How to Reduce the Electromagnetic Interference of Transformer to Amplifier

The power transformer can interfere with the amplifier through the magnetic field, electromagnetic induction and circuit, and is the largest source of interference in the audio machine. Therefore, it is necessary to deal with its working state and application environment, in order to effectively avoid the interference generated by the power transformer, so that the amplifier can get excellent sound effects. I will discuss this with you below.

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 at the same time, the voltage and current changes generated by the amplifier are reflected into the power grid. In order to cut off the electrostatic field and capacitive coupling between the windings, isolation and common mode suppression of the resulting interference, to avoid the common mode voltage in the power grid or circuit coupled to the secondary or primary, the winding of the audio power transformer with Faraday electrostatic shielding is very critical. This kind of shielding can be copper foil alternating between layers, or it can be a complete composite structure. In short, the more windings (especially the primary windings) are surrounded, the better the common mode suppression.

The magnetic field interference generated by the power transformer has always been a problem that has plagued the quality of the amplifier. Even if there is a pure power supply, the magnetic field induction from it can 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 seldom use the magnetic shielding isolation cover to cut off the magnetic interference of the transformer. Many of them only use 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 use multi-layer manganese swim alloy and crude copper layer phase structure, the transformer surrounded, on the one hand, the use of manganese swim alloy high resistance, high magnetic conductivity characteristics of magnetic short circuit, on the other hand through the copper layer caused by the eddy current to generate a magnetic field opposite to the interference magnetic field to offset magnetic interference, thus greatly reducing the magnetic field leakage of the transformer. Amateur conditions are difficult to obtain manganese swim alloy cover, but can also be 1.5mm soft iron plate and copper plate made of multi-layer structure of the magnetic shield cover.

3. When the primary impedance of the transformer is equal to the parallel value of the source resistance and the reflected 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. But this is not 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, the secondary load power is small, the magnetic flux density of the iron core will also be reduced, so that the inductance is reduced. In general, the power of the power transformer can be selected between 1.4-2 times the secondary power supply, which is more appropriate.

4, high-quality transformer core permeability is very high, magnetostrictive effect is also very high, the external magnetic field, pressure, vibration sensitive, can therefore produce additional voltage, causing interference. For this purpose, during assembly or installation of the transformer.

To take the following measures:

The core or shield must be demagnetized before assembly.

Avoid short circuit of iron core, generate eddy current, reduce magnetic flux and reduce inductance.

The transformer shall be vacuum impregnated so that the laminations cannot move with each other.

The transformer should be installed on the damping base, and any magnetic field source should also be installed with damping.

If the installation space allows, the transformer shall be acoustically isolated.

The form of the transformer is also important to reduce interference. In general, the ring type or O type transformer has high efficiency and small magnetic leakage,

However, the magnetic flux is easy to saturate, which is not conducive to resisting the interference of the power grid. The EI type is the opposite, and because there is a certain air gap, the magnetic permeability of the core can be stabilized. The R type is somewhere in. Due to the serious pollution of China's power grid, many people choose EI transformer as the sound power supply.