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INTRODUCTION
This service
manual
is
intended
to
assist trained
and
qualified
technical
personnel
in
verifying
the
performance
of,
adjusting,
and
repairing the
ADCOM
GFA-585
power
amplifier.
The
procedures described
here are
not
intended
for
persons
unfamiliar with the
appropriate safety
and
lest
procedures.
A
WARNING
A
THERE ARE
POTENTIALLY LETHAL
VOLTAGES
WITHIN
THE
GFA-585 AMPLIFIER
WHICH
WILL BE
ACCESSIBLE
ONCE
ITS
TOP
COVER
IS
REMOVED. DO
NOT ATTEMPT
FAMILIARIZATION
WITH,
INSPECTION
OF.
OR ANY PROCEDURE WHATSOEVER UNLESS YOU HAVE DISCONNECTED THE
GFA-585
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 PROVIDED
FOR USE ONLY
BY
COMPETENT
TECHNICAL PERSONNEL.
DO NOT UNDERTAKE ANY
SERVICE
PROCEDURES
IN
THE
GFA-S8S
UNLESS
YOU ARE
TECHNICALLY
QUALIFIED
TO DO
SO.
CIRCUIT
DESCRIPTION
The
ADCOM
GFA-585
is
a
stereo
power
amplifier rated
at0.02%THDfrom20Hz
to
20kHz
with
250
watts
into
8
ohms, and
400
watts
Into
4 ohms.
The
output stage
is
capable
of
greater
than
60
amps
into
low-impedance
loads.
The
amplifier
employs
a
discrete
dual-differential
cascodeClass-A
front-end followed
by
a
dual-cascode Class-A
voltage-gain stage
which
amplify the
input signal to the
voltage requirad
at
the output
of
the
amplifier.
Th<s 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, the
input signal
passes
through network
Cl
01
,
Cl
02,
R1
02,
and
R1
03 which
provide a
3dB
bandwidth
of
O.THz
to
700kHz
to
the 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
Ihe preamplifier.
WE
DO
NOT RECOMMEND THAT
C101
BE SHORTED
OUT.Q101,
0102. 01
03.
and Q1 04
form
one
differentlal/cascode input
stageandQl09,Qil0,Qiii,andOii2formmeomer.Ri06.Ri08.Ri4i.and R144
provide
local
feedback
to
control the
input-stage
open-loop
gain.
Open-loop
gain
is
defined
by R1
05,
Ri
07.
and Cl
03 on
one
side,
and
R1
42,
R1
43,
and Cl
17
on
the
other.
The
next
voltage-gam stage
consists
olOiOS
and
0106
on one
side
and Qi
18
and
01
1
9
on
the
other.
DC
bias
Is
set
by
R1
18,
D1
05,
0106, 0108,
DUO,
D118. Di
19.
R123. R130, and R210. Open-loop
gain
is
defined
byRIIS.
R124, R131, and R210,
with
R120, R135, R313, R413, C301,
and
C403
providing
high-frequency compensation.
Feedback
is
provided from
the output
to
(he
bases
of
0104
and
Q1
12 by
the
network R1
1
2,
R1
14,
and Cl
06.
Cl 06
provides
a high-frequency
rolloff
above 60kHz. DIOI
through
DI
04,
Di07, D109, D120,
DI
21
,0107,
0108,
R1
13,
R1
15,
R1
1
7,
RI
28.
RI
29 and Cl 07
provide
an
anti-
saturation
feedback
path tolhe
input stage.
For example,
when
the
cascode
voltage-gain stage
approaches
saturation
to
the positive supply.
Di
07
begins
to
conduct,
turning
01
07
on.
Current
flows
through D802,
the
INSTANTANEOUS
DISTORTION
ALERT
LED,
through Di
04and
Ri
1
7.
When
the
error
voltage
across
D101/D102 becomes
great
enough.
D101
begins
to
conduct,
forcing
the
input
stage
to
reduce
the drive
to
the voltage-
gain
stage.
The
input
stage
is
biased by Ri
46
through R14S,
R201
through
R207,
0113
through
01
17,
0502, D1151hrough Dll
7,
and D801.
0502
is
turned
on
by
the amplifier
bias-delay
circuitry
on
the
power
supply board. Acurrent
of
about
2mA
flows
through
the
thermal breakers
on
the
heatsinks
and
into
the
emitter
of
01
17
and
0167
(on the other
channel).
If
the
heatsinks overheat, the breakers
open and
the current flows
through
D801
,
the
THERMAL PROTECTION
LED
instead.
When
0117
is
carrying Ihe
current,
D117
is
biased
at
2.5V.
This creates about 1.8V across R202.
0116
then
sources
about
2mA
to
Di
1
5,
which develops about 2.2V across
it.
About
1
.6V
is
developed across
RI
46 and
RI
48.
01
1
conducts about
1
.3mA
to
D1
1
6
and
Qt
13 conducts about
3mA
to
Ihe
PNP
differential
input stage.
D1
1
6
develops about
2.2V across
it,
causing about
1.6V across
R147.
Q1
14 conducts aboul
3mA
to
the
NPN
differential
input stage.
If
the
negative
80V
supply
falls
or
its
fuse
opens,
0117
turns
off,
turning
of!
all
the
amplifier
bias
circuitry.
If
the
positive
80V
supply
fails
or
its
fuse
opens, again
0117
turns
off
and
the bias
circuitry
is
disabled.
Using
LEDs
for
the bias
circuitry
causes rough
temperature
compensation
of
the
amplifier
Class-A DC-blas
current.
Any
DC
imbalance
in
the amplifier
is
corrected by
R110, R111, C104, Cl
05,
and
IC101.
Any
DC
error
at
the
amplifier
output
is
servoed back through
1C
101
to
adjust (he
DC
current
through
the
input transistors.
DC
bias
is
nominally
1
5mA
through
0103, 0104, 0109, and 0112.
IC101
can
modify
this
by up
to
0,3mA
to
bring the amplifier
into
balance.
The
bias
network
of
RI
22,
RI
25,
RI
26,
R127,
R132,Q1
20,
and 0401
form
alemperalure-compensated DC-bias
voltage
to
the input
of
the triple-Oarlington-follower
output stage.
Mid-
and
high-frequency bypassing
is
provided by
Cl
1 1
R134
and
Cl 13
provide
a
load
for
the amplifier
at
high frequencies,
stabilizing
the amplifier
under
varying load
conditions.
D301 and D401
provide
a
high current
return
to
the
power
supply
for
backlash
current
from
the
load.
1
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