Revox A77 Service Manual

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121

-

31,113

—42dB

k

5k

10

k

131

2 k 5 k

10

k

20 kHz 20 50

100

200

000

Record

Processor

Characteristics

Fig.

12.1.-2.

If

a

higher

audio

signal

causes

the

D.C.

voltage

on

the

gate

to

exceed

the

bias

point of the

source —

which

is

set

with

P

101

r(law

control) —

the

drain-

source

path

will

become

conductive.

This then

results

in

a

change

of

the

turn-

over

frequency

of

the

second

high

pass

branch

formed by

C12

and

the

drain-

source

resistance.

With

increasing

input

level,

the

proportion

of the

side

channel

signal

added

to

the

main-

channel

signal

will

become

smaller,

resulting

in

a

frequency

characteristic

as

shown

in

Figure

12.1.-2.

At

0

VU,

the

side

channel

component

has

become

so

small

that

the

main

signal

will

appear

practically

unaltered

at

the

output,

that

is, it

has

a

linear

frequency

characteristic.

The

variable

filter

action

is

shown

in

Figure

12.1.-3.

It

shows

the

amplitude

characteristics

of

test

signals

at

differ-

ent

frequencies

and

at

a

low

level (-

42

VU,

which

is

below

the threshold

of

the

compressor)

in

the

presence

of

0

VU

signals

of

various

frequencies

which

drive

the

compressor

into

gain

reduction.

When

comparing

this

with

a

filter

of

fixed

bandwidth,

one

will

note

that

marked

noise

reduction

is

obtained

at

high

frequencies

in

the

presence

of

high-level

signals

of

lower

frequency.

A

two-

stage

non-linear

integrator

is

used

in

the

side

chain

to

avoid

the generation of

modulation

products

on

transi-

ents.

For

small

signal

level

variations,

diode

D5

is

non-

conducting,

so that

the

time

constant

of

the

RC

combination

R39,

C24,

provides

considerable

smoothing

effect.

On

a

sudden

increase,

however,

D5

becomes

conductive

and

quickly

lowers the

gain

of the

side

chain.

For

large

amplitude

transi-

ents,

the

attack

time

will

be

reduced

to

1.5

milliseconds,

approximately.

To

avoid

overshoots

in

the

output

during

this

interval,

two

clipping

diodes

D2, D3,

are

provided

which

respond

to

extreme

transients

only

until

the

side

chain

has

returned

to

its

original

operating

point.

As

a

result,

the

signal

appearing

at

the

output

consists

of

a

large

amplitude

undistorted

main

signal

mixed

with

a

small

amplitudeclipped

side

chain

signal.

The

distortion

of

the

output

signal

is

therefore

small,

and

as

the

effect

lasts

for

so

short

a

time,

inaudible.

(4)

(5)

161

20

kHz

Low-level

Frequency

Response

Characteristics

of

Record

Processor

under

High-level

Signal

Conditions

Fig.

12.1.-3.

To

prevent

the

erase

oscillator

from

interfering

with

the

control

circuit,

the

gain of

08

is

greatly

reduced

at

the

frequency

of

the

erase

oscillator

by

the

parallel

resonant

circuit

L3,

C22.

Capacitors

C19

and

C20

give

additional

attenuation.

Playback

Processor

1.077.852

The

same

basic

circuit

is

used

for

the

playback

processor,

but

instead

of

the

side

chain

being

fed

from

the

input,

it

is

fed

from

the

output.

Furthermore,

the

phase

of

the

output

signal (

and

hence

of

the

side

chain

signal )

is

opposite

to

that

of

the

input

so

that

the

side

chain

forms

a

negative

feed-

back

element,

and

produces

replay

characteristics

which

are

complementary

to

the

record

characteristics.

The

playback

processors

for

both

channels

are

contained

on

one

plug-in

printed

circuit

board.

In

addition,

this

board

contains the

VU

meter

amplifier

09,

used

for

matching

the

the

100

m V

levels

to

the

VU

meters.

Potentiometer

P

103

p

adjusts

the

gain

of

that

stage.

The

signal

arrives

from

input

9 (

13 )

and

is

amplified

by

Q1,

Q2,

Q3

from

40

mV

to

the

Dolby

Level

of

100

mV.

The

voltage

divider

formed

by

R14,

R15,

matches

the

output

signal

to

the

A77

internal

operating

level

of

9

mV

which

then

appears

at

point

11 (

14).

In

order

to

be

able

to

work

without

Dolby

noise

reduction,

the

side

chain

signals

can

be

shorted

out

by

Q4,in

the

play-

back

processor

and by

Q3

in

the

record

processor.

Both

devices

are

activated

by

the

Dolby "

In-

Out-"

switch

eon

the

front

panel

of

the

recorder.

u

u

Li

u

12.1.2.

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Revox A77 Specifications

General

Wow and Flutter	0.08% (7 1⁄2 ips)
Frequency Response	30Hz to 20kHz (7 1⁄2 ips)
Signal to Noise Ratio	60dB
Type	Reel-to-Reel Tape Deck
Track System	4-track, 2-channel stereo/monaural
Reel Size	Up to 10.5 inch reel
Motor	2 x reel, 1 x capstan
Inputs	Mic, Line
Outputs	Line
Tape Speed	3 3⁄4 and 7 1⁄2 ips

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