BRUEL & KJAER 1022 - Output Amplifier Section

BRUEL & KJAER 1022

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The

output

voltage

from

the

fixed

oscillator

is

fed

to

the

grid

circuit

of

a

pentode,

the

grid

bias

of

which,

is

controlled,

by

means

o'f a

regulating

amplifier

.

To

obtain

a

higher

degree

of

control

the

working-point

of

the

pentode

is

chosen

on

the

non-linear

portion

of

the

Ia-Eg

characteds

1tic,

near

cut-off.

The

purpose

of

this

circuit

is

to

control

automatically

the

output

power

of

the

Beat-Frequency

Oscillator

by

an

A.F

.

control

voltage.

For

example,

th

e

voltage

from

a

standard

microphone

placed

in

the

sound

field

of

a

loud-

speaker

which

is

fed

from

the

Oscillator.

In

this

case

the

output

power

of

the

Oscillator

will

be

so

controlled

that

a

cons

tant

sound

pressure

is

main-

tained

on

the

standard

microphone.

The

A.F.

control

voltage

should

be

fed

onto

the

screened

socket

marked

COMPRESSOR

INPUT

on

the

front

panel

of

the

Oscillator.

A

variable

potentiometer,

marked

COMPRESSOR

VOLTAGE

is

inserted

in

the

input

circuit

of

the

regulating

amplifier

and

can

be

used

as

volume

control

for

the

output

power

fron1

the

Oscillator

when

automatic

regulation

is

employed.

Care

must

be

taken

,

not

to

use

a

modulated

DC

voltage

as

regulation

voltage

,

as

the

input

circuit

is

directly

coupled.

The

regulating

amplifier

has

a

linear

frequency

characteristic

from

20 to

20ooo

c/s

and

should

have

an

input

signal

of

approximately

o.5

volt

on

the

grid

of

.

the

A.F.

amplifier

tube

for

full

regulation.

The

input

impedance

,

measured

across

the

terminals

of

the

socket

marked

COMPRESSOR

INPUT

i,s

approximately

25 k.Q,

and

the

maximum

obtainable

range

of

regulation

is 50 dB.

The

amplified

A.F.

control

voltage

is

rectified

in

a

full-wave

rectifier,

designed

to

give a DC

output

voltage

prop

,

ortional

to

the

average

value

of

the

A.F

.-co

ntrol

voltage.

By

means

of

the

switch

marked

COMPRESSOR

SPEED

on

the

front

panel

of

the

oscillator

the

regulation

speed

can

be

varied.

Regulation

speeds

of

30-

100- 300

or

lo

co

dB/s

may

be

chosen

by

ehanging

the

Yalue

of

the

resistor

in

the

A-C

filtering

network

for

the

rectified

control

voltage.

When

the

switch

COMPRESSOR

SPEED

is

in

position

"comp.

off."

the

output

from

the

rectifier

is

short-circuited

thereby

disconnecting

the

automatic

regulation

circuit.

To

make

linear

regulation

of

the

output

level

possible,

even

when

maximum

output

power

is

required

from

the

Oscillator

,

the

level

of

the

high

frequency

voltage

from

the

120

kc

/s

fixed

Oscillator

is

raised

approx.

10

dB

when

the

automatic

compression

is

switched

in.

The

anode-circuit

of

the

pentode

in

the

variable-,u

amplifier

is

tuned

to

120

kc

/s,

forming

a

band-pass

filter,

the

output

of

which

is

fed

to

the

mixer.

In

the

mixer

tube,

which

is

one

half

of

a

twin

triode,

the

120

kc/s

voltage

is

mixed

with

the

output

voltage

from

the

variable

oscillator

.

The

frequency

0f

the

variable

oscillator

can

be

altered

continuously

from

120

to

100·

kc/s

by

means

of

a

specially

designed

variable

capacitor.

This

capacitor

is

made

with

a

high

degree

of

accuracy

and

a

maxi

r

mum

deviation

of

o.7

degrees

8