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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.
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