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The noise of active speakers mainly comes from three kinds: electromagnetic interference, wire interference and mechanical noise and thermal noise. This article will introduce how to prevent and reduce these noises.
First, electromagnetic interference
The main sources of electromagnetic interference are power transformers and spatial stray electromagnetic waves.
In addition to a few special products, active speakers are mostly powered by the mains, so the power transformer must be used. The working process of the power transformer is an "electric-magnetic-electric" conversion process. Magnetic leakage will inevitably occur during the electromagnetic conversion process. The magnetic leakage of the transformer is picked up and amplified by the amplifying circuit, and finally the alternating sound emitted by the speaker.
The common specifications of the power transformer are EI type, ring type and R type. Whether from the sound quality angle or the electromagnetic leakage angle, these three kinds of transformers have their own advantages and disadvantages, and cannot be easily judged.
The EI type transformer is the most common and widely used transformer. The magnetic leakage mainly comes from the air gap between the E and I cores and the self-radiation of the coil. The magnetic leakage of the EI type transformer is directional. As shown in the figure below, in the three directions of X, Y and Z axes, 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. Between Z and Z, so try not to make the Y axis parallel to the board in actual use.
Since the toroidal transformer has no air gap and the coil is evenly wound around the core, theoretically, the magnetic leakage is small and there is no coil radiation. However, since the toroidal transformer has no air gap and has poor anti-saturation capability, it is prone to saturation when there is a DC component in the mains, and a strong magnetic leakage is generated. In many domestic areas, the waveform distortion of the mains is serious, so many users use the toroidal transformer and feel no better or even worse than the EI transformer. The so-called toroidal transformer has no leakage, or it is misleading due to media, or because the manufacturer has made a request for commercial propaganda. The magnetic leakage of the toroidal transformer is extremely low. It is only established when the mains waveform is a strict sine wave. In addition, the toroidal transformer will also have strong electromagnetic leakage at the lead. Therefore, the leakage flux of the toroidal transformer also has a certain directionality. When the actual installation is performed, the toroidal transformer is rotated to obtain the highest signal-to-noise ratio at a certain angle.
The R-type transformer can be simply seen as a circular transformer with a cross-section round shape, but there is a difference in the winding method of the coil. The heat-dissipating condition is much better than that of the toroidal transformer. The iron core is expanded to be an involute and progressive type, and the R-type transformer is electromagnetic. The leakage is similar to a toroidal transformer. Since the length of each turn is shorter than that of the toroidal transformer and can be wound close to the core, the R type transformer of the above three types of transformers has the smallest copper loss.
If conditions permit, consider installing a shield for the transformer and properly grounding it. The metal cover can only be made of iron. Generally, metals such as copper and aluminum have only electrical shielding and no magnetic shielding. They cannot be used as transformers. Shield.
The above analysis is based on the selection of materials and excellent production of transformers. Actually, most of the commercially available transformer products are not strictly designed according to industry norms, and even cut corners due to cost pressure and competition. There are many unpredictable factors in analysis. The first is the quality of the core material. Many companies use H50 iron cores, scraps and even noisy soft iron with low magnetic permeability to make transformers, resulting in high no-load current of the transformer, excessive iron loss and serious no-load heat generation. Reduce the cost, and at the same time cover the excessive voltage regulation caused by the high iron loss, greatly reduce the number of primary and secondary coil turns, reduce the copper loss to reduce the voltage adjustment rate, which further increases the air Current carrying, while the no-load current is too large will directly lead to increased magnetic leakage.
The toroidal transformer problem is more complicated. The regular toroidal transformer core is tightly wound from a strip of equal width silicon steel. Or for cost reasons, most low-cost toroidal transformers use several or even dozens of silicon steel strips for splicing, even using edge jagged edges to wrap, after winding, the machine is flat, due to the toroidal transformer coil package Around the iron core, it is difficult to find without destructive anatomy. Machining has severely damaged the lattice arrangement of silicon materials and the insulation between adjacent silicon steel strips. Such a toroidal transformer will be greatly reduced in performance and magnetic leakage characteristics, and even if it is annealed, it cannot compensate for serious defects in quality. .
