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A way to eliminate noise

Correct connection of the system

In an audio system, there are usually many connected devices. Different devices have different interface forms and use different connectors. There are balanced and unbalanced input and output forms, in order to effectively shield the electromagnetic radiation interference from the outside world, it is necessary to use the shielded cable and use the right way to connect.

It is well known that when the audio signal transmission adopts the balanced transmission mode, the common-mode interference level generated by the external interference source to each of the 2 signal lines in the cable is almost equal to the ground loop. At the input end of the amplifier inside the device, the common mode voltages on the two signal lines will be replaced by differential mode voltages and cancel each other out, creating no interfering voltages. Use a balanced connection whenever possible.

When connecting with unbalanced output devices, the port of the balanced device and the port of the unbalanced device can be connected directly by a single core shielded cable. Instead of a balanced - unbalanced converter. The noise induced by the shielding layer is mixed into the audio signal to increase the noise. This will be a major way to introduce noise. The recommended approach is to use a dual-core shielded cable for both balanced and unbalanced transmissions, and to ground the shielding layer only at one end of the balanced output or input, as shown in Figure 1. When there are unbalanced connections at both ends, it is best to use a balanced -- unbalanced converter or audio isolation transformer to convert to a balanced transmission if the transmission distance is large, as shown in Figure 2. The current audio equipment is generally connected by voltage bridge. In other words, the circuit output of all audio equipment is low-impedance output, while the input end of the circuit as the load adopts high impedance input. In addition to the connection between the power amplifier and the speaker, there is generally no special consideration of impedance matching.

Good grounding treatment

In order to make the cable with shielding layer can shield the outside stray electromagnetic interference. The shielding layer must be properly connected and well grounded. In practice, levitation of all equipment is the most commonly used method in the absence of special ground wire.

However, this is an extremely unstable working state, which tends to produce unstable random noise. Therefore, the whole system should be well grounded. First of all, there should be a special ground wire, and the ground resistance is less than 4 ohms. The null line of the power supply cannot be used as the ground line of the audio system equipment. In the outdoor place, temporary ground wire can be considered to be buried. The simplest way is to insert a steel tube or aluminum alloy tube about one meter long into the ground, and do salt treatment, the effect is very good.

A common system is a link system with multiple devices connected by cables. It is easy to form a chain grounding system by its shielding system. When electromagnetic radiation or electrostatic induced noise is generated on a certain equipment, the whole system will generate induced voltage due to the shielding layer of the transmission line and the grounding system composed of the iron equipment housing. This kind of interference is especially obvious in the audio system with long link. Therefore, the system should avoid chain-type grounding and use star type grounding instead. That is, each device is connected to the unified connection point through a special ground wire, which requires that the shielding layer of the audio cable connected to all devices should be grounded at one end. The ground wire of each device at the connection shield is connected by a special wire connection point.

If the shielding layer at both ends of the signal transmission line is grounded, a ground circuit must be formed. When the circuit is interfered by electromagnetic radiation from other equipment, induced current is bound to occur in the shielding layer of the cable, resulting in serious interference noise and noise interference of the ground circuit, as shown in FIG. 4.

In order to ensure that the system does not have ground loop structure, it is required that there should be only one grounding wire interconnection between each equipment, in the case of not strict requirements. The unbalanced equipment can be levitated to share the ground wire of the next level equipment through the audio signal wire. That is to say, the use of chain grounding, but the series of chain grounding can not be too much, generally not more than 2 level. Otherwise, the noise will increase seriously.

Attention should also be paid to the connection between the chassis. For example, many devices are installed on the same rack. If each device is connected to the ground wire separately, two devices are connected to the chassis because they are installed on the same rack, and of course the ground loop is formed.

System isolation

In some large audio systems, it is usually composed of several sub-systems centered on multiple mixing stations or connected with the video equipment system. Sometimes it is necessary to transmit signals to remote audio and video systems, radio stations and even common telephone lines to transmit live audio broadcast signals. For these remote connections, different subsystems have their own independent grounding systems. Once each subsystem is connected to the ground wire, ground noise is bound to form, as shown in Figure 5. On the other hand, due to the long transmission distance, the grounding resistance of transmission line shielding layer will increase, and even the use of unshielded transmission line, etc., it is easy to introduce a lot of external electromagnetic field radiation interference noise. In practical applications, if each system works alone, noise can be controlled within the allowable level through reasonable wiring and grounding, but when multiple subsystems are interconnected, even if single-terminal shielded grounding or long line segmenting grounding processing is used, there is no way to solve the radiation interference noise caused by long distance transmission. Especially with a large telephone network, the best thing to do is to use quarantine. Audio isolation transformers are installed between systems to isolate them from each other. The ground wires of multiple subsystems should not be connected, so the method of optical isolation can completely isolate different subsystems, and the effect is better.

The power to purify

In order to isolate the interference and noise caused by the public power network, it is better or generally used to isolate the purification power supply or isolation transformer. The grounding end of the isolation transformer or purification power supply must be guaranteed to have reliable and good grounding. Otherwise the quarantine effect is not good. To be isolated from some high-power electrical appliances with strong interference, provide power separately, or install a filter at the power input end of audio equipment to filter out the interference and noise.

Sometimes it is also possible to find a connection interpolation method with the least noise by changing the position of the fire line and zero line input of single-phase power supply audio equipment. This can also reduce some noise interference, but also pay attention to the audio transmission line and the power line should not be parallel wiring, the audio line and the power line should be cross-wiring, also reduce ac noise interference.


This paper mainly discusses the external interference noise in the practical application of audio frequency and sound reinforcement equipment. Of course, some of the noise is thermal noise generated by the circuitry inside the device. Some are directly contained in the input source