TELETRONIX LA-2A Manuale utente

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LA-2A

(3-66)

LEVELING

AMPLIFIER

MODEL

LA-2A

INSTRUCTION

MANUAL

TELETRONIX

ENGHr2:B!RING

CO

•

. 308

MON~ER~1Y

ROAD

SOUTH

PASADENA,

CALIFOmaA.

www.SteamPoweredRadio.Com

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TELETRONIX

MODEL

LA-2.

11

..

LE7ELING

AMPLIFIER

INTRODUC':'I

ON

The

Teletronix

Leveling

Amplifier

will

automatically

lir.~t

audio

pea.ks

which

might

otherwise

over

drive

broadcast

~~

recording

equipment.

Automatic

gain

reduction

is

accomplished

by

the

use

of

an

electro-optical

vo.rio.ble

attenuator,

which

is

placed

a.head

of

the

first

amplifier

stage.

The

uttenurition

is

con-

trolled

by

the

amplitude

of

the

LA-2A

input

signal.

This

system

permits

up

to

40

DB

of

instantaneous

gain

r

.0

duction,

yet

co.uses no wave

form

or

harmonic

distortion.

The

amplifier

provides

sufficient

gain

und

output

level

(10

DBM

nominal)

to

be

used

as

a.

line

or

program

amplifier,

or

for

direct

connection

to

the

transmitter

in

the

case

of

radio

or

T.V.

operation.

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

broadco.sting.

l.

Guin

Reduction:

2.

Distortion:

:>.

Response:

.

Noise:

l.'.

•

t"

Ga.in:

,,,

.

6 ,

·

Output

Level:

Input

Level:

LA-2A

(J-66)

SPECIFICATIONS

up

to

40

DB.

less

than

0.35%

total

harmonics

o.t

+10

DBM,

o.nd

less

thc.n •

75f{,

total

harmonics

at

+16

DBM.

±0,1

DB

30

cycles

to

15

KC.

75

DB

below

+10

DBM

output

level

40

DB

zl

DB

+10

DBM

nominal

+16

DBM

maximum

pea.ks

+

16

DBM

maximum

P

o.

ge

-2-

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t,ta1Jk

Time:

9Q

Release

Time:

10.

Input

Impedance:

11.

Output

Impedance:

essentially

instantaneoua

C~J

micro-sec.)

approximately

0.06

seconds

for

50~

release,

0.5

to

5

seconds

for

com-

plete

release

depending

upon

amount

of

50,

150,

250, and

600

ohms.

balanced

or

unbalanced.

50,

150,

250, and 600 ohms.

balanced

or

unbalanced.

12.

··

output

Source

Impedance:

approximately

150 ohms.

at

800 CPS.

13.

Panel

Size:

14.

Depth

Behind

Panel:

15'.

Panel

Controls:

16.

Meter:

17.

Power

Requirements:

18.

Tube Complement:

19.

Fuse:

standard

19"

x

5¼"

7

,1.tt

. 4

Gain

(input

level),

Peak

Reduction

and

Meter

Selector

Switch.

DB

Gain

Reduction

and

DB

Output

115/230

volts

50-60

cycle

35

watts

(2)

12AX7A

(1)

12BH7A

(

l)

6AQ.5

JAG,

3/8

AMP

Slow Blow

for

115V;

JAG

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

br·eakaway

point

at

-30

DB

input

and up

to

-20

DB,

at

which

point

the

curve

becomes

horizontal

to

exhibit

limiting

action.

The

.

!nput

increases

an

additional

20

DB,

but

the

output

increases

less

than

l

DB.

The

leveling

amplifier

thus

combines

the

ohar-

act~ristics

ot

a

compressor

and

limiter.

A

reasonable

amount

~r

care

in

gain

riding

will

restrict

normal

operation

to

the

compression

region,

but

uncontrolled

output

levels

will

be

p~evented

by

the

limited

action.

LA-2A

(3-66)

,._.

Page

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~e

ilea.rt

of

t~1.e

leveling

amplifier

is

the

Eila

~

t1

::,

~

q

:d

:/

.0c

.l

3.-t,

tenuator

which

is

placed

ahead

of

the

first

a."'!'ln~

_

i.fi

e::-'

::.

t.P.

6

e.

The

actual

stage

gains

and

tube

operating

paramet

e

rs

,

.:.re

not

varied,

permitting

the

tubes

to

operate

at

optimun

e:

.

ondi

tions

regardless

of

the

amount

of

gain

reduction.

The

optical

attenuator

consists

of

a

photo

conductivP.

cell,

which

is

optically

coupled

to

an

electro-luminescent

light

source.

The

electro-luminescent

device

provides

a

light

intem;i

ty

which

is

proportional

to

the

audio

voltage

applied

to

its

terminals.

