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BRUEL & KJAER 2204 - Input Amplifier; Weighting Network; Output Amplifiers and Attenuators; Rectifier and Indicating Meter

BRUEL & KJAER 2204
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Input
Amplifier
and
Attenuator.
The
input
attenuator
follows
immediately
after
the
source
follower
stage
and
is
designed
for
accurate
attenuation
of
the
input
signal
in
steps
of
10
dB. A
great
amount
of
negative
feedback
is
introduced
in
the
amplifier
in
order
to
ensure
a
high
input
impedance
and
stable
operation.
For
calibration
purposes
the
amplification
of
this
stage
can
be
altered
a few
dB
by
means
of
a
potentiometer
which
changes
the
amount
of
negative
feedback
in
the
circuit.
Weighting
Networks.
The
weighting
networks
(A, B
and
C)
are
introduced
between
the
input
amplifier
and
the
first
output
amplifier.
They
are
built
into
the
instrument
and
can
be
switched
into
circuit
by
means
of
a
knob
on
the
front
plate.
Terminals
are
also
provided
for
the
connection
of
external
filters
such
as
octave
or
1/3
octave
filters
for
sound
analysis.
The
output
impedance
of
the
EXT.
FILTER
IN
terminals
is
approximately
25
ohm.
while
the
input
impedance
of
the
EXT.
FILTER
OUT
terminals
is 146
kohm
in
parallel
with
45
pF.
Output
Amplifiers
and
Atlenuators.
The
output
from
the
filter
circuits
is
fed
through
two
amplifier
stages
with
associated
attenuators
.
The
attenuation
can
be
varied
accurately
in
steps
of
10 dB.
Stable
operation
IS
ensured
by
means
of
a
large
amount
of
negative
feedback.
Reetlfler
and
Indicating
Meter.
After
frequency
weighting
and
amplification
the
signal
is
fed
to
a
rectifier
and
then
to
the
indicating
meter.
The
rectifier
is a
full
wave
rectifier
with
characteristics
as
required
in
the
IEC
standard
for
sound
level
meters
(Publication
179),
providing
a
rectified
signal
which
corresponds to
the
RMS
value
of
the
input
from
the
microphone.
This
rectified
signal
is
fed
to
a
moving
coil
indicating
meter
which
includes
two
different
degrees
of
damp-
ing,
"Fast''
and
"Slow",
both
in
accordance
with
the
IEC
standard
for
precision
sound
level
meters.
The
meter
itself
is
ribbon
suspended
in
order
to
make
it
less
sensitive
to
shock
and
vibration.
Power
Supply.
The
Sound
Level
Meter
is
powered
by
three
ordinary
1.5 V
torch
batteries,
and
to
avoid
bad
contact
due
to
corrosion
the
battery
clips
are
gold
plated,
ensuring
a
negligible
contact
resistance.
The
HT
is
obtained
from
a
highly
stabilized
transistor
oscillator
working
at
a
frequency
of
1
kHz.
The
same
generator
supplies
a
signal
which
is
used
as a
calibration
signal
for
the
amplifiers
and
meter
circuit,
and
also
one
that
is
rectified
and
ustrl
as
polarization
voltage
(200
V)
for
the
micro·
phone.
14

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