There are three main sources of noise from active speakers: 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 stray electromagnetic waves in space.
Except for a few special products, most active speakers are powered by city electricity, so power transformers 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 leakage is picked up and amplified by the amplifier circuit, and finally appears as the humming sound emitted 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 electromagnetic leakage, these three types of transformers have their own advantages and disadvantages, and the advantages and disadvantages cannot be simply judged.
The EI transformer is the most common and widely used transformer. The main source of magnetic leakage is the air gap between the E and I cores and the radiation of the coil itself. The magnetic leakage of EI transformer is directional. As shown in the figure below, in the three directions of X, Y, and Z axis, the interference in the Y-axis direction of the coil axis is the strongest, the Z-axis direction is the weakest, and the radiation in the X-axis direction is between Y , Z, so try not to make the Y axis parallel to the circuit board in actual use.
Since the toroidal transformer does not have an air gap and the coil is evenly wound on the iron core, theoretically, there is little magnetic leakage and there is no coil radiation. However, the toroidal transformer has poor anti-saturation ability due to the absence of air gaps, and it is easy to saturate when there is a direct current component in the mains, resulting in strong magnetic leakage. In many areas of the country, the mains waveform distortion is serious, 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 misled by the media, or fabricated by the manufacturer for commercial propaganda. The argument that the magnetic leakage of the toroidal transformer is extremely low is only valid when the mains wave type is a strict sine wave. In addition, the toroidal transformer also has strong electromagnetic leakage at the lead, so the magnetic leakage of the toroidal transformer is also directional. When the toroidal transformer is actually 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 there are differences in the coil winding method. The heat dissipation condition is far better than that of the toroidal transformer. The iron core is expanded to gradually open and close. 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 the toroidal transformer and can be wound close to the iron core, the copper loss of the R-type transformer among the above-mentioned 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. General metals such as copper and aluminum have only electrical shielding but no magnetic shielding, and cannot be used as a transformer. Shield.
The above analysis is based on the selection of transformer materials and excellent production. Actually, most of the commercially available transformer products are not designed in strict accordance with industry specifications due to cost pressure and competitive needs, or even cut corners. There are many unpredictable factors in the analysis. The first is the quality of the core material. Many companies use H50 cores, scraps and even soft iron with low permeability to make transformers, resulting in high no-load current, excessive iron loss, and serious no-load heat; this type of transformer is To reduce costs and at the same time to cover up the problem of excessive voltage regulation caused by high iron loss, greatly reduce the number of turns of the primary and secondary coils, and reduce the voltage regulation rate by reducing copper loss. This approach further increases the space Load current, and the no-load current is too large will directly lead to an increase in magnetic leakage.
The problem of toroidal transformers is more complicated. The regular toroidal transformer core is tightly wound from a silicon steel strip of equal width. Still for cost reasons, most low-cost toroidal transformers use several or even dozens of silicon steel strips to splice, and even use ragged edges to wind. After winding, they are flattened with a machine tool. Because the toroidal transformer coils are wrapped Around the iron core, it is difficult to find without destructive dissection. Machining can seriously damage the lattice arrangement of silicon materials and the insulation between adjacent silicon steel strips. Such toroidal transformers will greatly reduce their performance and magnetic leakage characteristics. Even after annealing, they cannot make up for serious defects in quality. .
The stray electromagnetic waves mainly come from the power output wires of active speakers, speakers and power dividers, wireless transmitting equipment and computer mainframes. The causes are not discussed in depth here. The stray electromagnetic wave is similar to the power transformer in the form of transmission and induction, and the stray magnetic field has a wide frequency range. It is reported that the active speaker inexplicably receives the local radio broadcast is a typical stray electromagnetic wave interference.
Another source of interference that needs attention is the rectifier circuit. After the filter capacitor enters the normal state after being turned on, the charging is only concentrated on the peak value 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 the larger and the larger Good, the wiring between the rectifier tube and the filter capacitor should be as short as possible, and at the same time as far away as possible from the power amplifier circuit. If the PCB space is not allowed, use the ground wire as much as possible.
The main prevention and control measures of electromagnetic interference:
1. Reduce the input impedance.
Electromagnetic waves are mainly picked up by wires and PCB traces. Under certain conditions, 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 low impedance of the amplifier is very beneficial to reduce electromagnetic interference. For example, if the input impedance of an amplifier is reduced from the original 20K to 10K, the induced noise level will drop to a 1/4 level. The sound sources of active speakers are mainly computer sound cards, walkmans, and MP3s. This type of sound source has a strong load capacity. Appropriately reducing the input impedance of active speakers has a very weak impact on sound quality and is not easy to detect. I tried to input impedance of active speakers during the experiment. It was down to 2KΩ, no change in sound quality was felt, and no abnormality was seen after long-term work.
2. Enhance high frequency anti-interference ability
In view of the characteristics that most stray electromagnetic waves are medium and high frequency signals, a magnetic sheet capacitor is added to the ground at the input end of the amplifier. The capacitance value can be selected between 47 and 220P. The frequency turning point of the capacitance of hundreds of picofarads is two higher than the audio frequency range. , Three orders of magnitude, the effect on the sound pressure response and sense of hearing in the effective listening audio segment is negligible.
3. Pay attention to the installation method of power transformer
Use a better quality power transformer, try to extend the distance between the transformer and the PCB, adjust the position between the transformer and the PCB, and keep the transformer and the sensitive end away from the amplifier; the EI-type power transformer has different interference strengths in all directions, so pay attention to avoid interference as much as possible The strongest Y-axis direction is aligned with the PCB.