Not

unlike

a

capacitor

in

construction,

the

electro-luminescent

lamp

consists

of

a

plate

of

glass

or

plastic

coated

with

a

clear

conducting

material

on

one

.

side

an~

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

phosr

~·

phors

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

controlling

element

is

the

photo

conductive

cell.

The

resistance

of

the

cell

decreases

with

an

increase

in

the

impinging

light.

Since

the

light

is

produced

directly

from

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

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

5o%

release,

and

then

a

gradual

release

over

a

period

of

1

to

15

seconds

to

the

point

of

complete

release.

Refering

to

Figure

2,

the

functional

block

diagram,

t

he

imput

signal

is

applied

directly

to

the

optical

attenuator

from

the

high

impedance

winding

of

the

input

transformer.

The amount

of

attenuation

introduced

by

the

optical

attenuator

is

controlled

by

the

audio

voltage

applied

to

it

by

the

6AQ5

(V4)

luminescent

driver

amplifier.

The amount

of

signal

applied

to

the

12AX7

(V

l)

voltage

amplifier

is

also

controlled

by

the

manual

gain

control

.

The

voltage

amplifier

stage

provides

a

gain

of

40

DB.

Over

a

ll

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

is

used.

This

output

stag

e

can

tolerate

great

amounts

of

output

impedance

mismatch,

but

1·etains

low

distortion

and

flat

frequency

response.

:

·6:'

·

.-:For

sterea

-~br.oadctµ1t,-.g··

a.pplmcat!ons

.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

LA-2A

(J-66)

Page

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to

the

stereo

bo.ln.nce

control

und

is

also

brought

out

tr;

a.

terminal

on

the

cha.sis.

For

stereo

operation

this

tern.-

inn.~.

is

connected

to

the

some

terminal

on

a.n

identica.l

a.mpli-

.::i,

:;;

r and

the

control

vol

to.ge becomes

common

to

both

uni

ts.

A

g,1

L1••reduction

control

voltage

generated

in

either

o.mplifier

will

co.use

equ~l

gain

reduction

in

both

units.

The

control

T.-

•c,l

toge

is

applied

through

the

stereo

bu.lo.nee

control

to

the

t·A

(c

~5

driver

amplifier.

This

sto.ge

provides

the

necessary

voltage

to

operate

the

electroluminescent

light

source.

OPERATION

The

LA-2A

Leveling

Amplifier

is

designed

to

prevent

o.n

increase

in

output

level

beyond

a.

pre-determined

point,

and

due

to

its

unique

design,

functions

a.s

a.

combined comp-

ressor

and

limiter.

The

effect

is

illustrated

in

Figure

l.

The

point

at

which

the

compressed

curve

breaks

a.way

from

the

straight

"No-Ga.in

Reduction"

line

is

determined

by

the

setting

of

the

"Peak

Reduction

11

control.

It

ca.n

be

seen

from

the

curve

tho.t

compression

occurs

o.nd

gro.duo.lly

increa.ses

over

the

first

10

DB

of

input

level

rise.

The

slope

of

tho

curve

then

becomes

horizontal

preventing

an

increase

of

output

level

re-

gardless

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

progrrun

material.

Setting

of

the

"Go.in" and "Peo.k

Reduction"

controls

are

independent.

However,

the

"Go.in"

control

should

be

set

to

provide

sufficient

output

a.f'ter

the

"Pea.k

Reduction"

oon-

trol

ho.s

been

adjusted.

The "Peo.k

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

dynrunic ro.nge

of

music.

Maximum

benefit

is

obtc.ined

by

running

4

to

8

DB

of

compression

continuously.

Thir,

will

usually

cause

full

limiting

to

occur

when 100% mod-

ulation

is

upproo.ched.

For

ease

of

control

and

to

prevent

overload

of

th

o

input

transformer,

sufficient

fixed

po.d

should

· be

pluccd

cch

cc.d

·

of

the

LA-2A

to

allow

normal

output

a.t

approximately

5o%

set

..

ting

of

the

go.in

control.

LA-2A

{3-66) Page

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I

The

VU

Meter

servos

two

functions:

it

indica.tes

out-

t:u:

L.

level

a.s

well

a.s

go.in

reduction

directly

in

DB.

When

the

.

11

c1

tor

selector

switch

is

pla.ced

in

the

"Output"

position

the

meter

will

indicate

output

level

a.cross

the

600

chm

terminals.

'

rhe

meter

is

culibra.ted

to

rend

O

VU

or

loo% when

the

amplifier

o-..1tput

if

+10

DBM

or

+11-

DBM,

depending

upon

the

switch

position.

The

position

marked

11

Ga.in

Reduction"

permits

the

meter

to

indica.te

the

a.mount

of

go.in

reduction

or

pea.k

limiting

directly

in

DB.

During

periods

of

no go.in

reduction

the

pointer

will

return

to

O

VU

on

the

meter

sea.le.

