The following describes the specific processing methods.
Eliminate noise caused by improper connection of the device
In the audio system, there is a common problem of interconnection of devices. If the connection is improper, the lighter causes the system index to drop, causing noise, and even when the device is not working properly. Make the following points when connecting:
1, impedance matching
In the audio system, almost all devices are connected in a bridging manner, 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. China Home Theater Network
2, level matching
Level matching is equally important when audio equipment is interconnected. 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. China Home Theater Network
3. Balance and imbalance
Audio equipment usually has two connections: balanced and unbalanced. 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 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 also certain technical requirements for this:
(1) 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 of the power supply of the power grid from entering the sound system.
(2) It is best to use a multi-strand textile thread for the grounding wire. The area of ​​the welding point should be large enough to ensure 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 boards.
(3) The audio equipment should not be grounded at the input and output terminals at the same time, forming a grounding circuit loop, which is easy to generate self-excitation.
(4) Grounding requires a single point of grounding at the input. Because the signal level is small at this point and the noise level is also minimal, grounding at this point will 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.
Second, eliminate the inherent noise of the device itself
The sound system is made up 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, you can turn off the sound gain knob of other microphones in turn
If the noise of the microphone channel is turned off and the noise disappears, 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 thus generated: if there is noise, the noise is not generated thereby, and other units can continue to be detected.
4. Turn off the power switch of the mixer to see if the noise disappears.
If the noise disappears, it is proven 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
Continue to shut down the peripherals such as the effect processor, exciter, compressor, equalizer, electronic crossover, power amplifier, and acoustic feedback suppressor to observe whether the noise 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 discovering the unit that generates noise, 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. Un-charged and live-tested components, and repairable to detect faulty or damaged parts. If there is no repair capability, replace the new unit.
Third, eliminate the interference noise of the power supply
1. Noise interference of light thyristor
On the stage, the biggest interference from power supply noise is the noise from the light thyristor, because the brightness and intensity of the theater and dance hall are not changed by changing the voltage of 0~220V, but by electronic circuit control. The conduction angle of the silicon tube is controlled, and the conduction area of ​​the sine wave of the alternating current is changed to control the change of the power transmission. Therefore, the sinusoidal waveform in the power supply is changed to a sawtooth-like shape, and a significant 50 Hz ripple is generated in the power supply, which is manifested in the sound system, and significant noise appears. In order to eliminate the interference noise of the thyristor, the most effective way is to provide two phases of the three-phase power supply to the light, and another phase as the sound system. This will avoid the interference of the light thyristor.
2. Clutter interference of grid power supply
In the utility power grid, equipment such as electric drills, impact drills, electric welders, and elevators in the nearby construction sites are often connected in parallel on the public power grid, as well as household appliances of residents in nearby communities, such as washing machines, refrigerators, and range hoods. Produce certain pollution. If you use a dual trace oscilloscope to test it. It can be clearly seen that the sine wave often has a lot of high frequency wave burrs. In order to eliminate the clutter noise interference of these external power sources, the following methods can be adopted:
(1) If the grid voltage is often low, use a voltage regulator with a voltmeter to adjust the voltage that is too low or too high, that is, adjust the operating voltage to 220V through the regulator. Due to the electromagnetic circuit of the voltage regulator, the noise in the power supply can be eliminated to some extent. China Home Theater Network
(2) Use a regulated power supply. The automatic regulated power supply recognizes the low or high voltage in the power grid through a comparison circuit in the electronic circuit, and then controls the voltage regulator to automatically start through the servo circuit to adjust the power supply voltage within the range of 220V. Electronic circuits and voltage regulators have a certain elimination effect on power supply noise.
(3) Use an isolation transformer. The isolation transformer is a high-power power transformer, the grid voltage is primary, and the voltage is the secondary coil. In this way, the primary voltage and the secondary user power supply are not directly connected, and the noise in the power supply is filtered cleanly, and the electricity is purified.
(4) Use a small purification power supply. Made of electronic circuits, it can be made compact, can be installed in a larger power supply box, provide power for wireless microphone receivers and mixers, and can also be used for effects processors, actuators, compressors, equalizers The electronic crossover provides power supply and has a good purifying effect on the power supply, thereby improving the sound quality level of the sound system.
Four conclusions
This paper mainly discusses the external interference noise in the stage performance and live sound amplification practice. Of course, some noise is the thermal noise generated by the circuit inside the audio equipment, and some are the field environment directly contained in the input signal source or picked up by the microphone. Noise, which is related to the conditions of the equipment and sound source, this article does not introduce.
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