Shure SR105 User Manual

SR105

Power

Amplifier

OPERATION AND

Manufactured by

SHURE BROTHERS

222

Hartrey Avenue

Evanston, Illinois

SERVICE

INC.

60204

U.S.A.

MANUAL

Copyright

27A888

1979,

(SJ)

Shure Brothers Inc.

(95B652)

WARNING

To reduce the risk of fire or electric shock, do not
expose this appliance to rain or extreme moisture.

Printed in U.S.A.

SR105

Power Amplifier

SPECIFICATIONS

The following specifications apply to both SR105A and SR105B Power Amplifiers except where noted. The SR105A Power
Amplifier provides both direct-coupled speaker output and transformer-coupled, constant-voltage, 70-volt output. The
SRlO5B Power Amplifier is equipped for direct-coupled speaker output only.

Amplifier Type All silicon transistor power

Power Output

Voltage Gain

Sensitivity
Frequency Response

Input Impedance

Total Harmonic

Distortion

Hum and Noise

Output Clipping Level. . 28.3 Vrms min. at 1 kHz (4-ohm

Load lmpedance:

Direct-Coupled

Speaker Output

70-Volt Output*

Model

SR105A

"Model

SR105B

.......

.......

.200 watts rms continuous to

.........

...........

.

.

....

.39 kilohms &30%, balanced or

........

only.
only.

.2% max. at 1 kHz, 200W (4-ohm

......

.80 dB below rated output (less than

.

.4 ohms or greater (4 ohms min.)

...

.I50 watts max. (33 ohms min.)

amplifier
4-ohm load

100 watts rms continuous to

8-ohm load

150 watts rms continuous to

70-volt line (33-ohm load)*

27 22 dB (4-ohm load at 1 kHz)

e3.5 dB (33-ohm load at 1 kHz

35

across 70-volt output)'

1.2 volts nominal (full rated output)

&I

.5 dB, 20-20,000 Hz (typical)

*2 dB, 50-15,000 Hz (typical,

70-volt output)"

unbalanced*

25 kilohms

unbalanced*

load, 28.3 Vrms output)

3% max. at 1 kHz,

load, 70.7 Vrms output)*

2.8
output with Volume Control at

load)

&30%, balanced or

*

150W (33-ohm

mV at direct-coupled speaker

0)

Load Regulation:

Di rect-Coupled

Speaker Output

70-Volt Output*

Phasing..

Power Supply

Temperature Range:

Operating
Storage

Dimensions
Weight:

SR105A

SR105B
Finish
Installation

Certifications

...........

........

.........

...........

.........

...........

...........

..............

..........

........

. .

Less than 1 dB from no-load to 4-

ohm load at 200 watts rms out­put

....

Less than 2 dB from no-load to

ohm load at 150 watts rms out­put

Pin 3 and phone jack tips of in-

puts in phase with speaker out­put phone jack tips, and 28V
and 70V terminals

120 volts, 50/60 Hz ac only. Power

consumption: 40 watts max. (20
watts typical) with no signal, 450
watts with

watts output; 500 watts maximum

-7' to 43OC (20' to 1 10°F)

-34' to 74OC (-30' to 165OF)

.I78 mm x 483 mm x 270 mm

(7 in. H x 19 in.

15.66 kg (34 Ib 8 oz)

12.23 kg (27 Ib)
Matte black
Equipped for standard 19" (483

mm) rack mounting; may be op­erated in optional
ing Case

Listed by Underwriters' Labora-

tories, Inc.; listed by Canadian
Standards Association as certi­fied

1

kHz signal and 200

W

x 10% in.

A105A Carry-

33-

D)

SR105

SPECIFICATIONS

(Curves

Power Amplifier

shown

are

typical)

OUTPUT POWER (WATTS)

TOTAL HARMONIC DISTORTION VS. OUTPUT POWER TO
4-OHM LOAD AT 50 HZ, 1 KHZ
ONLY)

&

10 KHz (DIRECT OUTPUT

LOAD RESISTANCE, OHMS

OUTPUT POWER (WATTS)

TOTAL HARMONIC DISTORTION VS. OUTPUT POWER (VAR­IOUS LOADS) AT
(70V OUTPUT ONLY)*

1

KHz AND CONSTANT 70.7V OUTPUT

FREQUENCY

OUTPUT POWER VS. FREQUENCY AT 1%
MONIC DISTORTION TO 4-OHM LOAD (DIRECT OUTPUT
ONLY)

(Hz1

81

2%

TOTAL HAR-

OUTPUT POWER (WATTS)

TOTAL HARMONIC DISTORTION VS. OUTPUT POWER TO
150-WATT (33-OHM) LOAD AT 1 KHz FOR INCREASING OUT­PUT VOLTAGES (70V OUTPUT ONLY)*

'Model

SRIO~A

only.

FREQUENCY ( H

OUTPUT POWER VS. FREQUENCY AT 1%
MONlC DISTORTION TO 33-OHM LOAD (70V OUTPUT ONLY)*

J

&

2%

TOTAL HAR-

iii

SR105

Power Amplifier

TABLE OF CONTENTS

Section Page

SPECIFICATIONS ii
DESCRIPTION
OPERATING INSTRUCTIONS

Functional Identification 3
General Operating Instructions
Mounting and Ventilation 4
Thermal Overload
Power Requirements 4
Functional Circuit Description 4
Input Connections
Output Connections 6

Amplifier-to-Speaker Impedance Matching
Output Voltage Meter

Basic Operating Hints 10

SPECIAL OPERATING INSTRUCTIONS

Adding
Bridging Connections 11

Adding Headphone Jack to Speaker Connections

SERVICE INSTRUCTIONS

Amplifier Service 12

SRlO5A Amplifiers in a

70-Volt System (Cascaded) 10

...............................

..................................

........................

..................

........................

.............................

...........................

...................

.............................

............................

.........

...........................

..........................

...................

..........................

...

11

..............................

1

3

4

4

8

9

Section Page

Replacement Parts 12
Fuse Replacement 12
Bottom Plate Removal 12

Cover Removal 12

Lamp Replacement 12
Dc Output Balance Adjustment 12
Output Voltage Meter Calibration

Printed Circuit Board Removal 13
Small Signal and Predriver Transistors

Driver Transistors 14
Output Transistors 15

Diodes
Diode Bridge Rectifier 15
Checking Transistors and Diodes
SR105B Amplifier Conversion for 70-Volt Output
Service lllustrations
Guarantee
Shipping Instructions

PARTS LIST 19
CONDENSED OPERATING INSTRUCTIONS
ARCHITECTS' AND

.......................................

....................................

....................................

ENGINEERS' SPECIFICATIONS

............................

.............................

..........................

................................

............................

...................

.................

...................

............

.............................

.............................

..........................

.................

............................

...........................

.........

...............

....

12
14

15
15

15
17
17
17

27
27

SR105

Power Amplifier

LlST OF ILLUSTRATIONS

Figure Page

1 SR105 Power Amplifier Front Panel 2

2 SR105 Power Amplifier Rear Panel

SR105 Power Amplifier Dimensional

3

Outline Drawing 4

SR105 Power Amplifier Block Diagram

4
5 Input Connections

Output Power vs

6

2 Ohms to 32 Ohms

70-Volt Output Power vs

7

Impedance from 4 Ohms to 100 Ohms

Recommended Wire Gauge vs . Amplifier-to-

8

Speaker Distance: Direct Output

Recommended Wire Gauge vs . Amplifier-to-

9

Speaker Distance: 70-Volt Output
(SR105A Only)

.........................

.........................

.

Load lmpedance from

......................

.

Direct Output Load

..........................

...........

...........

........

.......

...........

2

5
6

7
7

8

8

Figure Page

10 70-Volt System Attenuator

Bridged Amplifier Connections

11
12 Headphone Jack for Speaker Connections

SR105 Power Amplifier Top View, Cover

13

Removed

SR105 Power Amplifier Bottom View.

14

Cover Removed

70-Volt Conversion: Added Circuitry

15
16 70-Volt Conversion: Transformer Mounting 17
17 70-Volt Conversion: Internal Wiring 18
18 Main Circuit Board Parts Location Diagram 22
19 Filter Circuit Board Parts Location Diagram 22
20 Transistor Lead Codes 23
21 SR105 Power Amplifier Circuit Diagram 25

(SR105A Model Shown)

(SR105A Model Shown)

...................

...............

..........

..........

.....

.....

11
11
11

13
14

16

.....

...........

....

....

.....................

.......

SR105

(Shown

Carrying

Power

Amplifier

DESCRIPTION

in

A105A

Case)

The Shure SR105 Power Amplifier is a high-power ampli­fier designed for sound reinforcement applications. It is
capsble of delivering 200 watts rms to a 4-ohm load, and is
available in two models, the SR105A and SR105B. The

SRlO5A Power Amplifier provides both a direct-coupled
speaker output and a transformer-coupled constant-volt­age, 70-volt output, while the
equipped with a direct-coupled speaker output only. In
this manual "Amplifier" refers to both
models unless otherwise specified.

The Amplifier is of rugged construction and is conserva-

tively designed to provide reliable operation at high power
levels with minimum distortion and phase shift. Used in
conjunction with a suitable mixer-preamplifier or audio
console such as the Shure Model
quality, high-power, sound reinforcement system may be
assembled for use with a variety of speaker systems. Sev-

SR105 Power Amplifiers may be interconnected (de-

eral
scribed under Operating Instructions) to provide greater

power output for large installations and redundant opera-

tion. The basic direct-coupled speaker output capabilities
of the Amplifier are well suited to the source-oriented
speaker systems used in theatres, auditoriums, stadiums,
ballrooms, and convention halls, in short, wherever maxi­mum coverage with reserve power is required. The addi-

SR105B Power Amplifier is

SRI05A and SRlOSB

SR101, a versatile, high-

tional 70-volt output feature of the
makes it equally adaptable to source-oriented or distrib­uted speaker systems such as those used for background
music or announcements in industrial buildings, institutions
or public places.

An Output Voltage Meter in the Amplifier facilitates ad-

justment and output level monitoring.

Silicon transistors and similar solid-state devices are
used throughout the Amplifier. Short-circuit and thermal
overload protection are design features which eliminate
the need for special installation or operation precautions.
All components are of the highest quality and are operated
well within their respective ratings to assure maximum
reliability under normal use conditions.

The Shure

1.8m

(6

sory package consisting of four rack-mounting screws, one
Volume Control Cover, one Power On-Off Switch Cover, and
one spare 5-ampere fuse.

The
Underwriters' Laboratories, Inc., and are listed by Canadian
Standards Association as certified.

