How to reduce active speaker noise
The noise of active speakers mainly comes from three types: electromagnetic interference, wire interference, mechanical noise and thermal noise. This article will introduce how to prevent and reduce these noises. 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 city power, so a power transformer must be used. The working process of the power transformer is an "electricity-magnetism-electricity" conversion process. In the electromagnetic conversion process, magnetic leakage will inevitably occur. The transformer magnetic flux leakage is picked up and amplified by the amplifier circuit, and finally appears as a hum produced by the speaker. The common specifications of power transformers are EI type, ring type and R type. Whether from the perspective of sound quality or from the perspective of electromagnetic leakage, these three types of transformers have their own advantages and disadvantages, and it is not easy to determine the advantages and disadvantages. EI-type transformers are the most common and widely-used transformers. The main source of magnetic leakage is the air gap between the E and I-type cores and the coil's own radiation. The magnetic leakage of the EI transformer is directional. As shown in the figure below, in the three directions of the X, Y, and Z axes, the coil axis has the strongest interference in the Y axis direction, the weakest in the Z axis direction, and the X axis direction radiation is between Y Between Z and Z, so try not to make the Y axis parallel to the circuit board in actual use. Toroidal transformers, because there is no air gap, the coil is evenly wound around the iron core, theoretically, the magnetic flux leakage is very small, and there is no coil radiation. However, due to the absence of air gap, the toroidal transformer has poor anti-saturation ability, and it is easy to saturate when there is a DC component in the mains and produce strong magnetic leakage. In many areas of the country, the mains waveform is severely distorted, so many users feel that using toroidal transformers is not better or worse than EI transformers. The so-called toroidal transformer has no leakage, either because of media misleading, or because the manufacturer has made it for commercial promotion needs. The statement that the toroidal transformer has very low magnetic leakage is only established when the mains wave type is a strict sine wave. In addition, the toroidal transformer will also have strong electromagnetic leakage at the lead, so the magnetic leakage of the toroidal transformer is also directional. When the actual installation is installed, the toroidal transformer is rotated 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 the coil winding method is different. The heat dissipation conditions are much better than the toroidal transformer. The iron core is expanded to be involute and gradually closed. The R-type transformer is electromagnetic The leakage situation is similar to toroidal transformers. Since the length of each turn is shorter than that of a toroidal transformer and can be wound close to the iron core, the copper loss of the R-type transformer among the above three types of transformers is the smallest. If conditions permit, consider installing a shielding cover for the transformer and properly grounding it. The metal cover can only be made of ferrous materials. Generally, metals such as copper and aluminum only have the function of electrical shielding but no magnetic shielding, and cannot be used as a transformer. Shield. The above analysis is based on the selection and well-made transformer materials. In fact, most of the commercially available transformer products are not strictly designed according to industry specifications due to cost pressures and competition needs, or even cut corners. There are many unpredictable factors in the analysis. The first is the quality of the iron core material. Many companies use low permeability H50 iron cores, scraps and even doped soft iron to make transformers, resulting in high no-load current of the transformer, excessive iron loss and serious no-load heating. Reduce the cost and at the same time cover up the problem of excessive voltage regulation caused by high iron loss, greatly reduce the number of primary and secondary coil turns, and reduce the voltage regulation rate by reducing copper loss. This approach further increases the empty Carrying current, and large no-load current will directly lead to increased magnetic leakage. The problem of toroidal transformers is more complicated. The core of a regular toroidal transformer is formed by tightly winding a silicon steel strip of equal width. Still for cost reasons, most low-cost toroidal transformers are spliced ​​with several or even dozens of silicon steel strips, and even wound with uneven edges and corners. After being wound, they are flattened by the machine tool, because the toroidal transformer coil package It is difficult to find without destructive dissection around the iron core. Mechanical processing has severely damaged the lattice arrangement of silicon materials and the insulation between adjacent silicon steel strips. No matter the performance or magnetic leakage characteristics of such toroidal transformers will be greatly reduced, even after annealing treatment, they cannot make up for serious quality defects . Stray electromagnetic waves mainly come from the power output wires of active speakers, speakers and power frequency dividers, wireless transmitting equipment and computer mainframes, and the causes are not discussed in depth here. Stray electromagnetic waves are similar to power transformers in the form