A field coil driver uses a dedicated power supply to feed its field coil to produce the strong magnetic field that makes the diaphragm react to the incoming electrical signals and reproduce sound, the stronger the magnetic field the more efficient the driver would be and the higher and lower frequencies it will respond, up to the diaphragm physical and acoustical limits.

 A very strong magnetic field was achieved with a special permanent magnet called Alnico (Aluminum, Nickel, Cobalt), only surpassed in strength by the newer Rare Earth Magnets, however this powerful magnetic field decreases with many factors, age and physical disturbances such as heat, strong vibrations or shocks and to a much less extend to reactions to opposite external magnetic fields, and specially if the magnetic field is disrupted by opening the driver, removing the magnet from the driver just once, will decrease the magnetic field to almost nothing !. such is the case that many if not all Alnico magnets have lost moderate to significant strength in their field , some up to 90% and more! and the only way to restore them back to their full strength is by re-magnetization with a very strong special electromagnet.  Such thing will never happen to a field coil type of driver, since it depends directly of the external power supplied.
The magnetic fields generated will go down near zero when the power to the field coil is removed but will go up to it's full strength as soon as the power is applied, and will never diminish in strength unless the power is somehow lowered or the power supply fails.

Now days we have an advantage over the past, a very stable solid state power supply is affordable and will provide a very steady and quiet regulated  power to the field coil.  
Most transistorized regulated power supplies will be able to provide a steady power to the field coils providing the power to the coils is maintained withing operational parameters avoiding excessive heat generation. 

In theory a permanent magnet such as Alnico, Ferrite or any type as well is very stable when not affected by external magnetic fields that will oppose and add to their force field , however when a modulating magnetic field interferes, such as the ones generated by the electrical signal applied to the coil of the diaphragm,  the magnetic domains in the magnet structure change their orientations in a more or less degree depending on the strength of the applied disturbing field, when the field is diminished or changes in direction, the magnetic domains must realign again to return to the original magnetic orientation and strength, producing variations in the magnetic field that changes the response of the diaphragm, resulting in induced sound distortions and changes in pitch at it's most extreme excursions, with a field coil these distortions are highly diminished or not present since the energy needed for these changes and polarization in the soft steel  core of the  electromagnet will be provided by the power supply that will correct these instantaneous variations by changing the instantaneous variations in currents fed to the field coil, not so by changing or distorting the permanent magnet field, maintaining in this way a very steady magnetic field,  reproducing sound with the up-most sweetness and free from induced distortions.

• A special flattened rectangular magnet wire for bigger more compact coils, with improved heat transfer.

• Precision-wound coils with no overlaps to utilize at maximum the space available inside the drivers and to keep heat to a minimum, a wire rated to withstand 200 degrees* Celsius (392 F) potted in the driver with a special heat conductive epoxy forming a huge solid coil with no loose wires to vibrate or rattle, they are stand alone rock solid units with no bobbin or form, utilizing the crimped space inside the driver motors to it's maximum, generating the strong magnetic field needed for the voice coil with minimal heat generation.

For the ultimate in sound reproduction a field coil driver is unsurpassed !. Very musical, smooth and warm !

*The drivers do get warm to the touch only and the 200 degree wire will insure a lifetime of trouble-free service. 

     

A graphical response comparison before and after conversion of a 288 Alnico magnet driver.
The black curve shows the response before conversion , the red curve shows the response after the conversion with the recommended 15 V dc applied to the field coil.

Electrical measurements are taken at normal listening levels, at input and output coils, with no horn load at 1/24 octave spectra with an 8 Ohms diaphragm  at 0.25 Watt  (1.414 Volts RMS into 8 Ohms) sweep sinusoidal signal  from 200 Hz to 23,000 Hz., No dividing  network crossover was used.
 

The following graphics show a 1kHz tone at 0.25W RMS.

Top graphic is the input to the voice coil, the bottom is the output from the pick-up coil .
a 20mV p-p output modulation is detected from an input of 2V p-p (0.25W RMS).  

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