The
signed specifically for use with the

SR105 Power Amplifier is supplied with one

ft) audio input connecting cable and an acces-

SR105A and SRIO5B Power Amplifiers are listed by

A105A Portable Carrying Case accessory is de-

SR105A Power Amplifier

SR105 Power Amplifier.

FIGURE 1. SR105 POWER AMPLIFIER FRONT PANEL

FIGURE

2.

SR105 POWER AMPLIFIER REAR PANEL

(SR105A MODEL SHOWN)

SR105

Power Amplifier

OPERATING INSTRUCTIONS

FUNCTIONAL IDENTIFICATION (Refer to Figures 1 and 2,
Page 2).

-

1. OUTPUT VOLTAGE Meter
put voltage in percentage of maximum voltage.

2. VOLUME

3. THERMAL OVERLOAD Indicator Lamp
Amplifier shutdown due to excessive heat sink temper­ature.

4. POWER ON-OFF Switch
fier.

5. BALANCED BRIDGING
anced bridging, high-impedance input connection.

3AGl5A Ac Line Fuse - Protects Amplifier ac input

6.
line aaainst overload.

7. Ac Line Cord

supply.

8. DIRECT OUTPUT Jacks (Four) - Provide for output

connection to

9. DIRECT OUTPUT

connection to

10. 70V-OFF Switch
70-volt output transformer for use with distributed
speaker systems.

11. 70V OUTPUT Terminal Strip

Provides for output connection to 70-volt distributed

speaker systems.

12. SPARE FUSE

power

13. UNBALANCED PARALLELED

put Jacks (Two) - Provide unbalanced (one side

grounded) input connections for use with

pedance sources.

70-Volt Output Cover Plate (SRIOSB Amplifier only)

14.
Covers pre-drilled and marked area where Switch (10)
and Terminal Strip (11) are located.

GENERAL OPERATING INSTRUCTIONS

Control - Controls Amplifier output level.

-

-

Connects ac power to Amplifier power

direct-coupled speaker systems.

Terminal Strip - Provides for output

direct-coupled speaker systems.

(SRIO5A Amplifier only) - Activates

Fuseholder- Holds spare 3AG-5A

supply fuse.

lndicates Amplifier out-

-

lndicates

-

Applies ac power to Ampli-

lnput Jack - Provides for bal-

(SRI05A Amplifier only)

HlGH IMPEDANCE In-

-

high-im-

-

WARNING

Voltages in this equipment are hazardous to life. Make
all input and output connections with ac power dis­connected. Refer servicing to
sonnel.

DIRECT SPEAKER OUTPUT OPERATION
(MODEL

1. Install Amplifier before making electrical connections.

SRIO5A and SR105B AMPLIFIERS)

Using hardware supplied, secure
carrying case, allowing at least 51 mm
and behind case for ventilation. Use forced-air cooling
for multiple-amplifier installations.

qualified service per-

Amplifier in rack or

(2

in.) above

2. Set Switches

0.

to

3. Connect required speakers to Amplifier DIRECT OUT­PUT jacks (8)
wire size and arranging speaker connections for
speaker load impedance as close to 4 ohms as possible.
Remove DIRECT OUTPUT Cover (14) and use right­angle phone plugs to connect speakers to Amplifier.
When using DIRECT OUTPUT Terminal Strip (9), thread
wires through Cover grommet. Replace cover.

(4,lO) to OFF and VOLUME Control (2)

andlor to terminal strip (9), using proper

total

CAUTION

In multiple speaker installations, be sure not to ex-

ceed maximum power rating of any speaker.

4. Connect audio
Amplifier UNBALANCED PARALLELED HlGH IMPE-

DANCE
terconnections up to 15m (50 ft), use
tor, shielded, low-capacitance cable. For intercon­nections longer than 50 ft, use 600-ohm balanced line
into Amplifier BALANCED BRIDGING lnput Jack (5)
(professional, 3-pin, female audio connector).

5.

Connect additional power amplifiers or other auxiliary
equipment as required to remaining Amplifier input
jacks.

6. Connect
50/60 Hz source capable of supplying 450 watts.

7.

With VOLUME Control (2) set at 0, turn front-panel

POWER Switch (4) on. Adjust VOLUME Control to de-

amplifier operating level.

sired

CONSTANT-VOLTAGE 70-VOLT SPEAKER OUTPUT
(MODEL

1. Connect

2. Connect required speakers to 70V OUTPUT Terminals

3. Connect audio

4. Turn 70V-OFF Switch (10) to 70V.

5.

6.

SR105A AMPLIFIER ONLY)

eration in steps 1, 2, 4, 5 and 6 above.

(11) and, if necessary, DIRECT OUTPUT Connectors
(8,9). Total speaker impedance should be as close to 33
ohms as possible. Be

150-watt power output capability. (See Output Con­nections, Page 6.)

Amplifier UNBALANCED PARALLELED HlGH IMPE­DANCE Input Jack (13) (standard phone jack). For in­terconnections up to 15m (50 ft), use
tor, shielded, low-capacitance cable. For intercon­nections longer than 50 ft, use 600-ohm balanced line
into Amplifier BALANCED BRIDGING lnput Jack (5)
(professional, 3-pin, female audio connector).

Connect line cord (7) to grounded 120 Vac *lo%,
50160

Hz

With VOLUME Control
POWER Switch (4) on. Adjust VOLUME Control to de­sired amplifier operating level.

console or microphone mixer output to

lnput Jack (13) (standard phone jack). For in-

single-conduc-

line cord (7) to grounded 120 Vac &lo%,

auxiliary equipment as for direct speaker op-

careful not to exceed Amplifier

console or microphone mixer output to

single-conduc-

source.

(2)

set at

0,

turn front-panel

MOUNTING AND VENTILATION

The SR105 Power Amplifier is designed for rack-mount­ing in a standard 19-inch (483 mm) audio equipment cabinet
rack and is supplied with the necessary mounting hardware.

To insure adequate air circulation, a minimum clearance of

51 mm (2 in.) should be provided above and behind the
Amplifier (see Figures 3, Page 4). If the cabinet ambient
temperature reaches

43OC (llO°F), forced air ventilation
should be provided to avoid the possibility of thermal over­load (see following section). The Amplifier may be operated

-7"

in an ambient temperature range from

to 43°C (20"

to 110°F) in continuous duty without derating.

The Amplifier may also be operated while mounted in a

A105A Carrying Case.

Shure

P,

.

.

?

*

I

NOT INCLUDING CABLE AND LlNE CORD CLEARANCE DEPTH.

FIGURE

3.

DIMENSIONAL OUTLINE DRAWING

SR105 POWER AMPLIFIER

270

-00-518 IN.)-

mm'

THERMAL OVERLOAD

The SR105 Power Amplifier is equipped with thermal
sensing switches on the output transistor heat sinks. The
thermal switches are set to sh~it off ac power to the Ampli­fier when a temperature of 90°C (194°F) is attained on the
heat sinks; the switches automatically recycle and re-apply
power when the

(164°F). A THERMAL OVERLOAD light

heat sink temperature drops to 73°C

(3),

located on the

front panel of the Amplifier, indicates if thermal cycling

has occurred. Thermal cycling may occur if air is not al­lowed to circulate across the black finned heat sinks and

through the grilles of the Amplifier, or if there is a pro-

longed short-circuit on the output.

POWER REQUIREMENTS

The SR105 Power Amplifier is furnished with a three­conductor power cable and three-prong grounded plug
(7). Connect the power cable to an outlet which supplies

~10% volts ac, 50160

120

Hz

power. The maximum power
consumption at 120 volts under any normal operating con­dition is 450 watts (3.75 amperes at 120 volts).

Idle power

consumption with no input signal is nominally 20 watts.

If extension cords are required to supply power to the
Amplifier, a high quality 18-gauge or larger cord should
be used.

(4)

A POWER toggle switch on the front panel

the application of ac power to the Amplifier. The

controls

tamper-

proof cover supplied may be used to eliminate accidental
movement of this switch.

Main ac power fusing is provided by a 5-ampere, type

3AG, cartridge-type fuse (6) located on the rear panel. A

spare ac power fuse is supplied for mounting in the clip on
the rear panel (12).

Additional protection is provided by wired-in fuses in the
main ac power circuit (FI, 8 amperes) and in the ac pilot
lamp circuit (F3, 1 ampere). These fuses are located under
the chassis.

FUNCTIONAL CIRCUIT DESCRIPTION

(See Figure 4,

Page 5)

The inputs of the

SR105 Power Amplifier consist of two
unbalanced (grounded) phone jacks (13) and one balanced­bridging, three-pin, female audio connector (5). The bal­anced-bridging input is wired in parallel to the phone jacks
after passing through an isolation transformer. The input
signal then passes through the VOLUME Control
if the Amplifier is an

SR105A, either passes through a Filter

Assembly and 70V-OFF Switch (10) to the Power Amplifier

circuitry, or bypasses the Filter Assembly and goes directly
to the Power Amplifier circuitry. In the

SR105B Amplifier the
signal passes directly from the VOLUME Control (2) to the
Power Amplifier circuitry.

The Power Amplifier circuitry contains a dc balance ad-

justment to minimize idle power consumption and eliminate

dc offset at the direct-coupled

(28V) output. The balance
adjustment is generally only performed when active Power
Amplifier circuit components are replaced.

The metering circuit, located at the output of the Power

Amplifier circuit, contains a meter calibration adjustment,

a meter rectifier, and an output meter that indicates per­centage of output voltage. NOTE: 100% is the maximum

I

Amplifier output voltage prior to clipping regardless of load.
Maximum power output is obtained with a direct-coupled
speaker load impedance of 4 ohms, or a 70-volt output

speaker load of 150 watts (33 ohms), or a combination of
direct-coupled and 70-volt loads which results in optimum

loading for maximum power output.

The Power Amplifier circuit output goes to four phone
jacks (8) and one two-contact terminal strip (9). In the
SR105A Amplifier, the output goes through the 70V-OFF

Switch

(lo), a constant-voltage, 70-volt output transformer,

and a three-contact terminal strip (1 1).

A detailed description of the Amplifier circuits and con-

trols and their uses is provided in the following paragraphs
of this section.

INPUT CONNECT IONS

Three LlNE LEVEL INPUTS Connectors are located on

the rear panel of the Amplifier (see Figure 2, Page 2). A

professional, three-pin, female audio connector
vides a balanced bridging, high-impedance input connec-

tion. Unbalanced, high-impedance input connections are

1/4"

provided by two standard

phone jacks (13) wired in

parallel.

The Amplifier may be driven to full-rated output by any

audio console, preamplifier or microphone mixer capable
of delivering 1.2 volts across a 25-kilohm load.