4. The metal shell must be grounded
For HIFI independent power amplifiers, products with design specifications 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, it also serves as a radiator. The metal panel also needs to be grounded; the volume and tone potentiometer shells 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
The ground wire design of electronic products is extremely important. Both low-frequency and high-frequency circuits must follow the design rules. High-frequency and low-frequency circuit ground wire design requirements are different. High-frequency circuit ground wire design mainly considers the influence of distributed parameters, which is generally ring ground. Low-frequency circuit mainly considers the superposition of large and small signal ground potentials. Independent wiring and centralized grounding are required. From the perspective of improving signal-to-noise ratio and reducing noise, analog audio circuits should be classified as low-frequency electronic circuits, and strictly follow the principle of "independent routing and centralized 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 wire, and the small signal ground refers to the input signal and feedback ground wire. Small-signal ground and power ground cannot be mixed, otherwise it will cause strong hum: In strong-electric ground, due to the large charge and discharge currents of the filtering and decoupling capacitors (relative to the signal ground current), there must be a certain voltage on the circuit board traces. If the small signal ground coincides with the strong current ground, it 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 injecting a signal voltage at the amplifier input, and the ground potential change will be picked up by the amplifier and amplified to produce hum. Increasing the ground wire width and back tin processing can only reduce the ground wire interference to a certain extent, but the effect is not obvious. There are some PCBs that do not strictly separate the ground wire due to the wide ground wire, short traces, few amplification stages and small decoupling capacitor capacity, so the hum is still within the acceptable range. It is only a special case and no reference. significance.
It should be noted that the hum frequency caused by the electromagnetic interference of the transformer is generally about 50HZ, and the hum frequency caused by improper ground wiring is about 100HZ due to the frequency multiplication of the rectifier circuit, which can be noticed carefully.
The correct wiring method is to select the main filter capacitor pin as the centralized grounding point, and strictly separate the strong and weak signal ground lines and collect them at the total grounding 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 electromagnetic conversion process, in addition to magnetic leakage, the alternating magnetic field will cause the iron core to vibrate. When the old-fashioned 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 absorbed by the alternating magnetic field and causes vibration.
A well-made transformer, the iron core is tightly pressed, and at the same time, it must be processed by a vacuum dipping process before going offline. The vibration of the iron core caused by the alternating magnetic field is very small; such as the loose or uncompacted iron core of the transformer, and the vibration caused by the power on Will be stronger (imagine the hairdresser's electric clippers). Many low-cost transformers only use "dipping" paint instead of "vacuum dipping" in order to save man-hours, and the iron core vibration is more serious. The speaker cabinet has a certain role in assisting the sound cavity, and the air disturbance caused by the transformer vibration is conducted 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 checked the circuit and couldn't find the cause. I accidentally disconnected the speaker cable. The noise was hardly reduced. In the end, it was confirmed that the transformer was at fault.
This situation is common in active speakers. The quality of the transformer only affects the amplitude of the resulting amplitude. Even very expensive power transformers also have vibration. Therefore, the noise level of most active speakers is inferior to that of the main box. Deputy box.
The preventive measures for mechanical noise caused by the 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 rigorous workmanship 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 choose 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 not prone to magnetic saturation, have good long-term working stability, and generate relatively small heat.
There is also a common mechanical noise that comes from potentiometers. Most of the active speakers on the market use rotary carbon film potentiometers. With the passage of time, the metal brush of the potentiometer and the diaphragm will have poor contact due to dust deposition and diaphragm wear. A lot of noise is generated, and a potentiometer that is severely worn may make noise even when it is not rotating.
There are also some special dynamic noises that need to be briefly described: some active speaker cabinets are not firmly connected, or the user does not tighten the mounting screws after unpacking the box by themselves, and noise is generated when playing more dynamic music; Or due to imperfect processing methods, the box body has different degrees of air leakage; the two ends of the inverting tube are not made with double R or exponential openings, and the airflow is compressed and expanded sharply in large dynamics to generate noise.
(2) Thermal noise
The active speaker circuit part is composed of passive components such as resistors and capacitors and active components such as ICs and transistors. Electronic components will inevitably produce unique "background noise" of the components themselves under normal working conditions, which is commonly referred to as heat. noise. Thermal noise is a broad-spectrum thermal noise, mainly concentrated in the middle and high frequencies, and is generally reflected in the sense of hearing as the "hissing" sound emitted by the tweeter.
There are a large number of free electrons in the conductive part of passive devices, and the number of free electrons is directly related to temperature. The higher the temperature, the greater the number. The movement of free electrons can be regarded as disordered movement, which can be regarded as clutter compared with normal and ordered signal current. The number of free electrons in active devices such as ICs is much greater than that of passive devices. Active devices have an amplifying effect, so the thermal noise of active devices is higher than that of passive devices.
Thermal noise is also impossible to cure. The prevention and control measures are mainly to replace components and reduce component workload. 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, the thermal noise of carbon film resistors is lower than that of carbon resistors, and the thermal noise of low-noise, low-temperature drift ICs is better than general-purpose ICs. In addition, strengthening heat dissipation measures and reducing operating temperature are also effective means to reduce thermal noise and enhance working stability. Generally, the noise and zero drift of Class A power amplifiers are inferior to those of Class A and B power amplifiers. Excessive operating temperature is not only the increase in noise, but also the instability of leakage current and gain for active devices, which is detrimental to the long-term stable operation of the power amplifier.