Adcom GFA-555II Service Manual page 2

INTRODUCTION
This service
manual
is
intended
to
assist trained
and
qualified
technical
personnel
in
verifying
the performance
of,
adjusting,
and
repairing
the
ADCOM
GFA-555II power
amplifier.
The
procedures
described here are
not intended
for
persons unfamiliar with the
appropriate safety
and
test
procedures.
A
WARNING
A
THERE ARE POTENTIALLY LETHAL VOLTAGES
WITHIN
THE
GFA-555D AMPLIFIER
WHICH
WILL BE
ACCESSIBLE
ONCE
ITS
TOP COVER
IS
REMOVED.
DO NOT ATTEMPT
FAMILIARIZATION, INSPECTION
OR ANY PROCEDURE WHATSOEVER UNLESS YOU HAVE DISCONNECTED THE
GFA-555I1
FROM
THE
WALL AC OUTLET
OR
OTHER SOURCE OF AC
POWER AND
THE POWER-SUPPLY CAPACITORS ARE
COMPLETELY
DISCHARGED. PLEASE TAKE NOTE THAT THE
POWER-SUPPLY CAPACITORS
TAKE AS
LONG
AS
5
MINUTES
TO
DISCHARGE.
THESE INSTRUCTIONS ARE PROVIDEO
FOR USE ONLY
BY
COMPETENT
TECHNICAL PERSONNEL.
DO NOT UNDERTAKE ANY
SERVICE
PROCEDURES
IN
THE
GFA-555D
UNLESS YOU ARE TECHNICALLY QUALIFIED
TO
DO
SO.
CIRCUIT
DESCRIPTION
The
ADCOM
GFA-555II
is
a stereo
power
amplifier rated
at
less
than
0.04%
THD
from
20Hz
to
20kHz
with
200
watts into 8
ohms and 325
watts
into
4 ohms.
The
output
stage
is
capable
of
greater than 60
amps
into
low
impedance
loads.
The
amplifier
employs
a discrete
differential
Class-A
front-
end
followed by
a Class-A voltage gain stage which
amplify the
input signal to
the voltage required
at the
output
of
the amplifier.
This
high-voltage
signal drives the high-current
triple-
Darlington-follower output stage
which
amplifies
the
current
by a
factor of about 50,000.
Referring
to
the
accompanying
schematic, describing the
Left
Channel only, the input signal passes through network
Cl
01,
Cl 02 and
R103
which provide a 3dB bandwidth
of
1.7Hz
to
500kHz
to
fhe input
of
the amplifier.
Cl 01
is
an
extremely
high quality capacitor
and
serves to protect the amplifier
and
the speakers connected
to
it
from
DC
faults at the
output of the
preamplifier.
WE
DO NOT RECOMMEND THAT
C101
BE SHORTED
OUT. Q101 and 0102
form
the
differential input
stage
Open-loop
gain
is
defined
by R1 05 and
the bias current through
Q1
01
and 0102. The
small-signal gain
is
approximately
825/(2x25)
=
1
6.
The
next voltage gain stage consists
of
Q1 07
with
Q1 08 as
a
current-source
load.
DC
bias
is
set
by R1
1
6,
D103
and
D104 Open-loop
gain
is
defined by
R112 and
R113,
with
R201,
R301, Cl
05,
C201, and C301
providing
high-frequency
compensation.
Feedback
is
provided from the
output to
the
base
of
Q102
by the
network R123,
R124
and Cl
06.
Cl 06
provides
a
high-frequency
roll-off
above 200kHz, improving
stability
by taking
high-frequency feedback before the
triple
Darlington.
The
input
stage
is
biased by Rt
08,
R1
09,
R1
1
0,
R1
22,
R1
1 5,
R1
1 4,
R1
28,
R1
1 6,
Q1
03,
Q1
05,
Q1
08,
D1
01 through D1
05,
and
the
overtemperature
LED, D903. Q1 05
is
turned on
when
the
B+
supply
is
on.
A
current
of
about
4mA
flows through
the
thermal breaker on the heatsink
and
into
D
1
03 and
D1 04.
If
the
heatsink overheats, the
breaker
opens
and
the current
flows through D1
05 and
the
THERMAL
OVERLOAD
LE
D
instead.
When
the breaker
carries the current,
D1 03 and
D
1
04
are
biased
at
1.4V.
This creates about 0.7V across R114;
Q103
then sources about
2mA
to
Q101 and 0102,
the
differential input
stage.
If
the negative supply
fails
or
its
fuse opens,
Q103
saturates,
0101
turns
off,
turning
off
Q107,
D301 turns
on and
Q1 08
saturates. This
holds the
input to the triple
Darlington
to
near ground.
If
the positive
supply
fails
or
its
fuse
opens,
Q105
turns
off
and
the bias
circuitry
is
disabled.
Any
DC
imbalance
in
the amplifiers corrected
by R125.R1
26,
R127, Cl
07,
Cl lOandICIOI.
Any
DC
error
at
the amplifier
output
is
servoed
back
through IC101
to
adjust the
DC
current through the inputtransistors.
DC
bias
is
nominally
1
,0mA
through
Q101 and Q102.
IC101 provides
the
DC
bias current
to
Q101 and can swing
from
ground
to
+10V
to
bring the
amplifier
into
balance.
The
bias
network
of
R1
1
7 through R1
1
9 and
Q307
form a
temperature-compensated DC-bias voltage
to
the input of the
triple-Darlington-follower output stage.
Mid-
and high-frequency
bypassing
is
provided
by Cl
04.
R901 and C901 provide a load
for
the amplifier
at
high frequencies,
stabilizing
the amplifier
under varying load
conditions.
0201 and D301 provide a high-current
return to
the
power
supply
for
backlash current
from
the
load.
The output stage consists
of
two
sets
of 4
parallel transistors
operated as
emitter followers,
driven by another pair of
emitter followers.
This
configuration
minimizes distortion
caused by varying load
impedances.
The
output transistors
have 0.22-ohm
ballast resistors to
ensure
current sharing and
bias stability.
1