Yamaha P2075 User manual

YAMAHA

Power

Amplifier

P2075

Operating

Manual

CHANNEL

A

CHANNEL

В

PROTECTION

SIGNAL

CLIP

Bru

СЫР

о

o

18 16

14

12

ag

16,14

yo

20,

HOW

TO

USE

THIS

MANUAL

If

you

are

an

engineer

or

technician

who

is

familiar

with

sound

system

design,

much

of

this

manual

will

serve

as

a

review

for

you.

The

basic

features

are

|

presented

in

the

"BRIEF

OPERATING

INSTRUCTIONS”

section.

Check

this

and

the

“SPECIFICATIONS”

section,

and

you

will

see

most

of

what

you

need

fo

know.

If

you

would

like

to

know

more

about

AC

power

distribution

and

safety,

grounding,

speaker

systems,

crossovers,

and

so

forth,

this

information

is

also

presented.

Check

the

Table

of

Contents.

Due

to

common

usage

and

for

convenience,

we

will

sometimes

refer

to

dual-

channel

amplifiers

ағ

"stereo"

models

....

especially

because

they

have

a

rear-

panel

MONO/STEREO

switch.

Bear

in

mind,

however,

that

the

two

channels

may

be

used

for

dual

monaural

programs,

for

two

sections

of

a

bi-amplified

speaker

system,

or

for

bridged

monaural

operation

—

so

"stereo"

is

not

neces-

sarily

an

accurate

description

of

the

amplifier

in

every

application.

TABLE

OF

CONTENTS

ІМТНОПОСТІОМ...........................

2

How

crossover

networks

operate

and

what

they

do

..

25

BRIEF

OPERATING

INSTRUCTIONS

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

3

The

difference

between

low

and

high

level

Front

Рапе!.............................

3

CYOSSOVEIS

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

26

Rear

Рапе!..............................

4

The

pros

and

cons

of

low

level

and

high

level

Amplifier

Operation

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

5

СГОЗЗОМе!

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

26

Precautions

regarding

AC

power

source

for

amplifiers

.

5

The

meaning

of

various

amplifier

power

rating

INSTALLATION

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

6

methods

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

28

Input

Connections...

6

PROTECTION

FOR

SPEAKERS.

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

29

Output

Connections

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

7

Output

relay

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

29

MORE

INFORMATION

ON

SOUND

SYSTEM

WIRING

.

9

Current

limiting

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

-,

29

Balanced

and

unbalanced

wiring

.............-

9

Speaker

Ғи5ес...........................

29

Why

input

transformers

sometimes

are

used.

......

10

Compressor/limiters

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

ИО

30

Noise

and

losses

іп

low

impedance

and

high

Equalizers

and

filters

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

0000

ee

30

impedance

Нпез...............

ежа

‚

91

High

frequency

driver

protection

networks

Signal

levels,

dynamic

range

and

headroom

......,

11

(Якев)...............................

30

Сғоипдіпа.............................

14

SPECIFICATIONS

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

32

|

AC

outlet

wiring

affects

grounding

and

safety

.....

17

DIMENSIONS

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

32

|

Speaker

Мігіпе..........................

:19

.

BLOCK

DIAGRAM

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

33

|

.

Distributed

speaker

systems

and

constant

voltage

lines

22

PERFORMANCE

GRAPHS

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

24

|

How

to

calculate

amplifier

power

output

in

bridged

|

MONO

тоде......:....................

23

MULTI-AMPLIFIED

SPEAKER

SYSTEMS

(Biamping,

triamping

and

so

forth)

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

...

24

INTRODUCTION

-

This

manual

describes

the

P2075

power

amplifier.

The

P2075

is

dual-channel

amplifier

rated

at

50

watts

into

8

ohms

or

75

watts

into

4

ohms.

This

amplifier

is

equipped

with

features

that

make

it

well

suited

to

a

variety

of

sound

reinforcement,

recording

and

musical

instrument

applications.

Electronically

balanced,

differential

input

circuits

allow

the

use

of

long,

shielded

3-wire

input

cables

to

reduce

susceptibility

to

hum.

Each

input

appears

at

an

XLR

connector

and

a

pair

of

phone

jacks.

The

XLR

inputs

combine

the

advantages

of

electrostatic

and

electromagnetic

noise

rejection,

with

locking

terminals

that

won't

pull

out

accidentally

should

the

cable

be

tugged.

The

phone

jacks

provide

for

fast,

compatible

connection

to

equip-

ment

that

utilizes

phone

jack

outputs.

The

output

circuitry

is

fully

protected

from

overloads

and

short

circuits,

and

the

speaker

outputs

are

relay-protected

against

turn-on

transients

and

DC

offset.

Protec-

tion

circuitry

is

activated

and

the

PROTECTION

LED

blinks

when

the

heat

sink

temperature

exceeds

60°С.

For

General

model,

when

the

heat

sink

temperature

exceeds

85^C,

the

relay

is

turned

off

and

output

is

cut.

Output

connections

are

made

via

either

phone

jacks

or

5-way

binding

posts.

The

binding

posts

provide

more

secure

wiring,

better

current-handling

capability,

and

less

tendency

to

build

up

contact

resistance

than

typical

phone

jack

outputs.

Phone

jack

.

outputs

are

provided

for

convenience

in

fast

setup

situations.

The

amplifiers

are

rated

for

4-ohm

or

higher

impedance

loads.

When

mounting

the

P2075

in

any

standard

19”

electronic

equipment

rack,

cooling

fans

maybe

required

when

it

must

produce

extremely

high

average

power

output.

This

amplifier

is

professional

in

every

detail,

right

down

to

the

input

attenuators

—

recessed

controls

with

31

calibrated

positions

and

rubber

security

caps.

Rather

than

experiment

with

exotic

designs,

Yamaha

has

applied

field-proven

principles

to

design

amplifiers

which

are

highly

stable

and

which

should

deliver

the

kind

of

reliability

for

which

Yamaha

has

earned

a

solid

reputation.

As

with

any

product,

however,

proper

installation

and

use

is

essential

to

success.

This

manual

has

been

prepared

to

assist

you

in

getting

the

most

out

of

your

new

Yamaha

power

amplifier.

We

encourage

you

to

read

it

thoroughly.

BRIEF

OPERATING

INSTRUCTIONS

Front

Panel

PROTECTION

|

POWER

AMPLIFIER

P2075

©

INPUT

ATTENUATOR(S)

The

P2075

has

two

log-linear

input

attenuators.

Each

control

is

marked

in

31

dB-calibrated

steps,

and

detented

for

extra

accuracy.

The

attenuators

provide

a

smooth,

noise-free

transition

from

maximum

level

(zero

attenuation)

to

silence

(“infinite”

attenuation).

The

attenuators

are

recessed

(flush

with

the

front

panel)

to

avoid

damage

or

accidental

setting

changes.

Calibrated

input

attenuators

offer

numerous

advantages.

They

allow

predictable

and

repeatable

setups

in

portable

sound

systems,

as

well

as

in

permanently

installed

systems

which

are

pre-checked

in

the

shop

and

later

installed

at

another

location.

Whether

the

amplifiers

are

used

on

stage

in

portable

sound

systems

or

installed

in

studios,

factories,

meeting

rooms,

theatres,

arenas

or

elsewhere,

their

calibrated

attenuators

allow

easy,

accurate

input

sensitivity

adjustments.

The

calibrated

attenuators

permit

operators

to

adjust

the

level

of

two

channels

with

precise

tracking.

@

INPUT

LEVEL

INDICATORS

(SIGNAL,

CLIP)

A

pair

of

LEDs

above

the

input

attenuator

monitor

the

signal

level.

The

green

"SIGNAL"

LED

turns

on

whenever

the

signal

present

at

the

amplifier

output

is

2

volts

RMS

or

higher.

Since

this

is

equivalent

to

1

watt

into

a

4-ohm

load,

or

1/2

watt

into

an

8-ohm

load,

the

SIGNAL

LED

basically

indicates

that

some

program

material

is

present.

The

red

“CLIP”

LED

turns

on

when

the

output

stage

is

about

to

clip,

either

from

an

overdriven

input

or an

overloaded

output.

This

"flags"

the

operator,

suggesting

the

attenuator

be

turned

down

or

the

speaker

load

corrected

to

avoid

distor-

tion.

Fig.

1

POWER

SWITCH,

POWER

&

PROTECTION

INDI-

CATORS

An

alternate-action

switch

turns

the

AC

power

on

and

off.

When

the

power

is

ON,

a

red

LED

immediately

above

the

switch

is

illuminated.

Immediately

after

turning.

power

оп,

a

second

red

LED

above

the

PROTECTION

label

will

also

be

on,

indicating

the

speaker

output

is

disconnected

from

the

load

by

a

protection

relay.

After

a

few

seconds,

the

PRO-

TECTION

LED

should

turn

off,

indicating

that

the

outputs

is

activated

(the

slight

delay

allows

time

for

the

internal

amplifier

circuits

to

stabilize

and

thus

avoid

turn-on

thump).

The

PROTECTION

LED

blinks

when

the

heat

sink

tempera-

ture

exceeds

60°C.

For

General

model,

at

a

heat

sink

tem-

perature

of

85°

С

the

relay

is

turned

off

and

output

is

cut.

HINT

REGARDING

TURN-ON

OF

THE

POWER

AMPLIFIER

Unless

you

are

turning

on

all

the

equipment

in

the

sound

system

simultaneously

with

a

switched

strip

of

power

outlets,

be

sure

to

turn

on

everything

else

before

you

turn

on

the

amplifier(s).

By

turning

on

the

power

amplifier

last,

you

can

prevent

turn-on

“thumps”

generated

by

the

console,

graphic

EQ,

electronic

cross-

over,

or

other

accessory

signal

processors

from

possibly

damaging

speakers.

The

opposite

procedure

should

be

used

to

shut

off

the

system:

first

turn

off

the

ampli-

fiers,

then

the

rest

of

the

equipment.

This

power

amplifier

has

a

relay

which

is

timed

to

turn

on

the

speaker

outputs

after

the

amplifier’s

power

supply

is

fully

charged

up,

thus

preventing

any

turn-on

noise.

Timing

of

the

amplifier’s

turn-on

circuit

is

usually

sufficient

to

accommodate

all

the

turn-on

anomalies

from

other

pieces

of

gear

in

a

system,

making

it

accept-

able

to

use

a

single

switched

power

strip

in

a

perma-

nently

installed

or

“packaged”

portable

sound

system.

Rear

Panel

CAUTION

то

REDUCE

THE

RISK

OF

ELECTRIC

SHOCK,

DO

NOT

REMOVE

COVER.

NO

USER

SERVICEABLE

PARTS

INSIDE

REFER

SERVICING

TO

QUALIFIED

SERVICE

PERSONNEL

ATTENTION

arin

OE

REDUIRE

LE

RISQUE

DE

CHOC

ELECTRIQUE.

NE

PAS

ENLEVER

LE

COUVERCLE.

IL

NE

SE

TROUVE

A

L'INTERIEUR

AUCUNE

PIECE

POUVANT

ETRE

REPAREE

PAR

L'USAGER

SADRESSER

A

UN

REPARATEUR

COMPETENT.

