How to solve the problem of interference sound in the audio system?

Interference sound refers to the noise generated by external electromagnetic fields interfering with audio equipment. How is interference sound generated?

The transmission pathways of electromagnetic interference are mainly through space radiation and wire conduction. Space radiation is the electromagnetic induction generated by electric and magnetic fields in a closed loop of equipment. The larger the loop area, the higher the induced voltage. The induced voltage varies with the magnetic flux density vector or the angle between the direction of the electric field and the normal of the loop plane. At the same time, the higher the frequency, the higher the induced level, which means that high-frequency signals are more likely to interfere with the loop. Wire conduction is the interference signal that enters the audio equipment connection through electromagnetic field coupling. The conduction method is to transmit it to the affected equipment through circuits (including circuit components such as stray capacitance and mutual inductance that can be represented by lumped parameters), such as pulse interference and AC sound interference. When the level of the interference signal is higher than the sensitivity threshold level of the audio amplifier, it interferes with the audio system.

1. Mid low frequency interference

The noise interference of the audio system can be mainly divided into two categories: pulse interference and AC noise interference, in addition to the thermal noise of the equipment and transmission lines themselves and the continuous "white noise" superimposed on them. Pulse interference is caused by the coupling of strong electromagnetic fields generated by pulse devices into the human channel. Motors, air conditioners, spark plugs for car engines, switch power supplies, and controllable silicon for controlling lighting all generate interference of 60Hz to 2MHz, and the harmonic components of these interferences fall within the audio frequency band (2Hz to 20kHz). AC noise interference is mainly caused by the potential difference between grounding points due to different grounding systems, resulting in the formation of a loop of ground current. Its typical manifestations are 50Hz AC noise and low-frequency continuous buzzing in the 100Hz and 160Hz bands caused by it.

2. Medium to high frequency interference

The electromagnetic energy emitted by mobile phones and other high-frequency radio transmission devices, as well as the strong stray high-frequency electromagnetic energy radiated from certain devices, can cause interference to audio amplifiers. Especially the high-frequency radiation interference of mobile phones is the most serious. The 900MHz/1800MHz electromagnetic energy emitted by mobile phones (taking CSM as an example) acts on the input loop of the audio amplifier, producing intermittent or periodic interference signals. These interference signals contain rich harmonic components, some of which fall within the range of 300Hz to 3400Hz. It should also be mentioned that GSM mobile phones adopt the transmission mechanism of time-division multiplexing. GSM mobile phones transmit information to the base station by transmitting RF pulses with a pulse width of 577us, a period of 4.615ms, and a frequency of 216.7Hz. In addition to high-frequency radiation interference, CSM phones also suffer from low-frequency interference caused by a 216.7Hz switching frequency, resulting in the speaker emitting a "click click" interference sound of 216.7Hz harmonics.

Based on the type of interference sound and the transmission path of the interference, determine which interference method it belongs to, and then take corresponding solutions.

1. Reasonable grounding

Separate the signal ground wires between two devices with different ground potentials to avoid forming a ground loop by directly connecting them. If the balanced connection of the external shielding wire is only grounded at one end, or both ends are not grounded, etc.

2. Use floating grounding

The point has the same potential as ground, which is zero potential, but it is not directly connected to ground. It is the point where the equipment circuit has zero potential to ground. There is impedance between this' ground 'and the actual ground, and it is high impedance, which can overcome common mode interference.

3. Group connection of power supply for weak signal equipment and power supply for strong signal equipment

Weak signal sound source devices such as CDs, card holders, effectors, mixing consoles, compression limiters, and equalizers are connected to one power supply, while strong signal power amplifiers are connected to another power supply to avoid AC noise interference caused by conduction modes.

4. In the case of severe power interference, the equipment is connected in separate phases

Connect the power supply of the small signal device and the monitoring device together, select the phase with the smallest AC sound interference among the three-phase power supplies, and connect the other two phases to the large signal power amplifier to reduce AC noise interference from the power supply.

5. Connect a clean power source for power supply

When connecting to AC power supply, an isolated power transformer should be used. If there are no conditions, an independent set of power supply can be used to supply power separately from air conditioning, lighting and other equipment to avoid AC low-frequency interference caused by voltage drop of lighting.

All signal lines exceeding 2 meters shall be connected using balanced connections or balanced isolation transformers.