4. Protection circuit
Protection circuit of power amplifier of AV321T (RU) is designed on power board. The protection
means of L, R, C channel is fulfilled through disconnecting delay Y100 when starting up to disconnect
the output. SR, SL channel achieves protection function through mute. AV321T (RU) is with power-on
delay protection, central point over-voltage and over-current protection, standby protection.
(1) Power-on delay attracting protection circuit: the working of circuit is unstable when power on,
and the produced impulse current does great harm to speaker and power amplifying circuit, so delay
attracting protection circuit is set. Power-on delay attracting protection circuit is divided into 2 steps: the
first step: C, L, R channel and its working process is as follows: for AC outputted from transformer, after
rectification and filtering of Vd113, C110, +22V voltage is achieved, charges C115 through R108 to make
Vd111 struck through reversely and to make V105, V104 forwards on, relay Y100 is made attracting
finally to reach delay attracting effect. The second step: SR, SL channel performs power-on anti-impulse
protection through the following means: when power on, reset of machine system has not finished,
P_CMS may probably be high level to make triode V137 on to cause power-on impulse sound, now a
+VS level is added on B electrode of triode V137 through resistor R163 to make it in cutoff state at the
moment of power-on; a -VS level is added on pin 3 of TDA2052 through resistor R189 to make TDA2052
in mute state to avoid the impulse to loudspeaker; after machine system resets, MCU outputs a P_CMS
low level signal to still make V137 cut off, pin 3 of TDA2052 is still -VS, TDA2052 outputs mute. After
delay start up of machine is successful, output signal P_CMS of MCU switches into high level
immediately, now relay has attracted, RLY0 is grounded through relay to make V137 on, voltage of pin 3
of TDA2052 is about 5V higher than negative power voltage, SL/SR channel restores normal output.
TDA2052 chip, under the condition that load is not connected, will appear component DC, so false load
R121, R122 is added in power board surround channel.
(2) Over-voltage protection: on output end of each channel, an over-voltage sampling resistor is
connected, L channel is R116, R channel is R117, C is R118, SR and SL are R120 and R119 respectively,
only one channel’
s central point voltage is more than +3.5V or less than -3.5V, V101 or V102 is on to
make their C electrode voltage decrease, B electrode V103 is pulled down to make V103 on, relay
disconnects finally, and over-voltage protection finishes. The working process is shown in the figure
3.2.5.2:
Over-voltage
sampling
resistor is
connected to
each channel
When m o r e
than +3.5V
V101 is on
L channel R116
V102 is on
C electrode
voltage
decreases
V103 is on
Relay
disconnects
and protection
starts up
When less
than -3.5V
Figure 3.2.5.2 Over-voltage protection flow chart
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