Teletronix La-2a User manual

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INSTRUCTION

MANUAL

LEVELING

AMPLIFIER

MODEL

LA-2A

TELETRONIX

DIVISION

BABCOCK

ELECTRONICS

CORP.

2952

RANDOLPH

STREET

COSTA

MESA,

CALIFORNIA

TELEPHONE

(714) 5

40-4040

LA-2A

(3-66)

0.5

to 5

seconds

for

complete

release

depending

upon

amount

of

Page

2

[ J

1

E ]

[I

J

OPERATING

INSTRUCTIONS

[I

J

TELETRONIX

MODEL

LA-2A

LEVELING

AMPLIFIER

[I

J

INTRODUCTION

The

Teletronix

Leveling

Amplifier

will

automatically

reduce

audio

peaks

which

might

[I

J

otherwise

over

drive

broadcast

or

recording

equipment.

Automatic

gain

reduction

is

accomplished

by

the

use

of

an

electro-optical

variable

[rJ

attenuator,

which

is

placed

ahead

of

the

first

amplifier

stage.

The

attenuation

is

controlled

by

the

amplitude

of

the

LA-2A

input

signal.

[I

J

[I

This

system

permits

up to

40

DB

of

instantaneous

gain

reduction,

yet

causes

no

wave

form

or

harmonic

distortion.

The

amplifier

provides

sufficient

gain

and

output

level

(10 DBM

[I

nominal)

to

be

used

as

a

line

or

program

amplifier,

or

for

direct

connection

to

the

transmitter

in

the

case

of

radio

or

TV

operation.

[I

Provisions

are

made

for

interconnection

of

the

optical

attenuators

to

provide

equal

gain

reduction

in

both

channels

when

two

of

the

LA-2A

Leveling

Amplifiers

are

used

for

FM

stereo

broadcasting

.

[I

SPE

CIFICATIONS

1.

Gain

Reduction:

up to

40

DB

[I

J

[ 2.

Distortion:

less

than

0.35

%

total

harmonics

at

+10 DBM,

and

less

than

0.75

%

total

harmonics

at

[I

+16

DBM

output

3.

Response:

±0.1

DB,

30

cycles

to

15

KC

[I

4.

Noise:

75 DB

below

+10 DBM

output

level

[I

5.

Gain:

40

±1

DB

6.

Output

Level

: +10

DBM

nominal

+16 DBM

maximum

peaks

r

_I

7.

Input

Level:

+16

DBM

maximum

[ 1

~

8.

Attack

Time:

essentially

instantaneous

(10

usee)

[ 9.

Release

Time:

approximately

0.06

seconds

for

50%

release,

[;

[I

10.

Input

Impedance:

50, 150, 250,

and

600

ohms,

balanced

or

[I

unbalanced

11.

Out~ut

Impedance:

50, 150, 250,

and

600

ohms,

balanced

or

[f

unbalanced

12.

Output

Source

Impedance:

approximately

150

ohms,

at

800

CPS

[I

J 13.

Panel

Size:

standard

19" x

5-1/4"

[I

J 14.

Depth

Behind

Panel:

7-1

/

4"

15.

Panel

Controls:

Gain

(input

level),

Peak

Reduction

and

Meter

[I

Selector

Switch

16.

Meter:

DB

Gain

Reduction

and

DB

Output

[ J 17.

Power

Requirements:

115/

230

volts

50-60

cycle

35

watts

['J 18.

Tube

Complement:

( 2 ) 12AX7A

[ ( 1 ) 12BH7A

( 1 ) 6AQ5

19.

Fuse:

3AG, 3/ 8 AMP

Slow

Blow

for

115V;

[

.-

3AG 15/ 100 AMP Slow Blow

for

230V

[ -

CIRCUIT

DESCRIPTION

The

LA -

2A

Leveling

Amplifier

will

produce

essentially

instantaneous

gain

reduction

of

[

..

over

40 DB with no

increase

in

harmonic

distortion.

