CIRCUIT DESCRIPTION
Amplifier stages are fully direct coupled (DC coupling).
w
ith
a dual differential pair for the input, and a fully complementary
MOSFET output stage. Transistors Q3. Q4, Q5 and Q6 form the
differential input,
while
Ql and
Q2
are constant current sources
for the input pair. Potentiometer PI adjusts the output offset
voltage.
Transistors Q7, Q8 . Q10 and Q11 form the second gain stage,
and are the ‘predrivers’ for the output stage drivers Q12 and
Ql3.
An adjustable voltage reference is formed by transistor
Q9
and potentiometer P2. The signal ‘sees’ the bases of transistors
Ql2 and Q 13 tied together.
P2
adjusts the voltage to bias the
amplifier for Class AB operation, and therefore determines
the
quiescent (idle) current in the driver and output stages. Thermal
stability is assured by the positive temperature coefficient of the
MOSFETs.
There is none of the usual volt-amp limiting in this design.
Diodes D111.
D12
, D13 and D14 prevent the gates of the
MOSFETs from being ov erdriven in the event of excessive
signal levels. The output current is limited by the B + and B
-
fuses and the speaker fuses.
A protective relay circuit has a j-section low pass filter which
prevents signals above 10 Hz from triggering the relay. This
circuit allows direct current to pass to IC101. which senses any
offset above 3.5 volts and then opens the relay to protect
the
load. RI06 and R107 form the reference for the positive offset
detector portion of IC
10
1. R108 and R
1O9
act for the
negative
detector. The charging time of C104 is the delay time at turn on.
D1O6 and C105 produce a rapid discharge supply that opens the
relay instantly when the amplifier is turned off.
An overload protection system includes thermal breakers on
the heat sinks which will turn off the amplifier if overheating
The output stages are also fully complementary. Q401. Q402
and Q403 are N channel MOSFETs connected in parallel for the
positive half of the signal. Q404. Q405 and Q406 are P channel
MOSFETs for the negative half.
occurs, and thermal sensors which will bypass fan-limiting
resistors as the temperature rises. increasing the
fan
speed. A
current inrush limiter TR401 is located in the AC line input.
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