of transmission and induction. The frequency range of the stray magnetic field is very wide. It is useful for users to reflect that active speakers inexplicably receive local radio broadcasts as typical stray electromagnetic interference. Another source of interference that needs attention is the rectifier circuit. After the filter capacitor enters the normal state after power-on, the charging is concentrated only on the peak of the alternating current. The charging waveform is a strong pulse with a narrow width. The larger the capacitance, the greater the pulse intensity. From the perspective of electromagnetic interference, the filter capacitor is not larger. Well, the wiring between the rectifier and the filter capacitor should be as short as possible, and as far as possible away from the power amplifier circuit. If the PCB space is not allowed, try to use the ground wire to envelop. The main prevention measures of electromagnetic interference: 1. Lower the 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 derivation of P = U ＾ U / R, the induced voltage is inversely proportional to the square of the resistance value, that is, the realization of low impedance of the amplifier is very beneficial to reduce electromagnetic interference. For example, if the input impedance of an amplifier is reduced from 20K to 10K, the induced noise level will be reduced to 1/4. The active speaker sound sources are mainly computer sound cards, walkmans, MP3s. These sound sources have strong loading capacity, and the effect of appropriately reducing the input impedance of the active speaker on the sound quality is very weak and difficult to detect. The author tried to input the active speaker input impedance during the experiment. Dropped to 2KΩ, no change in sound quality was felt, and no abnormality was seen in long-term work. 2. Enhance the anti-interference ability of high frequency In view of the characteristics of stray electromagnetic waves that are mostly high-frequency signals, a magnetic capacitor is added to the ground at the input of the amplifier. The capacitance can be selected from 47 to 220P. The capacitance frequency turning point of hundreds of picofarad capacitance is two times higher than the audio range. , Three orders of magnitude, the effect on the sound pressure response and the sense of hearing in the effective listening audio segment is negligible. 3. Pay attention to the installation method of power transformer Use a good quality power transformer, try to open the distance between the transformer and the PCB, adjust the orientation between the transformer and the PCB, and keep the sensitive end of the transformer and the amplifier away; the interference intensity of the EI type power transformer is different in all directions, pay attention to avoid interference as much as possible The strongest Y axis is aligned with the PCB. 4. The metal shell must be grounded For the HIFI independent power amplifier, the design specification products have an independent ground point on the chassis. The ground point is actually used to reduce external interference by the electromagnetic shielding effect of the chassis; for common active speakers, it also serves as a radiator The metal panel also needs to be grounded; the volume and tone potentiometer shells should be grounded as far as possible. Practice has proved that this measure is very effective for PCBs working in the harsh conditions of the electromagnetic environment. 2. Ground interference The grounding design of electronic products is extremely important, both low-frequency circuits and high-frequency circuits must comply with the design rules. The design requirements of high-frequency and low-frequency circuit grounds are different. The design of high-frequency circuit grounds mainly considers the influence of distributed parameters, which is generally ring ground. The low-frequency circuits mainly consider the superposition of large and small signal ground potentials, which require independent wiring and concentrated 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 wiring, concentrated point grounding", which can significantly improve the signal-to-noise ratio. The audio circuit ground 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 inevitably cause a strong hum: due to the large charge and discharge current (relative to the signal ground current) of the filtering and decoupling capacitors in the strong power ground, there must be a certain voltage on the circuit board traces When the voltage drops, the small signal ground coincides with the strong electric 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 and the signal ground is equivalent to injecting the signal voltage at the input of the amplifier. The change in ground potential will be picked up and amplified by the amplifier, producing hum. Increasing the ground wire width and back tin treatment can only weaken the ground wire interference to a certain extent, but the effect is not obvious. Some PCBs that do not strictly separate the ground wire are due to the wide width of the ground wire, the trace is very short, the number of amplification stages is very small, and the capacity of the decoupling capacitor is very small. Therefore, the hum is still within the barely acceptable range. significance. It should be noted that the frequency of the hum caused by the electromagnetic interference of the transformer is generally about 50HZ, and the hum caused by the improper ground wiring is about 100HZ because of the frequency doubling effect of the rectifier circuit. It can be discerned carefully. The correct wiring method is to select the main filter capacitor pin as the concentrated ground point, and the strong and weak signal ground lines are strictly distinguished and summarized at the total ground point. 