Shure M268n Schematic

SHUme

THE

SOUND

OF

THE WFESSIONAUB

MICROPHONES

\VORLWIDE

AND

222

EVANSTON.

ELECTRONIC

HARTREY AVENUE

ILLINOIS

60202-3696 U.S.A.

COMPONENTS

DATA SHEET

MODEL M268

MICROPHONE MIXER

GENERAL

The Shure Model M268 is a highly versatile portable
microphone mixer designed for use with sound rein­forcement, tape recording and audio-visual systems. Its
excellent operational characteristics, compact size,
and functional capabilities make the M268 a fine choice
as a primary or add-on mixer in any sound system.

M268

Features:

Wide, flat frequency response and extremely low
distortion at full output

Very low line noise and radio frequency in­terference susceptibility

Four low-impedance balanced and four high­impedance unbalanced inputs

High-level auxiliary input for tape, tuner and
accessories

Individual feedback-type active gain controls for all
five inputs

Master volume control sets overall output level

Compact and lightweight, with rugged, abrasion­resistant case

Listed by Underwriters' Laboratories, Inc. and listed
by Canadian Standards Association as Certified

SPECIFICATIONS

Frequency Response

40 Hz to

Voltage Gain (at 1,000 Hz)

(Outputs terminated: mic 150 ohm133 kilohms;
aux 47 kilohms; mix bus 3.3 kilohms)

Low-Impedance
Microphone

High-Impedance
Micro~hone

Aux In
Mix Bus

Inputs

20,000 Hz, + 3 dB

INPUT

OUTPUT

Lo

Imp

Mic

+28 dB

!

+5dB r3ldB +55dB I OdB

-16dB

-8dB

Hi Imp

+54 dB

I I

+I0 dB
+18dB

Aux Out

Mic

+78 dB

+34dB
+42dB

Mix Bus

+23 dB

I

/

-21 dB

--

Phantom power for condenser microphones

High- (unbalanced) and low-impedance (balanced)
microphone-level output matches most amplifier
inputs

High-impedance auxiliary outputs for high-level

equipment inputs

Direct mix bus for simple mixer interconnection
("stacking")

Automatic muting circuit prevents speaker damage
during turn-on and -off

Regulated power supply is unaffected by line

voltage fluctuations

Copyright

1995,

27A8432

Shure Brothers Inc.

(OH)

IMPEDANCE

INPUT

Lo Imp Mic

Hi Imp Mic

Aux

Mix Bus

'

Depending on control setting

for

Bal or unbal

19 to 600 ohms

Unbal 10 to 50

100 ohms to 10

kilohms

level unbalanced

3.3 kilohms

Designed

use with

kilohms

high-

Actual

(Internal)

1,000 ohms

140 kilohms

43 kilohms

3.3 kilohms

Input Clipping

Level

-32 to -5
(25 mV to 0.56V)

-

I0 to + 18 dBV'
(0.32V to 7.9V)

+I4 to +30 dBV'

(5.OV to 32V)

+8 dBV

(2.5V)

Distortion

0.2% or less THD from 40 to 20,000 Hz with lo imp
output at 60 mV level, hi imp mic output at 1.5V

mic
level, and aux out at 7.OV level

Pr~nted ln U.S.A

dBV*

Outputs

OUTPUT

Low Imp

Mic

High Imp

Mic

Aux

Mix Bus

IMPEDANCE

Designed for

Use W

ith

Any low imp

(19 to 600

mic circuit

Unbal 10 to

50 k

10 k

greater

unbalanced

high-level cir-

 mic

circuit

cuits

3.3 k

)

 or



Actual

(Internal)

120

5 k

2.5 k



3.3 k



Output

Clipping Level

–22 dB

(10

0 m

V)

+4.5 dBV

(1.7 V)

18 dBV

(7.9 V)

–8 dBV

(0.4 V)

Dimensions

See Figure 1.

309

mm

(12 5/32 in.)

68.8mm

(2 23/

in.)

32

6.3
(1/4 in. )

mm

229 mm

(9 in.)

FIGURE 1

Weight

Net: 1.8
Packaged:

kg (4 lb 1 oz)

2.8 kg (6 lb 1 oz)

Certifications

Listed by Underwriters’ Laboratories, Inc., and listed

Canadian Standards Association as Certified

by

Noise

Equivalent

mic

150  - 300-20,000 Hz) at full gain

Input Noise:

–128

dBV (low impedance

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

(

maste

r up), (

inpu

t c

ontrol

s d

own

, 3

own

maste

, 2

00-20,00

r d

0-20,00

dBV

Output

Hum and N

oise

dBV (master up) (inpu

: –82 dBV (

t c

ontrol

s d

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
pin

3; aux input in phase with aux outputs and pin 2

inputs

and mix bus jack in phase with

Phantom Power

own)

0 H

0 H

, –

z)

60

CONTROLS AND CONNECT

ORS

WARNING

This

apparatus must be earthed (grounded)!