MONO

MODE

OPERATION-SEE

INSTRUCTION

MANUAL

FOR

CORRECT

SPEAKER

IMPEDANCE

POUR

LE

МООЕ

MONAURALCONSULTEZ

LE

MANUAL

O'OPERATION

POUR

VOUS

ASSURER.

DE

10

IMPEDANCE

ADEQUATE

DES

HAUT-PARLEURS,

MODE

CHANNEL

Bn

INPUT

à

©

INPUT

CONNECTIONS

Input

connectors

for

each

channel

include

one

female

XLR

connector

(electronically

balanced)

plus

1/4-inch

Tip/Ring/

Sleeve

phone

jack

(for

balanced

or

unbalanced

lines).

2

of

the

XLR

is

wired

as

the

“hot”

lead

satisfying

DIN/JIS

and

IEEE

standards.

This

corresponds

to

the

tip

of

the

T/R/S

phone

jacks.

Unbalanced

T/S

phone

plugs

may

be

used,

as

well

as

balanced

T/R/S

plugs.

©

MODE

SWITCH

(MONO/STEREO)

This

two-position

switch

selects

the

operating

mode

of

the

STEREO

or

MONO:

STEREO

mode

selects

independent

operation

of

each

amplifier

channel.

Facing

the

rear

panel,

the

right-hand

pair

of

(*)

and

(—)

terminals

carry

signal

for

the

Channel

A

speaker

load,

and

the

left

pair

of

(+)

and

(—)

terminals

to

the

Channel

B

load.

:

МОМО

mode

rewires

the

two

channels

for

bridged

mono

operation.

Only

the

Channel

A

input

attenuator

and

input

connectors

are

operative.

Connect

the

speaker

load

to

the

two

red

(+)

terminals

for

Channels

А

and

B;

the

Channel

А

(+)

terminal

has

the

same

polarity

as

2

of

the

XLR

or

the

(*)

input

terminal.

Do

not

allow

either

of

the

speaker

wires

to

contact

any

other

wire,

chassis,

rack

or

connector.

Q

SPEAKER(S)

OUTPUT

CONNECTIONS

Each

channel's

output

is

brought

to

three

connection

points

(all

wired

in

parallel):

a

pair

of

5-way

binding

posts

and

two

1/4-іпсһ

Tip/Sleeve

phone

jacks.

The

red

(+)

and

black

(—)

terminals

of

the

5-way

binding

posts

are

the

preferred

connections

for

normal

operation.

However,

many

speaker

systems

are

equipped

with

phone

jack

inputs,

and

the

amplifier's

phone

jack

outputs

provide

a

convenient

means

to

connect

to

such

speakers

with

standard

phone-to-phone

unshielded

cables.

While

all

three

output

connections

may

be

used

simul-

taneously,

make

sure

the

combined

load

is

no

less

than

4

ohms.

See

the

"Speaker

Wiring"

subsection

of

the

MORE

INFORMATION

ON

SOUND

SYSTEM

WIRING"

section.

TO

RAIN

OR

MOISTURE

МАХ.

RMS

OUTPUT

75W/4Q

+173,

SP

WIRING.CLASS

2

WIRING

MAY

BE

USEO.

STEREO.

MADE

IN

JAPAN

MAX.RMS

OUTPUT

М/С

(34

6V)

АИ

төледік.

|І

CHANNEL

=—=

A

(MONO!

CHANNEL

B

WARNING

то

REDUCE

THE

RISK

OF

FIRE

OR

ELECTRIC

SHOCK

,

DO

NOT

EXPOSE

THIS

PRODUCT

(ЮҮАМАНА

РОМЕН,

кенімен

PRIMARY

"A

FUSE

120V

250W

ЗООМА

БОН;

~

БА

125У

SPEAKERS:

1and2-8-160.

SPEAKER

1012

4-160

/

SPEAKER.

2

FIRE

REPLACE

ONLY

WiTH

SAME

TYPE

FUSE.

ATTENTION

AFIN

DE

REDUIRE

LE

RISQUE

DE

FEU.

REMPLACER

UNIQUEMENT

PAR

UN

FUSIBLE

DE

MEME

TYPE.

U.S.

&

Canadian

models

Fig.

2

CAUTION

NEVER

USE

COIL

CORDS

FOR

SPEAKER

HOOKUP.

Coiled,

and

even

many

non-coiled,

guitar-type

cords

usually

have

higher

internal

resistance

than

the

speakers

themselves.

High

resistance

is

due

to

the

thin

wires

used

to

keep

the

cords

flexible

(guitar

cords

are

not

intended

to

carry

a

lot

of

power,

so

they

don't

need

thick

wires).

Such

cords

prevent

most

of

the

power

from

reaching

the

speakers,

and

instead

heat

up.

In

high

power

operation,

a

guitar-type

cord

can

melt

and

cause

a

fire

hazard.

Use

heavy

gauge

speaker

cords

with

phone

plugs

if

you

must

use

phone

plug

connections

at

all;

shielded

cable

is

not

necessary

or

desirable

for

speaker

connections.

©

FUSE

Given

reasonable

ambient

temperatures,

and

free

air

circula-

tion

for

the

amplifier's

cooling

system,

the

output

transistors

and

heat

sinks

should

be

able

to

handle

maximum

power

output

indefinitely,

but

in

the

event

of

a

power

supply

problem,

or

should

there

be

a

problem

with

the

power

mains

themselves,

some

form

of

protection

is

required.

This

fuse

can

be

of

some

value

in

protecting

the

power

supply

against

power-line

surges

and

long-term

overloads

at

the

outputs.

The

amplifier

is

designed

to

avoid

unwanted

shutoff

during

a

live

performance,

but

it's

better

to

blow

a

fuse

and

replace

it

than

to

loose

the

amp

altogether.

NOTE:

A

thermal

breaker

is

included

within

the

amp,

and

reaches

85°С

for

General

model

and

for

all

models,

a

protec-

tion

LED

will

begin

to

blink

when

the

heat

sink

temperature

exceed

60°C,

If

there

is

no

output,

and

the

fuse

is

not

blown,

the

problem

may

be

overheating;

wait

for

the

unit

to

cool,

and

it

should

come

back

on

line.

|

WARNING

ONLY

USE

FUSES

OF

THE

SAME

RATING

AND

TYPE

AS

THE

ORIGINAL

FUSE

SPECIFIED

FOR

THE

POWER

AMPLIFIER.

This

information

is

prínted

on

the

amplifier

rear

panel,

and

is

repeated

here

for

convenience:

U.S.

&

Canadian

тодйе15...............

5A

125V

General

model

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

2.5A

250V

Amplifier

Operation

This

procedure

applies

to

mono

systems

(stereo

amp

in

bridged

mode),

as

well

as

to

stereo

sound

systems.

It

applies

to

full-range

speaker

systems

which

have

a

passive

high-level

crossover

(or

none

at

all).

If

you

are

using

the

amplifiers

іп

a

multi-amplified

system

with

an

electronic

or

low-level

passive

crossover,

the

Input

Attenuators

on

the

amplifier

are

generally

set

to

maximum

(zero

loss),

and

all

level

con-

trolling

is

done

at

the

crossover

(skip

step

10).

-—

.

Make

certain

all

equipment

is

OFF.

2.

Plug

the

amplifiers

into

a

grounded

120

Volt,

50—60

Hz

AC

power

outlet.

3.

Connect

the

wiring

from

the

signa!

sources

to

the

ampli-

fier's

input

(XLR

or

phone

jack).

4.

Select

the

appropriate

.setting

for

the

MODE

switch

(MONO

or

STEREO).

5.

Connect

the

speakers

to

the

output

terminals

or

phone

jacks.

If

used

in

the

MONO

mode,

DO

NOT

USE

THE

PHONE

JACKS;

connect

only

to

the

red

(4)

terminals

of

the

two

channels'

5-way

binding

posts.

6.

Adjust

the

input

Attenuators

to

their

minimum

level

(infinite

loss)

setting.

7.

Turn

on

the

entire

sound

system

except

the

amplifiers.

8.

Adjust

the

controls

on

the

signal

source

(typically

a

console)

for

"normal"

indications

on

the

source's

meter

or

level

indicator.

№

there

is

no

metering,

then

set

the

master

control

at

the

"nominal"

position

or

mark.

9.

Turn

the

amplifier

on.

After

a

short

delay,

the

PROTEC-

TION

indicator

should

turn

off

(you

may

hear

a

"click"

from

within

the

amplifier

as

the

output

relay

engages).

10.Gradually

increase

the

Amplifier

Input

Attenuators

until

the

CLIP

LED

just

turns

on;

given

a

*4

dBu

input

signal

and

a

typical

speaker

load,

this

should

occur

just

as

the

controls

reach

the

maximum

(zero

loss)

setting.

*

Immediately

turn

down

the

Amplifier

Input

Attenuators

by

the

number

of

dB

(per

their

calibrated

scale)

that

match

the

amount

of

headroom

you

wish

to

preserve

in

the

sound

system.

For

a

high

quality

music

reproduction

system,

20

dB

of

gain

reduction

(headroom)

is

about

the

most

normally

used.

For

a

live

"rock"

music

performance,

10

to

15

dB

of

gain

reduction

provides

for

a

higher

average

level

while

retaining

adequate

headroom.

For

paging,

or

other

voice-only

or

background

music

systems,

5

to

10

dB

of

gain

reduction

(headroom)

should

be

sufficient.

|

*|f

the

СЫР

LED

does

not

come

on

with

the

amplifier

controls

at

maximum

(zero

loss),

then

the

source

device

nominal

level

is

simply

lower

than

—4

dBu.

This

is

not

a

problem,

but

in

order

to

establish

the

desired

headroom

figure,

you

will

temporarily

have

to

increase

the

source's

output

level.

Turn

it

up

until

the

Amplifier's

CLIP

LED

just

comes

on,

and

note

how

much

extra

level

you

had

to

extract

from

the

source.

Now

bring

the

source

back

to

its

nominal

level.

Take

the

number

of

dB

of

headroom

you

wish

to

establish

for

the

system,

subtract

the

“added

level”

you

just

had

to

extract

from

the

source

from

this

total

figure,

and

turn

down

the

Amplifier

Input

Attenuators

by

the

resulting

figure.

Precautions

regarding

AC

power

source

for

amplifiers

High

power

amplifiers

can

draw

a

lot

of

AC

power.

Be

sure

the

AC

power

source

for

your

AC

distribution

system

has

adequate

current

capability

to

bear

the

entire

load

with

an

extra

margin

of

safety.

If

you

use

a

power

outlet

strip

with

a

built-in

fuse

or

circuit

breaker,

make

sure

the

breaker

is

rated

for

sufficient

current

to

handle

the

combined

load

of

all

equipment

plugged

into

the

strip.

E

In

multiple

amplifier

installations,

we

recommend

sequential

turn-on

(either

manually

or

via

timed

relays)

to

avoid

a

sudden,

major

drain

on

the

AC

line.

You

should

also

keep

in

mind

that

severe

reduction

of

power

line

voltage

affects

the

amount

of

power

you

can

get

FROM

the

amplifier.

If

you

need

to

run

long

AC

extension

cords,

make

sure

their

conductors

are

as

large

as

practical

(small

gauge

number).

