Shure M268 Schematic

222 HARTREY

AREA

TWX:

AVE..

EVANSTON.

CODE

31z/866-zzoo . CABLE: SHUREMICRO

910-231-0048 TELEX: 72-4381

IL.

60204 U.S.A.

DATA

SHEET

MODELS

~268

AND

MICROPHONE MIXERS

~268~

GENERAL

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.

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

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

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

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