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DC OUT CIRCUIT
The circuit shown in the figure corresponds to the DC OUT of channel L. The goal of this circuit is
protecting the loudspeakers when, because of a module fail, there is some DC appearing at the output. The
voltages indicated in the figure correspond to rest state and they are given by the dividers made of R320-R322
and R332-R323.
The resistances R323-R322 are linked by their extreme to the leg 7(Q) of the monostable IC310 (4538),
which has +10V at rest state. On the other hand R320-R321 are linked by their extreme to the L output, which, in
these conditions, has 0V respect to ground. If we apply Ohm's Law to these dividers we will obtain the above
mentioned voltages.
Let's remember briefly the function of a NOR gate like the HEF4001B.
Let's suppose there is a continuous voltage appearing at the module output, because of any malfunction.
This makes the voltage dividers lose balance, no matter if the above mentioned voltage is positive or
negative, the gate goes to 0V, the base Q302 loses the current stream and, as a consequence, the relay K301
opens. The aim of the zeners D309 and D310 is protecting the gates, avoiding the voltage in them to be higher
than 8.2V when the voltage is positive and lower than -0.6V when it is negative; as you can see, the zener plays
the role of a diode.
A
B
C
0
0
1
0
1
0
1
0
0
1
1
0
12
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