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Field coil dummy...

108.87.24.185
Posted on December 3, 2016 at 10:35:38
dee eye why
Audiophile

Posts: 1148
Location: so. ohio
Joined: March 20, 2003
 I am curious about field coil speakers. I have no idea how they work. Do you drive them with a standard amp with output transformers?
Can someone give me a basic understanding?
Thanks!



.
Freak out...Far out...In out....

 

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RE: Field coil dummy..., posted on December 3, 2016 at 15:45:33
Steve O
Audiophile

Posts: 12383
Location: SE MI
Joined: September 6, 2001
 Most field coil speakers would be driven at the voice coil in the normal manner: tube amp or SS amp is immaterial, assuming of course that the voice coil is a known and reasonably common impedance (4-16 ohm)...not all are, esp vintage. The big difference is that stator's magnetic field is generated by an electromagnet and so it must be energized by a relatively clean DC current/voltage source for proper operation. The details of what current/voltage to use are very specific to a specific speaker but there is some latitude to adjust w/o damage. Some may be 5-10 VDC at an amp or so. Others may be 100-200 VDC at many 10s of mA. Some field coils also served as a PS filter choke esp in vintage equip and as a result may have an additional "hum bucking" winding. If you get deep enough into this you'll find claims that some power supply designs "sound better" than others, above and beyond the intrinsic sound of the speaker/enclosure system.

There is a tonne of info on the web if you care to explore further. But remember: Just because you found it on the web doesn't make it gospel truth ;-)

 

RE: Field coil dummy..., posted on December 5, 2016 at 12:43:47
Alpha Al
Industry Professional

Posts: 2958
Location: N. Carolina
Joined: February 16, 2004
Contributor
  Since: December 3, 2015



Here's a novel way to replace a field coil speaker. It's a 1949 Gibson BR6.

A former owner replaced the field coil speaker with a permanent magnet speaker, but kept the field coil from the original speaker intact,mounted to the side.

He could have used a choke to replace the field coil.

 

RE: Field coil dummy..., posted on December 5, 2016 at 13:27:48
BofService
Audiophile

Posts: 741
Location: Atlanta
Joined: February 28, 2003
 In most vintage radios (consoles from the 1930s, for example) the field coils are indeed doing double duty as the magnet for the speaker and the choke for the power supply. The voltage may be as high as 300vdc so do beware. Sometimes the cone of the speaker gets damaged, in which case the suggested "keep the coil and use a permanent magnet speaker" approach works well. However, often the field coil itself fails due to aging and heat fatigue, or from failure modes in the radio/amplifier that caused high current draw. In that case you are kind of SOL. The coil in the field coil speaker cannot be easily repaired and I don't think anybody makes replacements.

Supposedly, field coils can create a stronger magnetic field than typical magnets, and therefore a better speaker. I have some doubt about this basic tenant though. A very powerful speaker magnet could be made with neodymium magnets, but I am not seeing such items in the marketplace.

 

Neodymium based speaker magnets are quite common these days..., posted on December 5, 2016 at 14:41:44
Steve O
Audiophile

Posts: 12383
Location: SE MI
Joined: September 6, 2001
 ...check out the "Parts Express" website for starters. Lots of cone/dome type and compression drivers avail with Neo magnets. I'm using a pair of BMS Neo drivers on the 811s in my modified Altec Carmels (4552s). Physically small and very efficient although I think the ferrite version (4550) might sound better.

 

RE: Neodymium based speaker magnets are quite common these days..., posted on December 6, 2016 at 21:00:30
Michael Samra
Dealer

Posts: 36118
Location: saginaw michigan
Joined: January 30, 2005
 Steve
You may have seen it but you didn't,get over to Classic Audio Reproductions near Pontiac.John builds a set of Field coil speakers that are quite possibly the best dynamic speaker I've ever heard. They run about 70 grand a pair but they sounded like Quad ESL57s on steroids..They were that good.
"For every complex problem there is an answer that is clear, simple, and wrong" H. L. Mencken

 

RE: Neodymium based speaker magnets are quite common these days..., posted on December 7, 2016 at 07:50:38
bean
 For me, they're the highlight of the Axpona show every year. And John is a great guy. If I had the $, I would have a pair.

He said last year may be his last Axpona; hope not.

 

Yes, I've heard them..., posted on December 7, 2016 at 10:32:00
Steve O
Audiophile

Posts: 12383
Location: SE MI
Joined: September 6, 2001
 ...driven by some Atmosphere OTL amps. Not my cuppa tea. YMMV

 

RE:field coils can create a stronger magnetic field, posted on December 7, 2016 at 11:29:01
Bill Fitzmaurice
Industry Professional

Posts: 5371
Location: New England
Joined: October 20, 2002
 You can only use so much magnetic strength before it becomes counter-productive. Qes values of less than 0.3 tend to choke off bass response, and even the original Alnico magnet D130 circa 1949 had a Qes of 0.27.

Neodymium magnet drivers have dominated the high end of pro-sound for over a decade. They aren't used to get lower Qes values than ceramic can provide, they're used to realize lighter weight and smaller magnet structures. They're also becoming popular in OEM auto sound drivers, for weight reduction. Most mainstream drivers still use ceramic, as it's cheaper than neo, mainly because the Chinese have a virtual monopoly and they're keeping the price up.

 

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