10k Ω, voltage amplification factor 1~20 continuously adjustable; d. Low frequency noise voltage (below 20kHz) ≤ 10mV, measured when the voltage amplification factor is 10 and the input terminal is short circuited to ground; e. The power amplifier efficiency (output power/total power coumption of the amplifier) measured at the output power of 500mW is ≥ 50% (2) High efficiency: A signal conveion circuit with a magnification of 1 is designed and made to convert the signal output from both ends of the power amplifier into a single end output, which is filtered by RC for external test itruments. (3) Low non-linear distortion: Design and make a device to measure the output power of the amplifier. It is required to have a 3-digit digital display with an accuracy of better than 5%. The maximum undistorted output power is ≥ 1W5、 Circuit design When designing the power amplifier circuit, the circuit should be designed simply. If the original taste of music is puued, the links between the signal input circuit and the power amplifier circuit, such as tone, balance or other functional circuits, should be reduced or abandoned as far as possible. If you like to modify music, in addition to the above circuits, you can also add circuits such as BBE, 3D and SRS that can greatly modify music signals. Please follow the following principles in arranging the location and wiring of components on the circuit board: 1. Reasonably arrange the position of each component on the circuit board according to the signal tramission path from small to large, and try to shorten the distance between each component (jumper can be used when the distance is too large or obstacles are encountered) to reduce the introduction of external interference. 2. In the power supply line, it is not allowed to place the high current printed circuit or wire in the middle or near the low current path, so as to avoid interference to the low current line. 3. The path (printed line or wire) of high current on the circuit board shall be designed to be wider (thicker) so that the current can pass through smoothly.6、 Manufacturing process To make a power amplifier, not only the circuit should be reasonable, but also the production process should be careful, otherwise the speaker will have a large interference noise or howling noise caused by self-excited oscillation. When guiding students to make high-power discrete components and integrated success rate amplifie, I analyzed and summarized the above two problems, and improved the circuit structure and layout, and achieved good results. In general, the following aspects should be paid attention to when making power amplifie: 1. Select a stable and solid chassis, preferably of metal structure, to avoid short circuit of internal circuit or intrusion of external interference noise due to chassis deformation. 2. Reasonably arrange the position of each unit in the chassis and pay attention to the center of gravity of the whole machine. The power supply part shall be far away from the small signal amplifier circuit. It is better to isolate the power supply part from the amplifier circuit by placing the power supply part in an independent small room composed of metal to reduce the impact of the power supply on the amplifier circuit. 3. Before each component is loaded onto the board, the oxide layer on the lead wire shall be scraped off with tools, and the actual value shall be measured with a digital meter to see whether it is coistent with the nominal value, and whether the error is within the allowable range. After the elements are put on the plate, carefully check whether there are wrong soldering, missing soldering or soldering, and if there are any, timely eliminate them. 4. During welding, an electric soldering iron with appropriate power shall be selected according to the size of the components, and high-quality soldering tin and low corrosivity flux shall be used to eure that the solder joints are of appropriate size, smooth and bright, and free of false soldering and false soldering. 5. Each plate in the machine (including signal input and output sockets) cannot be grounded separately, but the whole machine shall be grounded one point at a time, that is, after finding the grounding point on the shell, use a thick black wire to lead out from the ground wire of each plate and collect at this point. When the grounding point is separated from the housing, the resistance between any point on the circuit board and the housing measured with a multimeter should be infinite. 6. After each unit is fixed in the chassis, all connecting harnesses shall be fastened with nylon fastene and fixed accordingly. After the completion of the whole machine, use a multimeter to measure the resistance between each key point and the ground. The resistance between the corresponding point of each sound channel and the ground should be the same or very close. If there is any abnormality, it is generally caused by the wrong soldering, missing soldering or incorrect value of the components used in the circuit. After careful ipection or measurement, it is not difficult to find the problem. If no problem is found in the above ipection, power on the whole machine (the speaker cannot be connected at this time) and observe whether there is any abnormality (such as overheating of components, smoking, whether the protective relay can be normally closed, etc.)." />
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Methods and precautions for making power amplifier

Time:2022-10-08 Views:1716
    The power amplifier (power amplifier for short) circuit in the general audio-visual circuit further amplifies the low-frequency signal after the voltage amplifier to obtain a larger output power, which is finally used to drive the loudspeaker to play or provide deflection current in the television.


