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@ Class
A — Class AB automatic switching circuit
3.
P.W
BOARDS
Quarter
A allows
the
main
amplifier
to operate as a
pure class A amplifier for outputs of up to 30W, which
is a quarter of the rated output 120W at 8Q, at the switch
to class AB for outputs larger than 30W.
The transition
from class A to class AB or vice versa is carried out by
switching the bias current and the power supply voltage.
As can be seen from the circuit diagram the chassis of Mode
Sm-8 consists of the following units. Each unit mounted or
a
printed
circuit
board
is descrided
within
the
square
enclosed by a bold dotted line on the circuit diagram.
The automatic switching circuit monitors the input volt-
1. PowerAmp.
.......
mounted
on P.W. Board P60C
age of the main amplifier and the output current of the
2. Voltage Amp.
.....-
mounted on P.W. Board P70C
speaker in order to detect output transitions.
Note that
3. Test Point.........
mounted
on P.W. Board PJOC
the use of a phase correction circuit inside the detector
4. A-B Power Switch. .. . mounted
on P.W. Board PKOC
prevents waveforms including those with a steep rising
5. A-B Detector.......
mounted on P.W. Board PLOC
edge from being distorted.
The automatic switching
6. ASO Protector ......
mounted on P.W. Board PNOC
circuit
can
be easily
used for amplifier
configurations
7. Connection
........
mounted
on P.W. Board PPOC
uniquely designed by Marantz
by merely connecting the
8. Speaker &
input of the detector to the signal system.
Meter Switch .......
mounted on P.W. Board PSOC
9. Output Network.....
mounted
on P.W. Board PWOC
10. Meter & Supply
.....
mounted
on P.W. Board PXOC
@ Large-sized
peak power
meter
of the logarithmic com-
pressed type
Large-sized power meters that are suitable for high quality
power
amplifiers
are available.
This power
meter
can
be used to check the output level and the balance between
the
right and
left channels.
It operates
by detecting
signal
peaks, but
it displays
rms values which
directly
represent the outputs of the amplifier to an 8Q load.
In addition,
since a circuit generating logarithmic com-
pressed
displayes
is used,
outputs
of
small
to
large
quantities can be monitored without changing the range.
This power
meter can also be used as a peak holder by
setting appropriate switches on the panel.
@ Protective circuit which does not degrade tone quality
The overload
protective circuit detects both overvoltage
and
overcurrent
conditions,
while
in the area of safe
operation
(A.S.O.)
does
not
degrade the operation
of
the drive transistors.
The protective circuit has adaptable
characteristics
so that
its operation
does not
interfere
with the program and its operation is always constant.
e Plus
The power output circuit is equipped with high efficiency
heat sinks to cool the power transistors, thus providing
continuous high output power.
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