The Shure Model M268 is a five-input portable
microphone mixer designed for use with sound reinforcement, tape recording and audio-visual systems. Its
excellent operational characteristics, compact size,
and functional versatility make the M268 a fine choice
as a primary or add-on mixer in any sound system.
The Model M268E is similar to the M268 except that it
is designed for connection to a 210- to 250-volt ac power
line. All information applies to both units except for
references to ac operating voltage, fuse and power line
cord.
M268
Features:
Wide, flat frequency response and extremely low
distortion at full output
Very low line noise and radio frequency interference susceptibility
Four switch-selectable low-impedance balanced or
high-impedance unbalanced inputs
High-level auxiliary input for tape, tuner and
accessories
Compact and lightweight, with rugged, abrasionresistant case
M268 (only): Listed by Underwriters' Laboratories,
Inc. and listed by Canadian Standards Association
as Certified
SPECIFICATIONS
Frequency Response
40 Hz to 20,000 Hz, +3 dB
Voltage Gain
(Outputs terminated: rnic 150 ohm133 kilohrns;
aux 47 kilohms; mix bus
(at 1,000 Hz)
3.3
kilohms)
Individual feedback-type active gain controls for all
five inputs
Master volume control sets overall output level
Simplex power for condenser microphones
High- (unbalanced) or low-impedance (balanced)
microphone-level output matches most amplifier
inputs
High-impedance auxiliary output for high-level
equipment inputs
Direct mix bus for simple mixer interconnection
("stacking")
Ac or external dc operation. Noiseless automatic
switchover to external power in case of ac line
failure
Automatic muting circuit prevents speaker damage
during turn-on and -off
Regulated power supply is unaffected by line
voltage fluctuations
Copyright
27A80ll (AE)
1981,
Shure Brothers Incorporated
Inputs
I
INPUT
Lo Imp Mic Bal or unbal
I
I
Mix Bus
Depending on control setting
Distortion
0.2% or less THD from 40 to 20,000 Hz with lo imp
rnic output at 80
level, and aux out at 8.OV level
IMPEDANCE
Designed
for use
with
1
(l~~~~~~l)
I
19
to
600
kilohms
100
ohms to
kilohms high-
unbalance(
level
3.3
kilohms
mV level, hi imp rnic output at 1.5V
,000
ohms
140
I
10
43
1
3.3
ohms
kilohms
kilohms
kilohms
I
1
1
Input Clipping
Level
+8
(2.5V)
Printed in
dBV
U.S.A.
Page 2
Outputs
IMPEDANCE
OUTPUT
Lo Imp Mic
HI
Imp MIC
Aux
Mix Bus
Designed
for
use
with
Any lo imp
(19 to 600 ohms)
mic c~rcuit
Unbal 10 to 50
kilohms mic
circuit
10 kilohm or
greater unbal
high-level
circuits
3.3 kilohms
Actual
(Internal)
50 ohms
kilohms
5
2.2 kilohms
3.3 kflohms
Output
-
(100 mV)
+
+
-8 dBV
Clipping
Level
20 dBV
4.5
(1.7V)
17 dBV
(7.1V)
(0.4V)
Noise
Equivalent input noise:
-
128 dBV (lo imp mic 150
ohms-300-20,000 Hz) at full gain
Equivalent input hum and noise: - 125 dBV (lo
imp mic 150 ohms-20-20,000 Hz) at full gain
dBV
Dimensions
See Figure
1
OVERALL DIMENSIONS
FIGURE
1
Weight
Net: 1.9 kg (4
Packaged: 2.8 kg (6
Ib 2 oz)
Ib 2 oz)
Certifications
M268 (only): Listed by Underwriters' Laboratories,
Inc. and listed by Canadian Standards Association
as Certified
Output noise: -90
-65
dBV (master up) (input controls down,
dBV (master control down),
300-20,000 HZ)
Output hum and noise: -82
-60
dBV (master up) (input controls down,
dBV (master down),
20-20,000 HZ)
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 and mix bus are
in phase; aux input is in phase with aux output, but
out of phase with pin 3 of mic connectors
Simplex Power
30 Vdc open circuit, 3.3 kilohms series resistance
Operating Voltage
Ac operation
M268: 105-125 volts, 50160 HZ, 5W
M268E:
210-250 volts, 50160 HZ, 5Wt
Dc operation
A2688 Battery Power Supply (optional accessory): 27 volts nominal at 12.2
no-signal, 12.5
mA typical full output; 16.2 volts
mA typical
minimum; approximately 40 hours battery life
with alkaline batteries, 4 hourslday duty cycle,
simplex off
External power: 30 Vdc, 12.6
mA full output
12.8
mA no-signal,
Temperature Range
-
Operating:
Storage:
t
Can
be
rewired
for
18O to 57OC (0° to 135OF)
-
29O to 74OC
105-125
Vac
operation
(-
(see Service
20° to 165OF)
section)
CONTROLS AND CONNECTORS
I
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.
