How to avoid the noise caused by improper connection of professional audio equipment

In the audio system, there is a general problem of equipment interconnection. If the connection is improper, the lighter will cause the system index to drop and cause noise. In severe cases, the device may not work properly. The following points should be done when connecting:

From the perspective of an R & D engineer and system engineer, you can judge from the following points:

1. Impedance matching

In the audio system, almost all devices use a jumper, that is, the output impedance of the device is designed to be very small, and the input impedance is large. This is because in the system, unless the signal is transmitted over a long distance, it is generally treated as a short line. Moreover, the signal level is low, which requires that the signal can be transmitted with high quality, and the change of the load basically does not affect the quality of the signal. When the signal source is designed as a constant voltage source, or the load is much greater than the internal impedance of the signal source, the above requirements can be met. In fact, the impedance of professional audio equipment is designed according to the above principles, and the equipment interconnection uses a cross-connect method, which is the impedance matching of audio equipment. When designing a sound reinforcement system, that is, when a signal source drives several loads, an active or passive audio signal distributor must be used to meet the equipment impedance matching requirements (if it is two devices, it can generally be directly and in the front stage The output end of the equipment) The power amplifier and the speaker are connected according to the nominal output impedance and the input impedance of the speaker. The output impedance of the amplifier is 4Ω and 8Ω, which can be connected to 4Ω speakers or 8Ω speakers. When connected to a 4Ω speaker, the output power of the amplifier is larger than when it is 8Ω. Two 8Ω speakers can be connected in parallel to the output of the power amplifier, in a 4Ω working state. It must be noted that when the speakers are connected in parallel, the impedance will decrease its parallel equivalent impedance is not less than the nominal minimum output impedance of the power amplifier, otherwise it will cause the load to be overloaded and cannot work normally. When using a 4Ω load impedance, the required transmission line impedance is twice lower than 8Ω. In a high-quality sound system, the transmission impedance at 4Ω output does not exceed 0.2Ω (not counting the internal resistance of the amplifier). If the transmission is less than 100m, the cross-sectional area can be reduced by half. It is also required that the contact resistance at both ends of the transmission line is small. When preparing the sound system for a performance, it was found that the noise was large and the connectors at both ends of the transmission line were replaced with better connectors, and the noise was significantly reduced.

2. Level matching

When professional audio equipment is interconnected, level matching is equally important. If the matching is not good, either there will be insufficient excitation, or an overload will occur and severe distortion will occur, which will cause the system to not work properly. To achieve level matching, it is necessary not only to match under the rated signal state, but also when the signal spikes, no overload occurs. The crest factor of a high-quality system is at least 10dB. Modern audio equipment is designed according to the standard, and only needs to be paid attention to when selecting the model and tuning the system to meet the requirements of level matching.

3. Balance and unbalance

Professional audio equipment usually has two connection methods: balanced and unbalanced. When there is common mode interference, the interference signals received on the two balanced terminals have the same value and opposite polarities, so the interference signals can cancel each other out on the balanced transmission load. Therefore, the balanced circuit has better anti-interference ability. In important performance activities, try to use balanced input and output.

4. Shield

The metal shell of the device should be properly grounded, the grounding resistance should be less than 4Ω, and interference noise should not be introduced due to grounding. Strong external high-frequency radio waves can also cause high-frequency radio interference to the sound system, especially when using wireless microphones, external radio waves such as radio waves from public security, firefighting, and taxi calling systems can interfere with the reception of wireless microphones. And produce noise. It is recommended to choose a wireless microphone whose transmission frequency and reception frequency can be adjusted freely.