When using the Shure

SR101 Audio Console as the

mixer-preamplifier, connect the cable supplied with the

Amplifier from one of the PROGRAM OUTPUTS Jacks

labeled LlNE LEVEL on the

SR101 to one of the LlNE LEVEL

INPUT Jacks (13) labeled UNBALANCED PARALLELED

HIGH IMPEDANCE on the Amplifier (see Figure 5A, Page

6). If a longer interconnecting cable is desired, a
conductor, shielded, low-capacitance cable (such as Bel­den #8401, 8410, or 8411) should be used.

'Des~gned to mate with Cannon

(0

G

)

Series, or

equivalent

XL

Ser~es, Sw~tchcraft

connector

A3

(2), and

(5)*

pro-

single-

INPUTS

METER

RECTIFIER

VOLT-

METER

(70)

0''

-

UNBALANCED

BALANCED
BRIDGING

!&

INPUT
XFMR

SRIOSA AMPLIFIER ONLY. I

a

I

VOLUME

/

FIGURE

?

4.

LOW-CUT 70

FILTER

ASSEMBLY

I

-

SR105

POWER AMPLIFIER BLOCK DIAGRAM

In applications where the Amplifier is located at a con-

siderable distance from the mixer-preamplifier, electrical

interference or excessive hum pickup may be encountered.

Hum pickup may also be encountered over shorter distan­ces as a result of ground loop conditions between the units.
Under these circumstances, a balanced line, such as is

used for low-impedance microphones, should be used for
interconnections. A low-capacitance, two-conductor,
shielded cable (such as

Belden #8412 or 8422) is recom­mended. A professional, three-pin, female audio connector
is required for

wnnection to the mixer-preamplifier, and a
similar three-pin male connector is necessary for connec­tion to the Amplifier (see Figure 5B, Page 6). When wiring
these connectors, connect the cable shield to pin 1 in each
connector, one of the cable conductors to pin 2, and the
other conductor to pin 3 in each connector. For balanced
line operation, use the cable as described above to connect
the mixer-preamplifier PROGRAM OUTPUTS three-pin con­nector labeled

LlNE LEVEL to the Amplifier LlNE LEVEL

INPUTS connector labeled BALANCED BRIDGING (5).

Since the two input jacks of the Amplifier are wired in
parallel, one jack may be used as an auxiliary, high-level,
unbalanced output to feed the signal to another Amplifier
(SR105 or similar equipment) or the high-impedance input
of a tape recorder. The three-pin input connector (5) is con­nected through an isolation transformer in parallel with the

two input jacks (13). If not otherwise in use, the three-pin

connector may be used as a balanced, high-level, signal

source for a second Amplifier or other auxiliary equipment

(see Figure

5C, Page 6).

I'

METER
ADJUST

/

V

!&

I
I

I

I

I

I

I

I
I
I
I

I

I
I
I
I

I

L

- - - - - - - - - - - - - -

f

BALANCE

1

1

Additional SR105 Power Amplifiers may be added to the
system for a fail-safe type operation where complete re­dundancy is required. Refer to the section on Bridging
Connections (Page 11).

Certain input equipment, such as passive equalizers,
step attenuators and matching networks, may require a
600-ohm termination for optimum performance. (This is
not required with any Shure products.) When a 600-ohm
terminated input is required, a 620-ohm,
sistor must be connected to the terminals of the input cable
connector (see Figure 5D, Page 6). This resistor must be
connected between pins 2 and 3 of the three-pin connector,
or between the tip and sleeve terminals of the phone plug.
The resistor will fit inside the sleeve of the three-pin con­nector or phone plug. Be careful not to damage the wires,
connections, sleeve or resistor when making this installa­tion.

An occasion may arise where it is desirable or expedient
to feed the Amplifier from a 70-volt line. This could be in
connection with the expansion of an existing installation,
or as a result of efforts to minimize cable runs in a new in-

stallation. An input signal for the Amplifier may be derived
from a 70-volt line through the use of an attenuator (see
Adding Amplifiers in

a

operating from a 70-volt line, the VOLUME Control (2) on
the Amplifier should be set to the "7" position. With the
recommended attenuator, this setting will produce approxi­mately 28.3 volts at the DIRECT OUTPUT terminals
70 volts at the 70V OUTPUT terminals
(1 1) when the 70-volt input line is at 70 volts.

d

-

- -

-

-

-1

70-Volt System, Page 10). When

DIRECT
SPEAKER
OUTPUTS

11

=

'0

XFMR*

I

5%, %-watt re-

(SRlO5A Amplifier)

(8,9),

*

or

@

UNBALANCED PARALLELED

0

0

MIXER-

PREAMPLIFIER

I

MIXER-

PREAMPLIFIER

POWER

AMPLIFIER

I

I

\/

I

I

\A

UNBALANCED PARALLELED

HIGH-IMPEDANCE INPUTS

A

+

-

SR105

POWER

AMPLIFIER

-

BALANCED
BRIDGING

INPUT TO POWER AMPLIFIER

I

TO POWER AMPLIFIER

-

OR HIGH-IMPEDANCE

TAPE RECORDER INPUT

OR OTHER AUXILIARY
EQUIPMENT

@

6-TT-

570,114 W TIP

OUTPUT CONNECTIONS

Four standard

barrier strip (9) on the rear panel of the Amplifier provide
direct-coupled speaker output connections (see Figure 2,
Page 2). All connectors are wired in parallel and pro­vide for simultaneous connection of a number of speaker
systems. A three-terminal barrier strip
OUTPUT
tion to the 70-volt output of the Amplifier (see Figure 2,

Page
Switch

The Amplifier utilizes direct coupling between the output
stage and the direct-coupled speaker output load. The
full-rated 200-watt output of the Amplifier is obtained when
the speaker load connected to the DIRECT OUTPUT ter­minals
conditions is 28.3 volts. Speaker loads of less than four

(SR105A Amplifier only), provides for connec-

2).

This output is present only when the 70V-OFF

(10)

(8,9) is four ohms. The output voltage under these

1/4"

phone jacks (8) and one two-terminal

is set to the 70V position.

5

FIGURE

(ll), labeled 70V

5.

INPUT CONNECTIONS

SLEEVE

ohms should not be used with this Amplifier. If less than
four ohms is inadvertently used, no damage to the speak­ers or Amplifier will occur, but a reduction in power output
and an increase in distortion will be experienced. Thermal
shutoff of the Amplifier may also result.

It should be noted that various speaker loads will affect

6,

the output power of the Amplifier. See Figure
to determine Amplifier output power for a given speaker
load. Refer to the section entitled, Amplifier-to-Speaker

8)

Matching (Page
load.

In the event that an
application where the 70-volt output is not used, the
OFF Switch (10) should be turned off. This disconnects the
70-volt transformer which would otherwise be operating
without a load. If the transformer is allowed to operate in a
no-load condition, it may activate the output transistor

for assistance in calculating speaker

SRlO5A Amplifier is operated in an

Page 7,

70V-

pro-

2

3

FIGURE

4 5 678910

LOAD IMPEDANCE (OHMS)

6.

TYPICAL OUTPUT POWER VS. LOAD IMPEDANCE

FROM

2

I1 KHz.

2%

OHMS TO

TOTAL HARMONIC DISTORTION)

(DIRECT OUTPUT ONLY)

32

OHMS

20

30

40

tection circuitry and cause low-frequency breakup or dis­tortion of the Amplifier audio output.

The 70-volt output of the

SRlO5A Amplifier is trans­former-coupled to facilitate the use of balanced lines in
the long cable runs associated with distributed speaker
systems. A center-tap connection to the secondary of the
output transformer (CT) is provided on the 70-VOLT OUT­PUT terminal strip (11). This terminal may be grounded to
provide speaker lines which are balanced to ground. This
is done by connecting a wire from the center-tap terminal to
the ground terminal. This arrangement can be effective in
eliminating hum induced in the speaker lines by strong
external fields. It can also be effective in eliminating cross­talk and a tendency for system oscillation in applications
where the 70-volt speaker output and low-level audio input
are run in the same raceway.

The full-rated output of 150 watts from the

SRI05A Ampli­fier at the 70-volt output terminals (11) will be realized with
a 33-ohm load. The output voltage under these circum­stances will be 70.7 volts. In order to activate the 70-volt
output of the Amplifier, turn on the 70V-OFF Switch (10).
When using this output, care should be taken to see that
the sum of the individual loads

(i.e., each speaker in a dis­tributed speaker system) does not exceed the 150-watt
power output capability of the Amplifier.

It is possible to utilize both the direct-coupled speaker

output and the 70-volt output of the

SR105A Amplifier
simultaneously, if desired. In this case, reference should
be made to Figure 7, Page 7 to determine the power avail­able to the 70-volt output as a function of the load on the
direct speaker output. As an example, assume an 8-ohm
speaker load is to be connected to the direct-coupled
speaker output, and the 70-volt output is also to be used.
Following the 8-ohm vertical line on the graph (Figure 7,
Page 7) to the point where it intersects the curve, note
that the horizontal line also passing through this point is
80 watts. This is the power available on the 70-volt output
with a simultaneous 8-ohm direct output load.

CAUTION

Parallel connection of the DIRECT OUTPUT Con-

nectors (8,9), or the speaker cables between two
SR105 Power Amplifiers, or the SR105 Amplifier and
any other amplifier, should not be attempted. Bal­ancing paralleled amplifiers is virtually impossible,
and the circuitry mismatch cannot be balanced. The
same precaution should be observed with regard to
the 70V OUTPUT Connectors (11) of two
Amplifiers, or the SR105A Amplifier and any other
amplifier. Such a connection may result in damage to
one or both amplifiers and is not covered by the
Guarantee. This restriction does not extend to the

series amplifier connections described in Bridging

Connections, Page 11.

As indicated on the rear panel of the Amplifier, Class
wiring* may be used for speaker connections to the
DIRECT OUTPUT terminals

(8,9) of the Amplifier as well as
the interconnection between speakers in the direct-coupled
distribution circuit. Class 2 wiring may be used for speaker
connection to the 70V OUTPUT terminals
fier or interconnection of speakers within this circuit.

In order to avoid appreciable power loss in the cables
used for speaker connection, it is necessary to consider the
length of cable required, the total impedance of the speak­ers being connected, and the Amplifier output (direct or
70-volt) being used. The graph presented in Figure 8, Page

8, for direct speaker output indicates the recommended

wire gauge for various speaker loads and amplifier-to­speaker distances. As an example, assume two 16-ohm
speakers are to be connected in parallel at a distance of

18.3m (60 ft) from the amplifier, and the direct output of

the Amplifier is to be used. The total impedance of the
speakers connected in parallel is eight ohms. Following

the 8-ohm horizontal line on the graph (Figure 8) to the
point where it intersects a vertical line representing
(60 ft), note that the nearest solid diagonal line to this
point is labeled AWG 18. This is the recommended wire

*As

defined

by

U.S.