[ -A

typical

gain

reduction

curve

for

this

system

is

illustrated

on

Figure

1.

Compressor

action

occurs

from

the

breakaway

point

at

-30

DB

input

and

up to

-20

DB,

at

which

point

the

curve

becomes

horizontal

to

exhibit

limiting

action.

The

input

increases

an

additional

20 DB,

[ .

but

the

output

increases

less

than

1 DB.

The

leveling

amplifier

thus

combines

the

characteristics

of a

compressor

and

limiter.

A

reasonable

amount

of

care

in

gain

riding

will

restrict

normal

[ -

operation

to the

compression

region,

but

uncontrolled

output

levels

will

be

prevented

by the

limited

action

.

r•

The

heart

of

the

leveling

amplifier

is

the

electro-optical

attenuator

which

is

placed

ahead

of

the

first

amplifier

stage.

The

actual

stage

gains

and

tube

operating

parameters

are

not

varied,

permitting

the

tubes

to

operate

at

optimum

conditions

regardless

of

the

amount

of

gain

reduction.

The

optical

attenuator

consists

of a

photo

·conductive

cell,

which

is

optically

coupled

to

an

electro-luminescent

light

source.

The

electro-luminescent

device

provides

a

light

intensity

[

which

is

proportional

to

the

audio

voltage

applied

to

its

terminals.

Not

unlike

a

capacitor

in

LA-2A

(3-66)

Page

3

[

~

J

[ 1

~

construction,

the

electro-luminescent

lamp

consists

of a

plate

of

glass

or

plastic

coated

with a

[I

clear

conducting

material

on

one

side

and a thin

layer

of

phosphor

on

the

other

side.

A

metallic

plate

contacts

the

phosphor

coating.

As

alternating

current

is

applied

to

the

conducting

plates

[\

the

phosphors

are

excited

by

the

voltage

across

the

dielectric

and

light

is

produced

.

The

amount

of

light

depends

upon the

applied

voltage

and

frequency.

The

gain

or

level

controll

ing

element

is

[I

the

photo

·conductive

cell.

The

resistance

of the cell

decreases

with

an

increase

in

the

impinging

light.

Since

the

light

is

produced

directly

fro111

the

audio

voltage,

the

response

is

instantaneous.

[

Rectification

and

filtering

of

the

audio

to

produce

a

control

signal

are

not

necessary

as

in the

case

of

conventional

limiters.

This

system

results

in

automatic

level

control

whose

speed

of

operation

[I

J

is

limited

only

by the

response

of

the

variable

resistance

photo

cell

used

.

[' A

cell

is

selected

which

provides

minimum

attack

time,

and

a

release

time

which

requires

about

60

milliseconds

for

50%

release,

and

then

a

gradual

release

over

a

period

of 1 to

15

seconds

to

the

pOint of

complete

release.

['

J

Referring

to

Figure

2,

the

functional

block

diagram,

the

input

signal

is

applied

directly

to

the

optical

attenuator

from

the high

impedance

winding

of

the

input

transformer.

The

amount

[I

] of

attenuation

introduced

by

the

optical

attenuator

is

controlled

by

the

audio

voltage

applied

to

it

by

the

6AQ5 (V4), which

is

the

luminescent

driver

amplifier.

The

amount

of

signal

applied

to

['

J the 12AX7

(Vi)

voltage

amplifier

is

also

controlled

by

the

manual

gain

control.

The

voltage

amplifier

stage

provides

a

gain

of 40 DB.

Overall

amplifier

feedback

of

approximately

20 DB

[ ]

provides

low

distortion,

flat

response,

and

gain

stability.

The

output

stage

is

somewhat

unconventional

in

that

a

totem

pole

or

double

cathode

follower

[ J

[

is

used.

This

output

stage

can

tolerate

great

amounts

of

output

impedance

mismatch,

but

retains

low

distortion

and

flat

frequency

response.

