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12.
INPUT AMPLIFIER: With program applied, set
the
Gain
control
so that the
green
"10"
LED lights
as
often
as
possible without
the red
"0"
LED turning on
at all.
If
the red
LED does turn
on, the
Gain setting should
be
reduced
.
CAUTION REGARDING FEEDBACK
Feedback
is
not only annoying
to the ear,
but
is
also dangerous
to
unprotected amplifiers
and
loudspeakers.
The Feedback
Suppressor section
of
the
Model
562
incorporates
a
protection
circuit which
is
activated
in the
Setup position
of the
front
panel toggle switch.
In
Setup mode, the output voltage
to the
power amplifier
is
limited
to
a
safer value by means
of
clipping excessive levels.
The
protective circuitry described
is
not
guaranteed
to
prevent damage
to
amplifiers or speakers.
Power amplifiers could
be set too
high and signal levels
at
the
protective circuit may
be too
low for
it to be
activated
before dangerous levels are achieved.
BE
CAREFUL
OF
YOUR
AMPLIFIERS AND SPEAKERS.
3.2
DISCUSSION
OF
FEEDBACK
3.2.1
GENERAL
The quality of amplified sound
in
a
room depends not only on
the
microphone, amplifier, loudspeakers, etc., but also
to
a
large
degree on
the
acoustical characteristics
of the
room itself.
Sounds reflected by walls
and ceiling add to and
subtract from
the
direct sound from
a
loudspeaker
as it
arrives
at
a
listener's
ear.
Various frequencies reflect
in
different amounts,
and resonant
peaks
and
dips will occur.
Peaks
in the
frequency response cause
coloration
of the
original sound.
They will mask adjacent frequen-
cies of the
program material.
Aside from adversely influencing the
quality
of
music
and the
intelligibility
of speech
reproduction,
these peaks
in
the
frequency response also reduce
the
maximum
obtainable average sound level throughout
the
audio spectrum.
The
average sound level
is
limited
by those
frequencies whose amplitude
approach or exceed
a
system gain
of unity.
This condition
is
marked
by
ringing and feedback.
Ringing
is
a
prolongation
of the
reverberation
time for those
frequencies
which are approaching unity gain.
Feedback
is
a
spontaneous
oscillation
at the
frequency where loop gain (including
the acoustic
environment) exceeds unity.
Reduction
of
acoustic resonances will produce
the following
desirable results:
a)
More sound level through higher achievable gain.
b)
More freedom for performers.
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11
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