Problems and preventive measures of PCB wiring of power amplifier circuit

An active speaker is a combination of a speaker and an amplifier. Therefore, the noise analysis of an active speaker is similar to that of a general amplifier. HIFI amplifiers can be used as a reference when analyzing and processing.

Noise and amplifiers are inevitable. The purpose of reducing noise here is to reduce it to an acceptable range, not to completely eliminate it. In other words, the signal-to-noise ratio can only be increased as much as possible, but not infinitely. Let's first briefly analyze the source and mechanism of noise generation, and then learn about some effective prevention and control measures that have been proven in practice.

1. Electromagnetic interference and prevention measures

1. Electromagnetic interference

The main sources of electromagnetic interference are power transformers and stray electromagnetic waves in space.

Except for a few special products, most active speakers are powered by the mains, so a power transformer is necessary. The working process of the power transformer is an "electric-magnetic-electric" conversion process. During the electromagnetic conversion process, magnetic leakage will inevitably occur. The transformer leakage magnetic field is picked up and amplified by the amplifier circuit, and finally manifested as the AC sound emitted by the speaker.

Common specifications of power transformers include EI type, toroidal type and R type. Whether from the perspective of sound quality or electromagnetic leakage, these three types of transformers have their own advantages and disadvantages, and cannot be simply judged.

EI type transformer is the most common and widely used transformer. Shenzhen major audio manufacturers basically use EI transformers. The main sources of magnetic leakage are the air gap between the E and I cores and the coil's own radiation. The magnetic leakage of EI type transformer is directional, as shown in the figure below. In the three directions of X, Y, and Z, the Y axis of the coil axis has the strongest interference, and the Z axis has the weakest interference. The radiation in the X axis is between Y and Z. Therefore, in actual use, try not to make the Y axis parallel to the circuit board.

Toroidal Transformer

Since there is no air gap in the toroidal transformer and the coil is evenly wound around the core, the leakage magnetic flux is theoretically very small and there is no coil radiation. However, since there is no air gap in the toroidal transformer, the anti-saturation ability is poor. When there is a DC component in the mains, it is easy to saturate and produce strong magnetic leakage. In many areas of China, the waveform of the mains is seriously distorted, so many users feel that the toroidal transformer is not better than the EI transformer, or even worse. The so-called toroidal transformer has no leakage, either due to media misleading or fabricated by the manufacturer for commercial publicity needs. The claim that the magnetic leakage of the toroidal transformer is extremely low is only true when the mains waveform is a strict sine wave. In addition, the toroidal transformer will also have strong electromagnetic leakage at the lead wire, so the leakage magnetic flux of the toroidal transformer is also directional. When actually installing the toroidal transformer, rotate the toroidal transformer to obtain the highest signal-to-noise ratio at a certain angle.

The R-type transformer can be simply regarded as a toroidal transformer with a circular cross-section, but it is different in the coil winding method. The heat dissipation condition is much better than that of the toroidal transformer. The iron core is unfolded into a gradual opening and closing type. The electromagnetic leakage of the R-type transformer is similar to that of the toroidal transformer. Since the length of each turn of the wire is shorter than that of the toroidal transformer, it can be wound close to the iron core, so the copper loss of the R-type transformer is the smallest among the above three types of transformers.

2. Main prevention and control measures for electromagnetic interference:

1) Reduce input impedance.

Electromagnetic waves are mainly picked up by wires and PCB board traces. Under certain conditions, the electromagnetic waves picked up by wires can basically be regarded as constant power. According to the deduction of P=U^U/R, the induced voltage is inversely proportional to the square of the resistance value, that is, low impedance of the amplifier is very beneficial to reducing electromagnetic interference.

2) Enhance high-frequency anti-interference ability

In view of the fact that most stray electromagnetic waves are medium and high frequency signals, a magnetic disk capacitor is added to the ground at the input end of the amplifier. The capacitance value can be selected between 47--220P. The frequency turning point of the capacitor with a capacitance of several hundred picofarads is two or three orders of magnitude higher than the audio range, and the impact on the sound pressure response and listening experience in the effective listening frequency band can be ignored.