[ -

For

stereo

broadcasting

applications,

a

portion

of

the

input

signal

is

fed

through

the

gain

reduction

control

to

the

12AX7

control

amplifier

(V3).

The

output

at

this

stage

is

applied

to the

[

..

stereo

balance

control

and

is

also

brought

out

to a

terminal

on the

chassis

.

For

stereo

operation,

this

terminal

is

connected

to

the

same

terminal

on

an

identical

amplifier

and

control

voltage

becomes

common

to both

units.

A

gain-reduction

control

voltage

generated

in

either

amplifier

will

cause

equal

gain

reduction

in

both

units.

The

control

voltage

is

applied

through

the

stereo

[

..

balance

control

R3 to

the

6AQ5

driver

amplifier.

This

stage

provides

the

necessary

voltage

to

operate

the

electro-luminescent

light

source.

OPERATION

The

LA-2A

Leveling

Amplifier

is

designed

to

prevent

an

increase

in

output

level

beyond

a

pre-determined

point,

and due to

its

unique

deSign,

functions

as

a

combined

compressor

and

limiter.

The

effect

is

illustrated

in

Figure

1.

The

point

at

which

the

compressed

curve

breaks

LA-2A

(3-66)

Page

4

J.

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[ J

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J

away

from

the

straight

"No-Gain

Reduction"

line

is

determined

by

the

setting

of

the

"Peak

Reduction"

control.

It

can

be

seen

from

the

curve

that

compression

Occurs

and

gradually

in-

creases

over

the

first

10 DB of

input

level

rise.

The

slope

of

the

curve

then

becomes

horizontal,

preventing

an

increase

of

output

level

regardless

of

input

increase.

CONTROL SETTINGS

It

is

recommended

that

the

"Peak

Reduction"

control

be

set

to

prevent

increase

in

output

level

beyond

the

100%

modulation

point.

This

setting

should

be

made

on

typical

program

material.

Setting

of

the

"Gain"

and

"Peak

Reduction"

controls

are

independent.

However,

the

"Gain"

control

should

be

set

to

provide

sufficient

output

after

the

"Peak

Reduction"

control

has

been

adj

usted.

The

"Peak

Reduction

"

control

should

be

set

for

the

desired

amount

of

gain

reduction

as

indicated

by

the

meter

.

Continuous

extreme

reduction,

such

as

20

or

30 DB,

does

tend

to

reduce

the

dynamic

range

of

music.

Maximum

benefit

is

obtained

by

running

4 to 8 DB of

compression

continuously.

This

will

usually

cause

full

limiting

to

occur

when

100

%

modulation

is

approached.

For

ease

of

control

and

to

prevent

overload

of

the

input

transformer,

sufficient

fixed

pad

should

be

placed

ahead

of

the

LA-2A

to

allow

normal

output

at

approximately

50%

setting

of

the

gain

control.

VU

METER

The

VU

Meter

serves

two

functions;

it

indicates

output

level

as

well

as

gain

reduction

directly

in DB.

When

the

meter

selector

switch

is

placed

in

the

"output"

position,

the

meter

will

indicate

output

level

across

the

600 ohm

terminals.

The

meter

is

calibrated

to

read

0

VU

or

100

%

when the

amplifier

output

is

+10 DBM

or

+4

DBM,

depending

upon

the

switch

position.

The

position

marked

"Gain

Reduction"

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 0

VU

on

the

meter

scale.

The

pointer

is

initially

set

to

this

position

by

means

of the

screw

driver

adj

usted

control

located

on

the

left

end

of

the

front

panel.

STEREO

If

two

LA-2A

Leveling

Amplifiers

are

to be

used

in

tandem

for

stereo,

the

gain

reduction

of

each

amplifier

can

be

made

equal,

regardless

of

which

channel

is

instigating

the

limiting.

This

is

accomplished

by

interconnecting

terminals

6

and

7 of

the

LA-2A

Leveling

Amplifiers.