Just

as

smaller

diameter

wire

causes

speaker

line

loss,

smaller

power

lines

cause

loss.

However,

the

effect

of

small

AC

lines

is

one

of

intermittent

clipping

under

severe

low-voltage

conditions.

WARNING

DO

NOT

CHANGE

A

THE

FUSE

OR

CIRCUIT

BREAKER

IN

THE

AC

POWER

DISTRIBUTION

SYSTEM

TO

A

HIGHER

VALUE

UNLESS

YOU

ARE

ABSOLUTELY

CERTAIN

THE

WIRING

IS

RATED

FOR

THE

HIGHER

CURRENT.

If

a

high-current

short

occurs,

the

wiring

becomes

the

"fuse,"

which

usually

starts

a

fire

before

the

wiring

can

"blow".

We

recom-

mend

that

you

refer

to

the

National

Electrical

Code

before

attempting

any

such

change.

INSTALLATION

Input

Connections

XLR

CONNECTORS

AND

PHONE

JACKS

The

amplifier

input

circuit

is

electronically

balanced.

Each

channel

is

equipped

with

a

T/R/S

phone

jack

wired

with

the

tip

for

signal

high

(+),

the

ring

for

signal

low

(—)

and

the

sleeve

for

chassis

ground.

These

jacks

are

wired

in

parallel

with each

other

and

with

an

XLR

connector

in

which

2

is

the

"hot"

or

signal

high

(+)

terminal,

3

is

signal

low

(—),

and

1

is

ground.

This

XLR

wiring

conforms

to

the

JIS/

DIN/IEEE

international

standards.

ATTENUATOR

ELECTRONICALLY

BALANCED

INPUT

AMPLIFIER

INVERTING

AMPLIFIER

ELECTRONICALLY

BALANCED

INPUT

AMPLIFIER

Fig.3

INPUT

As

of

this

date,

there

is

no

clear

standard

in

the

United

States

for

the

polarity

of

connections

on

the

XLR

connector.

If

opposite

amplifier

polarity

is

required,

or

if

the

equipment

driving

the

amplifier

uses

3

as

its

“hot”

lead,

the

input

cable

wiring

should

be

reversed

from

this

convention.

If

using

.

an

XLR-fitted

cable,

rewire

it

or,

preferably,

use

a

polarity-

reversal

adaptor.

Both

the

XLR

and

phone

jack

inputs

may

be

used

at

the

same

time

for

the

purpose

of

“daisy

chaining”

the

signal

to

another

input;

the

dual

input

connections

should

not

be

used

to

“mix”

two

signal

sources

to

feed

a

given

amplifier

channel.

(Sole

channel

of

mono

amp,

or

Channel

B

of

stereo

amp)

(Sole

channel

of

mono

amp,

or

Channel

A

of

stereo

amp)

CHANNEL

—

А

(MONO

INPUT

Patch

cord

between

channels

From

Signal

.

Source

Fig.

4A

“DAISY

CHAIN"

WITH

XLR

&

PHONE

JACKS

(Sole

channet

of

mono

amp,

ог

Channel

B

of

stereo

amp)

CHANNEL

B

INPUT

(Sole

channel!

of

mono

amp,

ог.

Channel

A

of

stereo

amp)

CHANNEL

=A

(мою)

Patch

cord

between

channels

From

Signal

Source

Fig.4B

1/4”

INPUT

WITH

XLR

TO

XLR

“DAISY

CHAIN"

Output

Connections.

The

amplifier

output

is

brought

to

both

a

1/4-inch

T/S

phone

jack

and

to

5-way

binding

posts

marked

"SPEAKER"

(or

"SPEAKERS").

With

regard

to

the

binding

posts,

each

amplifier

channel

has

one

red

(+)

and

one

black

(—)

terminal

for

connection

to

the

speaker

load.

In

normal

operation,

the

(+)

SPEAKER

terminal

has

the

same

signal

polarity

as

the

tip

of

the

input

phone

jack

or

2

of

the

XLR

input

(see

"MONO

MODE

CONNECTIONS"

for

an

exception.)

Refer

to

the

discussion

of

"Speaker

Wiring"

in

the

MORE

INFORMATION

ON

SOUND

SYSTEM

WIRING

section

of

this

manual

for

additional

information

on

calculating

load

impedances,

power

loss

in

cables,

and

minimum

wire

gauge

recommendations.

NORMAL

CONNECTIONS

Connect

the

speaker

load

as

illustrated

below.

The

minimum

nominal

load

impedance

in

this

mode

is

4

ohms.

OPTIONAL

SECOND

SPEAKER SPEAKER

——

SPEAKERS:

tand2

-8-160/

SPEAKER

Jor2=4-162,/

SPEAKER

2 2

—Í(MONO)8-320

em

(4+)

Fig.

BA

SPEAKER

OUTPUT

CONNECTIONS.

TOTAL

LOAD

IMPEDANCE

MUST

NOT

BE

LOWER

THAN

4

OHMS.

CHANNEL

B

SPEAKER(S)

TOTAL

LOAD

NOT

LESS

THAN

4

OHMS

FOR

#1,

#2

&

43

CONNECTIONS

CHANNEL

А

SPEAKER(S)

TOTAL

LOAD

NOT

LESS

THAN

4

OHMS

FOR

#1,

#2

&

#3

CONNECTIONS

SPEAKERS

land2

-8-160//

SPEAKER

10г2

>4-1690,/

SPEAKER

(MONO)8-322

Fig.

5B

NORMAL

OUTPUT

CONNECTIONS

(STEREO

MODE

OPERATION)

CHANNEL

B

SPEAKER(S)

TOTAL

LOAD

NOT

LESS

THAN

4

OHMS

FOR

#1,

#2

&

#3

CONNECTIONS

CHANNEL

А

SPEAKER(S)

TOTAL

LOAD

NOT

LESS

THAN

4

OHMS

FOR

#1,

#2

&

#3

CONNECTIONS

(—)

(MONO)8-320

eM

(

-H

Y

Fig.5C

NORMAL

OUTPUT

CONNECTIONS

(STEREO

MODE

OPERATION)

MONO

MODE

CONNECTIONS

The

following

connection

diagram

applies

to

the

P2075

when

operated

in

bridged

MONO

configuration.

SPEAKER(S)

TOTAL

LOAD

NOT

LESS

THAN

8

OHMS

(DO

NOT

USE

PHONE

JACKS)

SPEAKERS

Тапа2

=8-162/

SPEAKER

10г2

=4-160/

SPEAKER

2

A

©)

“27

CHANNEL

(—)—

——

(MONOJ8-320

Fig.

6

BRIDGED

MONO

OUTPUT

CONNECTIONS

The

rear

panel

MODE

switch,

when

set

to

MONO,

prepares

the

amplifier

for

bridged

operation.

In

this

context,

“bridged”

describes

the

special

wiring

of

the

two

channels

of

the

amplifier

so

that

the

outputs

add

together

and

produce

twice

the

voltage

of

one

channel

alone.

Since

power

is

pro-

portional

to

the

square.

of

the

voltage,

bridging

can

develop

up

to

four

times

the

output

power

given

the

same

load

impedance

...

assuming

that

the

amplifier

can

handle

it.

Typically,

however,

the

maximum

available

output

power

in

bridged

mode

is

somewhat

less

than

the

theoretical

4X

increase

due

to

power

supply

restrictions.

Since

amplifiers

generally

cannot

handle

four

times

their

maximum

unbridged

(normal

STEREO

mode)

power

output

when

in

bridged

(MONO)

mode,

a

more

useable

rule

of

thumb

is

that

an

amp

will

deliver

twice

its

“normal”

rated

power

into

twice

its

"normal"

rated

minimum

impedance

when

bridged.

By

using

both

channels

of

a

dual-channel

to

drive

a

single

speaker

load

in

the

bridged

(MONO)

configuration,

it

is

possible

to

achieve

the

wide

headroom

and

dynamic

range

needed

for

accurate

reproduction

of

peaks

and

transient

sounds.

When

a

stereo

program

must

be

reproduced,

then

a

pair

of

bridged

power

amps

can

be

used,

each

in

bridged

MONO

mode.

One

input

(Channel

A)

is

all

that's.

required

to

drive

the

amplifier

when

it

is

operated

in

MONO

mode.

The

rear

panel

MONO/STEREO

switch

"rewires"

the

input

circuits

so

Channel

A

feeds

both

channels

of

the

amplifier

simultaneous-

ly,

and

the

Channel

B

output

is

electronically

reversed

in

polarity

with

respect

to

the

Channel

A

output.

The

Charinel

B

SPEAKER

output

red

(+)

terminal

then

serves

as

the

"low"

(—)

side

of

the

bridged

output,

and

the

channel

A

SPEAKER

output

red

(+)

terminal

is

the

"high"

side

of

the

bridged

output.

Neither

Маск

(—)

SPEAKER

output

terminal

is

used

in

bridged

mode,

nor

are

the

1/4-inch

phone

jack

outputs.

For

MONO

operation,

connect

the

signal

source

to

the

Channel

A

input;

the

channel

B

input

should

remain

unused.

Connect

the

speaker

load

as

illustrated.

When

the

amplifier

is

set

to

MONO

mode,

only

the

Channel

A

input

connectors

and

controls

are

operative,

and

it's

a

good

idea

to

turn

down

the

Channel

B

Input

Attenuator

to.

avoid

confusion.

CAUTION

NEVER

CONNECT

THE

BLACK

(—)

SPEAKER

OUTPUT

TERMINALS

TO

ANYTHING

WHEN

THE

AMPLIFIER

IS

IN

BRIDGED

MONO

MODE.

In

this

mode,

both

of

the

red

(+)

SPEAKER

output

connectors

are

"hot".

Do

not

allow

them

to

short

together,

or

to

any

other

connections

in

your

sound

system.

WARNING

When

operating

in

the

bridged

mode,

the

extra

voltage

warrants

extra

care

to

avoid

touching

speaker

wiring

since

the

amplifier

can

easily

deliver

a

lethal

combina-

tion

of

voltage

and

current.

Make

sure

that

no

return

path

between

speaker

wiring

and

equipment

chassis

or

rack

cabinets,

which

are

probably

grounded,

exists

any

time,

especially

when

you're

using

the

amplifier

in

the

bridged

mode

of

opera-

tion.

In

bridged

mode,

the

red

(+)

output

terminals

of

the

amplifier

are

both

“hot”,

and

are

not

referenced

to

ground

but

only

to

each

other.

Thus

a

return

path

connected

to

ground

would

short

one

or

both

sides

of

the

amplifier,

could

cause

the

amplifier

to

shut

down,

and

might

even

damage

it.

In

mono

mode,

never

use

the

1/4-inch

phone

jacks

on

the

amplifier,

and

avoid

the

use

of

1/4-inch

phone

plugs

at

the

speaker

end

of

the

cable.

Phone

plugs

are

a

poor

choice

in

general

for

speaker

connectors,

especially

in

a

bridged

system,

because

these

plugs

cause

a

momentary

short

circuit

as

they

are

plugged

in

or

pulled

out.

MORE

INFORMATION

ON

SOUND

SYSTEM

WIRING

The

signal-carrying

cables

in

a

sound

system

are

as

much

an

audio

"component"

as

any

other

part

of

the

system.

