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External Voltage
Generator
Overload Relay
Fig.5.2. Input section block diagram
Ext.
Filter
To First Modulator
and
Circuit Selector
?77392
The input section has two alternative input sockets. The Dl RECT INPUT
socket is a standard B & K input socket and accepts the B & K plug type
JP 0101. The input impedance of the DIRECT INPUT is 1 MQ//80 pF. The
PREAMP. INPUT is a standard 7 pin microphone input socket which
provides all the necessary power supplies for B & K microphone preampli-
fiers and condenser microphones. This socket accepts the B & K 7 pin plug
JP 0701. The input impedance of the PREAMP. INPUT is 900 kQ//180 pF.
Both inputs lead to the INPUT SECTION ATTENUATOR which is
divided into two sections in order to obtain the best signal-to-noise ratio in
all switch positions. The attenuator steps down the input signal in accurate
10 dB steps up to a maximum attenuation of 90 dB. This enables a maxi-
mum input voltage of 300 V RMS to be measured in the calibrated mode.
However, if the extra attenuation of the GAIN CONTROL is also used,
input voltages up
to
700 V peak can be measured. When the GAl N CON
-
TROL is moved from its "Cal." position, the total attenuation is not a
specific number of 10 dB steps, and a small indicator lamp is lit indicating
this uncalibrated mode.
The input amplifier, similarly, is divided into two sections with an atten-
uator section in between. This ensures the best overall signal-to-noise ratio.
The first amplifier section contains a balanced input stage to regulate the
sensitivity of the two input sockets. The two SENS. potentiometers and the
40
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