National Electrical Code.

160

140

E

120

I-

I-

I

I00

-

f,

80

Z

g

60

I-

2

40

I-

s

20

0

I

2

3

456810 20 40 6080100

DIRECT SPEAKER OUTPUT LOAD (OHMS)

FIGURE 7. 70-VOLT OUTPUT POWER VS. DIRECT OUTPUT

LOAD IMPEDANCE FROM 4 OHMS TO 100 OHMS

(SRIOSA ONLY)

SR105A

(1 1) of the Ampli-

18.3m

2

AMPLIFIER-TO-SPEAKER DISTANCE (FEET1

36

-

32

0)

i

28

Y

2

24

Y

0

1

20

P

3

16

L

J

I2

0

FIGURE

200 400 600 800 1000 1200

8

4

0

50 100

AMPLIFIER-TO-SPEAKER DISTANCE

8.

RECOMMENDED WIRE GAUGE VS.

AMPLIFIER-TO-SPEAKER DISTANCE:

1% 200 250 300

(METERS1

DIRECT OUTPUT

gauge. This procedure should be followed for each speak­er cable connected to the DIRECT OUTPUT Connectors

(8,9) of the Amplifier. If a point plotted on the graph falls
between two of the wire gauge lines, the larger of the two

should be the wire gauge used. This will limit the power

loss in the cable to less than 10%.

The graph of Figure 9, Page 8, provides wire gauge
recommendations for use with the 70-volt output of the
SR105A Amplifier. A 70-volt line is usually used for dis­tributed speaker systems where many speakers and long
cable runs are involved. The wire gauge value determined
from the graph would be satisfactory for the entire cable
run in a distributed speaker system. However, it is possible

to reduce the wire size in increments as the speaker load

becomes less toward the end of the cable run or as

branches are split off the main line. The economy of such
an arrangement will vary depending upon the details of
each installation and should be evaluated accordingly.

AMPLIFIER-TO-SPEAKER IMPEDANCE MATCHING
DIRECT SPEAKER OUTPUT

In order to obtain maximum transfer of Amplifier power
to the speaker load at the direct speaker output and reduce
distortion, the total impedance of the speaker load should
be as near to four ohms as practical under the existing
circumstances (see Figure 6, Page 7). If the speakers
being used have a rated impedance of 16 ohms (such as
the Shure

SR102 and SR103), then four such speakers
connected in parallel present a total load impedance of
four ohms to the direct speaker output of the Amplifier.

This is

an,ideal match to the Amplifier. If, as often happens,
an exact match is not possible, it is desirable to have a
speaker load higher than the Amplifier minimum load im­pedance.

In general, if more than one speaker is to be connected
to the DIRECT OUTPUT Connectors
a determination should be made as to which method

(8,9) of the Amplifier,

-

series, parallel or series-parallel interconnection of speak-

-

will provide the best match to the Amplifier. To cal-

ers
culate the total impedance of a number of speakers con­nected in series, merely add the individual speaker im­pedance values together. To calculate the total impedance

of a number of speakers of identical impedance connected

in parallel, divide the impedance of one speaker by the
number of speakers in the group. For example, the total

impedance of three 16-ohm speakers connected in parallel

3,

is 16 divided by

or 5.3 ohms.

In the event that a group of speakers of different individ­ual impedances are connected in parallel, the total

impe-

a

W

AMPLIFIER-TO-SPEAKER

DISTANCE

(FEET

I

0 100 200 300 400 500 600 700 800 900 1000 1100 1200

AMPLIFIER-TO-SPEAKER

FIGURE

9.

RECOMMENDED WIRE GAUGE VS.

AMPLIFIER-TO-SPEAKER DISTANCE:

TO-VOLT OUTPUT

DISTANCE

(SRlOSA ONLY)

(METERS)

dance may be calculated by the formula:

1 1 1 1

-=-

ZT

where

ZT

is the total impedance,
first speaker,
and so forth, with a total number of fractional terms equal
to the total number of speakers in the group. Before at­tempting to add the individual fractions, a common denom-

inator for all fractions must be determined. For example, if
an 8-ohm and a 16-ohm speaker are connected in parallel,
the expression is written:

Examination indicates that 16 would be an appropriate
common denominator, allowing the expression to be re­written as follows:

and simplifying,

Taking the reciprocal of (inverting) the final expression

gives:

For a series-parallel interconnection of different speaker
impedances, determine the total parallel-connected speak­er impedance, and add it to the total of series-connected
impedances. The sum is the total load impedance.

In planning a multiple speaker installation, the maximum
power ratings of the individual speakers being used should
be kept in mind. This is particularly important where speak­ers of different impedance values are connected to the DI­RECT OUTPUT Connectors
8-ohm and a 16-ohm speaker are connected in parallel
across the Amplifier output and the Amplifier is operated
at full-rated output. Figure 6, Page 7, shows that approxi­mately 120 watts are developed across the 8-ohm speaker,
while only 60 watts are produced across the
speaker. The power ratings of the speakers employed must

be evaluated accordingly.
70-VOLT OUTPUT

When the 70-volt output of the

lized, matching the speaker load to the Amplifier output

impedance is not necessary. The only requirement is that
the total wattage of the speaker load connected to the
volt output does not exceed the power rating of the Ampli­fier.

In a 70-volt speaker system it is standard practice to cou-

ple each speaker to the 70-volt line with a line transformer.
These transformers are equipped with tapped secondary

windings to accommodate various speaker impedances.
The primary winding is also tapped and labeled with the

Z,

f-+-

ZI

is the impedance of the second speaker,

ZT

=

22

Z,

is the impedance of the

16

-

=

5.3 ohms

3

(8,9). For example, assume an

SR105A Amplifier is uti-

Z,

.

. .

.

etc.

lgohm

70-

wattage rating applicable to each tap. For example, the

A102A, a 50-watt transformer, has taps at 50, 25, 12

Shure
and 6 watts. (Transformers are available at various power

ratings to suit the requirements of the application.) The pri­mary taps allow adjustment of the power level delivered to
the speaker when the transformer is connected to the
volt line. Connection of the 50-watt tap to the 70-volt line

produces 50 watts across the speaker when the audio

signal level on the line is 70 volts. Similarly, use of the
watt tap produces 25 watts across the speaker.

The total load in watts presented by a 70-volt multiple
speaker system is the sum of the individual speaker loads.
The value of each individual speaker load is indicated by
the wattage rating of the transformer primary tap con-

nected to the 70-volt line. For example, assume only two
speakers are connected to a 70-volt line, the first using a
50-watt primary tap and the second using a 25-watt tap.
The total load across the 70-volt line is therefore 50 plus 25,
or 75 watts.

In the design of sound reinforcement installations, good

engineering practice allows for change or growth. An ex-

a

ample of this is
This system could be handled by two 70-volt lines of 150

watts each, connected individually to two

fiers. This system would be operating at full capacity with

no room for expansion. The preferred installation would be

three

70-volt lines of 100 watts each, connected individually
to three
tions or changes in each 70-volt line of up to 50 watts.

OUTPUT VOLTAGE METER

aid for adjusting and monitoring the Amplifier operating
level. The meter scale is calibrated in percent, with 100%
being equivalent to the maximum output voltage of the
Amplifier, prior to clipping. The performance of the meter

is similar to that of a VU meter and it responds to program
material in a similar manner.

sents a signal level of 28.3 volts rms across the DIRECT
OUTPUT
of 200 watts is obtained only when the total speaker load
is
than
power output,
100% reading indicates a power output of
that it is not possible to obtain a greater power output with
this load.

power indicated by a 100% meter indication for various di-

rect speaker loads. Note that while the meter is calibrated
for 28.3 volts output, the typical output power available is
somewhat higher and is indicated by meter readings
greater than 100%.

a signal level greater than 70.7 volts rms across the 70V
OUTPUT Terminals (11). The OUTPUT VOLTAGE Meter
(1) may be recalibrated for a 100% indication with
output. Maximum 70-volt amplifier power output is ob­tained with a 150-watt load (33 ohms) across the 70V
OUTPUT Terminals. Similarly, maximum power output

may be obtained with a combination of direct and 70-volt

loads (see Figure 7, Page 7).

(1) reading and the Amplifier direct-coupled output power

is illustrated in the following table:

SR105A Amplifiers. This arrangement allows addi-

The OUTPUT VOLTAGE Meter (1) provides a convenient

It should be noted that a 100% meter indication repre-

(8,9). The maximum rated amplifier power output

4

ohms across the DIRECT OUTPUT. With loads other

4

ohms, a 100% meter indication represents a lower

Figure 6 may be used as an approximate guide to the

SR105A Amplifier, a 100% indication represents

In the

The relationship between the OUTPUT VOLTAGE Meter

70-volt installation requiring 300 watts.

SR105A Ampli-

e.g., for a total speaker load of 8 ohms, a

100vvatts. Note

70-

25-

70.7V

Meter Reading

100% 1 X

86.7%

70.7% 1 /2X
50

%

33.3% 1/10X
10% 1/1oox

In most applications, the VOLUME Control
of the Amplifier is determined by the sound level require­ments of the area
made in the audio console feeding the Amplifier to set the
average program output level to a value convenient for

monitoring on the audio console VU meter, usually

This can be done either with program material or with an

internal tone oscillator as in the Shure
sole. With this signal as an input to the Amplifier, adjust
the VOLUME Control to provide the desired sound level
from the speakers connected to the Amplifier. At this point,

it is important to note the reading of the OUTPUT VOLTAGE

meter (1). If the average reading is 70% or higher, it is

likely that peak excursions in the program material will
exceed 100% and enter the clipping region of the Amplifier.

If the program material is of relatively constant level, as in

"hard rock" music, this operating condition may be ac-

ceptable or even desirable. It should be noted that a moder-

ate amount of clipping is not readily detectable and is toler-

ated by some users in the interests of maintaining a higher

average sound level at the amplifier output. If the dynamic

range of the program material is expected to be large, as in
a live orchestral or dramatic production, it is advisable to
set the output voltage level to 50% or less to reduce the

possibility of noticeable clipping on loud passages. In
either case, to achieve the best overall performance from
the sound system, the operator should keep in mind the
operating level he has set up for the Amplifier, and control
the console output accordingly.

The tamper-proof cover supplied is designed to replace

the pull-off VOLUME Control knob (2) and keep the VOL-

UME Control at a preset position.

