Problems in PCB wiring of power amplifier circuit and preventive measures
Time:2022-10-01
Views:1727
Active speakers are the combination of speakers and amplifiers. Therefore, the noise analysis of active speakers is similar to that of general amplifiers. HIFI amplifiers can be used for reference in analysis and processing.
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It is inevitable that the noise is accompanied by the amplifier. The purpose of this discussion is to reduce the noise to an acceptable range, not to eliminate it completely. In other words, the signal-to-noise ratio can only be improved as much as possible, but not infinitely. Next, we will briefly analyze the source and mechanism of noise, and then learn some effective prevention and control measures that have been tested in practice.
1、 Electromagnetic interference and prevention measures
1.1. Electromagnetic interference
The main sources of electromagnetic interference are power transformers and space stray electromagnetic waves.
Except for a few special products, most active speakers are powered by mains power, so power transformers must be used. The working process of the power transformer is a "electricity magnetic electricity" conversion process, in which magnetic leakage is bound to occur. The transformer magnetic leakage is picked up and amplified by the amplification circuit, which is finally expressed as the AC sound from the speakers.
Common specifications of power transformers include EI type, ring type and R type. From the perspective of sound quality or electromagnetic leakage, these three types of transformers have their own advantages and disadvantages, which cannot be simply determined.
EI transformer is the most common and widely used transformer. The major audio manufacturers in Shenzhen basically use EI transformers. The magnetic leakage mainly comes from the air gap between E and I type iron core and the radiation of the coil itself. The magnetic leakage of EI type transformer is directional, as shown in the figure below. In the three directions of X, Y and Z axes, the Y axis of the coil axis has the strongest interference, while the Z axis has the weakest interference. The radiation in the X axis is between Y and Z, so the Y axis should not be parallel to the circuit board in actual use.
Toroidal Transformer
Since there is no air gap in the ring type transformer and the coil windings the iron core evenly, theoretically the magnetic leakage is very small, and there is no coil radiation. However, because there is no air gap in the ring type transformer, its anti saturation ability is poor, and it is easy to produce saturation when there is DC component in the mains, resulting in strong magnetic leakage. In many areas of China, the electric wave distortion is serious, so many users feel that the ring type transformer is not better than the EI type transformer, or even worse. The so-called ring type transformer has no leakage at all, or is misled by the media, or is fabricated by the manufacturer for the need of commercial publicity. The statement that the magnetic leakage of the ring type transformer is extremely low is only true when the market wave type is a strict sine wave. In addition, the ring transformer will also have strong electromagnetic leakage at the lead, so the magnetic leakage of the ring transformer is also directional. When the ring transformer is actually installed, it will rotate to obtain the highest signal-to-noise ratio at a certain angle.
The R-type transformer can be simply regarded as a circular ring type transformer with a cross section, but there are differences in coil winding techniques. The heat dissipation condition is far better than that of the ring type transformer. The iron core is gradually opened and gradually closed. The electromagnetic leakage of the R-type transformer is similar to that of the ring type transformer. As the wire length of each turn is shorter than that of the ring type transformer and can be wound close to the iron core, the copper loss of R type transformer among the above three types of transformers is the smallest.
2. Main prevention measures for electromagnetic interference:
1) Reduce the input impedance.
The electromagnetic wave is mainly picked up by the wire and PCB wiring. Under certain conditions, the electromagnetic wave picked up by the wire can be basically regarded as constant power. According to P=U ^ U/R deduction, the induced voltage is inversely proportional to the square of the resistance value, that is, the low impedance of the amplifier is beneficial to reduce electromagnetic interference.
2) Enhance high frequency anti-interference capability
In view of the characteristic that most stray electromagnetic waves are medium and high frequency signals, magnetic sheet capacitors are added to the ground at the input end of the amplifier, and the capacitance value can be selected between 47 and 220P. The frequency turning point of capacitance with hundreds of pico capacitance values is two or three orders of magnitude higher than the audio range, and the impact on the sound pressure response and sense of hearing in the effective audio band can be ignored.
