Universal audio La 4 User manual

www.SteamPoweredRadio.Com

model

LA

4

COMPRESSOR/LIMITER

EFFECTIVE

WITH

SERIAL

#3196

UNITED

RECORDING

ELECTRONICS

INDUSTRIES

8460

SAN FERNANDO

RD

.,

SUN VALLEY, CALIFORNIA

91352

(818)

767-1000

■

[!J][g

compeny

www.SteamPoweredRadio.Com

BEFORE

PROCEEDING

WITH

COMPLETE UNPACKING

AND

SETUP,

CONSULT UNPACKING

AND

INSPECTION INSTRUCTIONS ON

PAGE

8

model

LA

4

COMPRESSOR/LIMITER

EFFECTIVE

WITH SERIAL

#3196

United Recording Electronics

Industries

8460

SAN

FERNANDO

ROAD,

SUN

VALLEY,

CALIFORNIA

91352

(818)

767-1000

Printed

in

U.S.A.

©

Copyright

1980,

UREI

www.SteamPoweredRadio.Com

■

.I

•

I.I

•

I

TABLE

OF

CONTENTS

SECTION I .

INTRODUCTION

1.1

FRON~

PANEL

1 . 2

REAR

PANEL

1.3

DESCRIPTION

1.4

DESIGNATION

OF

INPUT

AND

OUTPUT

LEVELS

1.5

DEFINITION

OF

COMPRESSOR

AND

LIMITER

1.6

SPECIFICATIONS

SECTION

II.

INSTALLATION

2. 1

UNPACKING

ANO

INSPECTION

2. 2

ENVIRONMENTAL

CONSIDERATIONS

2. 3

RACK

MOUNTING

A SINGLE UNIT

2

.4

RACK

MOUNTING

TWO

UNITS

2.5

LINE

VOLTAGE

SWITCH

2. 6

POWERING

2.7

EXTERNAL

CONNECTIONS

2.8

IMPEDANCE

AND

TERMINATION

2. 9 INPUT

AND

OUTPUT

LEVELS

AND

OVERLOAD

SECTION

III.

OPERATING INSTRUCTIONS

3.1

GENERAL

3. 2

CONTROL

SETTINGS

3. 3 STEREO OPERATION

3

.4

VU

METER

SECTION

IV.

MAINTENANCE

4. 1

GENERAL

4.2

GAIN

REDUCTION

METER

ZERO

ADJUSTMENT

4. 3 G. R.

METER

CALIBRATION

4.4

STEREO

TRACKING

4.5

REPAIRS

AND

WARRANTY

SECTION V. LA-4

SCHEMATIC

DIAGRAM

PAGE

l

2

4

5

6

8

9

11

13

14

15

16

17

18

19

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•

SECTION

I

INTRODUCTION

1.1

FRONT

PANEL

•t

~

..

~

...

.,

J

~

,.

.,•·

•

OUT!'u

r

<tvtl

0/.M'fl/8

•

·•

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FIGURE

1.

LA-4

FRONT

PANEL

A.

METER.

The

LA-4's

meter

is

factory

set

for

OVU

=

+4

dBm

(1.23

volts),

or

+8

dBm

(1.96

volts)

output

level.

In

GR

mode,

the

meter

should

read

O

dB

so

long

as

no

gain

reduction

occurs.

If

the

meter

rests

at

some

other

value,

it

may

be

calibrated

by

inserting

a

small,

slotted

screwdriver

through

the

hole

just

to

the

right

of

the

meter.

B.

STEREO

COUPLER.

This

switch

should

normally

be

OFF.

When

two LA-4's

are

interconnected

(by means

of

a

simple,

rear

panel

connection),

switching

both

STEREO

switches

ON

will

strap

the

gain

reduction

circuitry

in

both

units

to

maintain

a

stable

stereo

image.

c.

OFF.

POWER

.

This

switch

turns

the

LA-4's

AC

power

ON

and

No

signal

flows

when power

is

OFF.

D.

METER

FUNCTION.

The

outer

knob

switches

the

meter

function.

In

GR

mode,

the

meter

rests

at

O

VU

until

gain

reduction

occurs;

then

it

deflects

down-scale

corresponding

to

the

number

of

dB

of

compression.

In

+4

mode

or

+8

mode,

the

meter

indicates

output

level,

where O

VU

respectively

equals

+4

dBm

or

+8

dBm

output

level.

E.

OUTPUT

LEVEL.

The

inner

control

adjusts

the

gain

of

the

LA-4's

output

stage.

·

This

control

is

independent

of

the

THRESHOLD

and

RATIO

settings.

-1-

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F.

COMPRESSION RA'l'IO.

'1.'he

outer

knob

switches

the

amount

of

compress

ion

to

2,

4,

8,

12

or

20:

1.

The

LA-4

can

be

considered

to

be

limiting

at

crnnpression

settings

of

8:1

or

greater.

G. THRESHOLD.

The

inner

control

adjusts

the

level

above

which

compress

ion

or

1

imi

ting

occurs.

In

conj

unction

with

the

rear

panel

Input

Range

switch,

threshold

may

be

set

for

a

wide

range

of

input

signal

levels

and/or

amounts

of

compression.