BASIC OPERATING HINTS

Should any difficulty be encountered in Amplifier opera­tion, the problem may often be traced to some simple
source such as an error in interconnection. The following

is offered as a basic guide to problems of this sort.

Symptom: Amplifier is "dead" (no output, OUTPUT

VOLTAGE Meter lamps out)

being served. Initial adjustments are

Output

Power

3/4X
1 /4X

(2)

setting

0 VU.

SRlO1 Audio Con-

Symptom:

Check:

Symptom:

Check:

Symptom:

Check:

Symptom:

Check:

1. Check that ac power source is "live" and
that Amplifier is plugged in.

2. Check that POWER ON-OFF Switch (4) is
on.

3. Check to see that rear-panel

(6)

is good.

4.

Check that THERMAL OVERLOAD Lamp

(3) is not lit.

Amplifier is "dead" (no output, OUTPUT VOLT-

AGE Meter lamps lit)

1. Check that VOLUME Control (2) is set high
enough.

2. Check for defective input cable or source.

3. Check for defective output cables or
speakers.

4. Check for input and output cables re­versed.

No 70-volt output (direct output present)

(SR105A only)

1. Check that 70V-OFF Switch (10) is set to
70V.

2. Check for defective 70-volt output cables
or speakers.

Low or distorted output

1. Check for incorrect speaker load, shorted
or defective cables, and proper connec­tions.

2. Check that speaker power handling capa-

bility is adequate.

3. Check that 70V-OFF Switch (10) is not
turned on without a 70-volt load
direct output operation only).

4. Check that output cables are of sufficient
size (gauge) for load.

Hum or noise in output

1. Check for defective source or input cable
by disconnecting input cable.

2. Check need for balanced line on long cable

runs or to avoid ground loop condition be-

tween units.

3. Check speaker line for balance (should not
be grounded at speaker).

4. Check 70V OUTPUT Terminal Strip (11);

grounding the center-tap (CT) terminal may

reduce or eliminate condition
only).

3AG/5A Fuse

(SR105A;

(SR105A

SRl05

Power

SPECIAL OPERATING INSTRUCTIONS

The information contained in this section is provided to
more fully utilize the capabilities of the
Amplifier.

ADDING
70-VOLT SYSTEM (CASCADED)

For added power or when additional speakers are to be

installed, an additional

SR105A AMPLIFIERS IN A

SR105A Amplifier may be added to

SR105 Power

Amplifier

an existing 70-volt system. The Amplifier may be installed
either at the end, or anywhere along the system, by using

an attenuator such as that shown in Figure 10, Page 11.

Connect the attenuator to the 70-volt line as shown, and
connect the attenuator plug to the BALANCED BRIDGING
Input Jack (5) on the added Amplifier. Set the VOLUME
Control (2) on the added Amplifier to
output when the system voltage is at 70 volts. This will

7

to obtain a 70-volt

pro-

duce approximately 28.3 volts at the DIRECT OUTPUT
terminals

(11) when the 70-volt input line is supplying 70 volts.

BRIDGING CONNECTIONS

If greater power or optimum system reliability becomes
extremely important, two Amplifiers may be connected in
series (never in parallel!) to obtain an output of 400 watts
into an 8-ohm load, or 200 watts into a 16-ohm load (6 ohms
minimum load). To determine available power, divide the
actual load impedance by two and, referring to Figure 6,
Page 7, double the output power shown on the graph. This
connection also provides a fail-safe, or redundant, system:
if one Amplifier fails, the second remains functional at its
normal power output. This series connection of amplifier
outputs is commonly referred to as a bridged configuration.

Connect the Amplifier balanced inputs out of phase as
shown in Figure 11, Page 11, with the speaker load con-

nected between the two 28V terminals of the DIRECT OUT­PUT Terminal Strips (9). Connect the two

together.

If a 140-volt output is required
two
described above, except that the 70-volt outputs are used.
Connect the speaker load to the 70V terminals (11) of the
two
together. Use Class 1 wiring.*

ADDING HEADPHONE JACK TO SPEAKER
CONNECTIONS

If it is desirable to monitor the Amplifier direct output
through headphones, a headphone output may be fabri­cated as follows. Obtain parts listed below or their equiva­lent.

'As

def~ned

(8,9), or 70 volts at the 70V OUTPUT terminals

70-VOLT

LINE

*

ATTENUAm

.

l

r

-I

8

g

=

I

5

u

b

FIGURE 10. 70-VOLT SYSTEM ATTENUATOR

SR105A Amplifiers may be connected in series as

SR105A Amplifiers and connect the COM terminals

by

U.S.

r---i

I

Nat~onal Electrical Code

I

-

I

3-PIN MALE

AUDIO CONNECTOR

INPUT

GND

terminals

(SRlO5A Amplifier only),

Qty. Part Recommended Type

1

1 Phone Plug Switchcraft 280
1 Volume Control

1 Transformer, 25V Essex-Stancor A-8095

Connect the parts as shown in Figure 12, Page 11. Make

certain the "hot" speaker lead
watt or less tap on the 25-volt transformer; this connection
sets the maximum headphone volume. If only stereo head­phones are to be used, interconnect the tip and sleeve
contacts of the headphone jack. If it is desired to monitor
the 70-volt output of an
volt line to voice coil transformer for the 25-volt transfor-

mer listed above.

Phone Jack Switchcraft 12-8

Switchcraft

(L-Pad)

Line to Voice Coil or A-8096

(28V) is connected to a one-

SRlOSA Amplifier, substitute a 70-

IMPEDANCE..

POWER

TO
POWER TO 16n. ..200 W
VOLTAGE OUT

SR105 AMPLIFIER

BA

FIGURE 11. BRIDGED AMPLIFIER CONNECTIONS

666Pl or 656

..

8n

6fi MIN

en..

.

.4oo

.....

56.6 V

NOM

w

ER

PHONE

SPEAKER

TERMINAL

TO

PLUG

-3

NOTE:

TRANSFORMER

I W OR

LESS

1

COM

USE

JUMPER

HEADPHONES

VOLUME CONTROL

(%-OHM

8

OHM

COIL

COM

FOR

STEREO

ONLY.

FIGURE 12. HEADPHONE JACK FOR

SPEAKER CONNECTIONS

-

-

-

-

L-PAD)

NOTE

PHONE

TIP

-

-

RING

SLEEVE

JACK

SR105

Power Amplifier

SERVICE INSTRUCTIONS

AMPLIFIER SERVICE

SR105 Power Amplifier uses components of the high-

The
est quality, operating well within their respective ratings to
assure long life.

(See Guarantee)

WARNING

Voltages in this equipment are hazardous to life.

Make all input and output connections with ac power
disconnected. Refer servicing to qualified service
personnel.

REPLACEMENT PARTS

Parts that are readily available through local electronic
parts distributors are not shown on the accompanying
Parts List. Their values are shown on the Circuit Diagram
(Figure 21, Page 25). Commercial parts not readily avail-

arts

able and uniaue
be ordered diredtly from the factory.

The commercial alternates shown on the Parts List are
not necessarily equivalents, but are electronically and
mechanically similar, and may be used in the event that
direct factory replacements are not immediately available.

To maintain the highest possible performance and reliabil-

ity, Shure factory replacement parts should be used. When

ordering replacement parts, specify the Shure Replacement

Kit Number, description, product model number and serial
number.

FUSE REPLACEMENT

To replace line fuse F2 (with no apparent problems in the
unit), disconnect the line cord and remove the rear-panel
fuseholder cap. Replace the defective fuse only with a
3AG-5A fuse.

NOTE: The following symbol
on the rear panel denotes
operating fuse.

If trouble symptoms - overheating (thermal cycling),
erratic operation, etc. -were apparent before the
fuse blew, or if the replacement fuse blows, trouble­shoot the Amplifier carefully to find the source of the
trouble. Do not continue to replace fuses unless the
trouble has been corrected.

The Amplifier also contains two wired-in fuses, one in
series with rear-panel line fuse F2
in the meter lamp circuit (F3, SAG-1A). If replacement be­comes necessary, replace only with identical fuses.

BOTTOM PLATE REMOVAL

To remove the chassis bottom plate, turn the Amplifier
upside-down and remove the
edges of the chassis.

are shown on the Parts List and may

a

fast

CAUTION

(FI, 3AG-8A) and one

10 screws located at the

COVER REMOVAL

To service components on the top of the chassis, the
protective cover grille (MP7) must be removed. This is
done by removing one screw from the top, two screws from
each side, and eight screws from the back surface. To re­move, slide cover up and to the rear of the Amplifier.

LAMP REPLACEMENT

Two bayonet-base pilot lamps (PL2, PL3) illuminate the

scale of the OUTPUT VOLTAGE meter. These lamps are

mounted on the top surface of the chassis behind the meter

as shown in Figure 13, Page 13. To replace a lamp, remove
the grille cover and the nut on the lamp bracket mounting
stud. Swing the bracket up and out until the lamp is ac-

cessible. Push the lamp inward, twist it

clockwise, and withdraw it from its socket. After replacing
the lamp, reposition the assembly under the meter and
tighten the bracket nut. Replace the grille cover.

DC OUTPUT BALANCE ADJUSTMENT

In order to minimize the idle power consumption of the

Amplifier and eliminate dc offset at the direct-coupled

output, a dc balance control

vided. This control is located on the Main Circuit Board
under the chassis, as shown in Figure 14, Page 14, and may

be reached by removing the bottom cover. The dc balance

of the Amplifier should be checked and adjusted as neces­sary whenever any circuit components are replaced.

The dc balance adjustment is made with the Amplifier

VOLUME control
speaker load. If the Amplifier being adjusted is equipped
with a 70-volt output
should be turned off. Connect a dc voltmeter (preferably
with a 0.1 Vdc full-scale range) across the terminals of the
DIRECT OUTPUT terminal strip

(R41) full counterclockwise and no

(SR105A), the 70V-OFF Switch (S4)

(R4-DC BAL.) has been pro-

(TSI).

1/8-turn counter-

CAUTION

The polarity of any dc voltage at the output may be
either positive or negative and could be as much as

10 volts; care should be exercised in connecting the
meter such that an on-scale reading is obtained.

Carefully adjust the dc balance control for
at the output.

If the 70V-OFF Switch was moved to make this adjust­ment, return it to its original position. Replace the bottom
cover.