Improper

cables

between

the

signal

source

and

the

amplifier

can

result

in

exaggerated

or

deficient

high

frequency

response,

degradation

of

signal-to-noise

ratio,

and

other

problems.

Improper

'amplifier-to-speaker

wiring

can

degrade

amplifier

damping

factor,

reduce

power

delivered

to

the

speakers,

and

prematurely

trigger

protection

circuitry.

This

section

of

the.

manual

discusses

the

nature

of

balanced

and

unbalanced

signal

cables,

input

transformers,

.signal

levels,

grounding

techniques,

and

other

aspects

of

system

wiring.

It

is

not

an

all-inclusive

handbook,

although

it

should

provide

a

useful

background

so

that

problems

can

be

understood

and

either

avoided

in

the

first

place

or

more

easily

corrected

should

they

occur.

Balanced

and

unbalanced

wiring

In

a

general

sense,

there

are

two

types

of

signal

transmission

systems

for

low

to

medium

level

audio

signals:

the

balanced

line,

and

the

unbalanced

line.

Either

type

can

be

used

with

high

or

low

impedance

circuits;

the

impedance

of

a

line

bears

no

necessary

relationship

to

its

being

balanced

or

not.

The

UNBALANCED

LINE

is

simply

a

"two-wire"

system

where

the

shield

(ground)

acts

as

one

signal-carrying

wire,

and

the

center

(hot)

wire

enclosed

within

that

shield

is

the

other

signal-carrying

wire.

The

shield

is

typically

a

shell

made

of

fine,

braided

wires,

although

some

cables

have

“served”

(wrapped)

shields

instead.

The

BALANCED

LINE.

is

a

three-wire

system

where

two

signal

wires

carry

an

equal

amount

of

potential

or

voltage

WITH

RESPECT

TO

the

shield

(ground)

wire,

but

of

opposite

electrical

polarity

from

each

other.

The

shield

(ground)

in

a

balanced

line

does

not

carry

any

audio

signal,

and

is

intended

strictly

as

a

"drain"

for

spurious

noise

current

that

may

be

induced

in

the

cable

from

external

sources.

BALANCED

(3-CONDUCTOR)

SHIELDED

CABLE

INSULATING

Wine

c"

ISO

falis

BOPE

JACKET

SHIELD

UNBALANCED

(2-CONDUCTOR)

SHIELDED

CABLE

Fig.

7

BALANCED

AND

UNBALANCED

CABLES

Balanced

wiring

is

more

expensive.

and

complex

than

unbalanced

wiring.

|t is

often

used,

however,

because

it

offers

important

advantages.

There

is

nothing

inherently

"better"

or

more

"professional"

about

balanced

wiring;

the

application

dictates

whether

one

system

or

the

other

is

needed

to

yield

satisfactory

results.

In

electronics

laboratories,

where

critical

measurements

are

made

using

high-precision

test

equipment,

unbalanced

wiring

is

often

used.

Unbalanced

wiring

works

best

when:

high-

quality

wire

is

used,

the

cable

extends

over

relatively

short

distances,

and

one

leg

of

the

AC

power

system

feeds

all

the

gear.

Radio

transmitter

feeds,

computer

high-speed

data

transmission,

and

ultra-wideband

television

signals

are

usually

fed

over

unbalanced

lines.

In

short,

there

is

nothing

inherent-

ly

“unprofessional”

about

unbalanced

wiring.

In

electrically

“noisy”

environments,

balanced

wiring

helps

eliminate

noise

in

an

ingenious

way;

the

two

wires

of

the

"balanced"

cable

carry

the

same.

signal,

but

each

wire

is

opposite

in

signal

polarity

to

the

other.

Balanced

inputs

are

designed

to

recognize

only

the

DIFFERENCE

in

voltage

between

the

two

wires

(hence

the

term

“balanced

differential”

input”),

Should

any

electrostatic

interference

or

noise

cut

across

a

balanced

cable,

the

noise

voltage

will

appear

equally

—

with

the

same

polarity

—

on

both

signal-carrying

wires.

The

noise

is

therefore

ignored

or

“rejected’’

by

the

input

circuit.

(This

is

why

the

term

“common

mode

rejection”

applies;

signals

in

common

to

the

two

center

wires

are

rejected.)

Not

all

balanced

wiring

has

a

shield.

In

older

telephone

systems,

many

miles

of

cable

were

run

with

no

shielding

(that

much

shielding

is

too

expensive).

Out

in

the

open,

wires

are

subjected

to

radio

interference

and

hum

fields

set

up

by

power

lines.

Balancing

the

two

signal

hot

wires

with

respect

to

ground

gives

long

lines

immunity

to

external

interference.

Using

just

two

simple

wires

twisted

together

makes

the

two

signal

wires

subject

to

exactly

the

same

amount

of

bom-

bardment

from

hum

and

radio

sources,

so

a

balanced

input

(either

transformer

or

active,

differential

amplifier)

can

be

used

to

cancel

out

unwanted

signals

on

the

line

while

passing

the

desired

audio

signal.

Figure

8

illustrates

the

principle

of

balanced-line

interference

rejection.

PICTORIAL

VIEW

OF

BALANCED

(3-CONDUCTOR)

SHIELDED

CABLE

NOTE

THAT

EQUAL

NOISE

VOLTAGE

IN

BOTH

INNER

CONDUCTORS

CANCELS

HERE,

BUT

SIGNAL

IS

OPPOSITE

POLARITY

AND

DOES

NOT

CANCEL.

(4)

ELECTROSTATIC

OR

RADIO

NOISE

(+)

NT

—

SIGNAL

GE

NOISE

VOLTA

SIGNAL

(-.)

—

(-)

"7

SCHEMATIC

VIEW

OF

BALANCED

(3-CONDUCTOR!

SHIELDED

CABLE

TRANSFORMER

(WORKS

SIMILAR

TO

BALANCED,

DIFFERENTIAL

INPUT

AMP)

Fig.

8

NOISE

REJECTION

IN

A

BALANCED

LINE

LLL

———————

—Á———ML

a

atii

—

|)

The

ВЕ!

(radio

frequency

interference)

cuts

across

both

conductors,

inducing

equal

voltages

in

the

same

direction.

These

voltages

"meet"

in

the

transformer

(or

differential

amplifier),

and

cancel

out,

while

the

signals

generated

by

the

microphone

flow

in

opposite

directions

in

each

conductor,

and

hence

do

not

cancel

out.

Thus,

in

a

theoretically

perfect

balanced

system,

only

the

desired

signal

gets

through

the

transformer

or

differential

amplifier.

Why

input

transformers

sometimes

are

used

There

are

a

number

of

reasons

why

input

transformers

might

be

used,

We'll

discuss

just

a

few

of

them.

In

the

case

of

certain

audio

equipment

which

has

an

unbalanced

input

(not

these

amplifiers),

a

transformer

converts

the

unbalanced

input

to

a

balanced

input.

When

the

transformer

is

used

in

this

way,

primarily

for

ground

isolation

and

to

obtain

the

benefits

of

a

balanced

line,

it is

said

to

be

an

“isolation”

transformer

or

a

"matching"

transformer.

If

the

transformer

is

also

used

to

prevent

a

low

impedance

input

from

overload-

ing

a

high

impedance

output,

it

is

known

as

a

"bridging"

transformer

(not

to

be

confused

with

the

"bridged"

connec-

tions

of

a

stereo

power

amp

output

in

MONO

mode).

Since

they

already

have

balanced

inputs,

these

amplifiers

have

no

need

for

input

transformers,

There

are

instances

where

absolute

isolation

of

the

grounds

between.

the

power

amp

and

the

other

equipment

must

be

obtained,

and

in

such

cases,

there

is

no

viable

substitute

for

a

transformer;

in

such

cases,

the

commercial

versions

of

these

amplifiers

(“С”

suffix)

may

be

used

since

they

transformers.

Alternately,

an

external

input

transformer

may

be

used,

such

as

one

of

the

in-line

types

that

fit

in

a

case

not

much

larger

than

an

XLR

connector.

NOTE:

There

are

other

ways

to

achieve

isolation.

One

can

digitize

the

audio

signal

and

transmit

it

by

means

of

modulat-

ed

light

in

fiberoptics,

but

this

is

much

more

expensive

than

include

sockets

for

optional.

using

a

transformer,

with

no

great

performance

advantage.

`

One

can

use

the

audio

signal

to

modulate

a

light,

and

pick

up

the

light

with

an

LDR

(light

dependent

resistor),

thus

achieving

isolation

at

the

expense

of

increased

noise

and

distortion.

There

are

also

instances

where

they

is

an

extremely

large

amount

of

radiated

noise

energy

near

the

input

cables

to

the

amplifier

—

for

example

near

an

SCR-operated

lighting

dimmer

panel.

In

such

cases,

the

common

mode

rejection

of

the

balanced

differential

amplifier

input

may

not

be

adequate

to

cancel

out

the

very

high

peak

noise

voltages,

and

only

a

good

quality

input

transformer

can

do

the

job.

In

most

of

the

applications

for

which

these

amplifiers

are

intended,

you

don't

need

a

transformer,

and

you're

not

paying

for

К.

10

Noise

and

losses

in

low

impedance

and

high

impedance

lines

The

length

and

type

of

cable

can

affect

system

frequency

response

and

susceptibility

to

noise.

The

impedance

of

the

line

has

a

major

influence

here,

too.

Signal

cables

from

high

impedance

sources

(actual

output

impedance

of

5000

ohms

and

up),

should

not

be

any

longer

than

25

feet,

even

if

low

capacitance

cable

is

used.

The

higher

the

source

impedance,

the

shorter

the

maximum

recommended

cable

length.

For

low

impedance

sources

(output

impedances

of

600

ohms

or

less),

cable

lengths

of

100

feet

or

more

are

acceptable.

For

very

low

impedance

sources

of

50-ohms

or

less,

cable

lengths

of

up

to

1000

feet

are

possible

with

minimal

loss.

In

ай!

cases,

the

frequency

response

of

the

source,

the

desired

frequency

response

of

the

system,

and

the

amount

of

capaci-

tance

and

resistance

in

the

cable

together

affect

actual

high

frequency

losses.

Thus,

these

suggested

cable

lengths

should

not

be

considered

"absolute"

rules.

Susceptibility

to

noise

is

another

factor

which

affects

cable

length.

All

other

factors

being equal

(which

they

seldom

are),

if

a

given

noise

voltage

is

induced

in

both

a

high

impedance

and

a

low

impedance

cable,

the

noise

will

have

a

greater

.

impact

on

the

high

impedance

circuit.

Consider

that

the

noise

energy

getting

into

the

cable

is

more-or-less

constant

in

both

instances.

The

low

impedance

input

is

being

driven

primarily

by

power,

whereas

the

high

impedance

input

is

being

driven

primarily

by

voltage.

The

induced

noise

energy

must

do

MORE

WORK

when

it

drives

a

lower

impedance

—

it

does

not

have

much

power

—

so

less

noise

is

amplified

by

the

input

circuit.

The

induced

noise

energy

is

not

loaded

by

a

high

impedance

input,

and

so

it

is

amplified.

