BRUEL & KJAER 2619 - Page 6

The preamplifier 2619 has an input capacitance of approx. 0.7 pF and a
resistance of 4
Gnat 20
0
C. This gives a loss of voltage of only 0.3 dB for a
1/2" microphone. The calculated cut-off frequency is 2 Hz, and this is
about the same as the mechanical cut-off of the microphone due to the
pressure
equalization.
R
c
R
a b
Fig. 1. 1a. Equivalent circuit for microphone and preamplifier input cicuit.
Fig.1.1b.
Simlified equivalent circuit
5