The

interconnecting

wire

should

not

be

over

two

feet

in

length

and

should

be

shielded,

the

ends

of

the

shield

being

connected

to the #7

terminals

(ground).

Stereo

"set-up"

is

as

follows:

1.

Connect

the

input

terminals

of

the

left

and

right

channel

LA-2A

to an

audio

oscillator.

Make

certain

that

the

amplifiers

are

connected

in

phase

to

the

generator.

LA-2A

(3-66)

Page

5

~--------------------------------------------------------,

[

[.

-,

[, A

generator

frequenc

y of

400

or

1000

cps

is

satisfactory.

Generator

output

level

should

be set to the

average

lev

el to

be

applied

in

o

peration.

[, 2.

[

Place

the

meter

sele

c

tor

switches

in the +10

or

+4

position

and

adjust

the

LA-2A

"Gain"

controls

for

equal

output.

The

"

Peak

Reduction"

controls

must

be

set

to

full

counterclockwise

or

off.

3.

Mak

e

certain

that

the

screw

driver

adj

ustment

(R3)

on

the

rear

of e

ach

unit

are

[

-,

"full

on

"

(clo

c

kwise).

Place

each

meter

selector

switch

in

the

"

Gain

Reduction"

position.

[.

4.

Advance

the

"Gain

Reduction"

control

on

the

left

channel

amplifier

until

approxi-

mately

5 DB of

reduction

is

indicated

on

the

meters.

Note

which

channel

is

[

indicatin

g

the

most

gain

reduction.

Reduce

the

setting

of R3 on

this

unit

until

both

meters

show

equal

reduction.

l 5.

The

"Gain

Redu

c

tion"

controls

c

an

now

be

placed

at

any

desired

settin

g,

keepin

g

both

knob se

ttings

equal.

Gain

Reduction

will

now

be

e

qual

on

both

channels.

L' l

[

GAIN REDUCTION

FREQUENCY

RESPONSE

CONTROL

FM

broadcasting

and

TV

aural

transmission

systems

use

audio

pre-emphasis

in

the

transmitter.

The

standard

is

17

DB

increase

in

response

at 15 KC,

the

exact

curve

being

the

result

of a

75

microsecond

network.

The

program

frequencies

in

the

vicinit

y of 15

KC

will

l

modulate

the

carrier

17

DB

more

than

frequencies

below

1 KC.

Thus,

if

the

pro

g

ram

material

contains

a

large

amount

of

high

frequencies

,

over-modulation

may

occur

if

the

levels

had

been

1

previously

adjusted

for

program

material

with

less

high

frequency

co

ntent.

An

attempt

to

alleviate

this

problem

has

been

made

by

others

in

the

form

of

level

con-

[

trolled

high

frequ

e

ncy

cutoff

filters

and

high

end

peak

clippers.

Because

the

amount

of c

ontrol

over

the

remainder

of

the

spectrum

is

limited

and

because

of

the

high

distortion

creat

ed,

such

[

devices

have

found

only

limited

application.

[

The

Teletronix

Leveling

Amplifiers

are

capable

of

at

least

30 DB of

gain

reduction

or

limitin

g

with

less

than

0.5%

harm

o

nic

distortion.

For

most

applications,

su

ch

as

AM

broad-

casting

and

recording,

the

am

o

unt

of

gain

reduction

is

a

function

of

input

level

and

is

ind

e

pendent

[\

of

frequency.

By

increasing

the

gain

reduction

at

the

higher

frequencies,

the

over-modulation

caused

1( by

the

pre-emphasis

can

be

g

reatly

reduced

or

eliminated.

While

it

is

possible

to inc

rease

the

gain

reduction

sensitivity

on

an

inverse

of

the

pre-emphasis

curve,

this

usually

results

in

an

[ -I

insufficient

lev

e

ling

on

the

low

frequencies

. The

actual

amount

of

the

limiter

pre-emphasis

must

I

be

determined

according

to

the

amount

of high

frequency

content

in

the

program

mat

e

rial

.

