An analysis of the effect of the power applied to the field coil.



hh There are two major qualities of a vintage field coil loudspeaker that affect the sound it produces.  The first, and probably the most effective and the most obvious is the age of the paper cone.  Consider a paper cone speaker dating to the fifties, or even the forties.  These fellas are some seventy years old and much time takes its toll on paper, even under the best of conditions.  In short, it dries up and becomes brittle.  While still playable, moderation is of considerable importance as one good transient bass note can crack the cone or even shatter it like glass.  So that, in a nutshell is the main reason for the sound of vintage speakers.  Loaded with harmonics and spurious distortions caused by cone breakup, they are anything but accurate reproducers but, to many, they sound damn good.  It's called subjectivity, that human characteristic that determines why one prefers one flavour over another or blondes over brunettes over redheads.  'Nuff said.

The other characteristic is the motor, or magnet.  The stronger the field in the gap, the more accurately the cone and coil will move in unison with the incoming electric signal.  Inertia, due to mass, will make the diaphragm move after the signal and momentum will keep it moving in one direction when the signal changes direction.  Reducing cone and coil mass will alleviate this with consequences such as cone breakup.  Back then, exotic cone materials weren't available.   The alternative was a stiff but heavy cone, which required a stronger magnet.  Alnico magnets of practical size for speakers produced about one Tesla, 1T.  Today we have neodymium and cobalt-samarium, producing as much as 1.4T.  The alternative was an electric field coil. The first, before the forties, were actually used as a choke for the tubes DC supply as high voltage electrolytics were too large for radios.  So, the choke was placed around the pole-piece and voila, a magnet.  The problem was 120 hz AC hum, so a low resistance coil was placed above it just under the spider.  This demodulator coil was called a hum bucking coil, as it reduced the hum from the field coil.  Later, in the forties, separate DC supplies were used to power the field coils as this was probably cheaper than Alnico, especially with WW-II raging.   Today, field coil speakers are made but serve little purpose other than being a marketing thing.  However, they do have one good quality.  If the field coil is designed right, it can handle large current and generate a strong magnetic field.  Keep in mind that the huge magnets used in the LHC, Large Hadron Collider, are electric.  Therefore, by increasing the current, we get a stronger magnet.  There is a caveat, though, regarding vintage speakers.  The coils are old and originally designed for a specific current.  They will get hot and with a vintage speaker, this can be detrimental to its survival.  See:  Field Coil Data

So, that's what this study is all about.  How much does the strength of the magnetic field affect the speaker's sound?  The following tests do show some difference when comparing equal signals with the field coil energized with 8 watts and 15 watts.  Interpreting this as to how it affects the sound and whether or not it will be liked is another thing.



One of the usual suspects, a Jensen F12N field coil loudspeaker of which there are two, both with 1000 ohms field coils.





The following are alignments derived by BassBoxPro v6.  The RED and ORANGE are optimum alignments and T/S parameters with 10W (red) and 15W (orange) powered field coils.  Note the difference between the RED and ORANGE alignments, especially regarding box volume.

The YELLOW and GREEN are those alignments with a box volume of 4 cubic feet, which was picked by trial and error to obtain similar amplitude responses.


  RED                    ORANGE YELLOW                  GREEN   















MIC: Behringer ECM-8000 powered by an ART vacuum tube mic pre-amplifier.  Proximity effect was ignored as this test is to determine comparative results only.


The output of the mic pre-amplifier is fed into a PicoScope P-2205A PC USB oscilloscope.  The ART mic preamp is atop the Adcom preamp and behind it is the CLIO, which generates the required signal, which is monitored by the PicoScope.  The CLIO is controlled by the laptop; the PicoScope by a desktop under the work bench.  The displayed result is photographed as the persistence mode of the PicoScope doesn't give as fine an image.







The following results were measured near field with just under 1W applied to voice coil

The actual power transfers considering the impedance at each frequency is stated above each photo next to the power applied to the field coil

The odd appearing 51hz is the free air resonance of the speaker, the point at which the cone will oscillate the easiest, a worst case scenario.