1、 Concept and principle of power amplifier
    .1. Power amplifier concept
    Power amplifier (English name: power amplifier), referred to as "power amplifier", refers to an amplifier that can produce the maximum power output to drive a load (such as speakers) under a given distortion rate. The power amplifier plays a pivotal role of "organization and coordination" in the whole sound system, and to some extent, it determines whether the whole system can provide good sound quality output. The power amplifier, which can be said to be the largest family of all kinds of audio equipment, is mainly used to amplify the weak signal input by the sound source equipment and generate enough current to drive the loudspeaker to reproduce the sound.
    2. Operating principle of pow2 2. 2. Operating principle of power amplifier
    The current control function of triode or the voltage control function of FET is used to convert the power of the power supply into the current varying according to the input signal Because sound is a wave with different amplitudes and frequencies, that is, AC signal current, the collector current of the triode is always the base current β Times, β Is the AC amplification factor of the triode. If a small signal is injected into the base, the current flowing through the collector will be equal to that of the base β And then isolate the signal with a DC isolating capacitor to obtain that the current (or voltage) is the original β This phenomenon becomes the amplification effect of the triode. After constant current amplification, power amplification is completed.

2、 Characteristics of power amplification current
    The understanding. or evaluation of power amplifier circuit is mainly from three aspects: output power, efficiency and distortion.
    1. In order to obtain the required output power, the circuit must select a triode with sufficient collector power consumption, and the working current and collector voltage of the power amplifier are also high. In the circuit design and use, we should first consider how to give full play to the function of the triode without damaging the triode. Because the working state of the power amplifier tube in the circuit is often close to the limit value, the power amplifier current should be adjusted and used carefully, and should not be used beyond the limit.
    2. In terms of energy consumption, the output power of the power amplifier is ultimately provided by the power supply. For example, the power consumption of the power amplifier in the radio accounts for 2/3 of the whole machine. Therefore, great attention should be paid to improving the circuit efficiency, that is, the ratio of output power to power consumption.
   3. The input signal of the power amplifier circuit has been amplified for several levels with sufficient strength, which will make the operating point of the power amplifier tube move greatly, so the power amplifier circuit is required to have a large dynamic range. If the operating point of the power amplifier is not properly selected, the output will be seriously distorted.

2、 Principles of Common Power Amplifier Circuits
    The power amplifier circuit with single triode output has small output and low efficiency, which is rare in daily electrical appliances. At present, the push-pull circuit is often used.
    Figure 1 is a schematic diagram of a push-pull circuit using a coupling transformer. Its characteristic is that the static working current of triode is close to zero, and the power consumption of amplifier is small. When there is information input, although the working current of the circuit is large, most of the power is output to the load, and the loss itself is small, so the power utilization is high. In this circuit, each triode works only in half a cycle of the signal. In order to avoid distortion, two triodes work in coordination. The secondary of input transformer B1 in the figure has a grounded center tap. When the audio signal is input, two signals of equal size and opposite polarity in B1 secondary are sent to the transmitting junction of BG1 and BG2 respectively. In the positive half cycle time of the input signal, BG1 tube is cut off because of the reverse bias voltage. Only BG2 can amplify the signal and output it from the collector; In the negative half cycle of the signal, BG1 gets a positive high bias voltage, which can amplify and output the signal of this half cycle, while BG2 turns off. Although the two triodes in the circuit amplify half of the signal synchronization respectively, their output currents pass through the output transformer B2 successively, so the induced currents obtained at the secondary of B2 can form a complete output signal.
    In this power amplifier circuit, in order to solve the problems of impedance turn matching and signal phase, input and output transformers are indispensable. However, it is difficult to make a high-quality transformer in terms of materials and technology. It always consumes some energy and reduces the efficiency of the circuit. Moreover, the frequency characteristics of the transformer are not good, which makes the circuit output of signals at different frequencies very uneven and will cause distortion. Therefore, in order to improve the quality of power amplifiers, people use more transformer free (OTL) power amplification circuits.