L -

[L

LA-2A

(3-66)

Page

6

[I]

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[.

[

Adjustment

of

the

gain

reduction

frequency

response

is

accomplished

by

control

R37

which

is

located

on

the

rear

of the

LA-2A.

Increasing

the

resistance

of

R37

reduces

the

amplitude

of

the

low

frequency

voltage

applied

to

the

Peak

Reduction

control,

R2.

The

high

frequency

com-

ponents

are

not

affected

because

of the

low

reac

tance

of C12.

Thus,

if

the

control

is

set

to

the

"flat"

position

the

LA-2A

will

provide

equal

gain

reduction

on

all

frequencies.

If

the

control

is

moved

away

from

the

"flat"

position,

the

leveling

will

be

greater

on

the high

frequencies.

The

actual

setting

can

be

best

determined

on

program

material

for

a

compromise

between

low

and

high

frequency

limiting.

Maximum

high

frequency

response

will

provide

approximately

10 DB

more

reduction

at

15

KC

than

at

frequencies

below

1 KC.

230

VOLT

INPUT

The

LA-2A

is

set

for

a

power

line

voltage

of 115V

when

shipped.

In

order

to

change

this

for

230

volts,

open

the

front

panel

and

make

the

following

changes:

Locate

the

vertical

solder

terminal

strip

at

the

right

hand

end

of the

chassis.

Terminal

1

is

connected

t02,

and

3

is

con-

nected

to 4

for

115V

operation

.

Remove

these

jumper

wires

and

connect

terminal

2 to 3.

This

will

allow

operation

on 230

volts

±10%, 50/ 60

cycles.

LA-2A

(3-66

)

Page

7

'(

[

+20

[

+15

[.

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iIl

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INPUT

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OUTPUT

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PEAK

REDUCTION

CONTROL

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STEREO

VOLTAGE

BALANCE

AMPLIF1ER

[ J

STEREO

(J2AX7)

~

--

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V3

[ 1

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NO

GAIN

REDUCTION

V

OUTPUT

INCREASE

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GAIN

REDUCTION

V

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Figure

1.

-30

-25

-20

-15

-10

-5

o

+5

INPUT

LEVEL

DBM

Typical

Gain

Reduction

Plot

for

Model

LA-2A

Leveling

Amplifier

INPUT

TRANSFORMER

REDUCTION

FREQUENCY

RESPONSE

CONTROL

OPTICAL

ATTENUATOR

T4A

VOLTAGE

AMPLIF1ER

(J2AX7)

VI

CATHODE

FOLLOWER

(12BH7)

V2

GArN

NEGATlVE

FEEDBACK

ELECTRO-

LUMINESCENT

DRIVER

(6AQ5)

V4

OUTPUT

TRANSFORMER

[ I

LA-2A

(3-66)

Figure

2.

Limiter

Block

Diagram

Page

8

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ADDENDUM

-

LA-2A

INSTRUCTION

MANUAL

All

LA-2A

Leveling

Amplifiers

after

serial

number

572

incorporate

a

leveling

slope

control

switch.

The

purpose

of

this

switch

is

to

allow

s e

lection

of

either

limiter

or

compressor

action

in

the

LA

-2A.

Referring

to

Figure

1,

page

8

the

typical

gain

reduction

curve

is

illustrated

for

limiter

operation.

Beyond

the

break

away

point

the

output

does

not

increase

with

an

increase

of

input

l e

vel,

producing

a

plot

which

is

horizontal.

When

the

switch

is

set

for

compressor

operation,

the

output

will

increase

slightly

with

increasing

input

level.

The

resulting

curve

will

have

a

slope

of

approximately

3:1,

which

me

ans

that

an

inCl'

ea

se

of

input

level

of

100%

will

cause

an

output

increase

of

30%.

This

switch

is

located

on

the

chassis

adjacent

to

the

optical

a

ttenuator

(T

4A)

soc

ke

t.

LA-2A

(11-66)

TELETRONIX LA-2A User manual

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