BRUEL & KJAER 4111 - Page 11

8
lower.
This capacity ( Cf) is about 10-15 pF and the cartridge capa-
city (C ) is around 50 pF.
c
The added capacity of the cathode follower
will decrease the sensitivity · by the ratio of Cc to Cc + Cf"
The sen-
sitivity loss owing to this capacity in the cathode follower is around
1 1 I 2 - 2 db.
Secondly, the cathode follower will cause the drop
A/ A + 1 where A represents the amplification obtainable without feed-
back; this drop amounts to o. 5 db.
The sensitivity is measured with a polarization voltage as indicated on
the calibration curve of the cartridge.
If the microphone is attached to an analyzer the polarization of which
is different from that with which the microphone has been measured,
a correction for the difference in polarization voltage must be made,
the sensitivity being directly proportional to the polarization voltage.
The polarization voltage given by the Analyzer type 2105 should be
measured with a precision voltmeter with max. 50 fA current con-
sumption.
When this is done one e for all, the polarization voltage
should be marked on the condenser microphone's sensitivity curve, and
the condenser microphone's absolute sensitivity should be corrected to
the right polarization voltage.
If the analyzer's voltage regulator tube
V6 is replaced for any reason the polarization voltage must once again
be measured.
For practical reasons, the sensitivity is given as defined above, in-
stead of the open circuit voltage of the microphone cartridge when
working into an infinite impedance, as required by the American Stan-
dard Z 24. 4.
With the microphone used in combination with Micro-