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Technology sharing: solutions to common failures of power amplifiers

2020-05-25

As we all know, when we use power amplifiers in daily life, various problems may occur after a long time. 

So, what should be done if the power amplifier has problems? Gap is like a line, although not everyone can solve it, but knowing some related knowledge will make you easy to solve. The following describes three common faults and maintenance methods of the power amplifier.

 

1. The machine does not work

The failure of the whole machine does not work after the power is turned on, the amplifier has no display, the function keys are invalid, and there is no sound, just like when it is not powered.

The power supply 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? Some machines have added a temperature protection device, and a current fuse is connected to the primary circuit of the power transformer (usually installed inside the power transformer, you can see it by removing the insulation paper outside the transformer), it will also make the power transformer primary after damage The circuit is open.

 

If the resistance at both ends of the power plug is normal, it can be powered to measure whether the output voltage of the power circuit is normal. For amplifiers that use system-controlled microprocessors or logic control circuits, focus on checking whether the power supply voltage (usually + 5V) of the control circuit is normal. If there is no + 5V voltage, you should measure whether the input voltage of the three-terminal voltage regulator integrated circuit 7805 is normal. If the input voltage is abnormal, you should check the rectifier and filter circuit. If the voltage at the 7805 input terminal is normal and there is no 10V or low voltage at the output terminal, you can disconnect the load to see if the + 5V voltage can return to normal. If the + 5V voltage is normal, the fault is in the load circuit; if the + 5V voltage is still abnormal, the fault is in the 7805 itself. If the + 5V power supply voltage of the system control circuit is normal, you should check whether the clock and reset signal of the microprocessor are normal, and whether the key control and display drive circuits are damaged.

 

Second, the noise is large

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 back 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 back 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.

Induced noise is a complex and harsh AC sound, mainly caused by poor grounding of the transfer switch and potentiometer in the previous stage circuit or poor shielding of the signal wiring. The cracking sound refers to the intermittent "slap" and "click" sound. In the pre-stage circuit, check whether the signal input plug and the socket, the switch, the potentiometer, etc. are in poor contact, and whether the coupling capacitor has virtual welding or leakage. The post-amplifier circuit should check whether the relay contact is oxidized, whether the input coupling capacitor has leakage or poor contact. In addition, the soft breakdown of the differential input tube or the constant current tube in the subsequent stage circuit will also produce a "click" noise similar to sparks. White noise refers to the irregular continuous "rustle" sound, which is usually the background noise caused by the poor performance of the input stage transistors, field effect transistors or op amp integrated circuits in the front and back stage amplifier circuits. Replacement of components with specifications.

 

3. Light volume

 

The so-called light fault refers to the audio signal in the process of amplification and transmission, due to a certain amplification stage change or attenuation 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 all the above parts are normal, it should be determined 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 level of the speaker is unchanged, the fault is in the back-end circuit; otherwise, the fault is in the front-end circuit Circuit. The sound caused by the post-amplifier circuit is mainly due to insufficient output power and insufficient gain. You can use the method of appropriately increasing the input signal (for example, the signal output from the recorder to the speaker directly to the input end of the power amplifier circuit of the rear stage, changing the volume of the recorder, and observing the change in the output of the amplifier) to determine what caused the cause.

If the output sound is large enough after the input signal is increased, it means that the output power of the amplifier is sufficient, but the gain is reduced. It should be focused on checking whether the contact resistance of the relay contact increases, the input coupling capacitance decreases, and the isolation resistance increases. Negative feedback capacitor capacity becomes small or open circuit, negative feedback resistor resistance increases or open circuit, etc. If the input signal is increased, the output sound is distorted and the volume does not increase significantly, indicating that the output power of the post-stage amplifier is insufficient. You should first check whether the positive and negative power supply voltages of the amplifier are low (if only one channel is light , It is not necessary to check the power supply), whether the performance of the power tube or the integrated circuit has deteriorated, and whether the resistance value of the emitter has increased, etc. The sound caused by the transfer switch and potentiometer in the pre-stage circuit is light, and it is easier to find by visual inspection, and it can be cleaned or replaced. If it is suspected that a signal coupling capacitor has failed, it can be tested in parallel with the same value capacitor; the performance of the amplifier tube or op amp integrated circuit is poor, and it can also be checked by the substitution method. In addition, problems with negative feedback components can also cause the circuit gain to decrease.

In actual operation of the mixer, if it does not have certain relevant knowledge, it is very difficult to operate, and at the same time, its sound quality effect is often unsatisfactory. Therefore, in order to improve the output of the mixer, it is very important to learn and demodulate the relevant knowledge of the mixer.

1. The sound is harsh: Cause: The high-frequency energy is too strong; Solution: Attenuation 6KHz;

2. The sound is irritable: the reason: high frequency is not compliant; the solution: attenuate 8K-1OKHz;

3. Hair sounds: Reason: Ultra-high frequency energy is too strong; Solution: Attenuation 12K-16KHz;

4. The microphone is dry and the singing is tired: the reason: the energy of the microphone's intermediate frequency is not enough; the solution: increase the 800Hz frequency band;

5. Hard bass: Reason: The upper limit of the ultra bass is too strong; Solution: Attenuation 125-160Hz;

6. Bass dispersion: Cause: The energy of the lower limit of the low frequency is too large; Solution: Increase the lower limit frequency of the ultra bass.


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