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SECTION
IV
THEORY OF OPERATION
The
following descriptions
of
the
circuitry
in
the
amplifier
are presented
so
the
professional
user
may
have a
general understanding
of
how
the amplifier works.
They
are
not
intended as
a
guide
for service.
Service
on
this
product should
be performed
only
by
qualified
technicians.
THERE ARE
NO
USER
SERVICEABLE PARTS
INSIDE
.
In
the
following descriptions
the
component
designations
for
Channel
A
are used. Examination
of
the
accompanying
schematic
will
aid
in
understanding
the
following
circuit
descriptions.
4.1
INPUT DIFFERENTIAL AMPIIFIEB
The
input
differential
amplifier
is
comprised
of
IC1
C
and
D
and
associated
components.
Input signals
are
coupled
to
the
amplifier
after
passing through
one
of
three
connector types
and
passing across
RF
suppression
capacitors.
Two
amplifier
sections are
used as
unity
gain
inverters with
the output
of
one
summed
to
the
Input
of
the other
to
improve
common mode
cancellation.
Signals
which
appear
equally
on
both
Input terminals,
such as
common mode
noise
arid
hum,
will
be
cancelled.
Use
of
the
Inverting
mode
in
both sections
assures
high
speed,
good
common
mode
rejection
and
equal
impedance
at
both
input terminals.
This gives
a
substantial
advantage
over
less
expensive
single amplifier
topologies.
4.2
MODE
SWITCH
The
rear
panel
mode
switch allows the amplifier
to
operate
in
the Stereo,
Dual
Mono
or
Bridged
Mono
modes.
In
the
Stereo
mode, each
channel operates
normally.
In
the
Dual
Mono, Channel
A
input
is
routed
to
both
Channel
A
and
B
differential
amplifiers.
In
the
Bridged
Mono
mode,
the
Channel
A
input
goes
to
the
Channel
A
differential
amplifier,
and
is
simultaneously
inverted via
the
mode
switch
and
fed
to
the
Channel
B
differential amplifier.
Bridging the load
across outputs
A
and
B
results
in
twice the voltage
across
the load
and
four
times
the
power
as
compared
to
one
channel
driven
into
the
same
load.
The minimum
load
impedance
in
this
configuration
is
8 ohms.
4.3
POWER
AMPLIFEB
The JBL/UREI
Model 6290
Power
Amplifier
employs
discrete transistor
circuitry
in
the voltage
amplifier
and
output
stage.
A
symmetrical topology
was
chosen
to
take
advantage
of distortion
cancellation effects
and
equal
group
delay
for
each
half of
the
signal.
This allows simple
compensation, exceedingly wide open-loop frequency
response
and
excellent transient Intermodulation
performance.
Q20-23
are configured
in
matching
differential
amplifiers
loaded by
the emitters
of
Q7
and Q9. Note
that
no
inverting amplifiers
are used.
All
transistors
are
used
in
current-
mode,
non-inverting connections. This eliminates
bandwidth
limiting
due
to
Miller effect
(the effective multiplication of
collector-base
capacitance by
voltage
gain),
and
resultant
reduction
of
high
frequency response. Looking
at
the
lop
half"
of
Channel
A,
Q23
functions
as an
emitter follower
driving
Q22
In
the
common
base mode.
The
feedback
signal
at
R37
is
subtracted from the
Input at
this point.
Collector current
from
Q22
is
cascode connected
to
the emitter
of
Q7, which
Is
also
common
base
connected.
Q7
IV-1
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