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Generally, the equipment that needs electricity will be equipped with some protection measures. One of the short circuit and disconnection of the power supply as we know it is one of them. In addition to short-circuit protection, what other protective measures does the power amplifier have? And how to repair it in case of failure?
01 Soft start protection
In audio equipment with a large current draw, when the power is turned on, the current flowing through it can reach 4-10 times the average current value, which is an impact on the power grid and the equipment itself. In severe cases, the equipment will be damaged.
At this time, the soft-start circuit can suppress the influx of current at the moment of the device switching, so that it can smoothly reach the normal function of protecting the device and not causing fluctuations in the power grid. The negative temperature characteristic of the thermistor (NTC) is usually used to achieve this function.
02 DC protection
When the power amplifier output stage is damaged or the static offset is shifted, it is possible to deliver a DC signal. For the speaker, its working mode only produces impedance for the AC signal, and for the DC signal it does not produce any impedance (equal to zero impedance), then the current is infinite, so the coil of the speaker is equivalent to the DC signal A heating wire will be burned quickly.
Therefore, accurate and fast DC protection circuit is very important. The starting value of the DC protection of the power amplifier is usually set at 2V. When it is greater than or equal to this value, the power amplifier will cut off the output to protect the speaker. Of course, some power amplifiers will cut the output by burning the built-in DC fuse.
If the DC protection circuit of a power amplifier is normal, but the coil of the speaker is burned out, there are only two reasons: the power input to the speaker is too large, or the signal output from the power amplifier is clipped and becomes a square wave.
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03 Overcurrent protection
When the load of the power amplifier is too low but it has not reached the short-circuit state, the short-circuit protection will not operate at this time, but the output current will be very large beyond the safe use value of the power amplifier. At this time, the overcurrent protection circuit will intervene in the work. The usual practice Yes: Control the input voltage and output current so that the power amplifier always works within a safe range.
04 Overheating protection
The well-designed power amplifier will not cause overheat protection under normal use, and will only act when the external use environment is bad or the internal fault occurs. The hottest place of the whole power amplifier is the C pole (collector) of the output stage transistor, so the temperature sensor for overheat protection is generally installed in the place closest to the C pole of the transistor or the hottest place on the radiator.
The threshold for overheating protection is generally 95°C and 105°C. The extreme temperature of the transistor is 105°C.
05 Distortion Compressor
The input level of audio equipment has a specified range. If it exceeds this range, the signal will be truncated, and in severe cases it will become a square wave. The function of the distortion limiter is to ensure that the level of the input signal is always controlled within the linear working area allowed by the audio equipment. The general standard is to start when THD1%.
Common failure repair of power amplifier
1. The whole machine does not work
The failure of the whole machine is that the amplifier has no display after power-on, all function keys are invalid, and there is no sound, just like when it is not powered.
The power circuit should be checked first during maintenance. You can use a multimeter to measure the DC resistance value at both ends of the power plug (the power switch should be turned on). Normally, there should be a resistance value of hundreds of ohms. If the measured resistance value is much smaller, and the power transformer is seriously heated, it means that the primary loop of the power transformer has a local short circuit; if the measured resistance value is infinite, it should be checked whether the fuse is blown, the transformer primary winding is open, the power cord and the plug Is there any disconnection between them?
2. No sound output
Silent fault manifests as corresponding function display when operating each function key, but no signal output.
When overhauling an amplifier with a protection circuit, it should be seen whether the protection relay can pull in after starting up. If the relay has no action, it should be measured whether the output voltage of the midpoint of the power amplifier circuit is shifted and whether the overcurrent detection voltage is normal. If the midpoint output voltage shifts or the overcurrent detection voltage is abnormal, it indicates that the power amplifier circuit is faulty. Check whether the positive and negative power supplies are normal. If the positive and negative voltages are asymmetric, the load circuits of the positive and negative power supplies can be disconnected to determine whether the power supply circuit itself is abnormal or the power amplifier circuit is faulty. If the positive and negative power supplies are normal, check each amplifier tube in the power amplifier circuit for damage.
3. Light tone
The so-called light fault refers to the audio signal in the process of amplification and transmission, due to a certain amplification stage amplification or attenuated in a certain link, so that the amplifier's gain decreases or the output power becomes smaller.
When overhauling, you should first check whether the signal source and speakers are normal, and you can use alternative methods to check. Then check all kinds of transfer switches and control potentiometers to see if the volume can be increased.
If the above parts are normal, it should be judged whether the fault is in the pre-stage or post-stage circuit. For a channel with light sound, the signal output from the front-end circuit can be exchanged and input to the back-end circuit of the other channel. If the sound volume of the speaker does not change, the fault is in the back-end circuit; otherwise, the fault is in the front-end circuit Circuit.
4. Large noise
The noise of the amplifier includes hum, pop, induced noise and white noise.
When overhauling, you should first determine whether the noise comes from the previous stage or from the latter stage circuit. The signal connection plugs of the front and rear stages can be removed. If the noise becomes significantly smaller, it means that the fault is in the front stage circuit; otherwise, the fault is in the rear stage circuit.
The hum refers to the low-pitched, monotonous and stable 100Hz AC hum, which is mainly caused by the poor filtering of the power supply. It should focus on checking whether the power rectification, filtering and voltage-stabilizing components are damaged. The virtual welding or failure of the decoupling capacitors at the power supply terminals of the front and rear amplifier circuits will also produce a low-frequency oscillation noise similar to hum.
Distortion faults are caused by the shift of the working point of an amplifier stage or the asymmetry of the power amplifier's push-pull output stage. During maintenance, the specific fault location can be determined according to the change of amplifier output power and distortion.
If the tube amplifier is distorted and the output power becomes smaller (light sound), you should check whether a certain amplifier tube in the push-pull amplifier is aging, the operating point is incorrect, or the output transformer is partially short-circuited to cause its work to be unbalanced; if the distortion is distorted, the output power becomes larger It is mostly caused by the resistance change value in the negative feedback circuit, the capacitor failure or the bypass capacitor short circuit of the cathode self-generated bias.
Howling faults are caused by the existence of self-excitation in the circuit, and are divided into low-frequency howling and high-frequency howling.
Low-frequency howling is usually caused by poor power supply filtering or decoupling (it is often accompanied by hum at the same time). Check whether the power supply filter capacitor, voltage regulator and decoupling capacitor are open or invalid, so that the internal resistance of the power supply Increase. The performance of the integrated circuit of the power amplifier is poor, and there will also be a low-frequency howling failure. At this time, the operating temperature of the integrated circuit will be very high.
The high-frequency howling frequency is relatively high, which is usually caused by the failure of the high-frequency damping capacitor in the amplifier circuit or the deterioration of the performance of the preamplifier integrated circuit. It can be checked by connecting a small capacitor in parallel at both ends of the vibration suppression capacitor or decoupling capacitor of the amplifying circuit in the subsequent stage. In addition, when the negative feedback element is damaged, changed or de-soldered, it will also cause high frequency positive feedback and high frequency howling.
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