H. INPUT

OVERLOAD

INDICATOR.

This

LED

(Light

Emitting

Diode)

illuminates

when

the

input

level

reaches

+20

dBm

(7.75

volts),

high

enough

to

cause

distortion

in

the

input

amplifier.

Avoid

distortion

by

setting

the

output

level

of

the

source

or

by

using

an

attenuation

pad

so

the

LED

does

not

turn

on.

If

a

nominal

+4

dBm

source

causes

the

LED

to

be

constantly

ON,

be

sure

the

rear-panel

INPUT

RANGE

switch

is

in

the

HIGH

position.

1.

2

REAR

PANEL

A. FUSE.

For

110-125

V

operation,

use

MDL

1/4-amp

(slo-blo)

fuse.

For

220-250

V

operation,

use

MDL

1/8-amp

(slo-blo)

fuse.

B.

POWER.

For

safety,

the

IEC

style

receptacle

ties

the

mains

ground

(third

prong

on

the

plug)

to

the

chassis

ground.

In

some

instances

it

may

be

desirable

to

temporarily

break

the

mains

ground,

using

a

3-prong

to

2-prong

AC

adapter,

so

that

hum-causing

ground

loops

can

be

detected.

(See

Section

2.6.)

C.

MAINS

VOLTAGE

SELECTOR.

This

recessed

switch

can

be

set

with

a

screwdriver

for

nominal

115

V

or

230

V

operation;

be

sure

it

is

in

the

correct

mode

before

connecting

the

LA-4to

the

mains.

If

the

unit

is

switched

from

one

operating

voltage

to

another,

be

certain

to

exchange

the

fuse

for

another

of

the

correct

rating.

D. INPUT TERMINALS.

For

balanced

or

floating

sources,

connect

the

high

side

of

the

line

to

the"±"

terminal,

the

low

side

of

the

line

to

the

"COM"

terminal,

and

the

shield

to

the

"CHASSIS"

terminal.

For

unbalanced

sources,

connect

the

high

side

of

line

to

the"±"

terminal,

and

the

low

side

to

the

"COM"

and

"CHASSIS"

terminals.

E.

STEREO TERMINAL.

For

stereo

operation

of

two

LA-4's,

connect

a

short

cable

with

standard

phone

plugs

between

the

two

connectors

on

the

rear

chassis

of

the

two

units.

Older

model

LA-4's

(Black

front-panel)

do

not

have

this

connector.

Instead,

they

have

two

terminals

on

the

barrier

strip.

-2-

I

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■

II

•

Connect

the

center

conductor

of

the

single-conductor

shielded

cable

to

the

terminal

labeled

"STEREO"

and

the

shield

to

the

terminal

labeled

"CHASSIS".

On

the

front

panel,

the

switches

labeled

STEREO

must

be

switched

ON

to

link

the

gain

reduction

circuits

together.

Switching

either

switch

to

the

OFF

position

disconnects

the

link

between

the

limiters

for

independent

operation.

The

stereo

connecting

cable

may

remain

connected

as

long

as

one

or

both

of

the

STEREO

switches

is

in

the

OFF

position.

F.

OUTPUT

TEffi~INALS.

Connect

the

high

side

of

a

balanced

or

unbalanced

line

to

the

11

±

11

terminal.

Connect

the

low

side

of

a

balanced

line

or

the

shield

of

an

unbalanced

line

to

the

"COM"

terminal.

If

hum

develops

with

an

unbalanced

line,

connect

a

jumper

between

the

"COM"

and

"CHASSIS"

terminals.

If

a

high

impedance

device

is

connected

to

the

LA-4

output,

I

connect

a

620

ohm,

1/2

watt

load

resistor

across

the"±"

and

COM"

terminals.

G.

INPUT

RANGE

SWITCH.

This

switch

sets

the

LA-4's

input

sensitivity.

"HIGH"

position

corresponds

to

a

nominal

input

sensitivity

of

O dBm,

for

rnost

professional

lines.

"LOW"

posit

ion

corresponds

to

a

no1tiinal

input

sens

i

ti

vi

ty

of

-20

dBm,

for

preamplified

instruments,

very

"hot"

condenser

microphones,

or

semi-professional

or

hi-fi

type

equipruent.

-3-

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1.3

DESCRIPTION.

The

UREI

LA-4

Cornpressor/Linliter

is

a

single-channel

unit,

although

two

may

be

strapped

for

stereo

operation.

Making

use

of

UREI's

patented

Electro-Optical

Attenuator

technology,

the

LA-4

offers

smooth,

predictable

performance

coupled

with

ease

of

opera-

tion.

The

gain

reduction

is

accomplished

with

an

RIV!S

detector,

which

is

the

analog

of

human

hearing,

so

gain

reduction

is

smooth

and

natural

sounding.

The

detector

consists

of

a

light

dependent

resistor

(LDR)

and

an

electrolurninescent

light

source

which

is

a

se

m

iconductor,

so

it

does

not

change

or

deteriorate

with

age.

Compression

ratio

is

adjustable

by

an

easily

re-settable,

multi-position

switch

that

allows

effects

from

a

smooth

2:1

compression

through

a

super

tight-sounding

20:1

limiting.