M268

power supply is energized when the unit
connected to an ac source; disconnect mains
(power)

z)

Inputs

A

m

ribbon

plug from supply when not in use.

aximu

m o

f four low- and four h

o

r c

ondense

r m

icrophone

s can b

igh-impedanc

e c

onnecte
tacles marked INPUT 1, INPUT 2, INPUT 3, and INPUT 4.
The

i

nput

s are d

esigne

d for l

ow-impedanc

e m
19 to 600  impedance, or high-impedance microphones.
Both

low- and h

multaneously .

igh-impedance microphone

Crysta

l o

r c

erami

c m

icrophone

s can b

mended. The input receptacles are professional three-pin
XLR

A

udio-typ

e c

onnectors

, and 1/



p

hon

4

e j
ure 2 for low- and high-impedance connections to inputs.
Note that some condenser microphones produce very high
output

s

ignal

s w

hic

h may o

verloa

d the i

nput

s o
the M268’s feedback-type active gain controls virtually elimi ­nate th

e n

eed fo

r i

n-lin

e a

ttenuator

s t

o c

ompensat

“hot” microphones.

SHIELD

2

1

3

The

e d

d t

icrophone

e used s

s are not r

acks

. See F

f many m

e for t

SHIELD

is

ynamic,

o r

ecep­s w

ecom-

ixers;

hese

ith

i-

ig-

30 Vdc open circuit, 3.3 k series resistance

Operating V

120
for

oltage

or 240 volts ±10%, 50/60 Hz, 5W

120 V

ac operation (see Service section

operation)

Temperature

Range

Operating: –18o to 57o C (0o to 135o F)
Storage: –29o to 74oC (–20o to 165o F)

, supplied wired

for 240 V

ac

LOW IMPEDANCE OUTPUT

2

MICROPHONE

1

2

3

(BALANCED LINE)

MIXER

LOW IMPEDANCE
(BALANCED LINE)

MICROPHONE OUTPUT PLUG CONNECTIONS

INPUT

MICROPHONE INPUT CONNECTIONS

SHIELD

HIGH IMPEDANCE OUTPUT

FIGURE 2

MICROPHONE

HIGH IMPEDANCE

MIXER INPUT

SHIELD

The rear-panel phone jack marked AUX IN will accept

from a high-impedance, high-level source such as

output
a tape recorder or am-fm tuner.

Outputs

connectors

The

marked LOW IMP OUTPUT and HI IMP
OUTPUT are the “mixed” 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 profes­sional three-pin XLR audio connector and a
jack.

See Figure 2 for output connector configurations.

The

phone and phono jacks marked AUX OUT are high­impedance,
marily

high-level, unbalanced outputs designed pri

to feed a power amplifier requiring 0.1 to 2 volts in

1

/4I phone

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

a

dditio

n t

o the p

owe

r O

N/OF

F s

witc

h (an a

djacen

t L

ED
indicates
five
AUX,
put.
panel

p

ower-on in a

i

ndividua

and a M

l i

npu

ASTE

Note that the i

d

irectl

y b

ehin

c o

peration)

t gain c

ontrol

R gain c

npu

d t

ontrol fo

t c

onnectors ar

heir correspondin

s d

esignate

, the f

r the t

e l

ron

t p

ane

d 1 t

hroug

ota

l p

ocate

d o

g gain c

l c

ontains
h 4 a

rogra

n the r

ontrols.

m o

nd
ut-

ear

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 (clock­wise),

and

the MASTER control adjusted for the required

output.

If overload

high-level

sources, reduce the individual gain control set

distortion (clipping) occurs when using

tings. Unused individual gain controls should be kept at
the

minimum setting (counterclockwise).

Mix Bus

The rea

r p

ane

l MIX BUS p

hon

o jack f

acilitate

s the “

stack­ing” of mixers to obtain additional inputs without using any
of th

e M268 i

for

i

nstance

ing

two i
amplifiers
gain

will b
adversely
nections
the M26

nputs

. C

, d

irectl

ndependen

with 1

0 i

ndividuall

e r

educe

a

ffecte

d b

can also b

7 and the M

onnectin

y c

onnect

s t

t m

aste

r c

ontrol

y c

d b

y 6 dB, but n

y this i

nterconnection

e made with o

67.

g the mix b

hei

r m

ixin

s and two i

ontrolle

ois

the

d i

e s

r S

uses of two M

g s

ystems

solate

, p
d o

268s,

rovid-

nputs. Note that t

pecification

. Mix bus i

hur

e m

ixer

s are n

ntercon-

s such a

utput

he

ot

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 resis­tance

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
using unbalanced low-impedance microphones: objec-
tionable hum will result. Turn off the PHANTOM switch

condenser microphones are not being used.

when

Use only high-quality cable, as intermittent shorts be­tween broken shied wires and balanced conductors will
cause

of

fensive noise transients in the system.

