noise sources

 

The sources of noise in audio systems can be mainly divided into the following categories.

 

1The inherent noise of the system equipment itself

 

At present, the stand-alone technical indicators of broadcast broadcast control equipment are very high and have very low local noise indicators. But when multiple devices are connected, noise accumulates and increases. In practical applications, some low-endaudio equipmentDue to poor filtering of the internal power supply, the AC noise of the equipment itself will be very high, and sometimes serious noise will be formed in the sound system.

 

 

 

2Noise caused by external electromagnetic radiation interference

 

For example, high-frequency electromagnetic wave radiation interference from communication equipment such as mobile phones and walkie-talkies, electrical pulse radiation such as air conditioners, car ignitions, and welding in the surrounding environment, and radiation generated by silicon controlled rectification equipment used in studio lighting control will all be directly mixed into the transmission signal through the audio transmission line to form noise, or pass through the shell of poorly shielded machinery and equipment to interfere with internal circuits to produce interference noise.

 

 

 

3, power interference noise

 

For external interference from audio equipment, in addition to electromagnetic radiation, interference noise introduced into the power supply part will be another main cause of noise. Due to the common connection of various lighting equipment, power equipment, and control equipment, a very serious source of interference has been formed.

 

For example, lighting control equipment, air conditioners, motors and other equipment connected to the same power grid will generate spikes and surge circuits on the power line. Ripple voltages of different frequencies will enter the power supply of audio equipment through the power line, and there will always be a part of the interference noise cannot be effectively filtered through the power supply circuit of audio equipment, which will inevitably form noise inside the equipment. In particular, high-power equipment in the same power grid whose electromagnetic compatibility does not meet the requirements is the main reason for interference with audio equipment.

 

4, ground loop noise

 

In the audio system, the entire system must be required to have a good grounding, and the grounding resistance must be less than4ohm. Otherwise, the induced charge generated by the equipment in the sound system due to various radiation and electromagnetic induction will not be able to flow into the earth, thus forming a noise voltage superimposed on the audio signal.

 

If there is a ground potential difference between the ground wires of different equipment due to the difference in ground resistance, or if there is a loop in the internal grounding of the system, ground noise will be caused. When two different audio systems are interconnected, noise may also be generated, which is caused by the direct connection of the ground wires of the two systems.

 

solution

 

1, good grounding handling

 

In order for a cable with a shielding layer to shield stray electromagnetic interference from the outside world, the shielding layer must be correctly connected and well grounded. In practice, suspending all equipment is the most commonly used measure when there is no special ground wire.

 

But this is an extremely unstable working state, which often produces unstable random noise, so the entire system must be well grounded. First of all, a special ground wire should be provided, and the ground resistance should be less than4Ω。The neutral wire of the power supply cannot be used as the ground wire of the audio system equipment. In outdoor places, you can consider laying temporary ground wires. The easiest way is to insert a steel pipe or aluminum alloy pipe about one meter long into the ground and treat it with salt intrusion. The effect is very good.

 

A general system is a link system with multiple devices connected through cables. It is easy to form a chain grounding method from its shielding system. When electromagnetic radiation or electrostatic induced noise is generated on a certain equipment, an induced voltage will be generated in the entire system due to the grounding system composed of the shield layer of the transmission line and the iron equipment shell. In turn, the system generates a certain noise level, and this kind of interference is particularly obvious on audio systems with long links. Therefore, the system should try to avoid using chain grounding, Instead, star grounding should be used. That is, each equipment is connected to a unified ground point through a special ground wire, which requires that the shielding layer of the audio cable connecting all equipment must be grounded at one end. The ground wires of each equipment at the shield layer are connected through a special wire and one ground point.