3) Pay attention to the installation method of power transformer
The power transformer with good quality shall be used, the distance between the transformer and PCB shall be as far as possible, the orientation between the transformer and PCB shall be adjusted, and the sensitive end of the transformer and the amplifier shall be far away; The interference intensity of EI type power transformer is different in each direction, so try to avoid the Y axis direction with the strongest interference intensity aligning with the PCB
4) Metal enclosure must be grounded
For HIFI independent power amplifiers, the products with design specifications have an independent grounding point on the chassis, which actually reduces external interference by virtue of the electromagnetic shielding effect of the chassis; For common active speakers, the metal panel, which also serves as the radiator, also needs to be grounded; The housing of volume and tone potentiometers shall be grounded as far as possible if conditions permit. The practice has proved that this measure is very effective for PCBs working in harsh electromagnetic environment.
2、 Ground wire interference and prevention measures
1. Ground wire interference
The ground wire design of electronic products is extremely important, whether low frequency circuit or high frequency circuit must follow the design rules. The ground wire design requirements of high frequency and low frequency circuits are different. The ground wire design of high frequency circuit mainly considers the influence of distribution parameters, generally ring grounding. The low frequency circuit mainly considers the superposition of large and small signal ground potentials, which requires independent wiring and centralized grounding. From the perspective of improving signal-to-noise ratio and reducing noise, analog audio circuit should be classified as low-frequency electronic circuit, strictly following the principle of "independent wiring and centralized point grounding", which can significantly improve signal-to-noise ratio.
The ground wire of audio circuit can be simply divided into power supply ground wire and signal ground wire. The power supply ground wire mainly refers to filter and decoupling capacitance ground wire, and the small signal ground wire refers to input signal and feedback ground wire. The small signal ground and the power ground cannot be mixed, or it will cause strong AC sound: due to the large charging and discharging current of the filter and decoupling capacitor (relative to the signal ground current) in the strong electrical ground, there must be a certain voltage drop on the circuit board wiring. The small signal ground is overlapped with the strong electrical ground, which is bound to be affected by this fluctuating voltage, that is, the reference point voltage of the small signal is no longer zero. The voltage change between the signal input terminal and the signal ground is equivalent to the signal voltage injected at the amplifier input terminal, and the ground potential change will be picked up and amplified by the amplifier to generate AC sound. Increasing ground wire width and tin backing can only reduce ground wire interference to a certain extent, but the effect is not obvious. Some PCBs that do not strictly separate the ground wires have wide ground wires, short routing, few amplification stages, and small decoupling capacitance, so the AC sound is still within the barely acceptable range.
2. The correct wiring method is:
1) The main filter capacitor pin is used as the centralized grounding point, and the strong and weak signal ground wires are strictly separated and summarized at the general grounding point. The most common LM1875 (TDA2030A) is taken as an example, and the manufacturer‘s recommended route is used to explain:
2030 A Recommended Circuit Diagram
In the figure, R1 and R2 are input ground resistors, C2 are DC feedback capacitors, and the ground point is small signal ground, marked in blue,; C3, C4, C6 and C7 are decoupling capacitors, and the ground terminal is marked in red, which belongs to power supply ground. The correct grounding method is as follows: three small signal grounding points can be mixed on one ground wire, four power grounding points can be merged into another ground wire, and the power grounding point and small signal grounding point will converge at the general grounding point. Except the general grounding point, there shall be no other connecting points between the two types of grounding points!
2) The treatment method of the grounding point of the zobel phase-shifting network (R5, C5) at the output end of the power amplifier is special. If the grounding point is connected to the power ground, the ground wire voltage disturbance will be fed back to the inverted input end of LM1875 through R4, causing AC sound; If the signal is incorporated into the small signal ground, the quality of the music signal will be seriously degraded due to the inconsistency of the phase and strength of the signal. Therefore, if the space of the printed circuit board allows, it is better to route wires separately. The following is a detailed description based on several actual PCB diagrams:
TDA2030 PCB Diagram
In this PCB diagram, there is an obvious ground wire design error. The small signal ground is completely coincident with the power ground. Therefore, the board must have AC noise and is not controlled by the volume potentiometer. In the figure, C2, C3, C4 and C5 are decoupling capacitors, and the five grounding points of C7, R2, C6, JP1 first pin and JP2 third pin belong to small signal grounding points. After the large and small signal grounding points overlap, they are led to the general grounding points of C8 and C9 through jumpers. At the same time, the grounding point (the second pin C1) of the zobel phase-shifting network is also mixed on a ground wire, which inevitably makes the actual situation more complicated.
