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

Universal Audio LA 4 User manual

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