The

calibrated

ratio

switch

assures

consistent

and

repeatable

results.

Threshold

is

adjustable

over

a

wide

range,

giving

the

flexi-

bility

to

limit

only

the

highest

amplitudes,

or

to

compress

virtually

the

entire

program.

A

balanced,

differential

input

stage

and

a

transfonner-isolated

output

assure

trouble-free

patching

by

avoiding

ground

loop

problems.

The

input

sensitivity

is

switchable

to

accommodate

high

level

lines

(nominal

O

dBm

sensitivity)

or

lower

level

sources

(nominal

-20

dBm

sensitivity).

Input

level

is

monitored

by

a

bri9ht

LED

indicator

that

illuminates

whenever

the

incoming

signal

level

is

high

enough

to

cause

distortion

in

the

first

amplifier

stage.

Output

level

is

continuously

variable.

The

large,

illuminated

VU

Itteter

can

be

switched

to

indicate

dB

of

yain

reduction,

or

output

level;

front

panel

switching

sets

the

meter

sensitivity

for

O

VU=

+4

dBm

or

O

VU=

+8

dBm

output

level,

in

accordance

with

the

most

widely

used

recording,

sound

reinforcement

and

broadcast

standards.

In

gain

reduction

mode,

the

rneter

is

driven

by

a

crn~pletely

isolated

circuit,

so

distortion

is

minimized.

All

connections

are

made

to

a

standard

barrier

strip.

The

LA-4

is

ideal

for

all

phases

of

recording,

re-recording,

and

sound

reinforcement.

It

is

also

useful

for

broadcast

production.

However,

due

to

its

RMS-responding

gain

reduction

circuitry,

the

LA-4

is

not

recommended

for

overmodulation

protection

of

broadcast

transmitters,

disc

or

optical

recorders,

unless

followed

by

a

peak

limiter

such

a s

the

UREI

1176

LN.

-4-

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•

1.4

DESIGNATION

OF

INPUT

AND

OUTPUT

LEVELS

The

term

"dBm"

has

been

commonly

misapplied

when

describing

voltage

levels

in

hi(::Jh

impedance

1

ines.

In

fact,

"dBm"

refers

to

a

power

level,

where

O

dBm

is

equal

to

1

milliwatt,

or

0.775

V

across

a

600

ohm

ter1aination.

However,

in

high

impedance

circuits,

power

dissipation

is

negligible,

and

only

the

voltage

level

is

of

interest.

The

terrn

"dBV"

would

be

more

useful

here,

where

O dBV

is

equal

to

1

volt

(across

any

impedance).

Unfortunately,

because

O dBV

(1

volt)

across

a

600

ohm

line

is

equal

to

+2.2

dBi

11

,

the

term

"dBV"

can

be

misleading.

For

this

reason,

the

term

"dBm''

is

used

exclusively

in

this

manual,

with

the

understanding

that,

in

other

than

600

ohm

circuits,

the

voltage

level

is

the

reference,

not

the

power

level.

I

For

example,

if

an

input

sensitivity

is

rated

at

+4

dBm,

and

the

input

impedance

is

40

kohms,

the

true

sensitivity

is

1.23

volts

(1.23

volts

across

600

ohms

is

+4

dBm).

In

strictly

correct

technical

terms,

the

input

would

have

a

sensitivity

of

-14

dBm,

which

is

the

power

dissipated

by

1.23

volts

across

40

kohms.

However,

a

mixer

of

+4

dBm

nominal

output

provides

the

correct

voltage

to

drive

the

input,

so

the

input

sensitivity

is

rated

at

+4

dBm.

In

fact,

if

a

604

ohm

terminating

resistor

is

used,

the

input

sensitivity

becomes

a

true

+4

dBm.

The

same

criteria

apply

to

output

level

ratings.

1.5

DEFINITION

OF

COMPRESSOR

AND

LIMITER

Compressor:

A

variable

gain

amplifier

whose

gain

decreases

as

its

input

level

increases

past

the

threshold

point.

A

compressor

uses

low

compression

ratios

of

approxmiately

2:

l

or

4:

1.

Limiter:

A

compressor

with

a

high

compression

ratio

of

approximately

8:1

or

greater.

The

high

ratios

maintain

essentially

constant

output

lev

e l

despite

increases

in

input

level

above

the

threshold

.

Compression

Ratio:

The

ratio,

in

dB,

of

input

level

change

to

output

level

change,

above

threshold.

A

compressor

whose

output

level

changes

1 dB

for

a 2 dB

input

level

change,

above

threshold,

has

a

2:1

Compression

Ratio.

A

compressor

whose

output

level

changes

1 dB

for

a 4 dB

input

l e

vel

change

above

threshold

has

a

4:1

compression

ratio,

and

so

forth

.

- 5-

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Threshold:

The

level

at

which

compression

begins.

The

URE

I

LA-4

Compressor/Limiter

has

an

adjustable

threshold.

When

the

input

level

is

below

the

set

threshold,

the

unit

functions

as

a

1:1

amplifier

(a

unity

gain

device).

When

the

input

level

is

above

the

set

threshold,

the

unit

functions

as

a

compressor,

or

as

a

limiter,

depending

on

the

compression

ratio

selected.

