Back in the" Gold age"
of sound reproduction, an electromagnet was the only means of providing a
magnetic field to the voice coil of an electrodynamic loudspeaker driver for
sound reproduction, until the
permanent magnet made it's appearance, it was a great idea back then
to replace those bulky and expensive power supplies to feed the
electromagnet coils, but something very subtle happened to the sound
quality! somehow there was something wrong, the sound
deteriorated slightly but understandably reliability and substantial savings
the new design more than out-weighted the subtle degradation of sound.
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.
very strong magnetic field was
achieved with a special permanent magnet called Alnico
surpassed in strength by the newer Rare Earth Magnets, however this
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
way to restore them back to their full strength is by re-magnetization
with a very strong special electromagnet. Such thing will
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.
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
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
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.
The new field coil conversion includes:
- 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.
- A core of ultra-pure, low carbon magnetic iron, that undergoes a
special controlled heat treatment process under an Oxygen free atmosphere attaining a very high magnetic
Permeability with very low Coercivity resulting in a stronger magnetic field and less heat generation.
The new magnetic material does increase the "magnetic" size of the
core/coil near 78% ! keeping the core and field coil the same physical
- A pair of brand new all aluminum diaphragms, same as original Altecs in 8, 16 or 24 Ohms. (NO after market here)
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.