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Revox A77 Service Manual

Revox A77
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12.1.
REVOX
A77-
DOLBY
12.1.1.
Introduction
The
DOLBY
B
Type
Noise
Reduction
System
is
based
on
the
same
principles
as
the
professional
DOLBY
A
System,
but
is
simplified
and
economical
to
permit
its
application
in
consumer
tape
recorders.
The
system
is
designed
to
reduce
tape
hiss
by
increasing
the
level
of
low-level
signals
in
a
variable
range
of
high
frequencies
during
recording,
and
automatically
reducing
the
level
during
playback
of
the
same
locations
in
the
program.
Although
no
change
is
then
heard
in
the
original
signal,
the
attenuation
during
playback
results
in
a
reduction
of
tape
hiss.
The
amount
of
noise
reduction
achieved
is
approximately
10
dB (
DIN
weighting).
The
basic
Dolby
B
circuit
can
be
switched
to
make
it
operate
in
either
the "
record"
or "
play"
mode.
For
many
applications,
therefore,
it is
possible
to
work
with
one
switchable
record-
play
processor
per
channel.
However,
in
order
to
allow
simultaneous
recording
and
monitoring
of
Dolby-
processed
tapes
and
track-
to-
track
transfer
of
Dolby
recordings,
the
Dolby
version
of
the
Revox
A77
is
equipped
with
separate
recording
and
playback
processors
for
each
channel.
Because
of
the
limited
space
available
in
the
Revox
A77,
the
plug-in
power
amplifiers
cannot
be
accomodated
in
the
Dolby
version.
In
order
to
realize
the
full
subjective
improvement
in
signal-
to-noise
ratio
which
the
Dolby
System
can
provide,
it
is
essential
that
noise
and
hum
in
the
program
and
other
components
used
be
substantially
below
the
level
of
the
tape
hiss,
since
these
noises,
like
all
sounds
of
any
kind
which
are
part
of
the
original
program,
will
not
be
reduced
or
audibly
affected
in
any
other
way
by
the
Dolby
System.
Because
the
Dolby
B
System
reduces
noise
only
above
200
Hz,
it
is
particularly
important
that
low-
frequency
noise
and
hum
be
as
low
as
possible.
To
avoid
amplitude
distortion
when
working
with
the
Dolby
System,
the
tape
recorder
must
have
a
very
linear
frequency
response.
12.1.2.
Circuit
Operation
Record
Processor
1.077.853
In
addition
to
the
actual
processor,
this
printed
circuit
board
also
contains
amplifiers
and
filters
for
each
channel
plus
a
calibration
oscillator
which
is
common
to
both
channels.
This
oscillator
is
coupled
to
the
input
of
each
channel
by
separate
1
Mohm
resistors.
Power
is
supplied
to
the
oscillator
when
the
front
panel
switch
is
set
to
the
"calibrate"
position.
The
oscillator
level
is
adjustable
by
means
of
potentiometer
P
103
r
which
is
pre-set
at
the
factory.
The
oscillator
frequency
is
approximately
800
Hz.
The
signal at
input
5(
I
I)
is
amplified
by
01,
02
trom
Y
mV
to
200
mV.
-
34
d8
-
44
d8
-
54
dB
20
I
Record
—
Autnahme
_
Enregistrement
..........._._..._.....p.,Z.'--..."---".
Wren
Stier
Band
Enregistrement-lactu
Playback
Weider*.
Lecture
_
,
50
100
200
500
2k
5k
10
k
te
70
kHz
Low-level
Frequency
Response
Characteristics
of
Record
and
Play
Processors
Fig.
12.1.-1.
From
there
it
passes
through
the
filter
formed
by
Li,
L2,
C3—
C7
which
is
intended
to
suppress
interference
from
multiplex
signals
when
recording
stereo
FM
broadcasts.
Furthermore,
any
unwanted
signals
above
20
kHz (
e.g.,
residual bias
and
erase
oscillator
signals)
are
sufficiently
attenuated
by
the
active
low
pass
filter
built
around
transistor
04
that
they
cannot
upset
the
operation
of
the
dynamic
filter
section
of
the
noise
reduction
circuit.
If
the
multiplex
filter
with
its
cut-off
frequency
of
14.5
kHz
is
not
required,
it
may
be
disabled
by
a
relay
which
operates
on
both
channels.
The
audio
signal
at
the
output
of
the
low
pass
filter
Q4
is
divided
into
a
main
channel
and
a
side
channel.
The
main
channel
feeds
the
signal via
R42
and
the
emitter
follower
Q9
directly to
the
output
1 (
13).
In
the
side
channel,
the
signal
passes
through
the
dynamic
high
pass
filter
formed
by
C11,
R19,
C12
and
the
drain-
source
path
of
FET
05.
When
low-level
signals
are
present,
the
FET
is
non-conductive
and
has
a
high
drain-
source
resistance,
so
that
only
the
first
section of
the
filter
C11,
R
19
controls
the
response
of
the
side
chain.
The
output
from
this
filter
is
then
amplified
by
Q6,
07
and
recombined
with
the
main
signal at
09
via
the
resistor
R
41.
The
gain
of
the
side
chain
is
so
adjusted
that
at —
40 VU,
the
combined
signal
at
the
output
of
09
is
increased
by
10
dB
at
5
kHz.
The
graphs
in
Figure
12.1.-1.
show
the
frequency
response
for
low-level
signals.
The
side-
chain
output
signal
is
also
fed
to
transistor
08
where
it
is
amplified,
and
then
rectified
and
smoothed
by
the
non-
linear
integrator
D4,
D5,
R34,
R36,
C20, C21,
the
result-
ing
D.C.
voltage
being returned
to
the
gate of
FET
05.
12.1.2.

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Revox A77 Specifications

General IconGeneral
Wow and Flutter0.08% (7 1⁄2 ips)
Frequency Response30Hz to 20kHz (7 1⁄2 ips)
Signal to Noise Ratio60dB
TypeReel-to-Reel Tape Deck
Track System4-track, 2-channel stereo/monaural
Reel SizeUp to 10.5 inch reel
Motor2 x reel, 1 x capstan
InputsMic, Line
OutputsLine
Tape Speed3 3⁄4 and 7 1⁄2 ips

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