Model
M268E
Model M268E is supplied with a three-conductor
power-line cord without a plug. The plug should be installed by qualified service personnel. The brown lead
should be connected to the "live" or "hot" terminal of
the plug, and the blue lead to the neutral terminal. The
greenlyellow lead is the grounding conductor and
should be connected to the ground or earth terminal of
the plug.
Inputs
A maximum of four low- or high-impedance dynamic,
ribbon or condenser microphones can be connected to
receptacles marked MIC 1, MIC 2, MIC 3 and MIC 4. The
inputs are designed for low-impedance microphones
with 19 to 600 ohms impedance or high-impedance
microphones. Both low- and high-impedance
microphones can be used simultaneously. Crystal or
ceramic microphones are not recommended. Impedance is selected by a slide switch above each input.
The input receptacles are professional three-socket
audio connectors.* See Figure 2 for low- and
impedance connections to inputs. Note that some condenser microphones produce very high output signals
which may overload the inputs of many mixers; the
M268's feedback-type active gain controls virtually
eliminate the need for in-line attenuators to compensate for these "hot" microphones.
'
Designed
equivalent
to
mate
connector.
with
WARNING
Cannon
XL
series,
Switchcraft
A3
(Q.G.)
high-
series
or
I
2
Page 3
The rear-panel phono jack marked AUX IN will accept adversely affected by this interconnection. Mix bus inoutput from a high-impedance, high-level source such
as a tape recorder or am-fm tuner.
MICROPHONE
LOW
(BALANCED
MIC
OUTPUT
LOW
(BALANCED
SHIELD
LINE
SHIELD
-----7
\------A/
LINE
MIXER
1
/,
\
)
IMPEDANCE
MICROPHONE
IMPEDANCE
MICROPHONE
INPUT
INPUT
.
-
I
OUTPUT
FIGURE
MICROPHONE
MIC
SHIELD
HIGH
CONNECTIONS
OUTPUT
HIGH
PLUG
MIXER
IMPEDANCE
SHIELD-
- - - -
-
IMPEDANCE
CONNECTIONS
2
INPUT
-
-
Outputs
The connector marked MIC OUTPUT is a dualimpedance output selected by the switch above the connector. This output is the "mixed" output of all the input
sources and is designed to work into a balanced or un-
terconnection can also be made with other Shure
ers such as the M267 and
SE30.
Simplex Power
The M268 provides power for condenser microphones
such as the Shure SM81 and
SIMPLEX
OFFION switch controls the application of
SM85. The rear-panel
simplex voltage to all low-impedance inputs. With the
SIMPLEX switch on and the rear-panel
+
in the LO position,
30 Vdc is applied to pins 2 and 3 of
LOlHl switches
each input connector. Series current-limiting resistance
is 3.3 kilohms for each input. When using other condenser microphones with the
M268, verify that the
voltage and resistance requirements are compatible.
Balanced low-impedance microphones (dynamic, ribbon, self-powered condensers) can be used in combination with simplex-powered condenser microphones. IMPORTANT: Do not turn on the SIMPLEX switch when
unbalanced low-impedance microphones; objec-
using
tionable hum will result. Turn off the SIMPLEX switch
when condenser microphones are not being used.
balanced 25- to 600-ohm microphone line or into a highimpedance unbalanced amplifier or tape recorder
microphone input. The connector is a professional
three-pin audio connector.* See Figure 2 for output con-
Use only high-quality cable, as intermittent shorts
between broken shield wires and balanced conductors
will cause offensive noise transients in the system.
nector configurations.
BATTERY OPERATION - EXTERNAL POWER
The phono jack marked AUX OUT is a highimpedance, 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 control 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 highlevel 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 individually controlled inputs. Note that the gain will be
reduced by 6 dB, but noise specifications are not
'
Designed to mate with Cannon
equivalent connector.
XL
series, Switchcraft
A3
(Q.G.)
series or
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.
A268B contains three 9-volt transistor radio bat-
The
teries which will power the M268 at full rated output.
