Page 1
2 Outputs
IMPEDANCE
OUTPUT
Designed for
Use W
ith
Actual
(Internal) Output Clipping Level
Low Imp
Mic
Any low imp
(19 to 600 )
mic circuit
120 ±22 dB
(10
0 m V)
High Imp
Mic
Unbal 10 to
50 k mic
circuit
5 k +4.5 dBV
(1.7 V)
Aux 10 k or
greater
unbalanced
high-level cir-
cuits 2.5 k 18 dBV
(7.9 V)
Mix Bus 3.3 k 3.3 k ±8 dBV
(0.4 V)
Noise
Equivalent
Input Noise: ±128 dBV (low impedance
mic 150 - 300-20,000 Hz) at full gain
Equivalent Input Hum and Noise: ±125 dBV (low
impedance
mic 150 - 20-20,000 Hz) at full gain
Output Noise: ±90 dBV (master control down). ±65
dBV
(maste r u p), (inpu t control s d own , 3 00-20,00 0 H z)
Output
Hum a nd N oise : ±8 2 d BV ( maste r d own) , ± 60
dBV (master up) (inpu
t c ontrol s d own , 2 0-20,00 0 H z)
Common Mode Rejection
65 dB minimum with 100 mV input at 100 Hz
Control Interaction
Less than 1 dB with any control combination
Phase
All microphone inputs and outputs in phase; tips of
high-impedance
inputs and mix bus jack in phase with
pin 3; aux input in phase with aux outputs and pin 2
Phantom Power
30 Vdc open circuit, 3.3 k series resistance
Operating V oltage
120
or 240 volts +10%, 50/60 Hz, 5W , supplied wired
for 120 V ac operation (see Service section for 240 V ac
operation)
Temperature
Range
Operating: ±18 o to 57 o C (0 o to 135 o F)
Storage: ±29
o to 74 oC (±20 o to 165 o F)Dimensions
See Figure 1.
309 mm (12 5/32 in.)
68.8mm(2 23/32 in.)
6.3
mm(1/4 in. )
229 mm(9 in.)
FIGURE 1
Weight
Net: 1.8
kg (4 lb 1 oz)
Packaged: 2.8 kg (6 lb 1 oz)
Certifications
Listed by Underwriters' Laboratories, Inc., and listed
by
Canadian Standards Association as Certified
CONTROLS AND CONNECT ORS
WARNING
This apparatus must be earthed (grounded)! The
M268 power supply is energized when the unit is
connected to an ac source; disconnect mains
(power)
plug from supply when not in use.
Inputs
A
maximu m o f fou r low - a nd fou r h igh-impedanc e d ynamic,
ribbon
or condense r m icrophone s c an b e c onnecte d to r ecep-
tacles marked INPUT 1, INPUT 2, INPUT 3, and INPUT 4.
The
input s a re d esigne d for low-impedanc e m icrophone s w ith
19 to 600 impedance, or high-impedance microphones.
Both
low - a nd h igh-impedance m icrophone s c an b e u sed s i-
multaneously.
Crysta l or cerami c m icrophone s a re n ot recom-
mended. The input receptacles are professional three-pin
XLR
Audio-typ e c onnectors , a nd 1/4 phon e jacks . S ee Fig-
ure 2 for low- and high-impedance connections to inputs.
Note that some condenser microphones produce very high
output
signal s w hic h m ay o verloa d the input s o f m an y m ixers;
the M268's feedback-type active gain controls virtually elimi-
nate
the n eed for in-lin e a ttenuator s to c ompensat e f or these
ªhotº microphones.
1 2
3
MICROPHONEMIXER INPUT
SHIELD
LOW IMPEDANCE(BALANCED LINE)
MICROPHONE MIXER INPUT
SHIELD
HIGH IMPEDANCE
SHIELD SHIELD
LOW IMPEDANCE OUTPUT (BALANCED LINE) HIGH IMPEDANCE OUTPUT
MICROPHONE INPUT CONNECTIONS
MICROPHONE OUTPUT PLUG CONNECTIONS
1 23
FIGURE 2
Page 2
3 The rear-panel phone jack marked AUX IN will accept
output
from a high-impedance, high-level source such as
a tape recorder or am-fm tuner.
Outputs
The
connectors marked LOW IMP OUTPUT and HI IMP
OUTPUT are the ªmixedº output of all the input sources
and
are designed to work into a balanced (low impedance
only) or unbalanced 150- to 600- microphone line or into
a high-impedance unbalanced amplifier or tape recorder
microphone input. The output connectors are a profes-
sional three-pin XLR audio connector and a
1/4 phone
jack.
