Hammond Model A, Manual, Page: 122 / 298

THE HAMMOND VIBRATO
Hammond Organ consoles equipped with vibrato differ from tremulant models in the
omission of the tremulant switch, tremulant control, and non-vibrato preamplifier,
and in the addition of the vibrato line box, scanner, vibrato switch, and vibrato
preamplifier. Three degrees of vibrato are available and also a different degree of
chorus or celeste effect with each of the three degrees of vibrato. Console models
with the suffix -2 in their model designation have the selective vibrato feature,
with tilting control tablets permitting the player to place the vibrato effect on
either manual or both.
A conversion kit is available for installation in most earlier consoles not having
the vibrato. but it does not incorporate the selective feature.
PRINCIPLE OF OPERATION
The vibrato effect is created by a periodic raising and lowering of pitch, and thus
is fundamentally different from a tremolo, or loudness variation. It is comparable
to the effect produced when a violinist moves his finger back and forth on a string
while playing. Varying the frequency while maintaining constant volume.
Figure 1 - FUNDAMENTAL DIAGRAM OF VIBRATO EQUIPMENT
The Hammond Organ vibrato equipment (see simplified block diagram figure 1) varies
the frequency of all tones by continuously shifting their phase. It includes a phase
shift network or electrical time delay line, composed of a number of low pass filter
sections, and a capacity type pickup or scanner which is motor driven so that it
scans back and forth along the line.
Electrical waves fed into the line are shifted in phase by each line section (the
amount per section being proportional to frequency), so that at any tap on the line
the phase is retarded relative to the previous tap.
The scanning pick-up traveling along the line will thus encounter waves increasingly
retarded in phase at each successive tap, and the signal it picks up will
continuously change in phase. The rate at which this phase shift occurs will depend
on how many line sections are scanned each second.
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