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Author Topic: Field Coil Loudspeakers?  (Read 13523 times)

Jim McKeveny

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Field Coil Loudspeakers?
« on: November 28, 2012, 02:26:54 PM »

Field coil loudspeakers use an electromagnet to energize the gap, unlike the (ahem) permanent magnets of varying construction that are currently employed.

With the recent rare-earth price shocks, I wonder which manufacturers are taking a good look at revisiting field-coil technology. TrueExtent, the beryllium diaphragm people, work with a "classic audio" manufacturer that builds some interesting HF devices.

Field coil speakers pre-date microprocessors & DSP, but if they were incorporated into a broader loudspeaker system self-monitoring loop there could be useful merit.

With self-powered speakers taking an increasing share of the pro market there could be some opportunity here. Y/N?
« Last Edit: November 28, 2012, 03:02:54 PM by Jim McKeveny »
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Barry Singleton

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Re: Field Coil Loudspeakers?
« Reply #1 on: November 28, 2012, 03:13:24 PM »

FC won't beat ceramic for weight or cost. My bet is nope.
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Riley Casey

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Re: Field Coil Loudspeakers?
« Reply #2 on: November 28, 2012, 03:23:55 PM »

A class D power amp running a field coil in a push-pull closely feedback controlled self powered configuration wouldn't be a more effective solution than a lump of mud with some iron fillings mixed in it?  I'm curious as to why not.  Once the the electronics are in the box ( as they increasingly are ) and once the benefits of precision location of magnetic fields as implemented in rare earth magnet loudspeakers ( as they have been for several years now )  it seems like a worthwhile path of research.

FC won't beat ceramic for weight or cost. My bet is nope.

Ivan Beaver

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Re: Field Coil Loudspeakers?
« Reply #3 on: November 28, 2012, 06:57:09 PM »

Field coil loudspeakers use an electromagnet to energize the gap, unlike the (ahem) permanent magnets of varying construction that are currently employed.

With the recent rare-earth price shocks, I wonder which manufacturers are taking a good look at revisiting field-coil technology. TrueExtent, the beryllium diaphragm people, work with a "classic audio" manufacturer that builds some interesting HF devices.

Field coil speakers pre-date microprocessors & DSP, but if they were incorporated into a broader loudspeaker system self-monitoring loop there could be useful merit.

With self-powered speakers taking an increasing share of the pro market there could be some opportunity here. Y/N?
For one hting the field coil has to have POWER in order to operate.  So that is a downside.

HOWEVER as a result-it was a double bonus-back in the day.  The field coil was also used as a series choke in the tube power supply.  So it helped to smooth out the DC voltage and provided the magnetic strength for the loudspeaker.
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paul bell

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Re: Field Coil Loudspeakers?
« Reply #4 on: November 28, 2012, 07:14:47 PM »

FC speakers aren't very efficient-and they get hot.

And they are heavy. Although probably not as heavy as this cast iron one, one of the first 18's made.

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John Roberts {JR}

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Re: Field Coil Loudspeakers?
« Reply #5 on: November 28, 2012, 08:10:06 PM »

Field coil loudspeakers use an electromagnet to energize the gap, unlike the (ahem) permanent magnets of varying construction that are currently employed.

With the recent rare-earth price shocks, I wonder which manufacturers are taking a good look at revisiting field-coil technology. TrueExtent, the beryllium diaphragm people, work with a "classic audio" manufacturer that builds some interesting HF devices.

Field coil speakers pre-date microprocessors & DSP, but if they were incorporated into a broader loudspeaker system self-monitoring loop there could be useful merit.

With self-powered speakers taking an increasing share of the pro market there could be some opportunity here. Y/N?

I actually had an exchange with a guy who shall remain nameless, but knows about such stuff. He almost immediately lost me with the science (i couldn't keep up and didn't want to look as stupid as I am by asking him to explain it more simply). My sense is that yes you could (of course, they did before), but it was not a "great" idea.

From simple observation the coil to make the static field would be another source of heat in a place that doesn't need more. it seems in theory it could add another degree of control, but coils are not cheap. Driving the two coils differentially could have merit, but now we are talking 2x the amplifiers, unless similar coils could be reverse wired, but that sounds like another design can of worms to get them similar.

We are only having this conversation because rare earth magnets have some artificial pricing distortions due to Chinese hoarding and market supply manipulations. The most recent thing I recall reading about RE pricing was that it was softening (but i couldn't find it easily).

In any free market the unnaturally high price will lead to expanded investment in mining and processing to satisfy demand at those high prices. The natural pendulum swing of prices in free markets. Some old mines and facilities in the US closed by cheap chinese RE magnets may reopen. If the Chinese have distorted true supply demand balance, when they release full supply on the market prices could (should) fall.   

JR

PS The reason I asked my speaker expert about this was because i read about a Japanese car company looking at the same thing, more or less, to replace RE magnets in electric cars motors and alternators. This was probably a year ago or more so who knows. I may have the emails laying around but rather not publish private communications. i had no personal interest in this but though he might, since he is smart enough to make it work, and a big consumer of RE... He was not very interested as far as I could tell.

 
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Sam Feine

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Re: Field Coil Loudspeakers?
« Reply #6 on: November 28, 2012, 10:51:35 PM »

Not entirely sure how relevant it is but in a discussion I had with a professor who taught a class about power plant technology, (He actually worked at 3 mile island in some of the years prior to its incident), the topic of how the magnetic field used in the generator was produced came up. Apparently in large scale applications such as power plants, a smaller dc generator mounted on the shaft is used to create the magnetic field which is used for the production of power.

Of course this same sort of system could be used for loudspeakers, but as mentioned before, the heating would be an issue.

I wonder however what would happen if there were essentially two coils, the inner voice coil, and an outer field coil that were both wired to the same amplifier power output with their polarities reversed as to provide double the push / pull in either direction? In theory this would produce double the heat, but it would allow for a passive speaker design in which the only power needed would be that from the standard amp much like any other speaker.
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paul bell

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Re: Field Coil Loudspeakers?
« Reply #7 on: November 29, 2012, 08:15:08 AM »

I was asked who made this thing so I'll share:
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John Roberts {JR}

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Re: Field Coil Loudspeakers?
« Reply #8 on: November 29, 2012, 09:36:50 AM »

ERPI was actually the licensing division of Western Electric for movie and recording products.

Early big dog speakers were developed for movie theater playback.

JR
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GenePink

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Re: Field Coil Loudspeakers?
« Reply #9 on: December 03, 2012, 07:29:36 AM »


> I wonder however what would happen if there were essentially two coils, the inner voice coil, and an outer field coil that were both wired to the same amplifier power output with their polarities reversed as to provide double the push / pull in either direction? In theory this would produce double the heat, but it would allow for a passive speaker design in which the only power needed would be that from the standard amp much like any other speaker.


If you vary both the field strength and the VC voltage, they will multiply. The result would be a cone excursion that would equal the voltage squared, instead of having a linear relationship.

Might be interesting to hear, sort of a downward expander set to ridiculous settings.

Gene
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Re: Field Coil Loudspeakers?
« Reply #9 on: December 03, 2012, 07:29:36 AM »


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