1.
INTRODUCTION
1.1
GENERAL
The
1/2"
Preamplifier
Type
2619
is
designed especially for use with the
B&K
condenser microphone cartridges. It features a very high input
impedance field·effect-transistor input, which presents virtually no load
to
the
microphone cartridge. This gives
the
combination a wide frequency
range
of
2
Hz-200
kHz, and a wide dynamic range with a lower limit of
36
dB
re 2 x 10-
4
JJbar for
the
1/2"
microphone.
The
preamplifier can be operated from a
120
V
DC
supply, such as the
condenser microphone
input
of
the
B&K
microphone amplifiers, or from a
28
V
DC
supply, with slightly different specifications. A heater coil
is
built
into
the
tip
of
the
preamplifier
to
eliminate moisture
in
the
microphone
under conditions
of
high relative humidity.
Accessories included are: a gooseneck
UA
0196
to
facilitate measure-
ments
in
high
temperature
(150
0
C)
by taking
the
preamplifier away from
the
transducer, and for providing directional flexibility for the microphone,
an
adaptor
JJ
2615
for
the
connection
of
miniature cables such as from an
accelerometer, and containing a
50
pF capacitor
to
block
the
polarization
voltage,
and an
adaptor
for
1"
microphone cartridges,
DB
0375.
1.2 PREAMPLIFIER REQUIREMENTS
When a capaCitive
type
transducer such as a condenser microphone, or a
piezoelectric accelerometer
is
used for measuring purposes, it normally must
be connected
to
a high impedance,
to
preserve
the
low level signal. This
is
because
the
capacitance associated with
the
transducer
is
very small, and
the
capacitance of even fairly
short
cable lengths can load
the
transducer
to
seriously reduce
the
signal appearing
at
the
other
end
of
the
cable. An
equivalent circuit diagram of a microphone and preamplifier
input
circuit
is
shown in Fig.1.1a.
3
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