OUTPUT VOLTAGE METER CALIBRATION

In the event that the OUTPUT VOLTAGE Meter (MI) or
its associated circuitry is repaired, a meter calibration
check and adjustment should be performed. With the

Amplifier off, check the zero position of the meter. If it is

not on zero, use a small screwdriver to turn the meter zero

0 Vdc +-25 mV

R39 R40 Dl2 R36

T2

Y

SRIOSA MODEL ONLY

C14

C15

FIGURE 13. SR105 POWER AMPLIFIER TOP VIEW,

R3

1

Q8

COVER REMOVED

adjustment screw below the meter face until the needle is
properly positioned. With no load on the Amplifier (speaker
disconnected), connect an ac voltmeter across one of the
DIRECT OUTPUT Terminals. Turn the Amplifier on and ap-

3

ply a 1 kHz sine-wave signal at approximately

volts rms
to one of the Amplifier inputs. Adjust the VOLUME Control
for 28.3 volts at the output. Remove the bottom cover and
adjust the Meter Adjust resistor (R27-METER ADJ. located
on the Main Circuit Board under the chassis as shown in
Figure 14, Page 14) for a 100% reading on the OUTPUT
VOLTAGE Meter. No further adjustment is necessary.

Re-

place the bottom cover.

To calibrate the meter for a 100% indication with

70.7V

output, connect the voltmeter and an appropriate load

across the 70V OUTPUT terminals and proceed as above.

PRINTED CIRCUIT BOARD REMOVAL

The underside of the

SR105A Amplifier chassis contains

two printed circuit boards: a Main Circuit Board and a Filter

R35

(SRlOSA MODEL SHOWN)

Q7

R43 PL2 MI PL3

Circuit Board. (The

(XPL2) (XPL3)

SR105B Amplifier contains only a Main
Circuit Board.) The foil side of the boards may be made
accessible for servicing without disconnecting any leads
by removing the four Phillips head screws securing the
board. The boards may be completely removed as follows
(see Figure 14, Page 14). Remove the bottom cover. On the
Main Circuit Board, remove all

15

interconnecting leads

from the push-on board terminals, noting connections as

listed in the table below.

CAUTION

Similar wire colors are used in different circuits; make
sure proper re-connections can be made. Take care
not to bend or break the push-on terminals.

Remove the four Phillips head screws from the board and
remove the board from the chassis.

The Filter Circuit Board may be removed in the same

manner. Replace bottom cover after servicing boards.

13

Main Circuit Board

Letter

A

B
C

D

E

F

G

H

;

NOTE: Production variations may result in wire colors dif­fering from those in the table.

Wire Color

Blue
Black
Gray
Green/Black
Violet
Red
White
White

Filter Circuit Board

"

Yellow Yellow/Black

Letter

J

K

L

M

N

P

R

1;

-

-

Wire Color

Yellow/Black

Orange/Black
Orange

Brown
Yellow
Green
White

2;"

-

SMALL SIGNAL AND PREDRIVER TRANSISTORS

Transistors

Q203

are mounted on printed circuit boards. When re­placing these transistors it is imperative that proper lead
configuration be followed. A minimum of soldering heat
should be used to avoid damage to the transistor or printed

circuit board. Refer to the Notes to Circuit Diagram (Figure
20,

Page

DRIVER TRANSISTORS

Driver transistors
mounted on a bracket located on the top surface of the
Amplifier chassis. Before removing these transistors, write
down the lead color and location at each transistor solder
junction. When replacing transistors, apply Wakefield Type
120

thermal joint compound to each side of the insulating
wafer to provide good heat transfer from transistor to
bracket. After replacement and before connecting transis­tor leads, check the transistors between case and chassis
with an ohmmeter; there should be no continuity. Be sure
that these transistors are not interchanged in the circuit;
they are not identical devices.

Q8

is a PNP transistor. Refer to the Notes to Circuit Diagram

(Figure

R45 R46 R30 J6

Q1

through

23)

for the transistor lead codes.

20,

Page

23)

Q6, Q13, Q14,

Q7

and

Q8

for the transistor lead codes.

and

Q201

(Figure

Q7

is an NPN transistor and

13,

Page

through

13)

are

TI

R4

841

MPI M P4

Al R27 PLI R37 C14

C15

R3

DII MP5

W

I

F2
(XF2)

F

I

F3

*

SR105A MODEL ONLY

FIGURE 14. SR105 POWER AMPLIFIER BOTTOM VIEW,

COVER REMOVED

(SR105A MODEL SHOWN)

OUTPUT TRANSISTORS

Output transistors

are located on the black, finned heat sinks. The replace-

ment procedure is the same as that used for driver tran-

sistors Q7 and Q8.

NOTE: Output transistors
matched for current gain. When replacing output transis­tors, be sure to replace with devices which have the same
gain and part number as the original transistors. Shure
transistors have a letter suffix in the part number
86A360A) which serves as a gain code. Transistors with the
same part number and suffix letter have equal current gain.

DIODES

Diodes
the black, finned sinks with the output transistors. Special
care is required to insulate these diodes from the heat sink
while providing good heat transfer from sink to diode.
shrinkable tubing or "spaghetti" should be placed over the
diode and connecting leads; the diode should be securely
clamped to the heat sink with the clamp provided.

DIODE BRIDGE RECTIFIER

Silicon diode bridge rectifier

underside of the chassis as shown in Figure 14, Page 14.

When replacing this component, apply Wakefield Type 120

thermal joint compound between the heat sink base of the
rectifier and the chassis to provide good heat transfer to
the chassis. Note that the base of the rectifier is provided
with a locating pin. When installing the rectifier be sure
to position it such that the locating pin fits into the hole
provided for it in the chassis. The terminals are marked to
indicate the ac connections to the power transformer and
the
C14 and

Dl5 and Dl6 (Figure 13, Page 13) are located on

(+)

and

(-)

C15.

Q9 through Q12 (Figure 13, Page 13)

Q9 through Q12 must be

(i.e.,

Heat-

Dl1 is mounted on the

dc output connections to filter capacitors

with screws (Q7 through Q12) may be tested in place; how­ever, the base and emitter leads to these transistors must
be removed.

If all conditions in the following table are met, the tran-

sistors may be considered free of any major defect; if any
of the following conditions are not met, the transistor
should be replaced. See Notes to Circuit Diagram (Figure
20, Page 23) for transistor terminal codes.

Ohmmeter Connections

"Plus" Lead "Minus" Lead

Collector Emitter
Emitter Collector
Collector Base
Emitter Base
Base Collector
Base Emitter

SR105B AMPLIFIER CONVERSION FOR

70-VOLT OUTPUT

If it becomes necessary to equip a Model
Amplifier for 70-volt output (distributed speaker) operation
in addition to the existing 28-volt direct output, the conver­sion may be accomplished using the parts and procedures
described below. The required parts may be ordered
through a local Shure Franchised Dealer or directly from
the Shure Factory. No modifications to the metal chassis or
front panel are required.

Ohmmeter Reading

NPN PNP

Transistor Transistor

High High

High High

High Low

-

Low High
Low

Low

-

SRIOSB Power

WARNING

This conversion should only be performed by quali-

fied service personnel or the Shure Factory Service
Department.

CHECKING TRANSISTORS AND DIODES

Defective transistors and diodes may be located by use
of a standard ohmmeter such as a
(Polarity of the ohmmeter must be verified before these
checks are made.)

With a known diode orientation, measure the diode re-

sistance in the forward and reverse directions. The lowest

meter reading establishes the probe at the cathode end
(schematic symbol arrow points to cathode) as the "minus"
probe while the other probe is "plus." Some ohmmeters
are not polarized in this manner with relation to "volts
plus probe" and "volts minus probe." With the ohmmeter
"plus" probe on the anode end of a diode and the "minus"
probe on the cathode end, the ohmmeter should read ap­proximately 2
reversed, a reading of about 10 kilohms or more should
occur. If either of these conditions is not met, the diode
should be replaced.

When checking the bridge rectifier disconnect all leads

to the assembly, noting the terminals to which they were

connected. Check each diode leg of the bridge in the same
manner as described previously for individual diodes. Refer

to the preceding section, Diode Bridge Rectifier, for instal-

lation instructions if replacement is required.

To check transistors, the ohmmeter should be set to the

100- or 1,000-ohm scale. Small signal transistors

through Q6, Q13, (214, Q201 through (2203) must be re­moved from the circuit before testing. Transistors mounted

kilohms or less. With the meter probes

Simpson Model 260.

(Q1

Once the conversion is performed, the Amplifier is identical

SR105A Amplifier and all instructions and diagrams

to the

pertaining to the

The following parts, materials and tools are required to
perform the 70-volt conversion (NOTE: Figure 15, Page 16
shows the electrical circuitry involved. Reference to the
SR105A Amplifier Circuit Diagram, Figure 21, Page 25, will
aid in understanding the circuit modifications.)

Remove the line cord from the ac outlet and remove all

input and output cables. Remove the metal grille cover as

described previously in this section. Remove the two lock-

nuts securing the two lamp socket assemblies (see Figure
16, Page 17). Remove the lamp socket assemblies from the
mounting studs and move them away from the studs to

permit mounting of the 70-volt transformer. Place the trans­former over the four mounting studs, orienting the trans­former leads toward the center of the Amplifier chassis.
Replace the lamp socket assemblies, orienting them as
shown in Figure 16, and secure the transformer using the
two locknuts previously removed and the two supplied with
the transformer. Pass the transformer leads through the
unused grommet nearest the rear of the Amplifier chassis.

Turn the Amplifier upside-down and remove the chassis
bottom plate as described previously in this section. Re­move the rear-panel 70-volt conversion Cover (marked
SHURE MODEL
screws securing it to the chassis. Mount the terminal strip

SR105A Amplifier apply.

SRlO5B) (MP6) by removing the four

SR105B AMPLIFIER

CONVERSION

Qty.

1

1

1

1

Shure

Part No.

51 A244

55C131

56A204

90A2038

PARTS

Description

Transformer, 70V
Output

Switch, Slide, DPDT

64)

Terminal Strip, 3­Screw-Terminal (TS2)

Filter Circuit
Board Assembly

(A21

(T3)

Qty.

2

2

4

4

-

-

-

MATERIALS

Description

Locknuts, 10-32*

Machine Screws,
Phillips Round

x

%

"

'1'4 "*

3/16"

*

Head, 4-40

Machine Screws,
Phillips Head,

x

6-32

Machine Screws,
Phillips Head,

x

6-32
Wire, Hookup,

16 Gauge AWG,
Stranded Copper,
Vinyl Insulation

Wire, Hookup, 20
Gauge AWG, Stranded

Copper, Vinyl

Insulation
Solder, Rosin-Core

11

/I

I(

11

I I
I

I

TOOLS

Description

Pliers, Diagonal,
or Wire Stripper

Pliers, Long-nose
Screwdriver
Screwdriver, Phil-

lips Head
Soldering Iron,

Low Wattage

Wrench, Open-End,
Adjustable

Two locknuts

*'Four machine screws presently securing rear-panel 70-Volt Conversion Cover

FROM FILTER CIRCUIT

are presently securing

BOARD PIN

FROM MAIN CIRCUIT

BOARD PIN

lamp socket assemblies; two more are

r

TS

2

0

COM CT 70V

V

YEL/BLK

J

0

0

GRN RED

YEL

BLK

required.

