Tip 1: Eliminate the noise caused by improper connection of equipment In the audio system, the interconnection problem of equipment is common. If the connection is improper, the system will reduce the system index and generate noise. In severe cases, the equipment may not work properly. Make the following points when connecting:
1. Impedance matching: In the audio system, almost all devices use the bridging method, that is, the output impedance of the device is designed to be small, and the input impedance is large. This is due to the fact that in the system, unless the signal is transmitted over long distances, it is generally treated as a short line. Moreover, the signal level is low, and the signal is required to be transmitted at a high quality, and the change of the load does not substantially affect the quality of the signal. When the signal source is designed as a constant voltage source, or the load is much larger 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. The interconnection of devices is connected by means of jumper. This is the impedance matching of audio equipment. When designing a sound reinforcement system, it is generally unnecessary to consider the impedance problem. However, when the output of one device needs to connect multiple devices, that is, one source must drive an active or passive source signal distributor when driving several loads to meet the requirements of device impedance matching (if two devices, Generally available directly at the output of the preamplifier). The amplifier and speakers 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 that of 8Ω. Two 8Ω speakers can be connected to the output of the amplifier for 4Ω operation. It must be noted that when the speakers are connected in parallel, the impedance will be reduced, and the parallel equivalent impedance is not less than the nominal minimum output impedance of the power amplifier. Otherwise, the load will be overloaded and cannot work normally. When using a 4Ω load impedance, the required transmission line impedance is twice as low as 8Ω. In a high-quality sound system, the transmission impedance at 4Ω output does not exceed 0.2Ω (excluding the internal resistance of the amplifier). If the transmission is less than 100m, the cross-section is required to be no less than 9mm2. To reduce the cross section, you need to use an 8Ω output instead of a 4Ω output, in which case the cable cross-section can be halved. It is also required that the contact resistance at both ends of the transmission line is small. When preparing a sound system in a performance, it was found that the noise was large, and the connectors at both ends of the transmission line were replaced with better gold-plated connectors, and the noise was significantly reduced.
2, level matching: When the audio equipment is interconnected, the level matching is also important. If the match is not good, or there will be insufficient incentives, or an overload will occur and severe distortion will occur, the system will not work properly. To achieve level matching, it is not only necessary to match in the state of the rated signal signal, but also when the signal is spiked, no overload occurs. The peak factor of the quality system should be considered at least 10dB. Modern audio equipment is designed to meet the requirements of level matching by simply selecting equipment and tuning the system.
3, balance and imbalance: audio equipment usually has two ways of connection between balance and imbalance. When there is common mode interference, since the interference signals received on the two balanced terminals are similar and the polarities are opposite, the interference signals can cancel each other out in the balanced transmission load. Therefore, the balance circuit has better anti-interference ability. In important performance activities, try to use balanced input and output.
4. Shielding: The metal external passenger of the equipment should be properly grounded, the grounding resistance should be less than 4Ω, and the interference noise should not be introduced due to grounding. Strong external high-frequency radio waves can also cause high-frequency radio waves to interfere with the sound system, especially when using wireless microphones, external radio waves such as public security, fire, and taxi call systems will interfere with the reception of wireless microphones. And produce noise. It is recommended that the selected wireless microphone be freely adjusted for both the transmit and receive frequencies.
If the sound system is poorly grounded, it will cause noise to be sent to the next level of equipment, and sometimes it will sense the sound of the radio broadcast. Therefore, grounding has a major impact on noise cancellation. There are certain technical requirements for this: the grounding point of the sound system should not be connected to the ground of the power supply at the same point, so as to avoid the noise interference of the power supply of the power supply into the sound system; the grounding wire is preferably a multi-strand textile thread, the solder joint The area should be large enough to require good welding and avoid false welding, virtual welding, and desoldering. Do not use welding oil when welding. Use alcohol rosin. If soldering oil or the like is used, the solder joints and nearby circuits should be cleaned after soldering to prevent corrosion of components and circuit boards; the audio equipment should not be grounded at the input and output terminals at the same time to form a grounding loop, which is prone to self-excitation; Grounding requires a single point grounding at the input, because the signal level is small at this point and the noise level is also minimal. At this point, grounding is performed to short-circuit the noise to ground. If the output point is grounded, the signal level is large at this time, and the noise level is also large, so it is not easy to completely eliminate the noise.
Tip 2: Eliminate the inherent noise of the device itself. The sound system consists of multiple devices, such as microphones, DVD decks, mixers, effects, equalizers, compressors, actuators, electronic crossovers, power amplifiers. , speakers, etc., each device may be the source of noise. In order to find out and judge which unit the noise is generated from, it is necessary to perform the detection of the inherent noise for each unit. The specific inspection procedure is as follows.
1. Turn on the sound system: the order of booting is turned on step by step in the order of the signal flow. After the power is turned on, there is noise coming out of the speaker. First, you can turn off the power of the wireless microphone receiver and hear if there is any noise. If the noise disappears, it proves that the noise is generated by the wireless microphone receiver: if there is still noise, it proves that the noise is not generated by the wireless microphone receiver. Other units can be detected again.
2. The sound gain knobs of other microphones can be turned off in turn: If the knob of a certain microphone channel is turned off, the noise disappears, and it can be judged that the noise is generated by this path. If the noise is still there, then the other units will be detected again.
3. Turn off the power of the DVD player and observe if the noise disappears: If the noise disappears, the noise source is generated: if there is noise, the noise is not generated, and other units can continue to be detected.
4. Turn off the power switch of the mixer and discern whether the noise disappears: If the noise disappears, it proves that the noise is generated by the mixer: If the noise still exists, it is proved that the noise is generated by other units.
5, can turn off the peripheral equipment in turn to listen to: continue to the effect processor, exciter, compressor, equalizer, electronic crossover, power amplifier, acoustic feedback suppressor and other peripheral equipment to shut down, observe the noise Whether it disappears. If the noise disappears when a unit is turned off, it is proved that the noise is generated by this unit.
6. Specific inspection: After the unit that produces noise is found, the casing can be opened to check the structure of the machine body to check whether the circuit board jumper, socket and interface, and the joint are in good contact.
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