Several methods to teach you how to effectively

by:Gisen     2020-06-16
Technical column Power amplifier is abbreviated as power amplifier, commonly known as 'amplifier'. It is the most basic equipment in the audio system. Its task is to amplify the weak electrical signal from the signal source (from the mixer in the professional audio system) to drive the speaker makes a sound. During the tuning process, the noise problems of different programs are often encountered. For the circuit board that has been formed, the following methods can cure or reduce the noise. First, the current hum of the power amplifier board 1. Connect the speaker to the power amplifier, turn on the power, move the power transformer until the hum is reduced, and then fix it with a metal cover (which can be an iron shell). 2. If the secondary lead of the transformer is a flat cable, it should be disassembled and changed into a braided strand. 3. Solder the negative end of the speaker output lead on the circuit board, and find the lowest noise welding point on the large-area grounded copper foil after the filter capacitor. 4. Increase or replace the filter capacitor. This method is rarely used. The author has done many tests to prove that the power supply of ± 25V or more and the final current of the power amplifier is 2 ~ 7.5A, and the filter 3 capacitor value is not less than 3300μF, and no current hum will occur. 5. Change the installation position of the power amplifier board, and place the radiator horizontally between the transformer and the circuit board, which acts as a magnetic shield and reduces the current hum. 6. Properly change the height of the component pins, especially the feedback resistance and coupling capacitance. The voltage amplifying part composed of discrete components should also attract attention, and their pin height is preferably 2 to 5.5 mm away from the circuit board surface. Second, the hiss of the power amplifier 1. Take 1000pF ceramic dielectric capacitor and solder one on each diode in the rectifier circuit. After the filter capacitor, 1 to 3 100μF electrolytic capacitors and 0.1μF MKT capacitors are respectively incorporated between the positive and negative power supply branches and the ground. 2. Take a film capacitor with a capacity between 220 and 1500pF and merge it between the signal input terminal and ground to listen to it. Use a capacitor with the smallest sizzle; and play a familiar piece of music. Characteristics prevail. The above noise prevention method is performed by cutting off the front input. It can also be used for pre-amplification noise reduction. 3. The hum of the pre-amplifier 1. Disconnect the '+' end of the DC power supply line and connect it with an inductance of 100 ~ 300mH. Virtual soldering is strictly prohibited. 2. Use plastic rods or bamboo chopsticks to clamp the lead from the input end of the audio source to the preamplifier board, and find a place where the hum is the smallest. 3. Change the grounding points of the front and rear amplifier boards. If the two are connected by a shielded wire, the shielding net at one end of the shielded wire should be welded to the ground of the input end of the subsequent stage, while the other end is not grounded. The same is true for the wiring of the front end and the audio source input interface, only the ground of the audio source input end. In this way, there will be no ground loops and no unpleasant hum. Four, hiss before the power amplifier It mainly appears in feedback tone circuits, especially those welded by scaffolding, and the high-frequency hissing sound is serious. The solution is to shield it with a thin copper skin, or change it to a passive attenuation tone circuit, which can effectively reduce the hissing sound. The above several noise reduction measures are limited to the application in transistors and integrated circuit power amplifiers. Of course, the design of the circuit board is crucial, such as power supply, audio signal routing, analog ground and digital ground, etc.
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