The drive signal generated by the CLIO is a one cycle sine wave pulse.  At 51hz, this pulse is 19.6mS; at 1khz, it's 1mS; at 4 khz, it's 0.25mS and at 30hz, it's 33.3mS.

The use of the 30hz pulse was to show the loss of control, damping, on the cone by the magnetic field in the voice coil gap. SEE PHOTOS 5D AND 8D. These two show the cone runaway, damped oscillation which persists after the electrical signal stops.



51hz @ 22W   0.013W (13mW)

The pulse driving the speaker is similar to the red one shown in the last four images.

Brain pharght.




51hz @ 14W   0.013W

Careful observation among the series 5 photos will show that the negative and positive peaks of the cone are increasing in amplitude.  This is due to the decreased effect of damping by the magnetic field strength in the voice coil gap.


51hz @ 8W   0.013W

Here we see a distortion in the left negative peak, most likely caused by saturation of the field in the voice coil gap.  In the next photo, this distortion appears in all three peaks, the left one being barely perceptible; the top of which is slightly flattened.


51hz @ 2.5W   0.013W

The extra ripple at the far right is due to the un-damped cone continuing to oscillate due to the elasticity of the spider, the annulus and its mass.

Lowering the current through the field coil will weaken the magnetic flux in the voice coil gap.  This can be useful to a musician, especially one playing a bass guitar to change the sound of the instrument.  The caveat, though is a loss of sound pressure level which can be several dB.

See also photo 8D.


1khz @ 22W   0.44W

These four of the 6 series all appear the same but careful inspection will show the amplitude is decreasing as the field coil strength is decreased.  The ripples going right are due to traverse waves in the cone and cone breakup.  Cone breakup happens when different areas of the cone vibrate at different frequencies and phase relationships.

See the photo at the bottom of this page.




1khz @ 14W   0.44W


1khz @ 8W   0.44W


1khz @ 2.5W   0.44W


4khz @ 22W   0.23W

The RED trace in the following photos is the signal applied to the voice coil; the black one is the response of the speaker as picked up by the mic.  Note that there is a delay of a little more than a quarter of a millisecond between the peaks.  The electrical signal is gone before the speaker responds.  This is due to the inductive voice coil, the current lags behind the voltage.  At 4khz, the cycle duration is 1/4 mS (0.00025 second).  the extra lag time shown by the peak in the black trace vs the red one is due to the mass of the cone and coil assembly, inertia.

So, what has all that to do with field coil speakers?  Nothing, because the same phenomenon appears in permanent magnet speakers.  It was mentioned because the traces caught my interest and I couldn't resist the diatribe.  Details, baby. Details.

In the previous photos, I forgot to connect channel B on the scope to the voice coil.

Brain pharght.



4khz @ 14W   0.23W

In these four traces, the ripples at the far right all appear to be much the same.  They are most likely due to traverse waves in the cone and cone breakup.  At this high frequency, the cone is not moving in unison with the 4khz signal.  Most of what is heard is the 4khz vibration of the coil creating ripples (traverse waves) in the cone, much like the ripple seen in a garden hose laying on the ground after one end is whipped by hand.  This is similar to that which happens with a bullwhip but goes unnoticed, especially to one on the expending end of the aforementioned bullwhip.  By the way, the crack made by such a whip is due to the end of the whip breaking the sound barrier.


4khz @ 8W   0.23W


4khz @ 2.5W   0.23W


30hz @ 22W   0.3W


30hz @ 14W   0.3W


30hz @ 8W   0.3W


30hz @ 2.5W   0.3W

Here, at 30hz, the effect of un-damped oscillation is more pronounced as can be seen in the extra ripples at the far right.  The peaks are even higher than those of the incoming signal, again due to lack of control on the moving part of the system by the magnetic field.





Black 22W to field coil     RED 8W to field coil




Magnification 1   Black 22W to field coil     RED 8W to field coil




Magnification 2   Black 22W to field coil     RED 8W to field coil






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