Introduction of methods to reduce the interference of power transformer to audio power amplifier circuit
Time:2022-07-04
Views:2034
The power transformer can interfere with the amplifier through magnetic field, electromagnetic induction and circuit, and it is the largest interference source in the audio machine. Therefore, it is necessary to deal with its working state and application environment in order to effectively prevent the interference from the power transformer and make the amplifier get excellent sound effect. Now I will discuss this with you.
1. In addition to supplying power to the amplifier, the power transformer can also couple the amplifier with the power supply, so that the interference source in the power grid enters the amplifier, and also reflect the voltage and current changes of the amplifier into the power grid. In order to cut off the electrostatic field and capacitive coupling between windings, isolate and common mode suppress the resulting interference, and prevent the common mode voltage in the power grid or circuit from being coupled to the secondary or primary, it is critical to add the electrostatic shielding of the winding of the audio power transformer. This shielding can be copper foil alternating between layers or a complete composite structure. In short, the more windings (especially primary windings) are surrounded, the better the common mode suppression is.
2. The magnetic field interference caused by the power transformer has always been a problem that puzzles the quality improvement of the amplifier. Even if there is a pure power supply, the magnetic field induction from it can also cause a serious decline in the quality of the amplifier. Due to the high price of the magnetic shielding isolation cover (even higher than the transformer itself, which is also the reason for the high price of some imported transformers), the general domestic machines rarely use the magnetic shielding isolation cover to cut off the magnetic interference of the transformer, and many of them only use a simple iron cover for isolation, or even simply install the transformer exposed, so effective magnetic shielding cannot be carried out. Foreign high-quality transformers often adopt the structure of multi-layer manganese alloy and coarse copper layer, which surrounds the transformer. On the one hand, the characteristics of manganese alloy with high resistance and high magnetic conductivity are used for magnetic short circuit. On the other hand, a magnetic field opposite to the interference magnetic field appears through the eddy current caused by the copper layer to counteract the magnetic interference, so the magnetic field leakage of the transformer is greatly reduced. Under amateur conditions, it is difficult to get manganese alloy covers, but magnetic shielding covers with multilayer structure can also be made of 1.5mm soft iron plate and copper plate.
3. When the primary impedance of the transformer is equal to the parallel value of the source resistance and the reflection resistance of the load, low-frequency cut-off will occur, increasing the noise from the transformer, so the power transformer must also have sufficient inductance. However, this cannot be a reason to blindly increase the output power of the transformer. Because the primary inductance of the transformer changes with the magnetic flux density of the iron core, and when the secondary load power is small, the magnetic flux density of the iron core will also decrease, reducing the inductance. Generally, the power of the power transformer can be selected between 1.4-2 times of the secondary power supply, which is more appropriate.
4. The core permeability of high-quality transformer is very high, and the magnetostrictive effect is also very high. It is sensitive to the influence of external magnetic field, pressure and vibration, so it can cause additional voltage and interference. For this purpose, when assembling or installing the transformer.
The following measures should be taken:
- demagnetize the iron core or shield before assembly.
- prevent short circuit of iron core, eddy current, reduce magnetic flux and reduce inductance.
- the transformer shall be vacuum impregnated so that the laminations cannot move each other.
- the transformer shall be installed on the shock absorption base, and any magnetic field source shall also be shock absorption installed.
- if the installation space allows, the transformer should be acoustically isolated.
5. The form of transformer is also important to reduce interference. Generally, the ring type or O-type transformer has high efficiency and small magnetic leakage, but the magnetic flux is easy to saturate, which is not conducive to resisting the interference of the power grid. EI type is on the contrary, and because there is a certain air gap, it can stabilize the permeability of the iron core. R type is between the two. Due to the serious pollution of China‘s power grid, many enthusiasts choose EI transformer as the audio power supply.
1. In addition to supplying power to the amplifier, the power transformer can also couple the amplifier with the power supply, so that the interference source in the power grid enters the amplifier, and also reflect the voltage and current changes of the amplifier into the power grid. In order to cut off the electrostatic field and capacitive coupling between windings, isolate and common mode suppress the resulting interference, and prevent the common mode voltage in the power grid or circuit from being coupled to the secondary or primary, it is critical to add the electrostatic shielding of the winding of the audio power transformer. This shielding can be copper foil alternating between layers or a complete composite structure. In short, the more windings (especially primary windings) are surrounded, the better the common mode suppression is.
2. The magnetic field interference caused by the power transformer has always been a problem that puzzles the quality improvement of the amplifier. Even if there is a pure power supply, the magnetic field induction from it can also cause a serious decline in the quality of the amplifier. Due to the high price of the magnetic shielding isolation cover (even higher than the transformer itself, which is also the reason for the high price of some imported transformers), the general domestic machines rarely use the magnetic shielding isolation cover to cut off the magnetic interference of the transformer, and many of them only use a simple iron cover for isolation, or even simply install the transformer exposed, so effective magnetic shielding cannot be carried out. Foreign high-quality transformers often adopt the structure of multi-layer manganese alloy and coarse copper layer, which surrounds the transformer. On the one hand, the characteristics of manganese alloy with high resistance and high magnetic conductivity are used for magnetic short circuit. On the other hand, a magnetic field opposite to the interference magnetic field appears through the eddy current caused by the copper layer to counteract the magnetic interference, so the magnetic field leakage of the transformer is greatly reduced. Under amateur conditions, it is difficult to get manganese alloy covers, but magnetic shielding covers with multilayer structure can also be made of 1.5mm soft iron plate and copper plate.
3. When the primary impedance of the transformer is equal to the parallel value of the source resistance and the reflection resistance of the load, low-frequency cut-off will occur, increasing the noise from the transformer, so the power transformer must also have sufficient inductance. However, this cannot be a reason to blindly increase the output power of the transformer. Because the primary inductance of the transformer changes with the magnetic flux density of the iron core, and when the secondary load power is small, the magnetic flux density of the iron core will also decrease, reducing the inductance. Generally, the power of the power transformer can be selected between 1.4-2 times of the secondary power supply, which is more appropriate.
4. The core permeability of high-quality transformer is very high, and the magnetostrictive effect is also very high. It is sensitive to the influence of external magnetic field, pressure and vibration, so it can cause additional voltage and interference. For this purpose, when assembling or installing the transformer.
The following measures should be taken:
- demagnetize the iron core or shield before assembly.
- prevent short circuit of iron core, eddy current, reduce magnetic flux and reduce inductance.
- the transformer shall be vacuum impregnated so that the laminations cannot move each other.
- the transformer shall be installed on the shock absorption base, and any magnetic field source shall also be shock absorption installed.
- if the installation space allows, the transformer should be acoustically isolated.
5. The form of transformer is also important to reduce interference. Generally, the ring type or O-type transformer has high efficiency and small magnetic leakage, but the magnetic flux is easy to saturate, which is not conducive to resisting the interference of the power grid. EI type is on the contrary, and because there is a certain air gap, it can stabilize the permeability of the iron core. R type is between the two. Due to the serious pollution of China‘s power grid, many enthusiasts choose EI transformer as the audio power supply.
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