1.6

SPECIFICATIONS

ELECTRICAL

INPUT:

INPUT IMPEDANCE:

EQUIVALENT INPUT

NOISE:

MAXIMUM

INPUT LEVEL:

GAIN:

FREQUENCY

RESPONSE:

ATTACK

TIME:

RELEASE TIME:

COMPRESSION RATIO:

*

Ref.

0

dB=

0.775

V

rms.

Balanced

bridging

differential

amplifier.

40

kohms,

used

as

a

balanced

input.

20

kohms,

used

as

an

unbalanced

(single-ended)

input.

Less

than

-90

dBm

(15.7

kHz

band-

width);

input

and

output

termin

ated

with

600

ohm

load.

Determined

by

rear

panel

switch

to

allow

for

optimum

operation

in

different

level

environments;

HIGH

Range:

Maximum

+20

dB*

LOW

RANGE:

Maximum O dB*

Adjustable

with

front

panel

gain

control.

Maximum

20

dB

with

input

level

switch

in

LOW

p

osition,

or

40

dB

with

switch

in

HIGH

position.

±0.5

dB,

20

Hz

-

20

kHz.

l

to

10

milliseconds

for

63%

correc-

tion,

depending

on

signal

waveform.

100

milliseconds

to

1

second

for

63

%

return,

depending

on

duration

of

limiting.

2:1,

4:1,

8:1,

12:l

or

20:l;

switch

selectable

from

front

panel.

-6-

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THRESHOLD

OF

LIMITING:

Adjustable

with

front

panel

threshold

control

and

rear

panel

input

level

switch.

Minimurn

level

(±2

dB)

to

PHYSICAL

achieve

l dB

lirniting

is:

RATIO

LOW

RANGE

2:1

-61

dB*

HIGH

RANGE

-41

dG*

OUTPUT:

OUTPUT

LOAD:

POWER

OUTPUT:

POWER

REQUIREMENTS:

ENVIRONMENT:

4,8,12,20:l

-45

dB*

-2

5 dB*

Floating,

transfor

me

r

isola

ted

.

150

ohms

or

greater.

+24

dBm

into

a

600

ohm

load

(

12.

8

+20

dB*

into

a

150

ohm

load.

100

to

125

VAC

or

200

to

250

VAC,

50/60

Hz,

switch

selectable,

les

s

than

10

w.

Operating,

+0°C

to

+50°C.

Storage,

-

20°C

to

+60°C.

CONNECTIONS:

Input

and

output,

throu

gh

rear

chassis

barrier

strip.

Stereo

interconnect

throug

h

phono

jack.

Power

through

3-

wire

IEC

style

connector.

V) ;

INDICATO

RS:

StandarJ

VU

meter,

switch

selectabl

e

to

read

output

level

(0

VU

reference

+4

dBrn

or

+8

dBm),

or

amount

of

gain

reduction.

LED

indicator

for

input

overload.

LED

power

indicator.

DIMENSIONS:

216

mm

W x

89

mm

H

rack

panel

,

depth

behind

panel

is

203

mm

(8-1/2"

W x

3-1/2"

H x 8

11

D).

FINISH:

Panel

is

3.18

mm

(l/8")

brushed,

clear

anodized

aluminum

in

two

shades.

Chassis

is

cadmium

plated

steel.

WEIGHT:

2.95

kg

(6.5

pounds).

SHIPPING

WEIGHT:

3.63

kg

(8

pounds).

ACCESSORIES:

Single

Rack

Mount

Kit,

No.

SR-1.

Double

Rack

Mount

Kit,

No.

DR

-1

.

Model

301

XLR/QG

Adapter

kit

for

input

and

output.

-7-

I

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•

■

•

SECTION

II

INSTALLATION

2.1

UNPACKING

AND

INSPECTION

Carefully

examine

the

contents

of

the

shipping

carton

for

any

signs

of

physical

damage

which

could

have

occurred

in

transit.

Though

your

LA-4

was

carefully

packed

at

the

factory,

and

the

container

was

designed

to

protect

the

unit

from

rough

handling,

accidents

do

happen.

IF

DAMAGE

IS

EVIDENT,

DO

NOT

DESTROY

ANY

OF

THE

PACKING

MATERIAL

OR

THE

CARTON. IMMEDIATELY NOTIFY

THE

CARRIER

OF

A POSSIBLE CLAIM

FOK

DAMAGE.

SHIPPING

DAMAGE

CLAIMS

MUST

BE

MADE

BY

YOU,

THE

CONSIGNEE.

The

shipment

should

include

a

two-part

warranty

card

bearing

the

same

serial

number

as

the

LA-4,

an

instruction

manual,

and

the

LA-4.

IN

ORDER

TO

ACTIVATE

THE

ONE-YEAR LIMITED

WARRANTY,

THE

CARD

MUST

BE

COMPLETED

AND

THE

RETURN

PORTION MAILED

IMM

EDIATELY.

2.2

ENVIRONMENTAL

CONSIDERATIONS

The

LA-4

will

operate

satisfactorily

over

a

wide

range

of

ambient

temperatures,

from

10

to

50

°C

(45

to

121

°F).

