2
Connections
As shipped the microphone is connected for
an output impedance of 250 ohms. To connect
the microphone for an output impedance of 30
or 150 ohms, first lower the bottom cover by
removing the four machine screws around the
rim of the cover. Pull the cover down until
the transformer terminal board connections
are accessible. For the proper cable con-
nections, refer to figure 3.
Directional Characteristics
The adjustable shutter over the slot in
the tube leading to the acoustic labyrinth
may be rotated by means of a screwdriver ad-
justment extending through the rear screen
flush with a designation plate.
The plate is marked U, N and B, as symbols
for the uni -directional, non -directional and
bi-directional patterns. Three additional
markings L -1, L -2, and L -3 are used as
reference points for other directional
patterns which may be obtained. Refer to
figure 4 for the patterns associated with
each of the six symbols. Stops are provided
on the continuously-variable pattern selector
at the six marked positions, although the
shutter may be set at any intermediate
position.
Frequency Response
At the bottom of the lower shell is a
screwdriver -operated selector marked M (music)
V1 and V2 (voice). The voice positions
connect a reactor across the entire secondary
or part of the secondary of the output trans-
former, depending on the switch position (see
diagram, figure 3). Refer to figure 5 for the
frequency-response characteristics of each
setting. As can be seen from the curves, the
reactor attenuates the low- frequency response.
This is especially desirable when the micro-
phone is less than three feet from the source
of sound and the low-frequency response would
otherwise be exaggerated.
Phasing
The Type 77 -DX Microphone is phased so that
the red cable lead is electrically positive
when the sound pressure on the front of the
microphone is in the positive half of the
cycle.
When the outputs of two or more microphones
are connected into a mixing circuit, it is
necessary that the outputs of all such micro-
phones h'ave the same phase relation. Other-
wise, the output of one microphone will
oppose the output of another, resulting in a
reduction in output, and introducing varying
degrees of distortion.
To check the phasing of two or more micro-
phones, connect one microphone to the associ-
ated amplifier input and set the volume
control to obtain the desired output, while
talking into the microphone. Then, connect
the second microphone in parallel with the
first and, without changing the volume
control setting, hold both microphones close
together and talk into them. If the volume
decreases from the previous level, reverse
the connections of one of the microphone
cables at the microphone plug. Check each
additional microphone for phasing in this
manner, and, if necessary, reverse the cable
connections to correct the phasing to agree
with that of the microphone already connected.
When the sound source is directed toward
the back of the microphone, there will be a
large phase shift when changing the pattern
selector from bi- directional to non-direction-
al or the reverse. The safest way to avoid
undesirable directional effects resulting
from the above is to set microphones operating
close to one another on the same directional
response position, or at least avoid having
some on the non-directional pattern and the
others on the bi- directional pattern.
Directional Setting
The proper position of the pattern se-
lector depends upon the particular instal-
lation. The same holds true for the placement
of the microphone. Consult figure 4 for the
directional patterns of the six reference
positions.