Signal

levels,dynamic

range

and

headroom

STANDARD

OPERATING

LEVELS

There

are a

number

of

different

“standard”

operating

levels

in

audio

circuitry.

It

is

often

awkward

to

refer

to

a

specific

level

(i.e.,

+4

dBu)

when

one

merely

wishes

to

describe

a

general

sensitivity

range.

For

this

reason,

most

audio

engi-

neers

think

of

operating

levels

in

three

general

categories:

1.

MIC

LEVEL

OR

LOW

LEVEL

р

This

range

extends

from

no

signal

up

to

about

—20

dBu

(77.5

mV),

or

—20

dBm

(77.5

mV

across

600

ohms

=

10

millionths

of

a

watt).

It

includes

the

outputs

of

micro-

phones,

guitar

pickups,

phone

cartridges,

and

tape

heads,

prior

to

any

form

of

amplification

(i.e.,

before

any

mic,

phono,

or

tape

preamps).

While

some

mics

can

put

out

more

level

in

the

presence

of

very

loud

sounds,

and

a

hard-picked

guitar

can

go

20

dB

above

this

level

(to

0

dBu

or

higher),

this

remains

the

normal,

average

range.

2.

LINE

LEVEL

OR

MEDIUM

LEVEL

This

range

extends

from

—20

dBu

or

—20

dBm

to

+30

dBu

(24.5

V)

or

+30

dBm

(24.5V

across

600

ohms

=

1

watt).

It

includes

preamp

and

console

outputs,

and

most

of

the

inputs

and

outputs

of

typical

signal

processing

equipment

such

as

limiters,

compressors,

time

delays,

`

reverbs,

tape

decks,

and

equalizers.

In

other

words,

it

covers

the

output

levels

of

nearly

all

equipment

except

power

amplifiers.

3.

SPEAKER

LEVEL

AND

HIGH

LEVEL

This

covers

all

levels

at

or

above

*30

dBu

(24.5V)

*30

dBm

(24.5

V

across

600

ohms

=

1

watt).

These

levels

include

power

amplifier

speaker

outputs,

AC

power

lines,

and

DC

control

cables

carrying

more

than

24

volts.

Let's

discuss

these

levels

in

the

context

of

a

sound

system.

The

lowest

power

levels

in

a

typical

sound

system

are

present

at

the

output

of

microphones

or

phono

cartridges.

Normal

speech

at

about

one

meter

from

the

"average"

dynamic

microphone

produces

a

power

output

from

the

microphone

of

about

one

trillionth

of

a

watt,

Phono

cartridges

playing

an

average

program

selection

produce

as

much

as

a

thousand

times

this

output

—

averaging

a

few

billionths

of

a

watt.

These

signals

are

very

weak,

and

engineers

know

that

they

cannot

be

"run

around"

a

chassis

or

down

a

long

cable

without

extreme

susceptibility

to

noise

and

frequency

response

errors.

This

is

why

microphone

and

phono

preamps

are

used

to

boost

these

very

low

signal

levels

to

an

interme-

diate

range

called

"line

level".

Line

levels

are

between

10

millionths

of

a

watt

and

250

thousandths

of

a

watt

(1/4

watt).

These

levels

are

the

“dBm”

unit

of

measure-

ment

as

follows:

—20

dBm

=

10

microwatts

(0.00001

watts)

OdBm=

1

milliwatt

(0.001 watts)

+4

аВт

=

2.5

milliwatts

(0.0025

watts)

+24

dBm

=

250

milliwatts

(0.025

watts)

While

some

console

and

preamp

outputs

can

drive

lower

impedances,

primarily

for

driving

headphones,

typical

line

levels

(measured

in

milliwatts)

cannot

drive

speakers

to

useable

levels.

Not

only

is

the

power

insufficient

for

more

than

“whisper”

levels,

the

console

circuits

are

designed

to

operate

into

loads

of

600

ohms

to

50,000

ohms;

they

cannot

deliver

even

their

few

milliwatts

of

rated

power

to

a

typical

8-ohm

speaker

without

being

overloaded.

A

power

amplifier

must

be

used

to

boost

the

power

output

of

the

console

so

it

is

capable

of

driving

low

impedance

speaker

loads

and

delivering

the

required

tens

or

hundreds

of

watts

of

power.

UNDERSTANDING

DYNAMIC

RANGE

AND

HEADROOM

Every

sound

system

has

an

inherent

noise

floor,

which

is

the

residual

electronic

noise

in

the

system

equipment

(and/or

the

acoustic

noise

in

the

local

environment),

The

DYNAMIC

RANGE

of

a

system

is

equal

to

the

difference

between

the

peak

output

level

of

the

system'and

the

noise

floor.

A

concert

with

sound

levels

ranging

from

30

dB

SPL

(near

silence)

to

120

dB

SPL

(threshold

of

pain)

has

a

90

dB

dynamic

range.

The

electrical

signa!

level

in

the

sound

system

(given

in

dBu)

is

proportional

to

the

original

sound

pressure

level

(in

dB

SPL)

at

the

microphone.

Thus,

when

the

program

sound

levels

reach

120

dB

SPL,

the

maximum

line

levels

(at

the

console's

output)

may

reach

*24

dBu

(12.3

volts),

and

maximum

power

output

levels

from

the

amplifier

may

peak

at

250

watts.

Similarly,

when

the

sound

level

falis

to

30

dB

SPL,

the

minimum

line

level

falls

to

—66

dBu

(0.388

millivolts)

and

power

amplifier

output

level

falls

to

250

nanowatts

(250

billionths

of

a

watt).

The

program,

now

converted

to

electrical

rather

than

acoustic

signals,

still

has

a

dynamic

range

of

90

dB:

+24

dBu

—

(—66

dBu)

=

90

dB.

This

dB

SPL

to

dBu

or

dBm

corres-

pondence

is

maintained

throughout

the

sound

system,

from

the

original

source

at

the

microphone,

through

the

electrical

portion

of

the

sound

system,

to

the

speaker

system

output.

A

similar

relationship

exists

for

any

type

of

sound

reinforce-

ment,

recording

studio,

disco

or

broadcast

system.

Тһе

average

line

level

in

the

typical

commercial

sound

system

just

described

is

+4

dBu

(1.23

volts),

corresponding

to

an

average

sound

level

of

100

dB

SPL.

This

average

level

is

usually

called

the

"nominal"

program

level.

The

difference

between

the

nominal

and

the

highest

(peak)

levels

in

a

pro-

gram

is

the

HEADROOM.

In

the

above

example,

the

head-

room

is

20

dB.

Why

is

this

so?

Subtract

the

nominal

from

the

maximum

and

see:

120

dB

SPL

—

100

dB

SPL

=

20

dB.

The

headroom

is

always

expressed

in

just

plain

"dB"

since

it

merely

describes

a

ratio,

not

an

absolute

level;

“20

dB"

is

the

headroom,

not

"20

dB

SPL”.

Similarly,

the

electrica!

head-

room

is

20

dB,

as

calculated

here:

+24

dBu

—

(+4

dBu)

=

20

dB.

Again,

"20

dB"

is

the

headroom,

not

"20

dBu”.

Provided

the

amplifier

is

operated

just

below

its

clipping

level

at

maximum

peaks

of

250

watts,

and

at

nominal

levels

of

2.5

watts,

then

it

also

operates

with

20

dB

of

headroom

(20

dB

above

nominal

=

100

times

the

power).

If

another

sound

system

were

equipped

with

a

noisier

circuit

somewhere

along

the

line,

and

a

less

capable

line

amplifier

than

the

it

might

have

an

electronic

noise

floor

of

—56

dBu

(1.23

millivolts),

and

a

peak

output

level

of

+18

dBu

(6.16

volts).

The

dynamic

range

of

this

system

would

only

be

74

dB.

Assuming

the

original

program

still

has

an

acoustic

dynamic

range

of

90

dB,

it

is

apparent

that

16

dB

of

the

program

will

be

“lost”

in

the

sound

system.

How

is

it

lost?

There

may

be

extreme

clipping

of

program

peaks,

where

the

output

does

not

rise

higher

in

response

to

higher

input

levels.

Quiet

passages,

corresponding

to

the

lowest

signal

levels,

may

be

buried

in

the

noise.

Typically,

portions

of

that

16

dB

difference

in

dynamic

range

between

the

sound

system

capability

and

the

sound

field

at

the

microphone

will

be

lost

in

both

ways.

A

system

with

+24

dBu

output

capability

and

a

—66

dBu

or

better

noise

floor,

or

+18

dBu

output

capability

and

—82

dBu

noise

floor,

would

be

able

to

handle

the

full

90

dB

dynamic

range.

Thus,

for

high

quality:

sound

reinforcement

or

music

reproduction,

it

is

necessary

that

the

sound

system

be

capable

of

low

noise

levels

and

high

output

capability.

SOUNO

LEVEL

TYPICAL

GAIN

STRUCTURE

IN

ELECTRONICS

AT

MICROPHONE

MAXIMUM

SOUND

LEVEL

{SOUND

{THRESHOLD

OF

PAIN!

AUDIO

CLIPPING

PRESSURE)

120

dB

SPL

SIGNAL

POINT

+24

dBu

балада

12048

SPL

— —

+25

dBu

POINT

115

dB

SPL

—

+20dBu

+18

dBu

11048

SPL

—

105

dB

SPL

—

10048

SPL

—

95

dB

SPL

—

^15dBu

44dBu

2048

—

новь

NOMINAL

HEADROOM

|

1448

LEVEL

HEADROOM

2038

HEADROOM

—

+59Ви

100

dB

SPL

—.

OdBu

AVERAGE

90dBSPL

—

SOUND

—

-SaBu

85

dB

SPL

~

LEVEL

—

-10dBu

8008

SPL

—

-16

ави

75

dB

SPL

—

-20d8u

7038

РЕ

—

7048

30d8sPL

—

-25dBu

esdBspL.

.

S/N

AMBIENT

-80

dBu

RATIO

Nols:

=

60

98

SPL

—

chal

55

48

SPL

— —

-40dBu

50dBSPL

. —

-45dBu

45

dB

SPL

—

--

—50dBu

40 dB

SPL

——

—

-55

dBu

35

dB

SPL

—

——

-60dBu

30dBSPL

—

25

48

SPL

—

AMBIENT

20

dB

SPL

—

{№013

15dBSPL

—

ЕЕ

10dBSPL

—

5

dB

SPL

048

SPL

Fig.

9

DYNAMIC

RANGE,

HEADROOM

&

S/N

RATIO.

іп

the

special

case

of

an

analog

audio

tape

recorder,

where

the

dynamic

range

often

is

limited

by

the

noise

floor

and

distortion

levels

of

the

tape

oxide

rather

than

the

electronics,

there

is

a

common

method

used

to

avoid

program

losses

due

to

clipping

and

noise.

Many

professional

and

consumer

tape

machines

are

equipped

with

a

noise

reduction

system,

also

known

as

a

compander

(as

designed

by

firms

like

Dolby

Laboratories

and

dbx,

Inc.).

