Hammond Model A, Manual, Page: 138 / 298

A sound wave from the stirrup travels down the open spring at the far right to the
crystal pickup, where an electrical signal is produced and conducted to the power
amplifier. This is the first reflected signal, delayed about 1/15 second from the
part of the original signal which went directly to the power amplifier.
The same wave from the stirrup also travels down the second spring from the left,
which enters the short damping tube. At the bottom of this spring the wave is
reflected back along the spring, reduced in intensity by the damping action of the
fluid. At the stirrup the horizontal lever transfers the wave to the right-hand
spring, and it goes on to the crystal to produce a second reflected signal about
3/15 second after the direct signal.
Very little of the energy of each wave is absorbed by the crystal, and the rest
is reflected back along the spring. The first reflected signal traverses the
right spring, is transferred by the lever, and goes down the spring to the short
damping tube.
Here it is reflected in reduced intensity, retraces the same path to the crystal,
and produces a third reflected signal about 5 /15 second after the direct signal.
The second reflected signal is similarly repeated, and this process continues
over and over, giving a series of signals about 2/15 second apart, until the
vibration is dissipated by fluid friction in the short tube. Just above the short
damping tube a reflecting pin attached to the spring causes partial reflection of
high frequencies and helps to make the overall response more uniform.
The damper felt avoids undesirable transverse vibration of the springs.
A greater amount of fluid in the short tube will cause increased energy loss at
each reflection and thereby reduce the number of audible reflections. Adjusting
the level of fluid in this tube, therefore, changes the reverberation time and
simulates enclosures of different sizes.
A reverberation selector switch in the amplifier circuit following the crystal can
be adjusted to pass more or less of the reflected signal in proportion to the direct
signal. While this does not actually change the reverberation time, it is a
convenient way to change the amount of reverberation instantly. Generally,
therefore, the fluid level in the short tube is left constant, at the position
recommended on the tone cabinet instruction card, and the switch is used to select
the best amount of reverberation for each installation.
The photograph of the reverberation unit (figure 1) shows a reverberation
preamplifier of the type used in kits for installation in some non-reverberation
tone cabinets. In later reverberation type tone cabinets the power amplifier is
wired so that this preamplifier is unnecessary.
Amplifier circuits associated with the reverberation unit are shown in the
section containing amplifier diagrams.
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