Stray electromagnetic waves are mainly from the power output wires of active speakers, speakers and power dividers, wireless transmitters and computer mainframes. The reasons for this are not discussed in depth here. Stray electromagnetic waves are similar in transmission and induction to power transformers. The frequency range of stray magnetic fields is very wide. It is typical for spurious electromagnetic interference that useful speakers reflect that active speakers receive local radio broadcasts.
Another source of interference that needs attention is the rectifier circuit. After the filter capacitor enters the normal state, the charge is concentrated only on the peak of the AC current. The charge waveform is a strong pulse with a narrow width. The larger the capacitance, the greater the pulse strength. From the perspective of electromagnetic interference, the filter capacitor is not larger. Well, the trace between the rectifier and the filter capacitor should be as short as possible, and as far as possible away from the power amplifier circuit, PCB space is not allowed to use the ground envelope as much as possible.
Main prevention measures for electromagnetic interference:
1. Reduce the input impedance.
Electromagnetic waves are mainly picked up by wires and PCB boards. Under certain conditions, the electromagnetic waves picked up by the wires can be regarded as constant power. According to 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 beneficial to reduce electromagnetic interference. For example, the input impedance of an amplifier is reduced from the original 20K to 10K, and the induced noise level will drop to a level of 1/4. The active speaker sound source is mainly computer sound card, walkman, MP3, such sound source has strong carrying capacity, and the effect of properly reducing the input impedance of the active speaker on the sound quality is very weak and difficult to detect. I tried to input the active speaker input impedance during the experiment. It fell to 2KΩ, did not feel the sound quality changes, and there was no abnormality in long-term work.
2. Enhance high frequency anti-interference ability
For the stray electromagnetic wave, most of them are characterized by medium and high frequency signals. The magnetic capacitor is added to the ground at the input end of the amplifier. The capacitance can be selected between 47 and 220P. The capacitive frequency turning point of hundreds of skin capacitance values ​​is higher than the audio range. Three orders of magnitude have negligible effects on the sound pressure response and hearing in the effective listening audio segment.
3. Pay attention to the way the power transformer is installed.
Use a better quality power transformer, try to pull the distance between the transformer and the PCB, adjust the orientation between the transformer and the PCB, and keep the transformer and the sensitive end of the amplifier away; the interference intensity of each direction of the EI type power transformer is different, pay attention to avoid interference as much as possible. The strongest Y-axis direction is aligned with the PCB.
4. Metal case must be grounded
For the HIFI independent power amplifier, the product of the design specification has an independent grounding point on the chassis. The grounding point actually reduces the external interference by the electromagnetic shielding function of the chassis; for the common active speaker, it also serves as the radiator. The metal panel also needs to be grounded; the volume and pitch potentiometer housing should be grounded as far as possible, and it has been proved that this measure is very effective for PCBs working under harsh electromagnetic conditions.
Second, ground interference
The grounding design of electronic products is extremely important, and both low-frequency circuits and high-frequency circuits must follow the design rules. High-frequency, low-frequency circuit ground line design requirements are different, high-frequency circuit ground line design mainly considers the influence of distribution parameters, generally ring ground, low-frequency circuit mainly considers the size signal ground potential superposition problem, need independent routing, centralized grounding. From the perspective of improving signal-to-noise ratio and reducing noise, the analog audio circuit should be classified into low-frequency electronic circuits, strictly following the principle of “independent routing and centralized groundingâ€, 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 filtering and decoupling capacitor ground, and the small signal ground refers to the input signal and the feedback ground. The small signal ground and the power ground cannot be mixed, otherwise it will cause a strong AC sound: the strong electric ground due to the large charging and discharging current of the filtering and decoupling capacitors (relative to the signal ground current), there must be a certain pressure on the circuit board trace. Down, the small signal ground coincides with the strong electric ground, which is bound to be affected by the 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 a signal voltage at the input of the amplifier, and the ground potential change will be picked up and amplified by the amplifier to produce an hum. Increasing the ground line width and back tin treatment can only weaken the ground line interference to a certain extent, but the effect is not obvious. There are some PCBs that are not strictly separated by ground wire. Because the ground line is wide and the trace is short, the amplification stage is small, and the decoupling capacitance is small. Therefore, the hum is still barely acceptable. It is a special case, no reference. significance.
