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The rear-panel phono jack marked AUX IN will accept
adversely affected by this interconnection. Mix bus in-
output from a high-impedance, high-level source such
terconnection can also be made with other Shure mix-
as a tape recorder or am-fm tuner.
ers such as the M267 and SE30.
SHIELD
SHIELD
MICROPHONE
MIXER INPUT MICROPHONE
MIXER INPUT
LOW IMPEDANCE
HIGH IMPEDANCE
(BALANCED
LINE
1
MICROPHONE
INPUT CONNECTIONS
SHIELD
-----7
SHIELD-
- - - -
- -
-
/ ,
.
-
\
I
\------A/
MIC OUTPUT
MIC OUTPUT
LOW IMPEDANCE
HIGH
IMPEDANCE
(BALANCED LINE
)
MICROPHONE
OUTPUT
PLUG
CONNECTIONS
FIGURE 2
Outputs
The connector marked MIC OUTPUT is a dual-
impedance output selected by the switch above the con-
nector. This output is the "mixed" output of all the input
sources and is designed to work into a balanced or un-
balanced 25- to 600-ohm microphone line or into a high-
impedance unbalanced amplifier or tape recorder
microphone input. The connector is a professional
three-pin audio connector.* See Figure 2 for output con-
nector configurations.
The phono jack marked AUX OUT is a high-
impedance, high-level, unbalanced output designed
primarily to feed a power amplifier requiring 0.1 to 2
volts input, or the auxiliary or tuner input of an amplifier
or tape recorder. This output will also drive the input of
a Shure Model M63 AUDIO MASTERQ. Interconnecting
cables should be limited to a maximum length of about
25m (75 ft).
Controls
In addition to the power ONlOFF switch (an adjacent
LED indicates power-on in ac operation), the front panel
contains five individual input gain controls designated
MIC 1 through MIC 4 and AUX, and a MASTER gain con-
trol for the total program output. Note that the input
connectors are located on the rear panel directly behind
their corresponding gain controls.
The M268 has feedback-type active gain controls for
lower noise and greater dynamic range. In general, the
individual gain control adjusted for the required output.
If overload distortion (clipping) occurs when using high-
level sources, reduce the individual gain control set-
tings. Unused individual gain controls should be kept at
the minimum setting (counterclockwise).
Mix Bus
The rear-panel MIX BUS phono jack facilitates the
"stacking" of mixers to obtain additional inputs without
using any of the M268 inputs. Connecting the mix buses
of two M268s, for instance, directly connects their mix-
ing systems, providing two independent master con-
trols and two isolated output amplifiers with 10 in-
dividually controlled inputs. Note that the gain will be
reduced by 6 dB, but noise specifications are not
' Designed to mate with Cannon XL series, Switchcraft A3
(Q.G.) series or
equivalent connector.
Simplex Power
The M268 provides power for condenser microphones
such as the Shure SM81 and SM85. The rear-panel
SIMPLEX OFFION switch controls the application of
simplex voltage to all low-impedance inputs. With the
SIMPLEX switch on and the rear-panel LOlHl switches
in the LO position,
+
30 Vdc is applied to pins 2 and 3 of
each input connector. Series current-limiting resistance
is 3.3 kilohms for each input. When using other con-
denser microphones with the M268, verify that the
voltage and resistance requirements are compatible.
Balanced low-impedance microphones (dynamic, rib-
bon, self-powered condensers) can be used in combina-
tion with simplex-powered condenser microphones. IM-
PORTANT: Do not turn on the SIMPLEX switch when
using unbalanced low-impedance microphones; objec-
tionable hum will result. Turn off the SIMPLEX switch
when condenser microphones are not being used.
Use only high-quality cable, as intermittent shorts
between broken shield wires and balanced conductors
will cause offensive noise transients in the system.
BATTERY OPERATION
-
EXTERNAL POWER
In addition to ac operation, the M268 can be operated
from an external battery supply (Shure Model A268B) or
any well-filtered dc supply providing 30 Vdc. Current
drain is typically 12.5 mA at 8.OV output level. Battery
operation is recommended both for remote, on-location
operation, and as an emergency source in case of
failure of the ac power source.
The A268B contains three 9-volt transistor radio bat-
teries which will power the M268 at full rated output.
Alkaline battery life is approximately 40 hours at 4.OV
output, 4 hours per day use. Note that battery operation
with simplex-powered microphones will increase bat-
tery drain.
To power the M268 from an external 30 Vdc source,
obtain a power plug (Switchcraft S-760 or S-765 or
equivalent) and attach it to the power source leads,
observing proper polarity.
CAUTION: The 30 Vdc input circuit of the M268 is not
fused. An external 30 Vdc source should be provided
with a 0.25A, 250V in-line fuse as a safety precaution.
With a battery supply or external dc supply con-
nected, the M268 will automatically and silently switch
to battery operation should the ac line voltage fail.
ACCESSORIES
The Model A268B Battery Power Supply can be used
as a sole source of power for the M268, or as a standby
supply in case of ac power failure Attaches to side of
M268.
The Model A268R Rack Panel Kit consists of a 19 in. x
3% in. (483 mm x 89 mm) precut rack panel and
necessary hardware for rack mounting the M268 in a
standard 19 in. (483 mm) rack panel.
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