See Figure 2 for output connector configurations.
The phone and phono jacks marked AUX OUT are high-
impedance, high-level, unbalanced outputs designed pri -
marily
to feed a power amplifier requiring 0.1 to 2 volts in -
put, or the auxiliary or tuner input of an amplifier or tape
recorder. Interconnecting cables should be limited to a
maximum
length of about 25m (75 ft).
Controls
In
additio n t o the p owe r O N/OF F s witc h ( an a djacen t LED
indicates
power-on in a c o peration) , the f ron t p ane l contains
five
individua l inpu t gain c ontrol s d esignate d 1 t hroug h 4 a nd
AUX,
and a M ASTE R g ain c ontrol for the t ota l progra m o ut-
put.
N ote t ha t the i npu t c onnectors are l ocate d o n t he r ear
panel
directl y b ehin d t heir correspondin g g ain c ontrols.
The M268 has feedback-type active gain controls for
lower
noise and greater dynamic range. In general, the in -
dividual gain control should be set to 5 or higher (clock-
wise),
and the MASTER 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 rea r p ane l M IX B US p hon o jac k facilitate s the ª stack-
ingº of mixers to obtain additional inputs without using any
of
the M 26 8 inputs . C onnectin g the m ix b uses of two M 268s,
for
instance , d irectl y c onnect s t hei r m ixin g s ystems , p rovid-
ing
two independen t m aste r control s a nd two isolate d o utput
amplifiers
with 1 0 individuall y c ontrolle d inputs . Note tha t the
gain
will be r educe d b y 6 d B, but nois e s pecification s a re n ot
adversely
affecte d b y this interconnection . M ix b us intercon-
nections
can a lso b e m ad e w ith o the r S hur e m ixer s s uc h a s
the M26
7 a nd t he M 67.
Phantom Power
The M268 provides power for condenser microphones
such as the Shure SM81 and SM85. The rear-panel
PHANTOM OFF/ON switch controls the application of
phantom voltage to all low-impedance inputs. With the
PHANTOM
switch on, +30 Vdc is applied to pins 2 and 3
of each LO IMP connector. Series current-limiting resis-
tance
is 3.3 k for each input. When using other condens -
er
microphones with the M268, verify that the voltage and
resistance are compatible.
Balanced low-impedance microphones (dynamic, rib-
bon,
self-powered condensers) can be used in combina -
tion with phantom-powered condenser microphones.
IMPORTANT: Do not turn on the PHANT OM switch 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, as intermittent shorts be-
tween broken shied wires and balanced conductors will
cause
of fensive noise transients in the system.
ACCESSORIES
The
Model A268R Rack Panel Kit consists of a 483 mm
x 89 mm (19 in. x 3 1/2 in.) precut rack panel and necessary
hardware for rack mounting the M268 in a standard 483
mm
(19 in.) rack panel.
The Model RKC169 Rack Panel Bracket Kit enables
users of the Shure A68R Rack Panel Kit (originally de-
signed
for the Shure M67 and M68 Mixers) to rack-mount
the M268 with the A68R.
SERVICE
WARNING
Voltages in this equipment are hazardous.
Refer servicing to qualified service personnel.
Operation at 210-250 V ac
The M268 is supplied wired for operation at 105-125
Vac.
To rewire the M268 for operation at 210-250 V ac, pro -
ceed
as follows:
1. Disconnect the M268 from the ac line
2. Remove end caps and cover .
3. Locate Power Transformer T106 at right center of
printed circuit board. Remove two jumpers between
holes
marked ª120V ONL Y.º Add jumper between holes
marked ª230V ONL Yº at right of T206.
4. Replace ac connector with one designed for 210-250
Va c
source. If M268 is to be used outside U.S. and Can -
ada, local regulations may require replacing line cord
with
one having wire insulation colors as follows:
ªLiveº or
ªHotºNeutral
Earth or
Ground
U.S., Canada Black White Green
Europe Brown Blue Green/
Yellow
5.
Replace Fuse F 10 1 ( presentl y 0 .1A , 2 50V , slow-blow ) w ith
0.05A, 250V, time lag unit (Shure 80C380, Schurter
034.3104) using supplied fuse clips (Shure 80A8008,
Schurter
O G 7 51.0052).
6. Replace cover and end caps. Affix new label to rear
panel
to reflect new operating voltage range.