The

pointer

is

initia.lly

set

to

this

position

by mea.ns

of

the

screw

driver

a.djusted

pot

loca.ted

on

the

left

end

of

the

front

pa.nel.

STEREO

If

two

LA-2A

Amplifiers

a.re

to

be

used

in

tandem

stereo,

the

go.in

reduction

of

ea.ch

amplifier

ca.n

be

ma.de

regardless

of

which

cha.nnel

is

instigating

the

limiting.

is

accomplished

by

inter-connecting

terminals

6

a.nd

7

or

LA-2A

Amplifiers.

The

inter-connecting

wire,

should

not

over

two

feet

in

length

a.nd

should

be

shielded,

the

ends

shield

being

connected

to

tho

#7

terminals.

(ground.)

Stereo

"set-up"

is

as

follows:

for

equa.l,

This

the

be

of

tho

1.

Connect

the

input

terminals

of

the

left

and

right

channel

LA-2A

•s

to

a.n

a.udio

oscillator.

Ma.kc

sur

e

that

the

amplifiers

a.re

connected

in

phase

to

the

genera.tor.

A

genera.tor

frequency

of

400

or

1000

cps.

is

sa.tisfa.ctory.

Generator

output

level

should

be

set

to

the

o.vera.ge

level

to

be

applied

in

operation.

2.

Place

the

meter

selector

switches

in

the

+10

or

+4

position

a.nd

a.djust

the

LA-2A

"Ga.in"

controls

!'or

equo.l

output.

The "Pea.k

Reduction"

controls

must

be

set

to

·

full

counter-clockwise

or

off.

J.

Ma.ke

sure

thnt

the

screw

driver

o.djust4

pots

(R3)

on

the

rear

of

ea.ch

unit,

a.re

"full

on".

(clockwise)

Pla.ce

ea.ch

meter

selector

switch

in

the

"Ga.in

Reduction"

position.

4.

Advance

the

"Go.in

Reduction"

control

on

the

left

chrinne2.

omplifier

until

o.pproximo.tely

5

DB

of

reduction

is

indic-

ated

on

the

meters.

Note

which

cha.nnel

is

indicating

the

most

go.in

reduction.

Reduce

the

setting

of

R3

on

this

unit

until

both

meters

show

equal

reduction.

LA-2A

(3-66)

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5.

The

"Gain

Reduction"

controls

can

now

be

placed

at

any

desired

setting,

keeping

both

knob

settings

equal.

Gain

Reduction

will

now

be

equal

on

both

channels.

G~IN

REDUCTION FREQ.UENCY' RESPONSE

·uoNTROL

FM

broadc

·

asting

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

micro-second

network.

The

program

frequencies

in

the

vicinity

of

15

KC

will

modulate

the

carrier

17

DB

more

than

frequencies

below

1

KC.

Thus,

if

the

program

material

contains

a

large

amount

of

high

frequencies,

over-modulation

may

occur

if

the

levels

had

been

previously

adjusted

for

program

material

with

less

high

frequency

content.

An

attempt

to

alleviate

this

problem

has

been

made

by

others

in

the

form

of

level

controlled

high

frequency

cutoff

filters

and

high

end

peak

clippers.

Because

the

-amount

of

control

over

the

remainder

of

the

spectrum

is

limited

and

because

of

the

high

distortion

created,

such

devices

have

found

only

limited

application.

The

Teletronix

Leveling

Amplifiers

are

capable

of

at

least

30

DB

of

gain

reduction

or

limiting

with

less

than

0.5%

harmonic

distortion.

For

most

applications,

such

as

AM

broad-

casting

and

recording,

the

amount

of

gain

reduction

is

a

function

of

input

level,

and

is

independent

of

frequency.

By

increasing

the

gain

reduction

at

the

higher

frequencies,

the

over-modulation

caused

by

the

pre-emphasis

can

be

greatly

reduced

or

eliminated.

While

it

is

possible

to

increase

the

gain

reduction

sensitivity

on

an

inverse

of

the

pre-emphasis

curve,

this

usually

results

in

an

insufficient

leveling

on

the

low

frequencies.

The

actual

amount

of

the

limiteflpre-emphasis

must

be

determined

according

to

the

amount

of

high

frequency

content

in

the

program

material.

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

components

are

not

affected

because

of

the

low

reactance

of

Cl2.

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

determin

e,-:

on

program

material

for

a compromise

between

low and

high

frequency

limiting.

Maximum

high

frequency

response

will

provid

e

approximately

10

DB

more

reduction

at

15

KC

than

at

frequencies

below

l

KC.

.

..

LA-2A

(3-66)

Pago

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IMPUT

The

LA-2A

is

set

for

a

power

line

voltage

of

115

V 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

to

2 and 3

is

connected

to

4

for

115

V

operation.

Remove

these

jU?pper

wires

and

connect

Terminal

2

to

3.

This

will

allow

operation

on 230

volts

±_10%,

50/60

cycles.

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