Hammond Model A, Руководство пользователя, Страница: 35 / 298

THEORY OF OPERATION
The console of the Hammond Organ contains the entire tone-producing
mechanism, which is completely electrical in operation. Within it are
produced all the tones and tone combinations of the organ. The electrical
waves are made audible, as music, by one or more tone cabinets containing
suitable amplifiers and loud speakers. The block diagrams (Figures 13 and
14) show the chief components of the instrument.
Electrical impulses of various frequencies are produced within a unit known
as the tone generator, containing a number of phonic wheels or tone wheels driven
at predetermined speeds by a motor and gear arrangement. Each phonic wheel is
similar to a gear, with high and low spots, or teeth, on its edge. As the wheel
rotates these teeth pass near a permanent magnet, and the resulting variations in
the magnetic field induce a voltage in a coil wound on the magnet. This small
voltage, when suitably filtered, produces one note of the musical scale, its pitch
or frequency depending on the number of teeth passing the magnet each second.
A note of the organ, played on either manual or the pedal keyboard, generally
consists of a fundamental pitch and a number of harmonics, or multiples
of the fundamental frequency. The fundamental and eight harmonics available
on each playing key are individually controllable by means of drawbars and
preset keys or buttons. By suitable adjustment of these controls the player
is enabled to vary the tone colors at will.
The resulting signal passes through the expression or volume control and
through the preamplifier (where vibrato is introduced) to the tone cabinet.
Here reverberation is added electrically and a power amplifier feeds the
signal into loud speakers.
DESCRIPTION
A Hammond Organ console (Fig. 2) includes two manuals or keyboards: the
lower, or Great, and the upper, or Swell, and a pedal keyboard of 25 keys.
The concert models have a 32-key pedalclavier and are constructed to A. G. O.
specifications. Various controls have appeared on different models. The operation of
these controls is covered in the following paragraphs.
STARTING THE ORGAN
To start the organ, hold the start switch (Fig. 1)
in on position for approximately eight seconds.
Still holding it, push the run switch to on
position. After leaving both switches on for about four
seconds, release the start switch to return to its
normal position.
If the console is very cold, or if a frequency regulator
is used, it may be necessary to hold the start switch
slightly longer.
FIGURE 1
2