If

the

unit

is

installed

in

an

equiment

rack

that

also

contains

heat-

producing

equipment

(such

as

power

amplifiers),

adequate

ventilation

should

be

provided.

This

will

prolong

component

life

and

maximize

operational

stability.

Also,

while

circuitry

susceptible

to

hum

pickup

is

fully

shielded,

installation

should

nevertheless

be

planned

to

avoid

locating

the

LA-4

immediately

adjacent

to

large

power

transformers,

motors,

etc.

2.3

RACK

MOUNTING

A SINGLE UNIT

The

SR-1

kit

(optional)

allows

mounting

of

one

LA-4

in

a

standard

19"

USASI

rack.

The

unit

occupies

two

standard

spaces

(3-1/2"

high).

The

SR-1

kit

should

contain

the

following

items:

QUANTITY

2

4

DESCRIPTION

Rack

Adapter

Angles

Rack

Extensions

#6-32

Oval

Head

Screws

(black)

#6

Lock

Washers

#6-32

Nuts

#8-32

Pan

Head

Screws

#8

Lock

Washers

-8-

I

www.SteamPoweredRadio.Com

The

SR-1

Installation

Procedure

is

as

follows:

(Refer

to

Figure

2)

1.

Using

the

#6-32

screws,

lockwashers,

and

nuts,

mount

the

angles

to

the

back

side

of

the

rack

extensions.

Do

no

t

tighten

the

screws

at

this

time.

2.

Using

the

#8-32

screws

and

lock

washers,

mount

the

angle

and

rack

extension

assemblies

to

the

tapped

holes

in

the

sides

of

the

LA-4.

Do

not

tighten

the

screws

at

this

time.

I

3.

The

LA-4

front

panel

is

adjustable

to

allow

optimum

alignment

of

the

front

panel

with

the

rack

extensions.

For

precise

alignment,

loosen

the

front

panel

mounting

screws,

as

well

as

the

top

and

bottom

cover

screws

(1/2

turn

is

adequate).

Adjust

the

relative

positions

of

the

brackets

and

panels,

and

then

tighten

all

hardware.

2.4

RACK

MOUNTING

TWO

UNITS

The

DR-1

kit

(optional)

allows

mounting

of

two

LA-4 1s

side

by

side

in

a

standard

19"

USASI

rack.

The

assembly

occupies

two

standard

spaces

(3-1/2"

high).

The

DR-1

kit

should

contain

the

following

items:

QUANTITY

2

4

DESCRIPTION

Joining

Plates

Rack

Mounting

Angles

Spacer

Plates

#8-32

Pan

Head

Screws

#8

Lock

Washers

The

DR-1

Installation

Procedure

is

as

follows:

(Refer

to

Figure

3)

1.

Place

the

two

LA-4's

side

by

side,

with

front

panels

touching.

Remove

four

screws

from

the

top

of

each

unit,

as

illustrated,

to

permit

positioning

of

the

jo

ining

plate.

Replace

the

screws

through

the

plate

and

covers,

but

do

not

tighten

completely

at

this

tim

e .

2.

Turn

the

assembly

upside

down,

and

install

the

bottom

joining

plate

in

the

same

fashion

as

the

top

joining

plate

(step

1).

3.

Using

the

#8-32

screws

and

lock

washers,

mount

one

angle

and

one

spacer

plate

to

each

side

of

the

LA

-4

assembly.

The

spacer

plates

should

be

located

between

the

angles

a

nd

the

LA-4

chassis.

The

mounting

holes

in

the

LA-4

are

tapped,

so

nut

s

are

not

requir

ed

.

Do

not

tighten

the

screws

at

this

t

irne.

-9-

www.SteamPoweredRadio.Com

4.

The

LA-4

front

panels

are

adjustable

to

allow

optimum

alignment

of

the

front

panels

with

the

rack

mounting

angles.

For

precise

alignment,

loosen

the

front

panel

mounting

screws,

as

well

as

the

top

and

bottom

cover

screws

(1/2

turn

is

adequate).

Adjust

the

relative

positions

of

the

angles

and

panels,

and

then

tighten

all

hardware.

18-32

Pan

Head

Screw

18

Lock

Washer

16-32

Nut

16

Lock

Washer

16-32

Oval

Head

Screw

cover

screws

Angle

Rack

Extension

front

panel

mounting

screws

FIGURE

2.

MOUNTING

DETAILS

FOR

SR-1

RACK

MOUNT

KIT.

6)

(911®0

6)

cover

screws

11

Joining

Plate

II

11

0

~:®

18-32

Screw

II

48

Lock

Washer

II

®''0

front

panel

mounting

screws

FIGURE

3.

MOUNTING

DETAILS

FOR

DR-1

RACK

MOUNT

KIT.

-10-

Spacer

Plate

Angle

I

www.SteamPoweredRadio.Com

I

II

2.5

LINE

VOLTAGE

SWITCH

Unless

a

tag

on

the

line

cord

specifies

otherwise,

the

LA

-4

was

shipped

ready

for

operation

with

nominal

115

VAC

power

1nains.

In

order

to

change

this

for

nominal

230

V

(50

or

60

Hz),

slide

the

VOLTAGE

SELECTOR

switch

on

the

rear

panel

to

the

230

position.

