I would begin here; http://tubesandmore.com/dk_mk4.htm

Go to the "Books and DVD's" section.

I suspect the guys at this shop could direct you to which would be the most useful book and possibly any other easy to find sources. From my research there's nothing that's easy to find about field coils. The information exists as a by product of other discussions or in the heads of those who worked on the equipment. Technical information has all but disappeared from availability.

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Can you guys post some links where I can read about the history/technology of field coil drivers? Very curious. Books are fine, too. Thanks, y'all.

Field coil is inherently better (IF properly implemented) than a standard magnetic motor type loudspeaker driver (woofer),as the magnets do NOT have infinite stability, and in this... the transients are like motional mass.

Ie the delta of the transient is directly equatable to mass..and the magnet has FINITE mass..so..... therefore it follows that a standard magnet motor cannot produce perfect transients. The magnetic field does not react 'instantly' and it also bounces.

In the field coil design..the reaction is faster as BOTH structures, the signal in the coil and the magnet coil are BOTH energized and raring to go..INSTANTLY..and the strength/stiffness of the field/coil can be varied.

This field stability issue of standard designs is why I'm a huge fan of the Morel MDT33 tweeter, as it has a triple stacked magnet design. The accelerations in tweeters are HUGE..which means huge mass...and thus the magnets required to do transients correctly need be huge. Not tiny. sure, tiny magnet...it works..but not well.

This is why among those who know, the MDT33Se tweeter {and it's new variant) is considered as one of the finest (less than all fingers on one hand-it's a short list!) tweeters in existence.

A standard design also suffers from a minor but extant hysteresis in the magnetic field when the driver coil is energized and tries to change direction. Active coils suffer from less of this, as far as issues go. Most try to overcome these issues via brute force. High strength fields on the magnet is the trick. Neo magnets are useful as well. I have a midrange/wide range driver here that is 4 inches and has a 104db efficiency. It has a very high strength neo magnet in the motor.