jw5115]

Do you or Ralph (@Atmasphere) know the Web address for Classic Audio Loudspeakers?

It does not appear to be:
http://classicaudioloudspeakers.com which redirects to

Classic Audio Loudspeakers

There's a page there but tjat shows only a box with a blue border and no content.

That’s the correct website

 

[tima]

That’s the correct website

Yes, thank you. Turns out it is a rendering problem for the Firefox browser.

 

[Audiophile Bill]

The Hartsfield uses the same Woofer as the front Woofer in the T-1.5 and the T-3.4.
It is the Model 1501-8 or16 (CAL Underhung Field Coil Motor with a JBL K-145 Frame and NOS Jensen
all paper Cone 15LL

Any idea of pricing lists for these lovely drivers?

 

[AMR / iFi audio]

In order to drive any loudspeaker you need power. It isn't current vs voltage- if you can't make the voltage you can't make the current.

So the conversation is really about the output impedance of the amplifier driving the speaker. If it is very low with respect to the speaker, the amplifier can act as a voltage source since the amp can make constant voltage with respect to load impedance.

If the output impedance is very high, usually a multiple of the load impedance, the amplifier is a current source meaning the amp will make constant current regardless of the impedance of the speaker. The only speakers I know of needing this sort of drive are open baffles with some very specific drivers.

If the output impedance is close to or slightly less than the impedance of the speaker, the amp is likely a power source.

To create a voltage source you either need an inherently low output impedance such as is available from certain semiconductors, and/or you need enough voltage feedback to cause the amp to behave as a voltage source (though servo action of the feedback voltage) even if its output section is relatively high impedance otherwise. This is why tube amps can behave as a voltage source. The idea behind voltage drive is plug and play- no having to adjust the speaker to match the voltage response of the amplifier as with the prior art.

To create a current source the amp will need to have current feedback rather than voltage feedback. Usually this is done by placing a small resistance in series with the output load, over which a voltage drop will occur. This voltage is then fed back into the amplifier and will cause the output impedance to increase, again according to the rules of control (servo) theory.

To create a power source you either use a tube amp with no feedback (which will approximate a power source) or run both current and voltage feedback and balance the two exactly against each other. This was the predominant method of loudspeaker drive until the late 1950s when EV and MacIntosh introduced the idea of an amplifier being a voltage source and the speakers thus 'voltage driven', that latter bit in quotes because its a term with a meaning. This idea took a good 20 years to take over the industry.

There are still loudspeakers that are power driven (Power Paradigm) as opposed to voltage driven (Voltage Paradigm) made today. The former are arguably less than 1% of all speakers made.

I think you can ignore Wolf von Langa's background links since they are exactly that, unless you really want to delve into a lot of stuff that won't seem directly related to audio. FWIW, field coils are a lot different from voice coils in that they are used to produce a magnetic field of constant level. I see their advantage as allowing a cone speaker to have similar speed to an ESL, on account of the magnetic field not sagging as current is applied to the voice coil. To do that though requires some form a regulation as discussed earlier.

I agree. Current driving loudspeakers seem to be a good idea, but in reality, it doesn't provide too many pros. As stated, it might be beneficial in certain situations, when driving just a single driver directly. When drivers are coupled with a crossover, benefits disappear. That's why voltage driving is the standard now, as it has the best overall performance, independent of what you are connecting to the amplifier.

 

[Legolas]

I am also interested in this type of driver for speakers. If t gives both a performance hike + extra efficiency it is a great thing.

 

[Audiophile Bill]

I am also interested in this type of driver for speakers. If t gives both a performance hike + extra efficiency it is a great thing.

Hi,

A field coil doesn’t give more efficiency per se. A modern production neodymium magnet can easily provide enough flux density to saturate any metal poles including permendur without issue. Same goes for other magnets such as alnico. The theoretical maximum flux density achievable is 2.4T and numerous drivers have done this for many, many years with fixed magnets - Lowther PM4a would be an example.

The performance benefits largely reside in the incredible ability to tune the driver to the speaker by altering the magnet flux and consequent t/s parameters. Simplifying it a bit with an example of a field coil 8” full ranger - as you increase the voltage/current the magnetic force increases which leads to increased sensitivity and lower q. This results in a driver suitable mainly for horn loading applications but will also result in lower bass output. The same driver being fed a lower voltage/current will be less sensitive but its q will be higher meaning it will lend itself much more readily to open baffle application since the driver cone will be more readily able to move. This will give more bass output but at expense of sensitivity.

There are other arguments related to the field itself - that is the field coil (fed with appropriate power supply) magnetic field will be incredibly stiff and not fluctuating with increased heat like fixed ferrite magnets.

Then there is the area which is very hard to quantify relating to the sound itself. I have spent most of my time recently together with my Viking partner in crime (Leif) trying to extract the maximum from our field coil carts. People would tell us we lost the plot when we tell you that the type of capacitor used on the last filtration drastically alters the sound. Even though it is just a cap filtering dc for the magnetic field of the cartridge. If you read widely, there are all manners of opinion on which power supply types to use for best sound. Purists seem to opt for Tungar bulb type supplies, others on battery, other using various medical supplies and then there is the constant voltage and current crowd. I recently spoke to someone who said never to use constant voltage or current as it fights with the voicecoil.

Best regards.

 

[Zeotrope]

That's a great overview of the unique benefits of field coil magnets, Bill.

I can confirm significant improvements to sound quality when switching from constant voltage lab power supplies to unregulated (for the HV midrange), filtered, and constant current (for the woofers) power supplies.
Vocals sound softer, more relaxed. Imaging is also more focused, especially at different frequencies (e.g., guitarist's voice is now centered over the strings).