A

compander

noise

reduction

system

allows

the

original

program

dynamics

to

be

maintain-

ed

throughout

the

recording

and

playback

process

by

compressing

the

program

dynamic

range

before.it

goes

onto

'

the

tape,

and

complementarily

expanding

the

dynamic

range

as

the

program

is

retrieved

from

the

tape.

Compact

(laser)

discs,

and

digital

audio

tape

recording,

and

the

FM

recording

used

in

modern

stereo

VCR

soundtracks

are

additional

methods

of

recording

wide

dynamic

range

programs

which,

in

turn,

demand

playback

systems

with

wide

dynamic

range.

12

13

A

GENERAL

APPROACH

TO

SETTING

LEVELS

IN

A

SOUND

SYSTEM

Volume

control

and

fader

settings

throughout

a

sound

system

affect

the

noise

and

headroom

of

the

system.

To

provide

the

best

overall

system

performance,

level

settings

should

be

optimized

for

each

component

in

the

system.

One

popular

approach

is

to

begin

by

adjusting

levels

as

close

as

possible

to

the

signal

source

(i.e.,

at

the

console

input

or

microphone

preamp).

Set

the

input

pad

and/or

gain

trim

controls

for

the

maximum

level

that

will

not

produce

clipping

(overdrive);

this

can

be

seen

if

the

console

has

Peak

input

level

LEDs,

or

heard

by

listening

for

distortion

while

making

adjustments.

The

is

to

set

the

level

of

the

console

input

channel

(the

fader

or

send

control)

so

that

it

properly

drives

the

mixing

busses.

Consoles

with

VU

meters

should

indicate

bus

levels,

although

you'll

want

to

check

the

block

diagram;

sometimes

it

is

possible

to

overdrive

a

bus

summing

amplifier

and

still

have

the

bus

output

VU

meter

indicate

"normal"

levels.

If

line.

amplifiers,

electronic

crossovers,

equalizers

or

other

signal

processing

devices

are

inserted

in

the

signal

chain,

signal

levels

at

the

input

of

these

units

should

be

set

so

the

dynamic

range

of

each

unit

is

optimized.

In

other

words,

set

the

input

level

at

each

device

as

high

as

possible

without

producing

clipping,

and,

if

an

output

level

control

is

provid-

ed,

also

set

it

as

high

as

possible

without

clipping

the

output

—

and

without

causing

clipping

in

the

input

of

the

to

which

it

is

connected.

Check

the

operating

manual

of

each

piece

of

equipment

to

determine

the

specified

nominal

and

maximum

input

levels.

An

accurate

AC

voltmeter

is

often

helpful

for

verifying

levels.

As

a

rule,

keep

signal

levels

as

high

as

possible

throughout

the

system,

up

to

the

input

of

the

power

amplifiers.

Then

reduce

the

program

level,

as

required,

using

the

amplifier's

input

attenuators.

Input

attenuators

should

be

set

so

that

maximum

program

levels

from

the

source

equipment

won't

drive

the

amplifiers

to

clipping.

This

keeps

overall

system

noise

as

low

as

possible.

HOW

TO

SELECT

A

HEADROOM

VALUE

AND

ADJUST

LEVELS

ACCORDINGLY

Recall

that

headroom

is

the

amount

of

leve!

available

above

the

average

(nominal)

signal

for

peaks

in

the

program.

The

choice

of

a

headroom

figure

depends

on

the

type

of

program

material,

the

application,

and

the

available

budget

for

amplifiers.,

Рог

a

musical

application

where

high

fidelity

is

the

ultimate

consideration,

15

to

20

dB

of

headroom

is

desirable.

For

most

sound

reinforcement

applications,

especially

with

large

numbers

of

amplifiers,

economics

play

an

important

role,

and

a

10

dB

headroom

figure

is

usually

adequate;

in

these

applications,

a

limiter

can

help

hold

program

peaks

within

the

chosen

headroom

value,

and

thus

avoid

clipping

problems.

For

the

extreme

situation

(as

in

a

factory)

where

background

music

and

paging

must

be

heard

over

high

continuous

noise

levels,

yet

maximum

levels

must

be

restricted

to

avoid

dangerously

high

sound

pressure

levels,

a

headroom

figure

of

as

low

as

5

or

6

dB

is

not

unusual.

To

achieve

such

a

low

headroom

figure,

an

extreme

amount

of

compression

and

limiting

will

be

necessary,

causing

the

sound

to

be

somewhat

unnatural,

but

the

message

will

"cut

through".

Let's

go

through

an

actual

setup

procedure

for

a

high

quality,

music

reproductión

system.

First

choose

a

headroom

figure.

For

maximum

fidelity

when

reproducing

music,

it is

desirable

to

allow

20

dB

of

headroom

above

the

average

system

output.

While

some

extreme

musical

peaks

exceed

20

dB,

the

20

dB

figure

is

adequate

for

most

programs,

and

allowing

for

greater

headroom

can

be

very

costly.

A

20

dB

headroom

figure

represents

a

peak

level

that

is

one

hundred

times

as

powerful

as

the

average

program

level.

This

means

that

for

a

20

dB

headroom

figure,

even

an

amplifier

as

powerful

as

500

watts

has

to

operate

at

an

average

5

watts

output

power.

In

some

systems

such

as

studio

monitoring,

where

fidelity

and

full

dynamic

range

are

of

utmost

impor-

tance,

this

low

average

power

may

be

adequate.

In

other

situations,

such

as

70-volt

background

music

systems,

a

20

dB

headroom

figure

is

not

necessary

and

too

costly

due

to

the

number

of

amplifiers

required.

After

choosing

a

headroom

figure,

adjust

the

incoming

and

outgoing

signal

levels

at

the

various

devices

in

the

system

to

achieve

that

figure.

For

a

typical

system,

the

adjustments

for

a

20

dB

headroom

figure

would

be

made

as

follows:

1.

Initially,

set

the

attenuators

on

the

power

amp

at

maxi-

mum

attenuation

(maximum

counterclockwise

rotation).

Feed

a

sine

wave

signal

at

1000

Hz

to

the

console

input

at

an

expected

average

input

level

(approximately

—50

dBu

(2.45

mV)

for

a

microphone,

+4

dBu

(1.23

volts)

for

a

line

level

signal.

The

exact

voltage

is

not

critical,

and

1000

Hz

is

a

standard

reference

frequency,

but

any

frequency

from

400

Hz

to

about

4

kHz

may

be

used.

2.

Set

the

input

channel

level

control

on

the

console

at

its

marked

"nominal"

setting,

and

adjust

the

master

level

control

so

that

the

output

level

is

20

dB

below

the

rated

maximum

output

level

for

thé

console.

Suppose,

for

example,

the

maximum

rated

output

level

is

+24

dBu

(12.3

volts);

in

that

case,

the

output

level

should

be

adjusted

to

+4

dBu

(1.23

volts),

as

indicated

either

on

an

external

voltmeter,

or

on

the

console's

VU

meter

(typically,

O

VU

corresponds

to

*4'dBu

output).

3.

Assume

that

the

rated

maximum

input

level

for

the

graphic

equalizer

to

which

the

console

output

is

connect-

ed

is

+14

dBu

(3.88

volts).

Subtracting

*4

dB

from

+14

dB

leaves

only

10

dB

of

headroom,

so

a

10

dB

resistive

pad

should

be

inserted

between

the

console

output

and

the

equalizer

input.

The

signal

level

at

the

input

to

the

equalizer

should

now

be

—6

dBu

(388

mV),

which

can

be

confirmed

with

а

voltmeter.

4.

Assume

that

the

maximum

rated

output

level

of

the

equalizer

in

this

example

is

+18

dBu

(6.16

volts).

Adjust

the

master

level

control

on

the

equalizer

so

that

its

output

level

is

20

dB

below

the

rated

maximum,

or

—2

dBu

(616"

mV).

Since

the

equalizer

probably

Ваз.

no

VU

meter,

use

an

external

voltmeter

to

confirm

this

level.

5.

Finally,

starting

with

the

attenuators

on

the

amplifier

at

maximum

attenuation

(maximum

counterclockwise

rota-

tion),

slowly

rotate

it

clockwise,

observing

the

amplifier's

output

level.

When

the

POWER

output

is

1/100

of

the

maximum

rated

power

(1/10

of

the

maximum:

output

voltage),

the

amplifier

has

20

dB

headroom

left

before

clipping.

A

75

watt

amplifier

would

operate

at

nominal

0.75

watts,

on

average

level

passages

in

order

to

allow

20dB

for

the

loud

peaks.

To

operate

this

system,

use

only

the

controls

on

the

console,

and

avoid

levels

that

consistantly

peak

the

console's

VU

meter

above

the

"zero"

mark

on

its

scale,

or

that

peak

the

amplifier

meters:

above

a

safe

power

level

for

the

speaker

system.

Any

level

adjustments

in

the

other

devices

in

the

system

will

upset

this

established

gain

structure.

To

use

this

technique

with

any

sound

system,

first

design

the

required

speaker

system,

and

calculate

the

number

of

power

amplifiers

needed

to

safely

operate

the

speaker

system

with

adequate

headroom.

Then,

choose

the

console,

and

other

devices

that

feed

the

power

amplifiers,

and

set

up

the

system

according

to

the

above

instructions.

|

Grounding

is

an

area

of

"black

magic”

for

many

sound

technicians

and

engineers,

and

certainly

for

most

casual

users

of

sound

systems.

Everyone

knows

that

grounding

has

something

to

do

with

safety,

and

something

to

do

with

hum

and

noise

suppression,

but

few

people

know

how

to

set

up

a

proper

AC

power

distribution

system,

and

how

to

connect

audio

equipment

grounds

so

that

noise

is

minimized.

This

subsection

of

the

manual

won't

make

anyone

an

expert,

but

it

does

point

out

a

few

of

the

principles

and

precautions

with

which

everyone

should

be

familiar.

Whether

you

read

this

material

or

not,

before

you

start

cutting

shields

and

lifting

grounds,

read

this

warning:

:

WARNING

In

any

„audio

system

installation,

governmental

and

insurance

underwriters’

electrical

codes

must

be

observed.

These

codes

are

based

on

safety,

and

may

vary

in

different

localities;

in

all

cases,

local

codes

take

pre-

cedence

over

any

suggestions

contained

in

this

manual.

Yamaha

International

Corporation

shall

not

be

liable

for

incidental

or

consequential

damages,

including

injury

to

any

persons

or

property,

resulting

from

improper,

unsafe

or

illegal

installation

of

a

Yamaha

power

amplifier

or

of

any

neither

shall

Yamaha

be

liable

for

any

such

damages

arising

from

defects

or

damage

resulting

from

accident,

neglect,

misuse,

modification,

mistreatment,

tampering

or

any

act

of

nature.

(IN

PLAIN

WORDS...IF

YOU

LIFT

A

GROUND,

THE

RESULTING

POTENTIAL

FOR

ELECTRICAL

SHOCK

IS

YOUR

OWN

RESPONSIBILITY!

)

-

As

a

further

caution,

we

advise

you

never

to

trust

any

potentially

hazardous

system,

such

as

an

AC

power

system

of

any

type,

just

because

someone

else

tells

you

that

it's

okay.

Before

you

“trust

your

life”,

check

things

out

yourself!

Peopledo

get

killed

by

faulty

or

improper-

ly

wired

sound

equipment.

