The rear-panelphonojack markedAUX INwill 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-
impedanceoutput selected bytheswitchabovethe con-
nector.This output isthe "mixed" output of all the input
sources and is designed to work into a balanced or un-
balanced 25-to 600-ohmmicrophone lineor intoa high-
impedance unbalanced amplifier or tape recorder
microphone input. The connector is a professional
three-pin audio connector.* See Figure2for 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 alsodrive the input of
a Shure Model M63 AUDIO MASTERQ.Interconnecting
cables should be limited to a maximum length of about
25m (75 ft).
Controls
Inaddition to the power ONlOFF switch (an adjacent
LED indicates power-oninac operation), the front panel
contains five individual input gain controls designated
MIC 1through MIC 4 and AUX, and a MASTERgain con-
trol for the total program output. Note that the input
connectorsare locatedonthe rear paneldirectly 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 mixerstoobtainadditional inputs without
usingany of the M268 inputs. Connectingthe 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 M268providespower 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
inthe LO position,
+
30 Vdc isappliedto pins 2 and 3of
each input connector. Series current-limitingresistance
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-poweredcondensers) can be used incombina-
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
Inaddition to ac operation, the M268can beoperated
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 recommendedboth 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. Notethat 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-linefuse 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 linevoltage 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 ModelA268R Rack Panel Kitconsistsof 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.
