1. Good grounding treatment
In order for the shielded cable to shield the external stray electromagnetic interference, the shield must be properly connected and well grounded. Equipment suspension is a measure without special ground conditions, which is an unstable working condition. Random noise is often generated. Therefore, there must be a special ground wire. In the outdoor place, the ground wire can be temporarily buried. The easiest way is to insert the steel pipe or aluminum alloy pipe into the ground.
The general system is a link system in which a plurality of device is connected by cables, and it is easy to form a chain grounding method by the shielding system. When electromagnetic radiation or static induction noise is generated on a device, the grounding system consisting of the shielding layer of the transmission line and the iron equipment casing causes the entire system to generate an induced voltage, which in turn causes the system to generate a certain noise level. Therefore, the system should try to avoid the use of chain grounding, and use the star grounding method. This requires that the shield of the audio cable connecting all the devices be grounded at one end (to the shield), and the ground wire of each device is connected to a grounding point by a special wire.
In order to ensure that the system does not appear in the ground loop structure, only one grounding conductor can be interconnected between devices. In the case where the requirements are not strict, the unbalanced equipment can be suspended, and the ground wire of the next-level equipment is shared through the audio signal line, that is, the chain ground is used. The number of stages of this chain grounding should not be too much, generally no more than two levels, otherwise the noise will be seriously increased.
2. System isolation
In some large audio systems often composed of many subsystems. Most of these systems are remote connections and have separate grounding systems. Once the two systems are connected to ground, ground noise is inevitably formed. On the other hand, since the transmission distance is long and the grounding resistance of the transmission line shielding layer is increased, it is easy to introduce a large amount of external electromagnetic field radiation to interfere with noise.
In practice, if each system works alone, the noise can be controlled within the allowed levels by reasonable wiring and grounding. However, when two subsystems are interconnected, single-ended shielded grounding and long-line segmented grounding are used, and there is no way to solve the radiated interference noise caused by long-distance transmission. The best way to do this is to install an audio isolation transformer. An audio isolation transformer is installed between the two systems to isolate them from each other, and the ground lines of the two systems must not be connected.
Many systems are now connected by fiber optics, which works best. Long-distance audio transmission of optical fibers does not introduce thyristor dimming equipment and other stray electromagnetic waves to the audio system due to the skin effect of the cable, and can electrically isolate the audio system of each transmission end, thereby avoiding Interference between individual audio systems.
3. The correct connection of the system
There are many devices used in the audio system. Devices come in different interface forms with balanced and unbalanced input and output forms. In order to effectively shield external electromagnetic radiation and interference, shielded cables must be used uniformly and the correct connection method must be adopted.
When the audio signal transmission is balanced, the common mode interference level generated by the external interference power supply to the two signal lines in the cable is almost equal to the ground loop. At the input end of the internal amplifier of the device, the common mode voltage on the two signal lines will be replaced by the differential mode voltage and cancel each other, which will not form the interference voltage. Therefore, a balanced connection method should be used whenever possible.
When connected to an unbalanced output device. Instead of a balanced-unbalanced converter, use a single-core shielded cable to connect the ports of the balancing device to the ports of the unbalanced device. The noise induced by the shield is mixed into the audio signal, which increases noise, which is a major way to introduce noise. Therefore, for balanced or unbalanced transmissions, a double-core shielded cable should be used and the shield should be grounded only at the end of the balanced output or input.
When the two ends are unbalanced devices, if the transmission distance is long, it is better to use a balanced, unbalanced converter or audio isolation transformer to convert to balanced transmission.
4. The purification of the power supply
In order to isolate the interference noise formed by the public power grid, it is best to use an isolated purification power supply or an isolation transformer. The grounding end of the isolation transformer or the clean power supply must have a good grounding, otherwise the isolation effect is not good. It should be isolated from some high-powered appliances with strong interference. It is also possible to add a filter to the input of the audio device power supply to filter out the interference noise.
In practice, by changing the position of the live and neutral lines of a single-phase-powered audio device, one can find the least noise, and some noise interference can be reduced.
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