14

15

WHAT

IS

A

GROUND

LOOP,

WHY

IS

IT

BAD,

AND

HOW

IS

IT

AVOIDED?

The

Ground

Loop

is

perhaps

the

most

insidious,

widespread

problem

that

turns

up

in

one

sound

system

after

the

next.

A

"ground

loop"

is

a

multiple

electrical

path

between

two

or

more

components

—

a

path

formed

by

the

ground

wiring,

the

chassis

of

the

components

thernselves,

or

by

combinations

of

these

two

main

elements.

Electrical.noise

current

(induced

ВЕ!

and

60

Hz

hum)

that

flows

through

the

shield,

chassis,

and/or

AC

power

grounds

can

"loop'

around

from

one

piece

of

equipment

to

another.

Instead

of

going

directly

to

earth

ground

and

disappearing,

these

noise

currents

(which

act

like

signals)

travel

along

paths

that

are

not

intended

to

carry

signals.

The

currents,

in

turn,

modulate

the

potential

of

the

signal-carrying

wiring,

producing

hum

and

noise

voltages

that

can't

be

distinguished

from

program

signals

by

the

affected

equipment.

The

noise

is

thus

amplified

along

with

the

program

material.

Mixing

Console

00000000"

Each

chassis

is

tied

to

ground

via

its

AC

power

cord

(3-wire).

Chassis

grounds

of

console

and

amplifier

are

redundantly

tied

to

one

another

by

the

shield

of

one

or

more

audio

cables.

bee

ae

oe

om

en

The

ground

path

between

the

two

AC

plugs

provides

a

redundant

ground

(ground

loop)

since

the

audio

cable

shield(s)

already

does

the

job.

Witt

Wl

oo

1

The

shields

of

the

two

audio

cables

|

establish

redundant

paths

to

ground

between

the

two

chassis

(a

ground

loop).

1

|

Fig.

10

TYPICAL

GROUND

LOOPS

IN

SOUND

SYSTEMS.

Ground

loops

often

are

difficult

to

isolate,

even

for

experienced

audio

engineers.

Whenever

you

hear

hum

from

a

sound

system,

there

is

a

strong

possibility

that

it

is

being

caused.

by

a

ground

loop.

Sometimes,

in

poorly

designed

sound

equipment

(which

include

some

very

expensive

equip-

ment),

ground

loops

occur

INSIDE

the

chassis,

and

little

can

be

done

to

get

rid

of

the

hum

short

of

having

a

good

audio

engineer

re-design

the

ground

wiring

inside;

it's

better

to

avoid

this

kind

of

equipment.

One

myth

about

grounding

is

that

you

must

ground

the

equipment

to

prevent

noise

from

entering

the

system.

Anyone

who

owns

a

portable

cassette

machine

knows

that

simply

isn't

true,

The

main

reason

we

ground

a

sound

system

is

for

safety;

proper

grounding

can

prevent

lethal

shocks.

The

for

grounding

a

system

that

includes

AC

powered

equipment

is

that,

under

some

conditions,

proper

grounding

may

reduce

external

noise

pickup.

While

proper

grounding

won't

always

reduce

external

noise

pickup,

improper

grounding

can

unquestionably

increase

external

noise

pickup!

The

AC

power

cord

ground

(the

green

wire

and

the

third

on

the

AC

plug)

connects

the

chassis

of

electronic

equipment

to

a

wire

in

the

wall

power

service

that

leads

through

build-

ing

wiring

to

an

"'earth''

ground.

The

earth

ground

is

required

by

electrical

codes

everywhere,

and

can

contribute

to

ground

loops.

If

there

is

only

one

path

to

ground,

there

can

be

no

ground

loop.

However,

one

must

look

carefully.

For

example,

suppose

there

is

just

one

audio

cable

joining

a

console

to

a

power

amplifier...can

there

be

a

ground

loop?

Yes!

A

second

ground

connection,

through

the

AC

cables

and

the

chassis

of

the

two

units,

makes

the

“return”

connection

and,

along

with

the

audio

cable

shield,

constitutes

a

continuous

conducting

loop

for

noise

currents

to

flow.

One

commonly

used

method

to

break

this

ground

loop

is

to

“lift”

the

АС

ground

on

the

power

amplifier

with

a

two-wire

to

three-wire

AC

adaptor

(leaving

the

loose

green

wire

on

the

adaptor

unconnected).

This

practice

removes

the

AC

safety

ground,

relying

upon

the

audio

cable

to

provide

the

ground,

a

practice

that

can

be

hazardous.

AC

ADAPTOR

SHOULD

NOT

BE

USED

TO

CORRECT

GROUND

LOOPS.

PRACTICE

MAY

BE

DANGEROUS

AND

IS

ILLEGAL

IN

SOME

LOCALITIES.

IF

USING

ADAPTOR,

CONNECT

LEAD

TO

OUTLET

BOX.

Fig.

11

AC

ADAPTOR

Here

are

some

suggestions

to

minimize

the

safety

conflict

while

avoiding

noise

caused

by

ground

loops:

1.

Don't

lift

the

safety

ground

on

any

piece

of

equipment

unless

it

demonstrably

reduces

noise

levels.

2.

NEVER

defeat

the

AC

safety

ground

on

your

console

or

any

other

piece

of

gear

connected

directly

to

your

micro-

phones.

The

microphones

come

first

in

grounding

safety.

3.

Try

to

plug

all

affected

equipment

into

a

common

AC

service.

In

fact,

all

sound

equipment

and

sories

such

as

guitar

amps,

keyboards,

etc.,

should

be

connected

to

а

common

AC

system

to

avoid

safety

hazards.

Lighting,

air

conditioning,

motors

and

so

on

should

be

connected

to

a

completely

different

“phase”

or

"leg"

of

the

main

power

distribution

system

for

the

facility.

|

BALANCE

LINES

AND

GROUND

LIFT

SWITCHES

By

using

balanced

signal

lines

between

two

pieces

of

sound

equipment,

you

can

lift

(disconnect)

the

shield

at

one

end

(usually

at

the

output)

of

an

audio

cable

and

thus

eliminate

the

most

likely

path

that

carries

ground

loop

currents.

In

a

balanced

line,

the

shield

does

not

carry

audio

signals,

but

only

serves

to

protect

against

static

and

RFI,

so

you

can

disconnect

the

shield

at

one

end

without

affecting

the

audio

signal

on

the

two

inner

conductors

of

the

cable,

and

with

little

or

no

effect

on

the

shielding.

Unfortunately,

this

is

not

a

very

practical

solution

to

the

ground

loop

problem

for

portable

sound

systems

because

it

requires

special

cables

with

shields

disconnected

on

one

end.

CAUTION

Microphone

cases

typically

are

grounded

to

the

shield

of

the

cable,

and

connected

to

the

console

chassis

via

1

of

the

XLR

connector.

If

there

is

any

electrical

potential

on

any

external

equipment,

such

as

a

guitar

amp

chassis,

then

a

performer

who

holds

the

mic

and

touches

the

other

equipment

can

be

exposed

to

a

lethal

electrical

shock!

This

is

the

reason

one

should

avoid

"ground

lift”

adaptors

on

AC

power

connections

if

there

is

any

other

conceivable

way

to

eliminate

a

ground

loop.

Sometimes,

you'll

find

pieces

of

audio

equipment

or

accessories

that

are

designed

to

anticipate

ground

loops.

This

equipment

will

include

“ground

lift"

switches

апу

XLR

or

three-wire

(Tip/Ring/Sleeve)

phone

jack

outputs.

The

ground

lift

switch

makes

and

breaks

the

connection

between

the

connector's

shield

and

the

chassis

of

the

particu-

lar

device.

Ground

lift

switches

are

usually

found

on

"direct

boxes”,

which

are

used

when

an

electric

musical

instrument

is

to

be

plugged

directly

into

a

console

whose

inputs

are

not

designed

to

accommodate

direct

connection

of

such

instru-

ments

(a

direct

box

also

includes

a

transformer

or

isolation

amplifier).

Probably

the

best

way

to

keep

noise

out

of

a

microphone

input

is

to

start

with

a

high-performance,

low-impedance

microphone

and

to

connect

it

to

a

console's

low-impedance,

balanced

(or

"floating")

input

with

a

high-quality

micro-

phone

cable

that

utilizes

XLR

connectors.

Keep

microphone

cables

as

short

as

possible

within

the

constraints

of

a

per-

former's

needs,

and

keep

them

physically

separated

from

line-level

(console

output)

cables,

speaker

cables

and

AC

cables.

16

17

AC

outlet

wiring

affects

grounding

and

safety

Whether

you

are

a

technician

or

not,

there

are

two

items

you

should

carry

whenever

you

set

up

a

sound

system

in

a

new

location.

One

of

these

is

a

commercial

outlet

tester,

the

other

is

a

neon

lamp

type

AC

voltage

tester.

These

items

are

inexpensive

and

available

at

most

hardware

stores,

electrical

supply

houses

and

some

lighting

stores.

The

three-prong

outlet

tester

will

tell

you

if

the

outlet

is

properly

wired.

An

improperly

wired,outlet

may

have

its

two

AC

wires

reversed

(“polarity

reversal")

or

it

may

have

а

disconnected

ground.

ANY

FAULT

in

the

wiring

of

the

AC

outlet

is

potentially

hazardous.

Rather

than

take

a

chance

with

damage

to

equipment

and

possibly

lethal

shock,

it

is:

best

to

refuse

to

use

a

faulty

outlet

until

it

has

been

repaired

Бу

а

licensed

electrician.

HOW

TO

OBTAIN

A

SAFETY

GROUND

WHEN

USING

A

2-WIRE

OUTLET

Two-wire

AC

outlets

do

not

have

a

hole

for

the

"safety

ground”

prong

of

a

3-wire

power

cord.

To

use

one

of

these

two-wire

outlets

you

have

to

“adapt”

it

to

the

three-wire

АС

plug

on

your

sound

equipment

with

a

two-wire

to

three-wire

AC

adaptor

(Fig.

11).

These

adaptors

can

maintain

a

safe

ground

for

the

sound

system

IF

you

connect

the

loose

green

wire

on

the

adaptor

to

a

GROUNDED

screw

on

the

two-wire

outlet.

How

do

you

know

whether

or

not

the

screw

is

grounded?

|

1.

Connect

the

adaptor's

green

wire

to

the

screw

оп

the

two-

wire

outlet.

2.

Plug

the

adaptor

into

the

outlet.

3.

Plug

in

your

three-wire

AC

outlet

tester

into

the

adaptor.

If

the

screw

is

grounded,

your

AC

outlet

tester

will

tell

you.

(Most

three-wire

AC

outlet

testers

either

have

a

“good”

light,

or

they

don't

light

at

all

on

a

good

recep-

tacle.)

If

the

screw

is

not

grunded,

the

outlet

tester

will

indicate

this

too.

In

this

case,

you

must

connect

the

adaptor's

green

wire

to

some

other

grounded

screw

in

order

to

maintain

a

safe

ground

for

your

system.

If

the

outlet

tester

shows

a

good

ground

but

reversed

polarity

on

your

two-wire

to

three-wire

adaptor,

sometimes

you

can

reverse

the

adapter

in

the

outlet

by

pulling

it

out,

twisting

it

a

half

turn

and

reconnecting

it.

