Protection circuit and maintenance of household audio power amplifier

introduction

At present, the main channels of high-power household audio power amplifiers all use OCI circuits for power amplification. Since this part of the circuit works in a high voltage, high current, and high temperature environment, the failure rate is very high. When this circuit fails, the DC potential of its output often deviates from the zero level and a higher positive or negative DC voltage appears. When the output DC current flows through the speaker voice coil, the voice coil may be burned. In addition, in some very high-power amplifiers, due to the very strong output power, if the user operates improperly or the karaoke volume is too high, the output power of the channel will be much greater than its rated power, which may damage the power amplifier. Even the expensive speakers. Therefore, various power amplifiers will be equipped with protection circuits to protect the power amplifier and speakers from damage. The following introduces the common protection circuits on the market and their repair methods.

1 Signal shunt protection circuit

Figure 1 is the electromechanical diagram of the CAV-970 power amplifier. This is a fully symmetrical, dual differential, dual constant current source OCI power amplifier circuit. This article only introduces its protection circuit.
The protection circuit of CAV-970 abandons the traditional relay and uses transistors for protection. The functions include: short circuit protection, overload protection, and power-on delay protection. The author has conducted destructive tests on this machine: increasing the input signal amplitude, increasing the load (such as 2n load) or even short circuiting cannot damage the power amplifier stage of this machine. It can be seen that the designer of this machine is indeed outstanding.

1.1 Shunt overload protection circuit

This is the essence of the machine. It is composed of Q451, R3, C1, D2, Q450, R2, D1, C2 and other components. When working normally, due to the small resistance of the sampling resistors R4A1 and R4A2 (0.22Ω), the voltage drop of the output current on them is small, Q450 and Q451 are not turned on, and there is no shunting effect on the signal. If the input signal increases or the load impedance decreases, the output current increases, causing the voltage drop on the sampling resistors R4A1 and R4A2 to increase, Q450 and Q451 will be turned on. Q450 is connected to the base of the input terminal Q465 of the composite tube through the diode D1. Part of the signal is bypassed and shunted through the diode D1 and the transistor Q450, so that the amplitude of the signal input to the next stage is reduced. Maintain a certain level of output. Similarly, when Q451 is turned on, the signal on Q467 will also be shunted to stabilize the output. At this time, no matter how the input signal amplitude is increased or the load is reduced, the output current cannot be increased and is always limited to a certain level.

1.2 Short circuit protection circuit

The circuit consists of Q1, Q2, Q10 and their peripheral circuits. When the load is short-circuited, the output current suddenly increases and flows through R4A1, the emitter potential of Q461 rises, the base voltage of Q10 rises, and Q10 turns on, causing Q1 and Q2 to turn on. The input signal is bypassed to the ground, cutting off the signal source and protecting the power amplifier and speakers from damage.

1.3 Power-on delay circuit

The power-on delay circuit is composed of C3, C4, D6, R7, R12, etc. When the power is just turned on, the voltage on C3 and C4 is 0, D6 is turned on, Q1 and Q2 are turned on, the input signal is bypassed to the ground, and there is no sound in the speaker. In addition, the current slowly charges C3 and C4 through R7, R12, and D6, and the voltage on them slowly rises. When it reaches a certain level, D6 is turned off, and then Q1 and Q2 are turned off. The signal will not be bypassed to the ground. The amplifier enters normal working state.

1.4 Fault repair

1.4.1 If the rated power is not reached, the distortion will be very large

This is a fault unique to this machine. It is also a weakness of the shunt protection circuit. It is mainly caused by the mismatch of Q450 and Q451. When the output reaches a certain amplitude, Q450 and Q451 are not turned on at the same time. Instead, one is turned on while the other is not, or the two tubes are not turned on to the same degree. The positive and negative half cycles are shunted to different degrees, causing distortion. The repair method is to replace two strictly matched transistors (Q451, Q450).

1.4.2 Silence

This circuit can easily cause silent faults. Leakage of C3 and C4 and defective Q10 will cause Q1 and Q2 to be in the on state, causing the signal to be short-circuited to the ground and causing silence.

1.4.3 The startup delay is too long

This fault is actually the same as the "silent" fault. The root cause is also the leakage of C3 and C4. If the leakage of C3 and C4 is serious, the charging current through R7 is leaked, and the voltage of C3 and C4 cannot rise, which will cause the "silent" fault. If the leakage is not very serious, the charging current through R7 is partially leaked. But the voltage on C3 and C4 can rise after a long time, which will cause the startup time to be too long. In addition, the increase in the resistance value of R7 will also cause this fault.

2 Load cut-off protection circuit

2.1 Qisheng Av-2750 amplifier protection circuit

Figure 2 is the protection circuit of the Qisheng AV-2750 power amplifier. This circuit combines the single-chip microcomputer control technology to improve the traditional protection circuit, making the circuit have the advantages of fast response speed, high stability, simple circuit, and easy recovery. The main functions are: DC detection circuit, overload detection circuit, and power-on delay circuit. In the circuit, Q340, R394, R395, R327, R328, and C392 form an overcurrent protection circuit. The resistance of R327 and R328 is only 0.25 Ω/5w, which is very small. When the power amplifier works normally, the impact on the circuit is also minimal. However, when the volume is too large and the power amplifier is in the maximum power output state for a long time or the speaker connection line is short-circuited, etc., the emitter current of the power output tube increases significantly, and the current flows through. R327 or R328, the voltage generated at its two ends will increase, after R394, R4.395 voltage division, as long as the voltage at both ends of R395 is greater than 0.7 V, the duration is enough to make the capacitor C392 fully charged (i.e. delayed protection, changing the capacity of the capacitor can change the response speed of overload protection), Q340 will be turned on, its collector potential will drop, Q342 base potential will also be pulled down, Q342 will be turned on, and output a high level signal, which will be transmitted to the PRO port of the microprocessor (CPU) through R301 and R302. As soon as the microprocessor detects the high level indicating protection, it will immediately output a high level control voltage from MUTEl (mute control port 1) to control the relevant mute control circuit. One of the paths is controlled by R303 to turn on Q345, the base potential of the composite tube composed of Q343 and Q344 is pulled down, the composite tube is cut off, the relay RL301 loses current release, and the power output is disconnected from the speaker, thereby protecting the speaker and the power tube.

The DC detection circuit is mainly composed of Q339, Q336, R359, R362 and peripheral components. R355, R356 and C317, C318 form a low-pass filter. R355 and R356 are DC sampling resistors for the left (L) and right (R) channels, and also serve as input current limiting resistors for the DC detection circuit. C317 and C318 are connected in series to form a non-polar capacitor to bypass the audio signal. When the power amplifier is working normally, the alternating signals output by the left and right channels are coupled to the ground by C317 and C318 connected in series to form a non-polar capacitor after passing through R355 and R356, and the potential at the input end of the DC detector is almost OV.