How to reduce the noise of active speakers

The noise of active speakers mainly comes from three sources: 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 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. The main sources of magnetic leakage are the air gap between E and I type 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 axis, 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.

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.

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 only have electrical shielding effects but no magnetic shielding effects and cannot be used as transformer shielding covers.

The above analysis is based on the selection of transformer materials and excellent production. In fact, most transformer products on the market are not strictly designed according to industry specifications due to cost pressure and competition needs, and 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 with low magnetic permeability, scraps, or even mixed soft iron to make transformers, resulting in high no-load current, excessive iron loss, and serious no-load heating of the transformer. In order to reduce costs and cover up the problem of excessive voltage regulation caused by high iron loss, this type of transformer greatly reduces the number of turns of the primary and secondary coils to reduce the voltage regulation rate by reducing copper loss. This practice further increases the no-load current, and the excessive 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 made of a tightly wound silicon steel strip of equal width. Still for cost reasons, most low-priced toroidal transformers use several or even dozens of silicon steel strips to splice, or even use scraps with jagged edges to wind, and then use machine tools to flatten them after winding. Since the coil of the toroidal transformer is wrapped around the core, it is difficult to find without destructive dissection. Mechanical processing has serious damage to the lattice arrangement of silicon materials and the insulation between adjacent silicon steel strips. Such toroidal transformers will greatly reduce both performance and leakage magnetic characteristics, and even annealing cannot make up for the serious quality defects.

Stray electromagnetic waves mainly come from the power output wires of active speakers, speakers and power dividers, wireless transmitters and computer hosts. The causes are not discussed in depth here. The transmission and induction forms of stray electromagnetic waves are similar to those of power transformers. The frequency range of stray magnetic fields is very wide. Some users have reported that their active speakers inexplicably receive local radio broadcasts, which is a typical example of stray electromagnetic wave interference.

Another interference source that needs attention is the rectifier circuit. After the filter capacitor is turned on and enters the normal state, the charging is concentrated only at the peak of the AC power. 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 larger the filter capacitor, the better. The wiring between the rectifier tube and the filter capacitor should be shortened as much as possible and kept as far away from the power amplifier circuit as possible. If the PCB space does not allow, try to use the ground wire envelope.

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, the low impedance of the amplifier is very beneficial to reducing electromagnetic interference. For example, if the input impedance of an amplifier is reduced from the original 20K to 10K, the induced noise level will be reduced to 1/4. The sound source of active speakers is mainly computer sound cards, walkmans, and MP3s. These sound sources have strong carrying capacity. The effect of appropriately reducing the input impedance of active speakers on the sound quality is very weak and difficult to detect. During the experiment, the author tried to reduce the input impedance of the active speaker to 2KΩ, and did not feel the sound quality change, and there was no abnormality in long-term operation.

2. Enhance high-frequency anti-interference capabilities

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 can be selected between 47 and 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, keep the distance between the transformer and the PCB as large as possible, 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 different directions, 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

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 ground of the filter and decoupling capacitors, 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 the signal voltage at 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, short traces, few amplification stages, and small decoupling capacitor capacity, so the AC noise is still within the barely acceptable range. It is just a special case and has no reference significance.

It should be noted that the frequency of the AC noise caused by the electromagnetic interference of the transformer is generally around 50HZ, while the AC noise caused by improper ground wiring is about 100HZ due to the frequency doubling of the rectifier circuit, so it can be detected by careful distinction.

The correct wiring method is to select the main filter capacitor pin as the centralized grounding point, strictly separate the strong and weak signal ground lines, and summarize them at the total grounding point.

3. Mechanical noise and thermal noise

1. Mechanical noise

Active speakers integrate the speaker and amplifier together, so some of the noise is unique.

The most common source of mechanical noise is the power transformer. As mentioned above, the working process of the power transformer is the process of "electric-magnetic-electric" conversion. In the process of electromagnetic conversion, 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 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 vibration more serious. The speaker cabinet has a certain sound-assisting cavity effect. 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. 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.

The prevention and control measures for mechanical noise caused by power transformers are relatively simple. The following points can be used as reference according to actual conditions:

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. A transformer with a large power margin is less likely to have magnetic saturation, has good long-term working stability, and generates 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.

There are also some more special dynamic noises that need to be briefly described: the joints between the panels of some active speakers are not firm, or the user does not tighten the mounting 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.

2. Thermal noise

The active speaker circuit is composed of passive components such as resistors and capacitors and active components such as ICs and transistors. Under normal working conditions, electronic components will inevitably produce their own unique "background noise", which is often called thermal noise. Thermal noise is a broad-spectrum thermal noise, mainly concentrated in the mid- and high-frequency range, and is generally reflected in the "hissing" sound emitted by the tweeter.

There are a large number of free electrons in the conductive part of passive devices. 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 noise compared with the normal and orderly signal current. The number of free electrons in active devices such as IC is much larger than that in 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 incurable, and the main means of prevention and control are to replace components and reduce component workload. Replacing components means using low-noise components, such as metal film resistors with lower thermal noise than carbon film resistors, carbon film resistors with lower thermal noise than carbon resistors, and low-noise, low-temperature drift ICs with better thermal noise than general-purpose ICs. In addition, strengthening heat dissipation measures and lowering operating temperatures are also effective means of reducing thermal noise and enhancing operating stability. Generally, Class A amplifiers have lower noise and zero drift than Class A and Class B amplifiers. Excessively high operating temperatures not only increase noise, but also, for active devices, mean leakage current and gain instability, which is not conducive to the long-term stable operation of the amplifier.