The

voltage

is

visible

in

a

window

the

switch

slot.

Be

sure

to

change

the

fuse

to

the

correct

value,

1/8-amp

slo-blo

when

changing

to

230

V

operation

or

1/4-amp

slo-blo

for

115

V

operation.

A

small

screwdriver

should

be

used

to

move

the

recessed

switch.

2.6

POWERING

The

LA-4 may

be

operated

from

either

110

-

125

VAC

or

220

-

240

VAC

mains

(50

or

60

Hz,

single

phase).

As

indicate

d

in

section

2.5,

the

nominal

line

voltage

may

be

selected

with

a

rear

panel

switch.

BE

SURE

TO

VERIFY

BOTH

THE

ACTUAL

LINE VOLTAGE,

AND

THE

SETTING

OF

THE

VOLTAGE

SELECTOR

SWITCH

BEFORE CONNECTING

THE

LA-4

TO

THE

MAINS.

AC

mains

power

is

supplied

to

the

Model

LA-4

through

an

IEC-style

3-conductor

receptacle

mounted

on

the

rear

of

the

chassis,

and

a

ffiatching

3-wire

power

cable.

The

ground

of

t

he

connector

is

tied

to

the

chassis

of

the

Model

LA-4

in

order

to

comply

with

electrical

codes.

In

some

installations

this

may

create

ground-loop

problems.

Ground

loops

can

become

very

evident

(as

hum

and

buzz)

if

a

significant

potential

difference

exists

between

the

AC

conduit

ground

and

the

grounded

metal

enclosure

in

which

the

chassis

is

installed.

If

hum

is

experienced,

check

for

the

possibility

of

ground

loops

by

using

a

3-prong

to

2-prong

AC

adapter,

ungrounding

the

AC

plug

temporarily.

This

ungrounds

the

LA

-4,

and

may

cure

the

hum

or

buzz,

but

is

not

a

substitute

for

proper

system

grounding.

Be

aware

that

unless

the

Model

LA-4

Compressor/Limiter

is

AC

grounded,

a

safety

hazard

can

exist.

UREI

accepts

no

responsibility

for

legal

actions

or

for

direct,

incidental

or

consequential

damages

that

may

result

from

violation

of

any

electrical

codes.

2.7

EXTERNAL

CONNECTIONS

Input

and

output

signal

wires

should

be

shielded

cable,

and

connected

in

accordance

with

standard

wiring

practice,

as

indicated

on

the

rear

panel

barrier

strip.

Section

1.2

contains

wiring

information,

which

is

augmented

by

the

diagrams

in

Figures

4

and

5.

When

strapping

two

units

together

for

stereo

o~eration,

use

low

capacitance

shielded

cable,

and

restrict

the

length

to

under

ten

feet.

If

the

LA-4

is

connected

to

a

high

impedance

circuit,

we

recommend

shunting

the

11

±11

and

"COM"

output

terminals

with

a

620

ohm,

1/2

watt

resistor.

This

assures

proper

loading

for

the

LA-4.

(See

section

2.8

regarding

input

termination).

-11-

I

www.SteamPoweredRadio.Com

In

situations

where

Input

and

Output

will

be

connected

and

dis

-

connected

frequently,

we

suggest

th

e

use

of

the

Model 3

01

XLR/QG

Adapter.

This

is

a

small

box

contaiing

one

male

and

one

female

XLR

connector

for

Input

and

Output.

It

is

c

omplet

e

ly

wired

and

co

nn

ects

quickly

to

the

r

ea

r

pane

l

barrier

s

tr

ip

.

It

mounts

ove

r

the

barrier

strip

on

the

rear

panel

using

two

screws.

The

Model

30

1 i s

available

I

through

your

UREI d

ea

ler.

OUTPUT

GRD

INPUT

±

COM

±

FIGURE

4.

CONNECTING

THE

LA-4

WITH

BALANCED

INPUT

AND

BALANCED

OUTPUT

CIRCUITS.*

OUTPUT

GRD

±

COM

INPUT

COM

±

FIGURE

5.

CONNECTING

TH

E

LA

-4

WITH

UNBALANCED

INPUT

AN

D

UNBALANCED

OUTPUT

CIRCU

IT

S

.*

*

Wi

th

a

ba

l a n

ced

input

and

unbalanced

output

,

or

vice

-

versa

,

just

u

se

th

e

appropria

t e

con

n

ections

sugges

t

ed

by

each

of

the

above

diag

r

ams.

-12-

www.SteamPoweredRadio.Com

2.8

IMPEDANCE

AND

TERMINATION

This

section

has

been

provided

to

better

explain

the

nature

of

matching

and

bridging

inputs,

the

use

of

terminating

resistors,

and

the

relationship

between

actual

input

impedance

and

nominal

I

source

impedance.

Most

electronic

outputs

work

well

when

"t

er

-

minated"

by

a

load

(connected

to

an

input)

having

the

same

or

a

hi~her

actual

impedance.

Outputs

are

usually

overloaded

when

terminated

by

an

impedance

that

is

lower

than

the

source

i

mpedance

.

When

the

input

impedance

is

nearly

the

same

impedance

as

the

source,

it

is

known

as

a

"matching

"

input.

