HAMMOND ORGAN
The next items to look at are the typical loudspeakers that the Hammond company used in many of their speaker cabinets, or tone cabinets to use the correct terminology. A very common example is the so-called electro dynamic speaker which I show in cross section below. The other type of speaker which they used, especially in their newer speaker cabinets is the permanent-magnet speaker. The only real difference between the two is that in the electrodynamic speaker, the necessary magnetic field to make the speaker work is created by direct current flowing through a multi-turn coil which is part of the field structure on the speaker. Permanent or PM speakers use a permanent magnet and dispense with the field coil and also the so-called hum-bucking coil. Here is a cross section of an electrodynamic speaker. To make this simpler, I have left out the supporting frame or "basket" which holds everything in correct alignment and the speaker cone center sus-pension.
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In
operation, a steady direct current flows through the field coil, magnetizing
the field structure as shown. The audio signal is applied to a small coil
(called the voice coil) of wire wrapped on an insulated tube which is an extension
of the speaker cone at the center as shown.
The arrangement is such that the voice coil
lies in the gap between the north and south magnet poles. When current flows
in one direction through the voice coil, it creates a magnetic field which
reacts with the field in the gap and makes the cone move slightly to the right.
When the audio signal current reverses direction, it makes the cone move to
the left. The audio signal is always an alternating current, so as the current
fluctuates in response to the power amplifier's output signal, the speaker
cone faith-fully reproduces the variations of the audio signal as mechanical
motion, and the large surface area of the cone effectively generates a sound
wave which is a replica of the audio AC signal.
If there is even the slightest amount of 60Hz
AC power ripple in the DC that is applied to the field coil, this would react
with the voice coil and produce a slight 120 Hz hum in the audio output of
the speaker.
Figure 28. Diagram of electrodynamic speaker as used in many Hammond tone cabinets. This is a "cut in half" diagram, thus the coils are shown as tiny circles, as though you sawed the coil down the middle and looked at the actual ends of the wires that make up the various coils in the speaker.
In
order to keep this from happening, there is a small coil of wire right behind
the speaker field coil, and it is in series with the voice coil. If there
is any AC "ripple" in the speaker field current or the speaker's
magnetic field, this will, by transformer principle, induce a corresponding
ripple current in this "hum-bucking coil." The hum-bucking coil
is wired so that its output is applied to the voice coil in opposition to
any speaker cone motion that might be induced by AC ripple in the field current.
This effectively cancels the action of the slight field current ripple voltage
and eliminates the otherwise noticeable resulting AC power line humming from
the speaker.
In the newer PM speakers, there is no ripple
in the field of a permanent magnet and thus no need for a hum-bucking
coil. To my knowledge, electrodynamic speakers are no longer manufactured
because the new special alloys and compositions for magnets are much better
now than they were many years ago. Nevertheless, you can produce very
powerful magnetic fields by applying DC to a coil wrapped around iron
or steel bars, and many Hammond aficionados and preservationists will
have new replacement cones installed in existing electrodynamic speakers.
Below are two pictures of a Jensen model A-12, one of the preferred ED
speakers which Hammond used for many years in their tone cabinets.
Notice also that there is no need for any treble
speakers or tweeters in Hammond tone cabinets. Because the highest available
frequency is only 6kHz and also because Hammond didn't want to have the key
clicks emphasized, there was no reason to install speakers to handle any frequencies
in excess of 6 kHz.
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Figure 29. Jensen A12 electrodynamic speaker used in most early Hammond tone cabinets. Overall view showing hermetically sealed field assembly.
Figure 30. Interior close-up of the Jensen A12's field assembly showing main field coil (with label bearing patent numbers) and the hum-bucking coil, [white arrow]. Roll mouse cursor over picture for details.
Figures
twenty-nine and thirty give you an idea of what a typical electrodynamic
speaker looks like. These units were excellent, could take a lot of power,
were very conservatively rated and handled the entire range of a typical
Hammond without any problems. These speakers were always mounted in speaker
cabinets, however.
A little thought on how a speaker operates
will tell you that the cone produces a sound wave from both sides, and
by the very nature of the speaker and its operation, these two sound waves
are 180 degrees out of phase, which means that they will very effectively
cancel each other. If the wavelength of the soundwave is short, [less
than the radius of the speaker cone] cancellation will be markedly reduced,
however in the case of bass tones which have soundwaves many feet long,
no bass could be produced at all unless the speaker was enclosed in a
suitable cabinet to provide significant separation of these two out-of-phase
sound waves. This rule holds true even with today's modern speakers, which
is why speakers are always enclosed in some type of cabinet. In the typical
Hammond speaker cabinet, these speakers can produce bass that you can
feel as well as hear; the secret lying significantly in the design of
the enclosing tone cabinet.
The next and final item which we will look
at concerning the typical Hammond organ is the power amplifier which takes
the minute signal from the console and raises its power to a sufficient
level to drive these speakers to the point that they can rattle walls
and windows if you really want to push up the Hammond's volume.