ACCESSORIES

Model A268R

The
x 89 mm (19 in. x 31/2 in.) precut rack panel and

-

hardware for rack mounting the M268 in a standard 483

-

mm

(19 in.) rack panel.

Rack Panel Kit consists of a 483 mm

The Model RKC169 Rack Panel Bracket Kit enables
users of the Shure A68R Rack Panel Kit (originally de­signed
the

for the Shure M67 and M68 Mixers) to rack-mount

M268 with the A68R.

SERVICE

WARNING

V

oltages in this equipment are hazardous.

Refer servicing to qualified service personnel.

Operation at 210-250 V

­The M268 is supplied wired for operation at 105-125

T

o rewire the M268 for operation at 210-250 V

Vac.
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 “120V ONLY.” Add jumper between

marked

4. Replace
Vac

“230V ONL

ac connector with one designed for 210-250

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:

s

U.S., Canada

Europe Brown Blue Green/

5.

R

eplace Fus

e F101 (

0.05A, 250V, time lag unit (Shure 80C380, Schurter

034.3104) using supplied fuse clips (Shure 80A8008,

O

G 7

-

Schurter

6. Replace
panel

to reflect new operating voltage range.

51.0052).

cover and end caps. Affix new label to rear

ac

.

Y” at right of T206.

“Live” or

Neutral

“Hot”

Black White Green

presentl

y 0

.1A

, 2

50V

, s

necessary

ac, pro

Earth or

Ground

Yellow

low-blow

when

holes

) w

ith

-

-

-

3

PRINTED CIRCUIT

TOP VIEW

BOARDS

REPLACEMENT PARTS LIST

Reference Designation

C105,106,116,117,
120,121,201

1~1

I

~101-108, Dl 14-Dl 16 86A415

/

Dl 09-Dl 13

I

F101

1

J201
1~202 4205
(~206-211
101, ~2c~215 / 80A365R

/

R168, R174

86A630

1

8688402

1

86B404

(

80F159

1

95B8012

1

95B8011

1

95x8025

(

4688001

Part No.

Description

Capacitor, Electrolytic
470

WF,

35 V

Capacitor, Electrolytic

Capacitor, Electrolytic
220

wF, 60 V

1

Diode, Light-Emitting

I

Diode, Silicon, Computer 75 V I Motorola IN4148

/

Silicon Rectifier, 100 V,

1

Fuse, Slow-Blow, 0.1 A, 250

/

Connector, 3-pin

/

Connector, 3-Socket

/

Phone Jack

1

Ferrite Bead Ring

1

Knob

1

Transistor, Silicon, NPN

Potentiometer, 100 k

(

Potentiometer, 200 k

'I2

A

V

Commercial Alternate

NIC NAE4R7M50V5X13TR

NRSA471 M35V1 OX20

NIC

1

GI

MV5075C

/

Motorola IN4002

1

Littlefuse 313000 Series

I

Switchcraft Y3MPC

1

Switchcraft Y3FDPC

(

Switchcraft L-112BPC

/

Panasonic EXC-ELSA35

1

None

(

Motorola 2N5210
None

(

None

I

1
1

1
1
1
I
1
1

I

1

s1

1

S206

I

TI

01 -TI 05

1

TI

06

U103

W1

1

55A53

1

55B8001

1

90A4323

/

51 A8009

86A8953

90A8045

/

Switch, Slide, DPDT

/

Switch, Slide, DPDT

1

Transformer, Input

1

Transformer, Power
Integrated Circuit,

Voltage Regulator
Integrated Circuit, Quad

Op Ampl (Selected for NF)
Integrated Circuit, Dual

Amp (Selected for NF)
Line Cord

Op

/

None

/

None

(

None

1

None
National Semiconductor

LM317AT
Motorola MC33179P

Motorola

None

MC331788

1
I

I

1

210V

=

SAME

LINE

A.C

CDNOiTlON

EXCECT

TEST

Y,

NOTE

0--TION.

A5

V4C

+to-250

-

EWPm

SUPPLY

mula

MSG8

e.

Mli.

WiTH

BLL

COMTaOL

INPUT

AuX

SYCEPT

e

+

AUX

lil

MtN

NOTE

TO

A5

SET

NEASUFIED

A4E

MAX.

7C

CONDITIONS

VOLTAGE

SET

15

A.C

SAME

CONTROLS

5.

K

-0

ZIO-Z~OVAC

I

MODEL M268 CIRCUIT DIAGRAM

YOITS

x

MFD

IN

-5,PED-

SHOWN

UNLFSS

*X1.PE

OTHERWISE

OR

UPeFITmE

VOLTS

YNL-

50

11N0

LLECTIlOLITlC

Y+MT

LFD

N

ZC-.

SEClFKD

ESISIPPS

CIIAPCI-

PlL

OmSRWN

141U

2

lZOV.

-

LINE

1V;

WIT#

WLTACES MEASURED

IW

4

UNTEEIMIkA-D.

BUS

MIX

20%-

f