When

an

input

is

10

times

the

source

impedance,

or

more,

the

input

is

considered

to

be

a

"bridging"

input.

The

UREI

LA

- 4

has

actual

input

impedances

of

40

kohms

(40,000

ohms)

when

used

in

a

balanced

configuration,

and

20

kohms

when

used

in

an

unbalanced

configuration

(they

are

high

impedance

inputs).

This

makes

the

LA-4's

inputs

suitable

for

use

with

virtually

any

nominal

source

impedance,

low

or

high.

The

LA-4's

inputs

will

bridge

150

ohm

or

600

ohm

(low

impedance)

sources

and

will

match

10

kohm

or

greater

impedance

(high

impedance)

sources.

In

most

cases,

it

is

not

necessary

to

place

a

600

ohm

"terminating

resistor"

across

the

input

of

the

LA-4.

In

fact,

most

600

ohm

sources

operate

normally

when

bridged

by

a

high

impedance,

acting

as

though

no

load

were

connected

to

the

source

device.

A

604

ohm

termination

resistor

is

required

at

the

LA

-

4's

output

only

if

it

is

connected

to

a

high

impedance

line.

The

only

instance

where

an

input

termination

resistor

may

be

required

is

when

the

manufacturer

of

the

source

device

specifically

states

that

a

terminating

resistor

is

necessary.

In

such

cases,

there

is

usually

a

special

type

of

output

transformer

in

the

source

device,

or

the

source

device

is

made

of

precision,

passive

components

(no

transistors

or

tubes).

An

example

of

a

unit

requiring

a

termina-

ting

resistor

is

a

passive

equalizer.

The

ter

m

inating

resistor

assures

optimum

frequency

response

in

that

device.

If

a

150

ohm

or

600

ohm

resistor

is

specified

for

the

source

device,

it

should

be

installed

at

the

end

of

the

cable

nearest

the

LA-4

in

order

to

minimize

possible

hum,

noise

or

signal

losses

in

the

cable.*

*

600

ohms

is

not

a

standard

resistor

value.

A

604

ohm

precision

resistor

may

be

used,

but

620

ohm

or

680

ohm

resistors

are

close

enough

in

value

to

600

ohms

and

are

more

widely

available.

-13-

www.SteamPoweredRadio.Com

•

I

II

2.9

INPUT

AND

OUTPUT LEVELS

AND

OVERLOAD

The

LA-4's

differential

input

amplifiers

are

capable

of

being

driven

by

signals

up

to

a

level

of

+20

dBm

(input

range

switch

in

I

HIGH

mode),

or

O

dBm

(input

range

switch

in

LOW

mode),

above

which

clipping,

distortion

and/or

slew-rate

limiting

occurs.

The

input

overload

LED

turns

on

when

peak

signals

exceed

the

clipping

level,

and

will

remain

on

long

enough

to

be

seen

even

if

the

excess

signal

is

only

a

brief

transient.

To

avoid

distortion

when

the

LED

indicator

flashes

more

than

occasionally,

either

lower

the

output

level

of

the

source

feeding

the

LA-4,

or

use

an

external

attenuation

pad.

If

the

LED

is

illuminated

virtually

continuously,

check

the

input

range

switch;

if

it

is

in

LOW

mode

and

the

input

source

is

+4

dBm

nominal

level,

move

the

switch

to

HIGH

mode.

The

output

amplifier

is

capable

of

delivering

+24

dBm

into

a

600

ohm

load

(12.3

volts)

or

+20

dB

into

a

150

ohm

load

(7

.

75

volts,

which

is

actually

+26

dBm

referenced

to

1

milliw

att).

Average

output

levels

can

be

monitored

with

the

LA-4's

VU

meter,

but

remember

that

transient

peaks

may

exceed

the

rneter

indication

by

as

much

as

20

dB.

The

actual

output

headroom

depends

on

the

program

material,

the

threshold

,

and

the

compression

ratio

selected.

-14-

www.SteamPoweredRadio.Com

I

II

I

SECTION

III

OPERATING INSTRUCTIONS

3.1

GENERAL

The

LA-4

Compressor/Limiter

is

designed

to

prevent

an

increase

in

output

level

beyond

a

pre-determined

point

(the

threshold),

and

due

to

its

unique

design,

the

unit

functions

as

a

compressor

or

a

limiter.

The

effect

is

illustrated

in

Figure

6.

The

point

at

which

the

compressed

curves

br

eak

away

from

the

straight

"No

Gain

Reduction"

line

is

determed

by

the

setting

of

the

"Threshold"

control.

Incidentally,

this

point

is

sometimes

known

as

the

"rotation

point"

or

"threshold

point."

It

can

be

seen

from

the

curves

that

at

low

or

moderate

settings

of

the

"Ratio"

control

(2:1

or

4:1),

the

output

level

increases

very

gradually

as

the

input

level

rises

above

the

threshold

point.

The

2:1

setting

automatically

lowers

the

threshold

for

a

gentle,

overall

dynamic

range

compression.

It

can

also

be

seen

that

at

higher

settings

of

the

"Ratio"

control

(8:1

through

20:

1),

there

is

virtually

no

incr

_

ease

in

output

level

as

the

input

level

rises

above

the

threshold

point.