 

 

 

 

If the shielding layers at both ends of the signal transmission line are grounded, a ground loop will inevitably be formed. When the loop is interfered by electromagnetic radiation from other equipment, induced current will inevitably appear in the shielding layer of the cable, resulting in serious interference noise and formation of ground loop noise interference

 

 

 

 

In order to ensure that the system does not have a ground loop structure, only one ground conductor is required to be interconnected between each equipment. In occasions where requirements are not strict, unbalanced equipment can be suspended and the ground wire of the next level of equipment can be shared through the audio signal line, that is, chain grounding is used. The number of stages of this chain grounding cannot be too many, generally no more than two levels, otherwise the noise will be seriously increased.

 

The connection problem between casings should also be paid attention to. For example, many devices are installed on the same rack. If each device is individually connected to a ground wire,2Because the equipment is installed on the same rack, the casings are connected, which of course forms a ground loop.

 

2Isolation of the system

 

In some large audio systems. It is often composed of many subsystems. Most of these systems are connected remotely and have independent grounding systems.2Once each system is connected to ground, ground noise will inevitably be formed. On the other hand, due to the long transmission distance and the increase in the ground resistance of the transmission line shield layer, it is easy to introduce a large amount of external electromagnetic field radiation interference noise.

 

 

 

 

In practice, if each system works separately, noise can be controlled within allowable levels through reasonable wiring and grounding. However, when multiple subsystems are interconnected, even if single-ended shielded grounding and long-line segmented grounding are used, there is no way to solve the radiated interference noise caused by long-distance transmission. The best way at this time is to install an audio isolation transformer. Install audio isolation transformers between multiple systems to isolate them from each other. The ground wires of multiple systems must not be connected. The effect is better to completely isolate different systems by using light isolation.

 

 

 

 

3Correct connection of the system

 

In an audio system, there are generally many connected devices. Different devices have different interface forms and the connectors used are different. There are balanced and unbalanced input and output forms. In order to effectively shield external electromagnetic radiation interference, shielded cables must be used uniformly and connected in the correct way.

 

When the audio signal transmission is balanced, external interference power supply affects the2The level of common mode interference generated by each signal line is almost equal to the ground loop. At the input end of the internal amplifier of the equipment,2The common mode voltages on the signal lines will be replaced by differential mode voltages to cancel each other out, and no interference voltage will be formed. Therefore, a balanced connection method should be used as much as possible.

 

When connecting to unbalanced output equipment, directly use a single-core shielded cable to connect the port of the balanced equipment to the port of the unbalanced equipment, instead of using balancing—unbalanced converter. Noise induced by the shielding layer is mixed into the audio signal, thereby increasing noise, which is a major way to introduce noise. Therefore, whether it is balanced or unbalanced transmission, a dual-core shielded cable should be used, and the shielding layer is only grounded at one end of the balanced output or input.

 

 

 

 

When both ends are unbalanced equipment, it is best to use balance if the transmission distance is long—Converts to balanced transmission with an unbalanced converter or audio isolation transformer.

 

 

 

 

Nowadays, the connection of audio equipment generally uses voltage jumper connections. That is to say, the line output of all audio equipment is a low-impedance output, while the line input terminal as a load uses a high-impedance input. Except for the connection between the power amplifier and slightly, impedance matching generally does not need to be specifically considered.

 

4Purification of power supply

 

In order to isolate interference noise formed by the public power grid. It is best to use an isolated purification power supply or an isolation transformer. The grounding terminal of the isolation transformer or purification power supply must be well grounded, otherwise the isolation effect will not be good. It must be isolated from some high-power electrical appliances with strong interference. Separate power supply. You can also install a filter at the input of the audio equipment power supply to filter out interference noise.

 

In actual work, you can also find a connection method with the least noise by changing the positions of the live and neutral line inputs of single-phase powered audio equipment. This can also reduce some noise interference. Also pay attention to the fact that the audio transmission line must not be wired in parallel with the power line. The audio line and the power line must be wired across to reduce AC noise interference.

 

The above analysis of the noise problem generated in the audio system. Corresponding solutions are proposed according to the causes of noise. In actual work, if the cause of the noise cannot be ascertained in a short period of time, effective means such as noise gates, noise reducers, and filters can also be used to reduce the noise.