3、 Selection of components and power amplifier circuits
    When making power amplifier, the selection of components and power amplifier circuits can only start from the following aspects:
    1. Power transformer
    The power transformer shall be reasonably selected according to the total power consumption of each circuit in the machine. In order to make the whole machine have enough power reserve, the power of the power transformer shall be at least twice of the total dissipated power of each circuit in the machine. If the power amplifier is a Class A amplifier circuit, the power of the power transformer shall be doubled on this basis. Whether E type, C type, R type or ring type power transformers are of high quality and sufficient power, they can be selected. If conditions permit, you can use the switching power supply to try, and I believe you will get unexpected results, because the author has also made several power amplifiers with switching power supply and achieved good results.
    2. Resistance, capacitance and connecting wire
    1) Resistance
    The precision five color ring metal film resistance shall be selected as far as possible, and the error shall be controlled below 1%. The design power margin shall be as large as possible, and the general power shall be more than 1/4W. The resistance power of some special parts (such as the emitter resistance of the power tube or the sampling resistance of the negative current feedback circuit) shall be between 1/2W and 5W to improve the stability of the whole circuit during operation.
    2) Capacitance
    Capacitors shall be selected as far as possible, and the imported high-quality capacitors shall be the first choice. For large capacity (such as power filter and decoupling capacitor), special audio electrolytic capacitor with withstand voltage higher than power voltage and maximum capacity shall be used to improve filtering effect. Capacitors with small capacity, such as high-frequency decoupling of power supply, signal coupling, and DC isolation capacitor in negative feedback network, should try to use CBB capacitor or tantalum capacitor with high quality and low dielectric loss, which can minimize the attenuation of high and low end of audio signal, so as to fully extend the high and low band information of music.
    3) Connecting wires
    The choice of connecting wires is often ignored when making power amplifiers. The connecting conductor of large current (such as power line, power output line and ground wire) shall be copper conductor with good insulation performance, thick wire diameter and multiple strands of fine wires twisted as far as possible to overcome the skin effect when the current passes through the conductor. The smaller wire diameter can be used for connecting wires with small current. Wires of different colors shall be used to distinguish each part of the wiring. Generally, red wire is used for positive power supply, green or blue wire is used for negative power supply, and black wire is used for ground wire. The signal path in the amplifier shall use high-quality double core shielded wire, and it is absolutely not allowed to use single strand wire or flat wire to replace it, so as to prevent the invasion of external interference noise.
    3. Power amplifier circuit
    Relatively speaking, when making power amplifier, the choice of power amplifier circuit should be said to be more important. Firstly, the structure of the selected power amplifier circuit (Class A, Class A and Class B or other types) and the model of each device or element (capacitor, resistor, amplifier tube, etc.) in the circuit should be determined according to your listening environment and preference for the playback timbre (cold and bright, warm or both).
    Then decide whether to make the power amplifier circuit or buy the finished circuit board according to your knowledge of the structure and principle of the power amplifier. If you have a deep understanding of the structure of the power amplifier circuit and a certain degree of motorcycle skills, you should choose high-quality components that you trust or are suitable for taste to make or buy finished power amplifier circuits. If you are not too strict with the timbre of music, have a certain hands-on ability, and do not know the structure and principle of power amplifier circuit, then I advise you to buy a power amplifier board with mature circuit structure and composed of discrete components or a power amplifier circuit board with integrated circuit as the core. At present, the power amplifier circuit composed of integrated circuits such as LM3886, LM4766 and TDA7294 is satisfactory in terms of output power, sound reproduction quality and working stability. Compared with the power amplifier board assembled by discrete components, it has the characteristics of simple circuit, easy debugging and low cost.