It should be noted that the frequency of the ac vibration caused by the electromagnetic interference of the transformer is generally about 50HZ, and the humming sound caused by the improper grounding wiring, because the frequency multiplication frequency of the rectifier circuit is about 100HZ, careful distinction can still be detected.
The correct wiring method is to select the main filter capacitor pin as the concentrated grounding point, and the strong and weak signal ground lines are strictly separated and summarized at the total grounding point.
Third, mechanical noise and thermal noise
(1) Mechanical noise
Active speakers integrate the speakers with the amplifier, 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 “electric-magnetic-electrical†conversion. In the electromagnetic conversion process, in addition to the magnetic leakage, the alternating magnetic field will cause the core to vibrate. When the old-fashioned ballast fluorescent lamp is working, the ballast will beep, and the sound will increase after a long time of use, because the iron core is shocked by the alternating magnetic field.
Well-made transformers, the core pressure is very tight, and at the same time before the offline line is subjected to the vacuum dip coating process, the core vibration caused by the alternating magnetic field is small; if the transformer core is loose, not compacted, the vibration caused by the energization It will be stronger (imagine the hair clipper of the barbershop). Many low-cost transformers only do "蘸" paint to save man-hours and do not do "vacuum dipping" treatment, the core vibration is more serious. The speaker cabinet has a certain sound-assist cavity function, and the air disturbance caused by the vibration of the transformer is transmitted to the diaphragm of the speaker, which sounds very similar to the noise caused by electromagnetic interference. Years ago, I repaired a set of active speakers with serious humbs. I couldn't find the reason for the circuit. I accidentally broke the speaker cable, and the noise was almost not reduced. Finally, it was confirmed that the transformer was a stranger.
This situation is ubiquitous in active speakers. The quality of the transformer only affects the amplitude of the final amplitude. Even the very expensive power transformers have vibrations. Therefore, the noise level of most active speaker main boxes is inferior. Deputy box.
The mechanical noise prevention and control measures caused by the power transformer are relatively simple, and can be used as a reference according to the actual situation:
1. Select a transformer with good quality and strict technology to reduce the vibration of the transformer itself. This 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. Select 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 prone to magnetic saturation, have long-term stability, and generate relatively little heat.
Another common mechanical noise comes from potentiometers. Most of the commercially available active speakers use a rotary carbon film potentiometer. As time goes by, the potentiometer metal brush and the diaphragm may have poor contact due to dust deposition and diaphragm wear. When turning the potentiometer A lot of noise is generated, and the worn-out potentiometer will have noise even when it is not rotating.
There are also some special dynamic noises that need to be briefly described: some of the active speaker cabinets are not securely connected, or the user does not press the mounting screws after unpacking, and there is noise when playing a slightly louder music; Or because the processing means is not perfect, the cabinet has different degrees of air leakage; the two ends of the inverted tube do not have double R or exponential type openings, and the airflow is rapidly compressed and expanded to generate noise during large dynamics.
(2) Thermal noise
The active speaker circuit is composed of passive components 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. noise. Thermal noise is a broad-spectrum thermal noise, mainly concentrated in the mid-high frequency, which is reflected in the sense of hearing is generally the "beep" sound emitted from the tweeter.
There are a large number of free electrons in the conductive part of the passive device. The number of free electrons is directly related to the temperature. The higher the temperature, the more the number. The free electron motion can be regarded as disordered motion, which can be regarded as clutter compared with the normal ordered signal current. The number of free electrons of active devices such as IC is much larger than that of passive devices. The active devices have amplification, so the thermal noise of active devices is higher than that of passive devices.
Thermal noise is also incurable, and the main means of prevention and control are to replace components and reduce component workload. Replacement of components refers to the use of low-noise components, such as metal film resistance thermal noise is lower than carbon film resistance, carbon film resistance thermal noise is lower than carbon resistance, low noise, low temperature drift IC thermal noise is better than general IC. In addition, strengthening heat dissipation measures and lowering the working temperature are also effective means to reduce thermal noise and enhance work stability. Generally, Class A power amplifier noise and zero drift are inferior to Class A and Class B power amplifiers. Excessive operating temperature is not only an increase in noise. For active devices, it also means that leakage current and gain are unstable, which is detrimental to the long-term stable operation of the power amplifier.
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