120

VOLTS

ACROSS

"HOT"

AND

“NEUTRAL”

5

120

VOLTS

ACROSS

"HOT"

LEAD

AND

BOX

GROUND

0

VOLTS

ACROSS

“NEUTRAL”

|

AND

BOX

GROUND

|

Рів.

12

TESTING

A

2-WIRE

OUTLET.

IMPROPERLY

WIRED

AC

OUTLETS:

LIFTED

GROUNDS

A

“lifted

ground”

condition

exists

if

the

ground

or

green

wire

from

the

outlet’s

safety

ground

is

disconnected

or

missing.

In

older

wiring,

the

heavy

green

wire

was

sometimes

omitted

from

internal

wall

wiring

in

favor

of

letting

the

metal

flex

conduit

or

pipe

suffice

as

the

ground

path

from

the

electrical

service

entrance.

Inspectors

usually

accept

this

approach,

and

normally

there

is

no

problem

as

long

as

the

metal

conduit

in

the

wall

is

intact

and

all

the

screws

holding

the

joints

together

are

secure.

However,

a

single

loose

screw

in

a

conduit

joint

inside

a

wall

can

remove

the

safety

ground

of

the

box

in

the

line

(and

all

the

subsequent

boxes

on

that

same

line).

IMPROPERLY

WIRED

AC

OUTLETS:

LIFTED

NEUTRAL

If

the

neutral

becomes

lifted

at

a

power

outlet,

it

is

possible

that

items

plugged

into

the

outlet

will

be

fed

the

full

220

to

240

volts

available

from

the

power

service

instead

of

the

desired

110

to

120

volts.

440

VOLTS

OR

HIGHER

&

SERVICE

SERY

TWO

SEPARATE

OUTLETS

POWER

TRANSFORMER

(TYPICALLY

ON

POLE)

120V

Broken

or

Disconnected

NEUTRAL

Neutral

Conductor

EARTH

GROUND

CONNECTION

|

GROUND

=

Fig.

13

SCHEMATIC

OF

OUTLETS

WITH

LIFTED

NEUTRAL

Such

outlets

may

operate,

but

the

voltage

can

swing

from

0

volts

to

220

or

240

volts

AC

(depending

on

the

maximum

voltage

at

the

service

entrance),

creating

a

shock

hazard

and

possibly

damaging

your

equipment.

If

a

power

amplifier

is

plugged

into

one

socket

of

one

of

the

two

outlets

with

lifted

neutral,

and

a

rack

of

signal

pro-

cessing

equipment

is

plugged

into

the

other,

fuses

would

probably

blow

upon

turning

on

the

system,

and

some

of

the

sound

equipment

could

be

destroyed.

If

you

detect

any

voltage

between

the

larger

slot

(white

wire)

in

an

outlet

and

the

ground

terminal

(round

third

pin)

when

there

is

no

load

on

that

line,

you

should

contact

a

licensed

electrician

to

check

it

out.

WARNING

IN

AC

POWER

WIRING,

BLACK

IS

HOT,

WHITE

IS

NEUTRAL—the

opposite

of

most

audio

signal

wiring

and

speaker

wiring.

It

is

safer

to

consider

all

AC

wiring

as

potentially

lethal.

It

is

possible

someone

miswired

the

system,

or

that

a

short

circuit

has

developed.

Test

the

voltages

yourself,

and

be

safe.

Although

the

white

wires

(neutral)

and

the

green

wires

(ground)

in

the

AC

wiring

are

technically

at

the

same

potential

(voltage),

and

should

measure

the

same

potential

using

a

voltmeter,

the

ground

prong

connec-

tions

at

the

outlets

are

connected

to

the

grounding

bar

that

was

driven

into

the

earth

as

an

additional

safety

precaution

in

case

something

should

happen

to

the

wires

running

from

the

service

entrance

transformer

to

the

building

or

within

the

equipment

itself.

[f

a

short

should

occur

within

the

equipment,

hopefully

the

electricity

will

find

its

way

to

ground

via

the

safety

ground,

instead

ОҒ

via

a

person's

body.

When

checking

AC

power

lines

at

the

outlet,

be

sure

you

have

proper

testing

tools

and

"some

familiarity

with

the

danger

of

shock

hazards

from

AC

power.

Follow

the

diagram

shown

here,

being

careful

not

to

touch

metal

with

your

hands

or

short

the

test

leads

together.

NEUTRAL

"HOT"

(120V)

SAFETY

GROUND

(EARTH

GROUND)

“Third

prong”

socket

ground

and

center

screw

of

outlet

are

internally

connected

and

grounded,

Fig.

14

TEST

TO

DETERMINE

PROPER

AC

OUTLET

WIRING

AC

SAFETY

TIPS

1.

Get

an

AC

outlet

tester

for

your

tool

box.

2.

Be

conscientious.

Use

your

tester

on

power

lines,

and

use

a

neon

voltage

tester

to

check

for

voltage

present

between

microphone

and

guitar

amps,

microphones

and

electric

keyboard

chassis,

and

so

forth.

3.

Use

common

sense.

For

instance,

a

clip-on

ground

lead

may

do

the

job

on

a

lab

bench,

but

it

certainly

is

not

safe

for

a

sound

system

ground.

AC

power

must

be

regarded

with

respect.

4.

Don't

use

questionable

power

outlets;

anticipate

the

worst,

and

always

carry

a

long

extension

made

of

heavy

gauge

wire.

A

good

extension

should

be

made

of

7412-3

(12

gauge,

3

wires),

and

no

longer

than

15

meters

(50

feet).

5.

Н

you

can't

find

suitable

power

at

a

venue,

refuse

to

plug

your

equipment

in.

Besides

posing

a

hazard,

it

could

destroy

your

equipment.

Don't

risk

it.

Speaker

Wiring

USE

THE

APPROPRIATE

WIRE

For

speaker

cables,

use

the

largest

practical

wire

size

(the

lowest

gauge

number).

Speaker

cables,

especially

for

portable

use,

should

be

rugged.

The

type

of

wire

normally

used

for

heavy-duty

AC

power

cables

ànd

utility

extensions

(such

as

for

240-volt

industrial

power

tools).

is

a

good

choice;

it

has

heavy

rubber

or

vinyl

outer

insulation,

stranded

conductors,

and

is

usually

fairly

flexible.

Wire

gauge

is

the

most

impor-

tant

consideration,

especially

for

long

speaker

cables.

Remember

to

always

calculate

the

resistance

based

on

twice

the

distance

to

the

speaker

since

the

signal

must

travel

up

one

conductor

and

back

the

other

to

complete

the

circuit.

Table

1

shows

nominal

losses

(in

dB)

for

a

30

meter

(100

ft)

cable

run

of

different

gauges

driving

a

4,

8,

and

16

ohm

load.

(Note:

since

there

are

2

conductors,

and

the

signal

flows

through

both,

the

actual

round

trip

cable

run

is

60

meters

or

200

feet.)

For

an

"ideal"

unchanging

load,

these

relationships

are

logarithmic.

However,

for

typical

speaker

impedances

and

cables,

a

linear

calculation

will

yield

a

reasonably

close

approximation

of

the

signal

loss.

Thus

15

meters

of

wire

(50

feet)

would

give

about

half

the

loss

shown

on

the

chart,

and

so

on.

For

example,

ten

feet

of

10-gauge

wire

driving

an

8-ohm

studio

monitor

will

produce

a

loss

of

0.022

dB.

If

the

wire's

resistance

becomes

significant

compared

to

the

overall

load

impedance,

heating

will

occur

in

the

wire

during

high

power

operation.

|

Large

diameter

(small

gauge

number)

wire

is

expensive,

and

long

cables

made

from

it

are

heavy.

Rather

than

running

long

speaker

cables,

it is

better

to

locate

power

amplifiers

near

speakers

and

run

a

line-level

signal

cable

over

the

long

distance

to

the

amplifier.

This

approach

eliminates

most

of

the

signal

loss

due

to

speaker

cable's

resistance

so

the

speakers

will

be

fed

al!

the

amplifier's

power

without

the

need

for

heavy

cables.

It

can

actually

save

money

in

many

instances.

Where

speakers

and

power

amplifiers

are

located

far

away

from

the

signal

source

(be

it

a

console

or

a

ргеатр),

"balanced

line"

signal

cables

are

a

wise

choice.

19

Always

use

stranded

wire

for

three

reasons:

(1)

It

is

more

flexible

and

less

prone

to

metal-fatigue

breakage.

(2)

If

an

end

is

nicked

while

insulation

is

being

stripped

for

connection,

only

one

or

two

strands

will

break,

not

the

entire

wire,

апа...

:

(3)

There

is

some

evidence,

though

disputed,

that

higher

frequency

audio

signals

flow

along

the

outside

of

each

conductor

(skin

effect);

if

this

is

so,

the

more

strands,

the

lower

the

effective

cable

resistance

to

high

fre-

quencies.

CAUTION

NEVER

USE

COIL

CORDS

FOR

SPEAKER

HOOKUP,

not

even

temporarily.

Coiled

guitar

type

cords

usually

have

higher

internal

resistance

than

the

speakers

them-

selves.

High

resistance

is

due

to

the

thin

wires

used

to

keep

the

coil

cords

flexible.

These

cords

will

prevent

most

of

the

power

from

reaching

the

speakers.

іп

high

power

operation,

a

coil

cord

can

melt

and

cause

a

fire

hazard.

As

a

general

rule,

guitar-type

connecting

cords,

both

straight

and

coiled,

make

poor

speaker

cables.

CALCULATING

SPEAKER

SYSTEM

LOAD

IMPEDANCES

Several

speaker

components

having

the

same

power

handling

capability,

sound

dispersion

angle,

efficiency

or

other

characteristic

are

often

connected

in

groups

to

make

a

system

that

goes

beyond

the

capability

of

the

individual

units

in

that

system.

There

are

two

basic

ways

to

hook

up

speakers

so

that

power

is

uniformly

distributed

and

it

remains

relatively

easy

to

calculate

the.

resulting

load

on

the

amplifier.

The

easy

way

to

figure

how

much

power

a

group

of

speakers,

or

drivers,

can

handle

is

to

multiply

the

power

handling

specification

of

one

of

the

drivers

by

the

number

of

drivers.

That

part

of

the

process

is

always

the

same

as

long

as

all

the

connected

drivers

are

identical.

It

is

not

recommended,

nor

is

it

common

practice,

to

place

different

types

of

bass

drivers

in

a

single

enclosure,

or

to

connect

different

types

of

bass

drivers

in

series,

or

parallel

because

the

power

will

not

be

distributed

evenly

among

the

drivers.

This

same

reasoning

also

applies

to

the

connection

of

various

systems

which

have

different

rated

impedances.

A

group

of

speakers

that

are

connected

in

series

presents

a

higher

impedance

to

the

amplifier

than

any

one

of

them

alone;

the

total

impedance

is

the

sum

of

the

individual

impedances.

A

higher

total

impedance

represents

a

lesser

load

to

the

amplifier,

and

consequently

draws

(or

receives)

less

power.

Yamaha P2075 User manual

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