3. Mechanical noise and thermal noise (1) Mechanical noise Active speakers integrate speakers and amplifiers, so some 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 "electricity-magnetism-electricity" conversion. In the process of electromagnetic conversion, in addition to the magnetic leakage, the alternating magnetic field will cause the iron core to vibrate. The ballast of the old-style ballast fluorescent lamp will hum when working, and the sound will increase after a long time of use. It is because the iron core is attracted by the alternating magnetic field to cause vibration. A well-made transformer has a tightly pressed iron core and undergoes a vacuum dipping process before going offline. The vibration of the iron core caused by the alternating magnetic field is very small; if the iron core of the transformer is loose and uncompacted, the vibration caused by power Will be stronger (imagine the hair clippers in the barber shop). In order to save man-hours, many low-cost transformers only do "dipping" paint without "vacuum dipping" treatment, and the core vibration is more serious. The speaker box has a certain role as an auxiliary sound cavity. The air disturbance caused by the vibration of the transformer is transmitted to the speaker diaphragm, which sounds very similar to the noise caused by electromagnetic interference. Years ago, I repaired a set of active speakers with severe hum. I could n’t find the reason through the circuit. I accidentally disconnected the speaker wires. The noise was almost not reduced. It was finally confirmed that the transformer was the fault. This situation is common in active speakers. The quality of the transformer only affects the resulting amplitude. Even the very expensive power transformers have vibrations, so the noise level of the majority of active speakers is inferior to the main box. Deputy box. The prevention measures of mechanical noise caused by power transformer are relatively simple, and the following points can be used as reference according to the actual situation: 1. Choose a transformer with better quality and strict process to reduce the transformer's own vibration, which is also the most effective measure 2. Add a shock-absorbing layer between the transformer and the fixed plate, and select elastic soft materials such as rubber and foam 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 not prone to magnetic saturation, have good long-term stability, and have relatively low heat generation. Another common type of mechanical noise comes from the potentiometer. Most of the commercially available active speakers use rotating carbon film potentiometers. Over time, the metal brush of the potentiometer and the diaphragm will have poor contact due to dust deposition and diaphragm wear. When rotating the potentiometer, there will be A lot of noise is generated, and the potentiometer with serious wear will have noise even when it is not rotating. There are also some special dynamic noises that need to be briefly described: some active speaker boxes are not firmly joined, or the user does not tighten the installation screws after unboxing, and there is noise when playing slightly more dynamic music; Or due to imperfect processing methods, there are different levels of air leakage in the box; double R or exponential openings are not made at both ends of the inverter tube, and the air flow is greatly compressed and expanded to generate noise during large dynamics. (2) Thermal noise The active speaker circuit is composed of passive devices such as resistors and capacitors, and active devices such as ICs and transistors. Under normal working conditions, electronic components will inevitably produce "background noise" that is unique to the components themselves, which is often called heat. noise. Thermal noise is a broad-spectrum thermal noise, mainly concentrated in the middle and high frequencies, which is generally reflected in the sense of hearing as the "hissing" sound from the tweeter. There are a large number of free-state electrons in the conductive part of the passive device. The number of free-state electrons is directly related to the temperature. The higher the temperature, the greater the number. The movement of free state electrons can be regarded as disordered movement, which can be regarded as clutter compared with the normal and orderly signal current. The number of free-state electrons in active devices such as ICs is much larger than passive devices. Active devices have an amplification effect, so the thermal noise of active devices is higher than that of passive devices. Thermal noise is also incurable, and the main prevention measures are component replacement and component workload reduction. Replacement of components refers to the use of low-noise components. For example, the thermal noise of metal film resistors is lower than that of carbon film resistors, and the thermal noise of carbon film resistors is lower than that of carbonaceous resistors. In addition, strengthening heat dissipation measures and lowering the operating temperature are also effective means to reduce thermal noise and enhance working stability. Generally, the noise and zero drift of Class A amplifiers are inferior to those of Class A and B amplifiers. Excessive operating temperature is not only an increase in noise, but for active devices, it also means that the leakage current and gain are unstable, which is detrimental to the long-term stable operation of the power amplifier. 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