What are field coils?

[robertbartsch]

I see these for sale now and again. I assume these drivers have motors that do not have fixed magnets but rahter, their motors are powered by an extrenal electrical power source.

Is this correct?

Why are these designs necessary and what are their advantages and disadvantages Vs. fixed motors?

[louped garouv]

yep, they have different supplies for audio (to the VC) and dc (for energizing the coils) signal...

generally, they are older designs... before the permenant magnets came around...

supposed to sound very good...

even see pages of folks converting alnico magnets to FC online....

hth

[jerry_rig]

JM Focal Labs use a field coil for the 16" low frequency driver in their top-of-the line Grande Utopia EM. According to their site:

"The EM Electro-Magnet is made of a 15.4lb (7kg) copper coil. The motor including the coil weighs 48.5lb (22kg). The 16” reaches a total weight of 52.9lb (24kg). “A monster” in the literal meaning of the word, where manufacturing costs cannot be compared to a standard magnet and are only onceivable on exceptional loudspeakers."

[Ducatista47]

http://audioheritage.org/vbulletin/showpost.php?p=126115&postcount=13