TECHNICAL
DESCRIPTION
©
Cyrus
Audio
Ltd
Jun
2014
9
Cyrus
6
DAC,
Cyrus
8
DAC
service
manual
Issue
1
Volume
control
The
output
signal
from
the
input
selector
buffer
(IC103)
is
coupled
to
the
input
of
the
digitally
controlled
volume
IC
(IC100).
The
output
of
the
volume
control
IC
is
then
fed
to
the
preamplifier
output
and
the
power
amplifiers.
Power
amplifier
stage
The
power
amplifier
is
a
discrete
voltage
feedback
design.
RV201
allows
adjustment
of
the
output
transistor
bias
current.
T208
and
T210
are
the
driver
transistors.
T206,
T208
and
T210
are
bolted
together
to
ensure
good
thermal
coupling.
T209
and
T211
are
the
output
transistors.
DC
offset
at
the
output
of
the
amplifier
may
be
set
under
conditions
of
no
signal
by
adjusting
RV203.
Once
set,
DC
working
conditions
are
stabilised
within
the
power
amplifier
by
a
DC
servo
(IC200),
measuring
the
DC
offset
at
the
amplifier
output
and
adjusting
the
bias
voltage
applied
to
T202B
to
compensate.
Power
amplifier
standby
control
is
achieved
via
the
CCS
point.
With
this
line,
the
microcontroller
can
switch
the
power
amplifiers
on
or
off
via
the
level
shifting
and
buffer
circuit
on
the
‘Charge
Pump’
schematic
sheet.
Headphones
The
drive
for
the
headphones
comes
from
a
dedicated
power
amplifier
IC901.
The
headphone
amplifier
is
muted
by
the
microcontroller
when
the
power
amplifiers
are
on
and
activated
when
the
power
amplifiers
are
off.
The
PHONES_IN
signal
enables
the
microcontroller
to
detect
when
a
3.5mm
jack
plug
is
inserted
in
the
headphone
socket.
Power
supply
The
front
panel
control
circuit
(with
the
exception
of
the
display
backlight)
is
powered
from
a
small
power
transformer
mounted
on
the
main
PCB.
When
exiting
standby,
the
control
system
switches
relay
RL401,
applying
mains
power
to
the
main
power
transformer.
This
enables
the
control
system,
including
microcontroller,
to
consume
very
little
power,
particularly
when
set
to
Standby.
Power
for
the
display
backlight
and
the
digital
input
PCB
is
provided
separately
from
a
low
voltage
winding
of
the
main
power
transformer.
The
power
supplies
for
the
main
amplifier
(
VCC)
are
42V
(Cyrus
8
DAC)
or
39V
(Cyrus
6
DAC).
These
are
protected
by
fast
blow
fuses
FS401
and
FS402
on
the
main
PCB.
In
addition
a
regulated
42V
is
derived
from
a
charge
pump
circuit
and
supplies
the
low
current
section
of
the
power
amplifiers.
A
regulated
power
supply
of
15V
is
derived
from
the
power
supply
for
the
main
amplifier
via
regulators
VR402
and
VR403.
The
15V
supplies
are
used
for
the
input
selector,
buffers
and
parts
of
the
protection
circuits.
5V
is
derived
from
the
15V
supplies
for
the
volume
control
IC100.