3) Pay attention to the installation method of the power transformer

Use a power transformer with good quality, try to increase the distance between the transformer and the PCB, adjust the orientation between the transformer and the PCB, and keep the transformer away from the sensitive end of the amplifier; the interference intensity of the EI type power transformer is different in each direction, so try to avoid aligning the Y-axis direction with the strongest interference intensity with the PCB.

4) The metal casing must be grounded

For HIFI independent power amplifiers, products with standardized design have an independent grounding point on the chassis. This grounding point actually uses the electromagnetic shielding effect of the chassis to reduce external interference; for common active speakers, the metal panel that also serves as a heat sink also needs to be grounded; the volume and tone potentiometer casings should be grounded as much as possible if conditions permit. Practice has proved that this measure is very effective for PCBs working in harsh electromagnetic environments.

2. Ground Interference and Prevention Measures

1. Ground interference

The ground design of electronic products is extremely important. Both low-frequency and high-frequency circuits must follow the design rules. The ground design requirements of high-frequency and low-frequency circuits are different. The ground design of high-frequency circuits mainly considers the influence of distributed parameters, which is generally a ring ground. The low-frequency circuit mainly considers the superposition of ground potentials of large and small signals, and requires independent routing and centralized grounding. From the perspective of improving the signal-to-noise ratio and reducing noise, analog audio circuits should be classified as low-frequency electronic circuits. Strictly following the principle of "independent routing and centralized grounding" can significantly improve the signal-to-noise ratio.

The ground of the audio circuit can be simply divided into power ground and signal ground. The power ground mainly refers to the filter and decoupling capacitor ground, and the small signal ground refers to the input signal and feedback ground. The small signal ground and the power ground cannot be mixed, otherwise it will cause strong AC noise: due to the large charging and discharging current of the filter and decoupling capacitors (relative to the signal ground current), there must be a certain voltage drop on the circuit board trace. The small signal ground coincides with the strong power ground and is bound to be affected by this fluctuating voltage. In other words, 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 injecting a signal voltage into the input terminal of the amplifier. The ground potential change will be picked up and amplified by the amplifier, generating AC noise. Increasing the ground line width and back-tinning can only reduce the ground interference to a certain extent, but the effect is not obvious. Some PCBs that do not strictly separate the ground lines have wide ground lines and short traces, and at the same time, there are few amplification stages and small decoupling capacitor capacity, so the AC noise is still within a barely acceptable range.

2. The correct wiring method is:

1) The main filter capacitor pin is used as the centralized grounding point. The strong and weak signal ground wires are strictly separated and gathered at the main grounding point. The following takes the most common LM1875 (TDA2030A) as an example and explains the manufacturer's recommended circuit:

2030A Recommended Route Map

In the figure, R1 and R2 are input grounding resistors, C2 is a DC feedback capacitor, and the grounding point is the small signal ground, marked in blue; C3, C4, C6, and C7 are decoupling capacitors, and the grounding end is marked in red, which belongs to the power ground. The correct grounding method is: the three small signal grounding points can be mixed on one ground line, and the four power grounds are combined into another ground line. The power ground and the small signal ground meet at the total grounding point. Except for the total grounding point, the two grounds must not have other connecting points!

2) The grounding point of the Zobel phase-shift network (R5, C5) at the output of the power amplifier is handled in a special way. If this grounding point is connected to the power ground, the ground voltage disturbance will be fed back to the inverting input of the LM1875 through R4, causing AC noise; if it is connected to the small signal ground, the quality of the music signal will be seriously degraded due to the inconsistency of the signal phase and intensity. Therefore, if the space on the printed circuit board allows, it is best to route it separately. The following is a detailed explanation combined with several actual PCB board diagrams:

TDA2030 PCB Diagram

In this PCB diagram, there is an obvious ground design error. The small signal ground and the power ground completely overlap, so the board must have AC noise and it is not controlled by the volume potentiometer. In the figure, C2, C3, C4, and C5 are decoupling capacitors, and the five grounding points, including C7, R2, C6, the first pin of JP1, and the third pin of JP2, are small signal grounds. After the large and small signal grounds overlap, they are led to the total grounding point of C8 and C9 through jumpers. At the same time, the grounding point of the zobel phase shift network (the second pin of C1) is also mixed on a ground line, which will inevitably make the actual situation more complicated.