Alkaline battery life is approximately 40 hours at
output, 4 hours per day use. Note that battery operation
with simplex-powered microphones will increase battery 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
0.25A, 250V in-line fuse as a safety precaution.
with a
With a battery supply or external dc supply connected, 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
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
19
standard
in. (483 mm) rack panel.
M268, or as a standby
mix-
4.OV
Page 4
The
Model RKC169
owners of the Shure
designed for the Shure M67 and M68 Mixers) to rackmount the M268 with the
This Shure product is guaranteed in normal use to be
free from electrical and mechanical defects for a period
of one year from date of purchase. Please retain proof
of purchase date. This guarantee includes all parts and
labor. This guarantee is in lieu of any and all other
guarantees or warranties, express or implied, and there
shall be no recovery for any consequential or incidental
damages.
SHIPPING INSTRUCTIONS
Carefully repack the unit and return it prepaid to:
Shure Brothers Incorporated
Attention: Service Department
1501 West Shure Drive
Arlington Heights, Illinois 60004
If outside the United States. return the unit to vour
dealer or Authorized Shure
The unit will be returned to you prepaid.
Rack Panel Bracket Kit enables
A68R Rack Panel Kit (originally
A68R.
GUARANTEE
service Center for repair.
PRINTED CIRCUIT BOARDS
SERVICE
WARNING
I
Voltages in this equipment are hazardous to life.
Refer servicing to qualified service personnel.
M268E Operation at 105.125 Vac
The M268E is supplied wired for operation at 210-250
Vac. To rewire the M268E for operation at 105-125 Vac,
proceed as follows.
1. Disconnect
2. Remove end caps and cover.
3. Locate Power Transformer
printed circuit board. Remove jumper marked
"230V ONLY" at right of T206. Add two jumpers
between holes marked
4.
Replace Fuse F101 (presently 0.05A, 250V, time
lag) with
Schurter 034.31 17).
5. Replace cover and end caps, and mark rear panel
to reflect new operating voltage range.
M268E from ac line.
TI06 at right center of
"120V ONLY."
O.IA, 250V, time lag unit (Shure 80B380,
I
Page 5
REPLACEMENT PARTS LIST
Reference
Designation
lacement
Re
Rit NO.*
-
-
Replacement Kit Consists Of:
Qty. Part No.
-
-
-
-
86L629
86K629
86D442 Diode, Light-Emitting
Capacitor, Electrolytic,
470
Capacitor, Electrolytic,
220
Description
uF, 35V
uF, 60V
Commercial
Alternate
Sprague
503D477G035
None
I
GI MV5075C
I
86A415
86A404
80C380
95A8012
95A8011
80A365
Diode, Silicon, Computer,
75v
Silicon Rectifier,
112A
Fuse, Slow-Blow, O.lA,
250V (M268)
Fuse, Time Lag, 0.05A,
250V (M268E at 210-250V)
1
Connector, 3-Pin
I
Connector, Three-Socket
I
Ferrite Bead Ring
100V,
TI 1 N4148
Motorola 1 N4002
Littelfuse
313000 Series
Schurter
034.31 04
I
ADC PCM-3-D
I
I
ADC PCF-3-D
I
Stackpole 57-3425
I
I
I
90A8028
86A350
46A8001
I
Knob
I
Transistor, Silicon, NPN
Potentiometer, 100k
I
None
I
Motorola 2N5210
I
I
I
Parts listed as
number is shown
46B8001
55A53
5588001
I
Potentiometer, 200k
I
Switch, Slide, DPDT
I
Switch, Slide, DPDT
I
None
I
None
I I
55B8008
I
Switch, Slide, 3PDT
I
90A8032
90A8051
5 1 A8008
51 A8003
86A8800
868808
86B811
86A808
90A8045
9088045
I
by that kit number. Any orders received for piece parts where RKC Kit
:ities.
I
Transformer, Input
1
Transformer, Output
Transformer, Power
I
Transformer, Power (M268E) I None
Integrated Circuit,
Voltage Regulator
Integrated Circuit, Quad
Op Ampl (Selected for NF)
Integrated Circuit, Dual
Op Ampl (Selected for NF)
Integrated Circuit, Quad
Op Ampl
I
Line Cord (M268)
I
Line Cord (M268E)
I
(M268)
I
None
I
None
I
None
SGS
L146C
Raytheon
RC4156DB
Raytheon
RC4559N B
Raytheon
RC4156DB
I
None
I
1
I
I
I
I
I
I
I
1
Page 6
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