(53A1319)

s4-

may be used.

T3

OFF

-

d

28V

0

GND

WHT
WHT

TS

J4 -J7

I

-

-

I

FROM FILTER CIRCUIT

BOARD PIN T

16

FIGURE

BLU

15.

70-VOLT CONVERSION: ADDED CIRCUITRY

0

0

70

V

in the space exposed by removal of the cover using four

3h1'

6-32 x

Phillips head machine screws. Be sure to orient
the screw contacts on the terminal strip in the same man­ner as the existing DIRECT OUTPUT terminal strip. Mount
the slide switch next to the terminal strip in the space pro­vided, using two Phillips round head 4-40 x 3/16" machine
screws. Mount the Filter Circuit board next to the Main

1/4"

Circuit board using the four 6-32 x

Phillips head ma-

chine screws previously used to secure the cover.

V (yellow preferred) to the upper left (viewed from terminal
side) terminal of switch S4; from pin

T

(blue preferred) to
the bottom left (viewed from terminal side) terminal of
switch S4; and from pin S (red preferred) to the positive

(red dot) terminal of capacitor C14.

Solder a length of 16-gauge hook-up wire (white pre-

ferred) from the 28V terminal of the 28-volt terminal strip

(TS1) to the bottom right (viewed from terminal side) ter­minal of switch S4 (see Figure 17, Page 18). Solder trans-

former T3 leads coming out of the chassis grommet as

follows:

TRANSFORMER

FIGURE

16.

TRANSFORMER MOUNTING

70-VOLT CONVERSION:

Remove the white lead from pin H on the Main Circuit
board and fasten it to pin Won the Filter Circuit board. Re­move the

yellowlblack lead from pin J on the Main Circuit
board and fasten it to pin Yon the Filter Circuit board. Cut
a length of 20-gauge hook-up wire

and solder one end to pin

J

(yellowlblack preferred)

on the Main Circuit board.

CAUTION

Be careful not to damage board or adjacent compo­nents when soldering.

Run the soldered wire roughly parallel to the long edge of

the Main Circuit board to the main wiring harness and along
the harness to the 70-volt slide switch

up wire to the left center terminal (viewed from terminal

side) of the switch (see Figure 17, Page 18). Cut a length of
20-gauge hook-up wire (white preferred) and solder one

H

end to pin

wire twice around the two

board
tiometer

the white wire to pin

on the Main Circuit board. Run the soldered

yellowlblack wires (Main Circuit

J

to switch S4, and Filter Circuit board Y to poten-

R41) to form a loose shield. Solder the other end of

X

on the Filter Circuit board.

Cut three lengths of 20-gauge hook-up wire and solder

them to pins on the Filter Circuit board as follows: from pin

(S4). Solder the hook-

Transformer

Lead Color

Red

Black

Yellow

Green

Blue

The Amplifier conversion is now complete. Perform

Part

Connection

28V Terminal

(TS1)

Strip

70V Terminal

(TS2)

Strip

70V Slide

Switch

(S4)

Part Terminal

Designation

GND

70V

CT

COM

Right Center

Terminal

a

dc
balance adjustment as described previously in this section.
Reassemble the Amplifier and return it to service.

SERVICE ILLUSTRATIONS

The pages that follow contain parts locating drawings
and an overall Circuit Diagram. The parts locating drawings
are of the Main and Filter Circuit boards. Foil circuit paths
are shown as shaded areas on the drawings. The Circuit

Diagram shows all printed circuit board and

chassis-

mounted parts.

GUARANTEE

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 your dealer

or Authorized Shure Service Center for repair. The unit

to

will be returned

you prepaid.

JI-J2

e

-

-

TS2

S

4

OFF

YEL

/

BLK

a

J

R4

VOLUME

1

H&

WHT

MAIN

CIRCUIT

BOARD

a

FILTER

CIRCUIT

BOARD

GROMMET

&-

TU

t

1

x

J

e

d

+

3

m

RED

FIGURE

17.

70-VOLT CONVERSION: INTERNAL WIRING

REPLACEMENT PARTS FOR SR105 POWER AMPLIFIER

-

Reference

Designation

Q8

Q9-Q12

R35-R40
R41
R43, R44
R45, R46

S 1

52, S3
S4
TI
T2
T3

TB1
TB2
W1
W2

XF1

XPLl
XPL2

Replacement

Kit.

NO.*

-

-

-

-

-

-

-

RKC37

-

-

-

-

-

-

-

-

-

-

-

Qty.

-

-

-

-

-

-

-

1

-

-

-

-

-

-

-

-

-

-

Part No.

86A362

86A360
45EB439D
46A041
45CB758C
45CB308C
55A96

95A551
55C131

90K2150

51 A256

51 A244

56A211

56A204

95A632

90A1522

95A429

95A598

95A599

Replacement Kit Consists

Transistor, Silicon, PNP, Driver,

BVCEO 120V min.

Transistor, Silicon, NPN

Resistor, Fixed, 0.43 ohms, 5%, 7W
Potentiometer,
Resistor, Fixed, 7.5 ohms, 5%, 1 W
Resistor, Fixed, 3.0 ohms,
Switch, Toggle, SPST, POWER

Thermostat, SPST
Switch, Slide, DPDT, 70V-OFF
Transformer Assembly, Balanced Input
Transformer, Power

Transformer, 70V Output

Terminal Strip, 2-screw, DIRECT OUTPUT
Terminal Strip,
Line Cord and 3-Conductor Ac Plug Assembly
Cable and Plug Assembly, 6-Foot, with

2 Phone Plugs

Fuseholder, 3AG-5A

Lampholder and Bracket Assembly

Lampholder and Bracket Assembly,

Nylon Sleeve

50K, VOLUME

Of:

Description

&screw, 70V (SR105A only)

5%,

1 W

(SRl05Aonly)

(SR105A only)

Commercial

Alternate

Motorola

(Selected)

RCA 2N3773
None
None
Allen-Bradley GB
Allen-Bradley GB
Cutler-Hammer

751

None

None
None
None
None
Cinch 2-141-Y
Cinch 3-141-Y
None

None

Littelfuse 342014

None

None

2N3741

OK1 2

MAIN CIRCUIT BOARD ASSEMBLY

C1

C2

C5

C6

D 1
D2, D3, D6

04, D5

D9

D8,

'Parts listed as

20

-

-

-

-

RKC23
RKC21

I

-

I

RKCl9

RKC

Kits should be ordered by that kit number. Any orders received for piece parts where RKC Kit number is shown will be shipped in RKC quantities.

-

86K628

-

50KC224

-

86D630

86H628

86A409

1

86A404

4

I

-

1

86A415

1 4 1

86A405

Capacitor, Electrolytic, 250 PF, 16V

Capacitor, Film, .22

Capacitor, Electrolytic, 10 PF, IOV

Capacitor, Electrolytic, 80 PF, 25V

Zener Diode, Silicon,
Silicon Rectifier, 100V, 1/2A

1

Diode,Silicon, Computer, 75V

1

Diode, Germanium.30V

PF, 100V

2.6V

Sprague
30A-TE11645;

Mallory MTA­250F15

CDE 1 P22-10

Sprague
30D-TE1128;
CDE NLW-10-16

Sprague
30D-TE1210;

CDE NLW-100-25

Motorola 1

Motorola 1 N4002

TI or GE 1 N4148

RCA 1N48, IN60

N4729A

REPLACEMENT PARTS FOR SR105 POWER AMPLIFIER

Reference

Designation

Kit. No.*

I

RKC9

-

1

-

RKC53

-

-

-

-

Part

1 4 1

1

86A349

-

86A355 Transistor, Silicon, NPN

-

/

86A354

1

86A333

-

86A363

-

86A364
46A042

-

45F6818B

-

45F6498B

Replacement Kit Consists

No.

I

/

Transistor. Silicon. NPN

1

Transistor, Silicon, Power, PNP
Transistor, Silicon, NPN
Transistor, Silicon, Power, NPN,
Transistor, Silicon, PNP,

Potentiometer, P.C. Bd. Mtg., 10K
Resistor, Metal Film, 1.3k, 1

Resistor, Metal Film, 68.1 ohms, 1

Resistor, Metal Film, 64.9 ohms, 1

Of:

Descri~tion

VI Limiting

FILTER BOARD ASSEMBLY (SRIOSA ONLY)

%,

1/2

VI Limiting

W

%,

112

W

%,

1/2

W

Commercial

Alternate

Motorola 2N5088;

TI 2N3711

Motorola MPS-A06

Motorola 2N5680

Motorola

Motorola MPS-A20'

Motorola MPS-A70**

IRC CEC-TO

I RC CEC-TO

2N5682

*

C201

C202, C203

C204

C205
C206

Q201, Q202

Q203

'Parts listed as RKC Kits should be ordered by that kit number. Any orders received for piece parts where RKC Kit number is shown will be shipped in RKC quantities
^*Use in emergency only. Select for high gain. May affect power output and reliability.

-

-

-

-

-

RKC9

-

-

-

-

-

-

4

-

50KB224
50KA104

86A630

50KA124
86K628

86A349

86A348

Capacitor, Film, .22 aF, IOOV

Capacitor, Film,

Capacitor, Electrolytic, 4.7 or 5 PF, 35V

Capacitor, Film,
Capacitor, Electrolytic, 250

Transistor, Silicon, NPN

Transistor, Silicon, Low Power, PNP

.I

aF, IOOV,

.I

2 PF, 1 OOV, 5%

PF, 16V

5%

CDE 1 P22-10

Sprague

225P10491;

CDE 1P1-10

Sprague

30D-TE1303;

CDE NLW-5-50

None

Sprague

BOD-TE1164.5;

Mallory MTA-

250F15

Motorola 2N5088;

TI 2N3711

Motorola 2N5087

FIGURE

18.

MAIN CIRCUIT BOARD

PARTS LOCATION DIAGRAM

FIGURE

19.

FILTER CIRCUIT BOARD

PARTS LOCATION DIAGRAM

2038-21525-2

SR105

Power

Amplifier

NOTES TO CIRCUIT DIAGRAM

GENERAL

Shure part numbers are not shown in the Parts List
accompanying the Circuit Diagram (Figure 21, Page 25)
if parts are readily available through local electronics parts
suppliers. In these instances, the Circuit Diagram shows
only the reference designation and value of the standard

parts.

