Adcom GFA-565 Service Manual page 4

R158
and Cl
11
provide a
load for
the amplifier
at
high frequencies,
stabilizing
the amplifier
under
varying load
conditions
D201 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
1 0
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 0.33-ohm
ballast resistors
for
current sharing
and
bias
stability.
The
AC
input
power supply board includes
a
power-on
delay
relay
to
reduce the turn-on current surge
in
the
AC
power
line,
jumpers
to set
the
supply voltage range,
and an
amplifier
bias delay.
CAUTION
DO NOT
use any
type
of
variable
AC
supply,
such
as
a variac,
to
slowly turn
on
the
G
FA-565
without
first
shorting
R506
(4.7
ohm/20
watt).
R506
is
an
integral part of the turn-on,
inrush-suppression
circuit
and
failure to
comply
will
burn out
R506.
TEST
PROCEDURES
All
tests
are performed
with a
1
20V,
low-distortion (less
than
2%
THD),
AC-power
source,
8-ohm
resistive
load (except
slew
rate),
and
a signal source
of
not
more
than
600 ohms.
Tests are
performed
after
warming
up the amplifier
at
100
watts
into
an
8-ohm
load
for at
least
10 minutes.
All
grounds
during testing are referred
to
the
ground
of
the black output
terminal.
80kHz
low-pass
filter is
employed
during
THD
distortion
measurements.
Signal-to-noise
measurements
are
A"
weighted.
Damping
factor
is
measured
by
comparing
the
20-watt-output voltage
with
and
without
an
8-ohm
load.
Slew
rate
is
measured
with
an
inductive load,
and
is
derived with a
dual-time-based
oscilloscope reading the slope
of
a
full-power
(135V
peak-to-peak)
5kHz
square wave.
To
avoid
damaging
output
network
R158 and C121,
DO NOT
OPERATE THE
AMPLIFIER AT
FULL-POWER
SINE
WAVE ABOVE
22kHz
OR FULL-POWER
(135V
PEAK-TO-
PEAK)
SQUARE WAVE ABOVE
5kHz.
IMPORTANT
BEFORE PROCEEDING
WITH ADJUSTMENTS,
MAKE SURE
AMPLIFIER
IS
AT
ROOM
TEMPERATURE.
BIAS
ALIGNMENT
1
.
With set-up as per
the
first
paragraph
of
TEST
PROCEDURES
and
with
NO SIGNAL
IN,
set bias control
(
R
1
43) to
midpoint.
2.
Connect
a
millivolt
meter across
TP201 and TP301
3
Turn
amplifier
on
and
allow a 3
to
5
minute
settling
period.
4.
Adjust
BIAS
control to obtain either
a
+ or-24mV
(±1
mV)
indication
on
the millivolt
meter.
5.
To check
for
proper
bias setting,
remove
millivolt
meter and apply
input signal
to
obtain
1
00
watts
into
8
ohms
for
1
minutes
with
cover
on.
6.
Remove
input signal
and connect
the
millivolt
meter as
in
Step
2.
Let amplifier
idle until
bias stabilizes
and
readjust
to
24mV
(±1 mV).
2