4、 Basic requirements for making power amplifier
    When making a power amplifier, there are also some basic requirements based on some characteristics of the power amplifier:
    (1) Enough output power:
    a. The 3 dB passband is 300~3400Hz, and the output sine signal has no obvious distortion;
    b. Maximum undistorted output power ≥ 1W;
    c. Input impedance>10k Ω, voltage amplification factor 1~20 continuously adjustable;
    d. Low frequency noise voltage (below 20kHz) ≤ 10mV, measured when the voltage amplification factor is 10 and the input terminal is short circuited to ground;
    e. The power amplifier efficiency (output power/total power consumption of the amplifier) measured at the output power of 500mW is ≥ 50%
    (2) High efficiency:
    A signal conversion circuit with a magnification of 1 is designed and made to convert the signal output from both ends of the power amplifier into a single end output, which is filtered by RC for external test instruments.
    (3) Low non-linear distortion:
    Design and make a device to measure the output power of the amplifier. It is required to have a 3-digit digital display with an accuracy of better than 5%. The maximum undistorted output power is ≥ 1W

5、 Circuit design
    When designing the power amplifier circuit, the circuit should be designed simply. If the original taste of music is pursued, the links between the signal input circuit and the power amplifier circuit, such as tone, balance or other functional circuits, should be reduced or abandoned as far as possible. If you like to modify music, in addition to the above circuits, you can also add circuits such as BBE, 3D and SRS that can greatly modify music signals. Please follow the following principles in arranging the location and wiring of components on the circuit board:
    1. Reasonably arrange the position of each component on the circuit board according to the signal transmission path from small to large, and try to shorten the distance between each component (jumper can be used when the distance is too large or obstacles are encountered) to reduce the introduction of external interference.
    2. In the power supply line, it is not allowed to place the high current printed circuit or wire in the middle or near the low current path, so as to avoid interference to the low current line.
    3. The path (printed line or wire) of high current on the circuit board shall be designed to be wider (thicker) so that the current can pass through smoothly.

6、 Manufacturing process
    To make a power amplifier, not only the circuit should be reasonable, but also the production process should be careful, otherwise the speaker will have a large interference noise or howling noise caused by self-excited oscillation. When guiding students to make high-power discrete components and integrated success rate amplifiers, I analyzed and summarized the above two problems, and improved the circuit structure and layout, and achieved good results. In general, the following aspects should be paid attention to when making power amplifiers:
    1. Select a stable and solid chassis, preferably of metal structure, to avoid short circuit of internal circuit or intrusion of external interference noise due to chassis deformation.
    2. Reasonably arrange the position of each unit in the chassis and pay attention to the center of gravity of the whole machine. The power supply part shall be far away from the small signal amplifier circuit. It is better to isolate the power supply part from the amplifier circuit by placing the power supply part in an independent small room composed of metal to reduce the impact of the power supply on the amplifier circuit.
    3. Before each component is loaded onto the board, the oxide layer on the lead wire shall be scraped off with tools, and the actual value shall be measured with a digital meter to see whether it is consistent with the nominal value, and whether the error is within the allowable range. After the elements are put on the plate, carefully check whether there are wrong soldering, missing soldering or soldering, and if there are any, timely eliminate them.
    4. During welding, an electric soldering iron with appropriate power shall be selected according to the size of the components, and high-quality soldering tin and low corrosivity flux shall be used to ensure that the solder joints are of appropriate size, smooth and bright, and free of false soldering and false soldering.
    5. Each plate in the machine (including signal input and output sockets) cannot be grounded separately, but the whole machine shall be grounded one point at a time, that is, after finding the grounding point on the shell, use a thick black wire to lead out from the ground wire of each plate and collect at this point. When the grounding point is separated from the housing, the resistance between any point on the circuit board and the housing measured with a multimeter should be infinite.
    6. After each unit is fixed in the chassis, all connecting harnesses shall be fastened with nylon fasteners and fixed accordingly.
    After the completion of the whole machine, use a multimeter to measure the resistance between each key point and the ground. The resistance between the corresponding point of each sound channel and the ground should be the same or very close. If there is any abnormality, it is generally caused by the wrong soldering, missing soldering or incorrect value of the components used in the circuit. After careful inspection or measurement, it is not difficult to find the problem. If no problem is found in the above inspection, power on the whole machine (the speaker cannot be connected at this time) and observe whether there is any abnormality (such as overheating of components, smoking, whether the protective relay can be normally closed, etc.).
   





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