LM4766 PCB Diagram

In the figure, C5, C11, and C12 are OP decoupling capacitors, and the ground terminal belongs to the power ground. The current direction is marked with a thin red line in the figure; and the ground terminal of the HPF circuit resistors such as R5, R6, R7, and R9 belongs to the small signal ground. If they share a ground line with the decoupling grounds such as C5, C11, and C12, the voltage drop caused by the working current of the decoupling capacitor and the internal resistance of the ground line will inevitably be superimposed on the ground terminal of R5, R6, R7, and R9, causing AC noise or even self-excitation.

3) A PCB diagram with correct ground wiring, as shown below:

Ground wiring PCB diagram

In this PCB, the grounds of large and small signals are strictly separated, and some other noise reduction methods are used. The signal-to-noise ratio is very high. When the input end is open, the residual noise at the output end is not higher than 0.3mV. There is no noise when the ear is placed on the speaker unit in the dead of night. For the convenience of viewing the picture, only the ground wire of one channel is drawn for demonstration. C9, R1, C10 and the ground terminal of the signal input socket are small signal grounds, which are connected to the main ground point through the red ground wire. The ground wire on the left is the speaker and zobel network ground, and the ground wire on the right is the power ground of the decoupling capacitor. The three ground wires meet at the 2nd foot of the main filter capacitor C4, realizing the true "one-point grounding".

3. Mechanical noise and prevention measures

1. Mechanical noise

Active speakers integrate speakers and amplifiers together, so some of the noise is unique. The most common source of mechanical noise is the power transformer. As mentioned earlier, the working process of the power transformer is the process of "electric-magnetic-electric" conversion. In the electromagnetic conversion process, in addition to magnetic leakage, the alternating magnetic field will cause the iron core to vibrate. When the old ballast fluorescent lamp is working, the ballast will make a buzzing sound, and the sound will increase after long-term use, because the iron core is attracted and repelled by the alternating magnetic field, causing vibration.

Well-made transformers have very tight cores, and they are vacuum-coated before going offline. The alternating magnetic field causes very little vibration to the core. If the transformer core is loose and not compacted, the vibration caused when powered on will be relatively strong (think of the electric clippers in a barber shop). Many low-priced transformers are only "dipped" in paint to save time, but not "vacuum-coated", which makes the core vibrate more severely. The speaker box has a certain sound-assisting cavity effect. The air disturbance caused by the transformer vibration is transmitted to the speaker diaphragm, which sounds very similar to the noise caused by electromagnetic interference. A few years ago, I repaired an active speaker with severe AC noise. I couldn't find the cause after checking the circuit. I accidentally broke the speaker connection, and the noise was almost not reduced. Finally, I confirmed that it was the transformer that was causing the problem.

This situation is common in active speakers. The quality of the transformer only affects the amplitude of the final vibration. Even very expensive power transformers have vibrations. Therefore, the noise level of the main box of most active speakers is lower than that of the sub-box.

2. Preventive measures for mechanical noise caused by power transformer:

1) Choose a transformer with good quality and rigorous workmanship to reduce the vibration of the transformer itself, which is also the most effective measure.

2) Add a shock-absorbing layer between the transformer and the fixed plate, and use elastic soft materials such as rubber, foam, etc. to cut off the vibration coupling channel between the transformer and the box.

3) Choose a transformer with a certain power margin. The closer the transformer works to the rated upper limit, the greater the vibration. Transformers with large power margins are less likely to experience magnetic saturation, have good long-term working stability, and generate relatively little heat.

Another common mechanical noise comes from the potentiometer. Most of the active speakers on the market use rotary carbon film potentiometers. As time goes by, the metal brush of the potentiometer and the diaphragm will have poor contact due to dust deposition and diaphragm wear. When the potentiometer is turned, there will be a lot of noise. Severely worn potentiometers will even make noise when they are not turned.

In addition, there are some more special dynamic noises that need to be briefly described: the joints between the box panels of some active speakers are not firm, or the user does not tighten the installation screws after unpacking the box, resulting in noise when playing music with slightly larger dynamics; or due to imperfect processing methods, there are varying degrees of air leakage in the box; there are no double R or exponential openings at both ends of the bass reflex tube, and the airflow is rapidly compressed and expanded here during large dynamics, producing noise.

The above article briefly analyzes the source and mechanism of noise generation, and proposes some effective prevention and control measures that have been tested in practice, in the hope that they can help those in need.