All capacitor values are shown in microfarads unless
otherwise designated. All non-electrolytic capacitors are

100 working volts dc or more unless otherwise specified.

x

Electrolytic capacitors are shown in microfarads

=

All resistor values are shown in ohms (k
tors are 10% tolerance unless otherwise specified. Resis­tors are

table replacements are as shown in the Parts List.
Chassis Ground

Circuit Ground
Printed Circuit Board Ground

TROUBLESHOOTING

Amplifier is completely "dead," check the ac power source,
fuses, and power supply output
Circuit board). If the indicator lamps are on but the output
is distorted, low or not present, apply an input signal as
described under Ac Voltage Measurements below, and

Vi-watt unless otherwise specified.

Transistor lead codes are as shown in Figure 20. Accep-

The following round symbols denote:

4

&

A

FIGURE

A general troubleshooting process is as follows: If the

20.

TRANSISTOR LEAD CODES

(52.5V at pin F of Main

1000). Resis-

volts.

determine that the input and output voltages to each board
assembly are correct. If an incorrect ac voltage is found
at any board output, perform Dc Voltage Measurements on

that board as described below to isolate the problem area.

AC VOLTAGE MEASUREMENTS

The numbers within rectangular symbols
Circuit Diagram denote the ac voltage at that point under
the following test conditions:

1. Voltage measured with respect to chassis unless other-

wise indicated.

2. Line voltage:

3. Test signal of

4.

Ac voltage measurements may vary t20% from values

shown.

5. Measurements made with ac VTVM of 1 megohm or
greater input impedance.

6. Four-ohm non-inductive 200-watt load across DIRECT

OUTPUT Terminal Strip
ments or
load across 70-volt OUTPUT Terminal Strip TS2
COM) for 70-volt output measurements, but not both.

7. Volume Control set to maximum (10).

8.

SR105A Only: 70V-OFF Switch set to OFF for all circuit
measurements except 70-volt circuitry (Filter Circuit
board and associated components). Filter Circuit board

rolls off the low frequencies; amplifier output should
be -3

-c-3 dB at 20 Hz.

DC VOLTAGE MEASUREMENTS

The numbers within elliptical symbols
diagram denote the dc voltage at that point under the
following test conditions:

1. Voltages measured with respect to chassis unless other-

wise indicated.

2. Line voltage:

3. No input signal applied.

4.

Dc voltage measurements may vary t20% from values
shown.

5.

Measurements made with VTVM of 11 megohms or

greater input impedance.

RESISTANCE MEASUREMENTS

With the ac line cord disconnected from the ac source
and the POWER ON-OFF Switch in the OFF position, the
following ohmmeter measurements may be made:

1. Transformers may be checked for continuity of each

winding.

2. To test transistors and diodes, see Page 15.

120V, 50/60 Hz.

0.3V, 1 kHz applied across connector J1.

TS1 for direct output measure-

(SR105A only) 33-ohm non-inductive 150-watt

-c-1 dB at 30 Hz referenced to 1 kHz and -12

120V, 50160 Hz.

0

0

on the circuit

on the

(70V-

LINE LEVEL INPUTS

120 VAC

50160 H

NOTE

r

I:

DIFFERENCES BETWEEN SRIOSA AND SR105B:

WIRING SHOWS SRlOSB CIRCUITS; SRIOSA CIRCUITS INCLUDE
ASSEMBLY
STRIP TS2.

A2,

SWITCH S4, TRANSFORMER T3, AND TERMINAL

"STRAIGHT-

THROUGH"

SRIOSB

REFERENCE

NO.

R

I,

C17,

R9

CIB

DESCRIPTION

WERE

ADDED

--

12kb

C13

ADDED

FIGURE 21. SR105 POWER AMPLIFIER CIRCUIT DIAGRAM

SR105

Power Amplifier

CONDENSED OPERATING INSTRUCTIONS

DIRECT SPEAKER OUTPUT

(MODELS SRlOSA and SRlOSBI

I.

'with line cord unplugged, install Amplifier in rack or

carrying case, allowing adequate ventilation.

2. Connect speakers to DIRECT OUTPUT Jacks
Terminal

3. Connect

PARALLELED
length greater than

anced line into BALANCED BRIDGING

(three-pin). (three-pin).

4.

connect additional power amplifiers or auxiliary equip-

ment as required.

5. Connect line cord to ac source.

6.

Turn on POWER ON-OFF Switch and adjust VOLUME

Control to desired level. Control.

Stri~. 2. Connect speakers to 70V OUTPUT Terminal Strip.

con'sole or mixer output to UNBALANCED

HIGH IMPEDANCE Input Jack. For cable

15m (50 ft), use 600-ohm bat- length greater than 15rn (50

SR105

and/or

Input Jack anced line into BALANCED BRIDGING Input Jack

Power Amplifier

CONSTANT VOLTAGE 70-VOLT SPEAKER OUTPUT

(MODEL

l.

for Direct Speaker Output operation.

3. Connect console or mixer output to UNBALANCED
PARALLELED

4.

fur,

5. Connect line cord to ac source.

6. Turn POWER ON-OFF Switch on and adjust VOLUME

ARCHITECTS' AND ENGINEERS' SPECIFICATIONS

SRlOSA POWER AMPLIFIER

The Amplifier shall be a rack-mounted, 120-volt,
Hz line-operated, all silicon transistor, 200-watt power
amplifier.

The Amplifier shall
put power at

distortion (DIRECT OUTPUT), and 150 watts rms continuous

output power with less than 3% distortion to a 33-ohm load

(70-VOLT OUTPUT). Typical frequency response shall be
20 to 20,000 Hz

PUT) and 50 to 15,000 Hz 2 db with a 33-ohm load

VOLT OUTPUT).

The Amplifier
minal strip speaker output for
minal strip for 70-volt output.

The Amplifier shall have

Amplifier from open circuit, short circuit, or mismatched
output loads, by using current-limiting, voltage-limiting,
temperature-sensing diodes, and thermal-sensing switches.

The automatic thermal-sensing switches and temper­ature-sensing diodes shall be mounted on the output tran­sistor heat sinks to protect the Amplifier from overheating

due to short circuits, mismatched output loads, or high
ambient temperatures. The thermal switches shall auto-

matically shut off the ac power in the event of overheating
and shall automatically restore the ac power when the
Amplifier has reached a safe operating temperature. When

the Amplifier has thermally cycled off, a THERMAL

LOAD indicator shall light.

The INPUTS shall accept signals from high impedance
sources. The balanced bridging input shall be a profes­sional three-pin audio connector. The unbalanced input
connectors shall be standard

1

deliver 200 watts rms continuous out-

kHz into a 4-ohm load with less than

t1.5 dB with a 4-ohm load (DIRECT OUT-

shall have four phone jacks and one ter-

25-volt output, plus one ter-

built-in circuitry to protect the

%-inch phone jacks.

50/60

2%

(70-

OVER-

The Amplifier shall have an independent VOLUME con­trol, POWER ON-OFF Switch and
to activate the 70-volt output transformer.

The Amplifier shall be enclosed in a metal, rack-mount-

ing cabinet, with a scuff-resistant vinyl-covered front panel.

The dimensions shall be 178 mm

(7 inches in height, 19 inches in width, and
in depth). The weight shall be no more than 15.66 kg
pounds 8 ounces).

The Amplifier shall have a maximum voltage gain of

27

-+2 dB at 1 kHz, and a nominal input sensitivity of

1.2 volts for 200 watts rms output power to a 4-ohm load.
Seventy-volt circuitry voltage gain shall be 35
at 1 kHz, and a 1.2-volt

watts rms output power to a 33-ohm load.

The

below 200 watts rms output with a 4-ohm resistive load.

The Amplifier

13 other

Any Amplifier not meeting all of the above specifications
shall be deemed unacceptable under this specification.
The Amplifier shall be a Shure Model

SRlOSB POWER AMPLIFIER

The Amplifier shall be a rack-mountable, 120-volt,

Hz line-operated, all silicon transistor, 200-watt powar

amplifier.

The Amplifier shall deliver 200 watts

power at 1 kHz into a 4-ohm load with less than 2% dis-

tortion. Typical frequency response shall be 20 to

21.5 dB with a 4-ohm load.
The

minal strip speaker output for 25-volt output.

SRlOSA ONLY)

Amplifier

.

.

70V-OFF

Amplifier hum and noise shall be at least 80 dB

silicon transistors, and 1 Zener diode.

Amplifier shall have four phone jacks and one ter-

and

connect

HIGH IMPEDANCE Input Jack. For cable

Switch

shall contain 4 silicon output transistors,

to

nominal input sensitivity for 150

70".

auxiliary

ft),

70-VOLTIOFF Switch

x

equipment

use 600-ohm bal-

483 mm x 270 mm

lO5/8

inches

%3.5 dB

SR105A.

50/60

rms continuous

20,DOO Hz

as

(34

The Amplifier shall have built-in circuitry to protect the
Amplifier from open circuit, short circuit, or mismatched
output loads, by using current-limiting, voltage-limiting,
temperature-sensing diodes, and thermal-sensing switches.

The automatic thermal-sensing switches and
ature-sensing diodes shall be mounted on the output tran­sistor heat sinks to protect the Amplifier from overheating
due to short circuits, mismatched output loads, or high

ambient temperatures. The thermal switches shall auto­matically shut off the ac power in the event of overheating

and shall automatically restore the ac power when the
Amplifier has reached a safe operating temperature. When
the Amplifier has thermally cycled off, a THERMAL OVER-

LOAD indicator shall light.

The INPUTS shall accept signals from high impedance
sources. The balanced bridging input shall be a profes­sional three-pin audio connector. The unbalanced input
connectors shall be standard

%-inch phone jacks.

temper-

The Amplifier shall have an independent VOLUME Con­trol and POWER ON-OFF Switch. The Amplifier shall be en­closed in a metal, rack-mounting cabinet, with a
resistant vinyl-covered front panel. The dimensions shall
be 178 mm x 483 mm

inches in width, and 105h inches in depth). The weight

shall be no more than 12.23 kg (27 pounds).

The Amplifier shall have a maximum voltage gain of

k2 dB at

27

1.2

volts for 200 watts rms output power to a 4-ohm load.

The Amplifier hum and noise shall be at least 80 dB

below 200 watts rms output with a 4-ohm resistive load.

The Amplifier shall contain 4 silicon output transistors,

10 other silicon transistors, and

Any Amplifier not meeting all of the above specifications
shall be deemed unacceptable under this specification.
The Amplifier shall be a Shure Model

1

x

270 mm (7 inches in height,

kHz,

and a nominal input sensitivity of

1

Zener diode.

SR105B.

scuff-

19