3.

2

FIGURE

6.

dB OUTPUT LEVEL

LA

· 4 COMPRESSOR / LIMITER

•15

•10

•5

-5

-

10

-

15

-20

,. I lL

' / I

lL

V

f'OM

iREss

ON

. •1

I ' / i-::-

~

-·--=

-e:1

t1MI

,,

. ·

12

,f

Timfs

w

;:.

m:1

-

V V ~ ·

:,r

I/

/ V

F0!-1

'

11'~"'

ON

,,

I/

_,,,v

l/

v

~

V

,!

dB

INP1/l

I

COII

PRES

~ION

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INPUT LEVEL

TYPICAL INPUT

VERSUS

OUTPUT

LE

VEL

CURVES

FOR

LA-4

AT

VARIOUS

COMPRESSION RATIOS.

CONTROL

SETTINGS

l.

The

rear-panel

"Input

Rang

e "

switch

s

hould

be

pre-set

a t

the

appropriate

position:

"H

igh"

for

nominal

+4

dBm

s

ources,

or

"Low"

for

nominal

-20

dBm

sourc

es

.

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I

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2.

The

"Stereo"

switch

should

be

in

the

"Off"

position;

even

if

stereo

operation

is

desired,

initial

setup

is

done

independently

for

each

LA-4.

3.

Turn

the

"Power"

switch

"

On,"

and

wait

a

few

seconds

to

assure

the

system

is

warmed

up

and

operating

with

complete

stability.

Set

the

"Meter"

switch

to

either

"+4"

or

"+8",

whichever

level

corresponds

to

the

nominal

operating

level

of

your

audio

system.

Feed

typical

program

material

to

the

LA-4

input,

and

observe

the

"

Input

Overload"

indicator

to

ensure

it

does

not

illuminate

more

than

occasionally.

If

the

indicator

is

frequently

or

continuously

illuminated,

lower

the

incoming

signal

level

(with

the

output

attenuator

of

the

source

device

or

with

an

attenuation

pad),

and

check

the

position

of

the

INPUT

switch

on

the

rear

panel.

4.

Set

the

"Ratio"

control

to

·

the

desired

amount

of

compres-

sion.

The

optimum

setting

varies

with

pro~ram

material,

and

some

experimentation

may

be

useful

(you

may

reset

the

"Ratio"

after

setting

the

"Threshold").

With

a

typical

input

signal

present,

adjust

the

LA-4's

"Output

Level"

control

for

average

meter

deflections

of

O

VU.

Peaks

may

occasionally

deflect

the

pointer

off

scale.

5.

Set

the

"Threshold"

control

at

"0"

(full

counterclockwise),

so

that

the

threshold

is

above

any

input

signal.

Set

the

"Meter"

switch

to

"GR"

mode,

and

observe

the

meter.

Gradually

rotate

the

"Threshold"

control

in

a

clockwise

direction,

lowering

the

threshold

point

until

the

meter

displays

3 dB

to

5 dB

of

gain

reduction

(average),

with

occasional

peaks

of

10

dB

to

20

dB

downward

deflection.

As

with

the

"Ratio,"

setting,

some

experimentation

may

be

useful

in

determining

the

optimum

"Threshold"

setting.

3.3

STEREO OPERATION

If

two

LA-4

Compressor/Limiters

are

to

be

used

in

tandem

for

stereo

programs,

the

gain

reduction

of

each

unit

can

be

made

equal,

regardless

of

which

channel

is

instigating

the

action.

The

channel

that

requires

the

most

gain

reduction

will

drive

the

other

channel

to

the

same

degree

of

gain

reduction,

thereby

maintaining

a

stable

stereo

image.

For

stereo

operation,

repeat,

the

procedure

in

Section

3.2

separately

for

each

unit,

and

then

turn

both

the

"Stereo"

switches

"On".

In

"Stereo"

mode,

the

meters

in

each

unit

will

independently

display

the

output

level

(

"Meter"

switch

in

"+4"

or

"+8"

position),

but

will

both

display

the

same

amount

of

gain

reduction

("Meter

switch

in

"GR"

position).

The

tracking

in

"GR"

mode

is

due

to

the

fact

that

the

level

detection

circuits

in

both

units

are

strapped.

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3.4

VU

METER

The

VU

meter

serves

two

functions:

it

indicates

output

level

as

well

as

gain

reduction

directly

in

dB.

When

the

"Meter"

switch

is

placed

in

the

"+4"

or

"+8"

position,

the

meter

indicates

the

output

level.

0

VU

or

100%

corresponds

to

a

voltage

output

of

+4

dBm

or

+8 dBm,

depending

on

the

switch

setting.

The

position

marked

"GR"

permits

the

meter

to

indicate

the

amount

of

gain

reduction

or

peak

limiting

directly

in

dB.

During

periods

of

no

gain

reduction,

the

pointer

will

return

to

O

VU

on

the

meter

scale.

The

pointer

is

initially

set

to

this

position

by

means

of

the

screwdriver

adjusted

control

accessible

through

a

hole

in

the

LA-4

front

panel.

-17-

I

Universal Audio LA 4 User manual

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