7 Power Amplification and Protection Circuits
The power amplification circuit is this unit's hard core which is working under high voltage
and large current volume, so its failure rate is very high. A protection circuit is added to the
power amplification's output to protect the amplifier and speakers' circuits. In addition, a
multi-channel switching circuit is also connected to the output of AB217. We take the R
channel as an ample to analyze the circuit as shown in the figure 10.
AC Negative
Feedback R413,
R408,C403
Compound Power
Amplification(NPN)
V430,V407,V409
Difference
Amplification
V401,V402
Temperature
Compensation
V406
Speakers
Mirror Image Constant
Current Source
V403,V405
VD401,VD402
Compound Power
Amplification(PNP)
V431,V410,V408
Protection
Circuit
Voltage
Amplification
V404
R
INPUT
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Figure 10
7.2.3 Short Circuit over-current Protection
The channel R's output end is parallel connected with a over-current sampling triode V421.
R415 and R416 are over-current sampling resistor. When current soars up sharply due to
short circuit, the potential difference between R415 and R 416 also increases. The current
passes to the base and emitter of V421 through R433 and R434. When their potential differ-
ence is beyond 0.7V, V421 is conducted and its collector's potential decreases. Finally, the
current passes VD405 and R443 to make V425 conducted and thus the relay is shut off.
Over
Current
Potential Difference
Between R415 and R416
Increases
V421
Conducted
V425
Conducted
Relay
Shut Off
7.3 Multi-channel Control Circuit
In the front panel circuit introductions, we have explained that when we choose the multi-
channel output mode, N911A's output end sends out high level which reaches the base of
a compound tube composed of V434 and V435 through R441. V434 and V435 are
conducted and there is current in the coil Y402. Y402 is switched on. Channels C, SR and
SL pick up signals from channels R and L and then send them out.
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