LM4766 PCB diagram
In the figure, C5, C11 and C12 are OP decoupling capacitors, and the grounding terminal belongs to power grounding. The current direction is marked with red thin lines in the figure; However, the resistance grounding terminals of HPF circuits such as R5, R6, R7, R9 belong to small signal grounding. If they share a ground wire with the decoupling grounding of C5, C11, C12, etc., the voltage drop caused by the working current of decoupling capacitor and the internal resistance of ground wire will inevitably be superimposed on the grounding terminals of R5, R6, R7, R9, causing AC sound and even self excitation.
3) A PCB diagram with correct ground wire routing, as shown below:
.Ground wire wiring PCB diagram
In this PCB, the large and small signals are strictly separated, and some other noise reduction methods are used at the same time. The signal-to-noise ratio is very high. When the input terminal is open, the measured residual noise at the output terminal is not higher than 0.3mV, and there is no noise when the ear is close to the speaker unit in the dead of night. For the convenience of drawing, just draw the ground wire of one channel for demonstration. The grounding terminals of C9, R1, C10 and signal input sockets are small signal grounding, which are connected to the general grounding point through the red ground wire. The left ground wire is the speaker and zobel network ground, the right ground wire is the power ground of decoupling capacitor, and the three ground wires converge at two pins of the main filter capacitor C4 to achieve "one point grounding" in the true sense
3、 Mechanical noise and prevention measures
1. Mechanical noise
Active speakers integrate the speakers and amplifiers together, so some noise is unique. The most common source of mechanical noise is the power transformer. As mentioned earlier, the working process of power transformer is a process of "electric magnetic electric" conversion. In addition to magnetic leakage, the alternating magnetic field will cause vibration of the iron core during the electromagnetic conversion process. The ballast of the old type ballast will make a buzzing sound when the fluorescent lamp is working, and the sound will increase after a long time of use, because the iron core is attracted and repelled by the alternating magnetic field, causing vibration.
The iron core of a well made transformer is pressed tightly. At the same time, it needs to be treated by vacuum impregnation process before going offline. The vibration of the iron core caused by the alternating magnetic field is very small; If the transformer iron core is loose and not compacted, the vibration caused by power on will be relatively strong (imagine the electric pusher in the barber shop). In order to save working hours, many low price transformers only do "dip" painting instead of "vacuum dip" treatment, and the core vibration is more serious. The speaker box has a certain role of sound aid cavity. The air disturbance caused by transformer vibration is transmitted to the loudspeaker diaphragm, which sounds very similar to the noise caused by electromagnetic interference. A set of active speakers with serious AC noise was repaired years ago. The cause could not be found by checking the circuit. The speaker connection was accidentally broken, and the noise was almost not reduced. It was finally confirmed that the transformer was responsible.
This kind of situation is common on active speakers. The quality of transformers only has an impact on the amplitude finally caused. Even the expensive power transformer also has vibration. Therefore, the noise level of the main box of most active speakers is lower than that of the auxiliary box.
2. Prevention measures for mechanical noise caused by power transformer:
1) It is also the most effective measure to select the transformer with good quality and rigorous technology to reduce the transformer‘s own vibration
2) Add a damping layer between the transformer and the fixed plate, select elastic soft materials such as rubber, foam, etc., and cut off the vibration coupling channel between the transformer and the box.
3) Select a transformer with a certain power margin. The closer the transformer works to the upper limit of the rating, the greater the vibration. The transformer with large power margin is not easy to appear magnetic saturation, has good long-term working stability and relatively small heat value.
Another common source of mechanical noise is the potentiometer. The vast majority of commercially available active speakers use rotary carbon film potentiometers. As time goes by, the contact between the metal brush and the diaphragm of the potentiometer will be poor due to dust deposition and wear of the diaphragm. When the potentiometer is rotated, there will be a lot of noise. Potentiometers with severe wear will also have noise even when they are not rotating.
In addition, there are some special dynamic noises that need to be briefly described: some of the active speakers are not firmly connected between the boards, or the installation screws are not pressed tightly after the user unpacks the box himself, so there is noise when playing a slightly dynamic music; Or due to imperfect processing means, the box has air leakage to varying degrees; There is no double R or exponential opening at both ends of the inverting tube, and the air flow rapidly compresses and expands here to generate noise in case of large dynamic.
The above is a brief analysis of the source and mechanism of noise, and proposes some effective prevention and control measures that have been tested in practice, in order to help the people in need.
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