4
Mix
Bus
Direct
access to
the mixing bus is provided through the
rear-panel MIX BUS phono pin jack. This provision is
made
primarily
to facilitate stacking or “multing” M267s to
achieve additional input capacity without losing any in-
puts.
With two M267s, for example, the two mixing buses
are
directly connected, providing two independent master
gain
controls and two isolated line amplifiers with eight in
-
dividually controlled inputs. Since the buses are directly
paralleled,
a 6 dB drop in gain will occur; and the master
or
input controls must be
increased to compensate. Noise
specifications
are not adversely af
fected by this intercon
-
nection. Mix bus interconnection can also be made with
other
Shure mixers, such as the M268, FP42. etc.
VU Meter
The
VU meter is factory-calibrated for use with a 600
terminated
line.
The VU RANGE switch on the rear panel
selects
either a +4 or +8 dBm output at 0 VU meter indica
-
tion.
(This switch changes the meter indication but does
not change the actual output level.) Microphone output
levels
are 50 dB below line
output. The +4 range is recom
-
mended
for normal use to provide approximately 14 dB of
headroom
from operating level to clipping level.
The
VU meter is
illuminated by two No. 86 lamps operat
-
ing
well under their normal ratings for a life expectancy of
greater
than 10,000 hours. The lamps are only lit
during
ac
operation. Consequently
, the illumination serves as a
visual alarm if the ac is interrupted and the unit has
switched
to battery
.
The VU meter is connected on the primary side of the
output
transformer to assure protection from any dc level
on
a telephone line.
Headphones
The headphone outlet appears at the front-panel jack
panel designated PHONES. The two-circuit phone jack
will
accommodate most stereo
or mono headphones. The
output
level
is suf
ficient to provide high volume for use in
noisy
environments.
Note that the headphone output level is also high
enough
to
use as an auxiliary unbalanced line feed to drive
a
tape deck or a power amplifier
.
The tip and ring connections of the headphone plugs
are
in phase with pin 3 of all input
and output connectors,
and
with the tip of the MIX BUS jack.
T
one Oscillator
The
highly stable, low-distortion tone oscillator provides
for line test and level checks.
The oscillator is instantly ac
-
tivated
by the front-panel INPUT
1/OSC 1 switch; its level
can then be controlled by the INPUT 1 gain control on the
front
panel. The tone oscillator frequency is 1,000 Hz, and
the signal appears on both the line and microphone out-
puts,
as well as the headphone and mix bus connectors.
The oscillator should be switched off (INPUT 1 position)
when
not in use.
Phantom Power
The
rear-panel PHANT
OM OFF/ON switch controls the
application of phantom power for condenser micro-
phones,
such as the Shure SM81 and SM85, to all inputs.
With
the switch on and the
rear-panel MIC/LINE switches
in the MIC positions, +30 Vdc is applied
to
pins 2 and 3 of
each
input connection. Series current-limiting resistance
is
3.3 k
for
each
input. When using other condenser mi
-
crophones
with the M267, verify that the voltage and resis
-
tance
requirements are compatible.
Note that the phantom power cannot normally be ap-
plied
to the inputs with the MIC/LINE switches in the LINE
position.
IMPORTANT
:
Do
not
turn the PHANT
OM switch on
when
using
unbalanced
low-impedance microphones; objec-
tionable hum will result. Turn off the PHANTOM switch
when
condenser microphones are not being used.
Use
only high-quality cable.
Intermittent shorts between
broken shield wires and balanced conductors will cause
offensive
noise transients in the system.
BATTER
Y OPERA
TION AND EXTERNAL POWERING
In addition to ac operation, the M267 can be operated
from
an internal battery pack. Current drain is typically 17
mA
at +8 dBm output
level and typically 15 mA at + 4 dBm.
Battery
power is recommended both
for remote, on-loca
-
tion operation, and as an emergency backup source in
case
of failure of the ac power
.
Access to the battery compartment is provided at the
bottom
of the
chassis. Three 9-volt alkaline batteries pow
-
er
the M267 at full rated output. Use alkaline batteries for
maximum life. Duracell MN1604A or Eveready 522 are
recommended. Battery life is approximately 20 hours at
+4 dBm continuos use. Note that battery operation with
phantom
power loads
and high level headphone monitor
-
ing
will increase battery drain.
With
batteries in the battery compartment, the M267 will
automatically and silently switch to battery operation
should
the ac voltage fall below a suitable level. If the ac
power fails completely, the VU meter lamps will go out,
providing
a visual indication of line failure.
Battery
condition can be determined by using the BA
TT
CHECK switch on the front panel. Activate the BATT
CHECK switch and observe the VU meter. A new set of
batteries
will give about a +2 VU indication. Battery condi
-
tion
is good if the reading
is above 0 VU; a lower reading
means that new batteries are required for proper opera-
tion.
Note that the M267 power switch must be turned on
to
check battery condition.
Telephone Interconnection
When
using the M267 connected directly to a telephone
line, check to see whether the local telephone company
requires an interface coupler between the M267 and the
