The power transformer can interfere with the amplifier through magnetic fields, electromagnetic induction and circuits, and is the largest source of interference in audio equipment. Therefore, it is necessary to deal with its working state and application environment in order to effectively avoid the interference caused by the power transformer and make the amplifier get excellent sound effects. Below I will discuss this with you.
1. In addition to supplying power to the amplifier, the power transformer can also couple the amplifier to the power source to allow interference sources in the power grid to enter the amplifier, and also reflect the changes in voltage and current generated by the amplifier into the power grid. In order to cut off the electrostatic field and capacitive coupling between the windings, isolate and common-mode suppression of the resulting interference, avoid coupling the common-mode voltage in the grid or circuit to the secondary or primary, add to the winding of the power transformer for audio Faraday static shielding is critical. This shielding can be a copper foil with alternating layers, or it can be a complete composite structure. In short, the more the windings (especially the primary windings) are enclosed, the better the common mode suppression.
2. The magnetic field interference generated by the power transformer has always been a problem that troubles the quality of the amplifier. Even if there is a pure power supply, the magnetic field induction from it can cause the quality of the amplifier to seriously deteriorate. Due to the high price of magnetic shielding shields (even higher than the transformer itself, which is also the reason why some imported transformers are expensive), ordinary domestic machines rarely use magnetic shielding shields to cut off the magnetic interference of transformers, and many only use simple iron sheets The shield is isolated, and the transformer is even installed bare, so effective magnetic shielding cannot be performed. Foreign high-quality transformers often use a multi-layer manganese alloy and thick copper layer structure to surround the transformer. On the one hand, they use the high resistance and high permeability characteristics of manganese alloy to conduct magnetic short circuit, on the other hand, they are caused by The eddy current generates a magnetic field opposite to the disturbing magnetic field to cancel the magnetic interference, thus greatly reducing the magnetic field leakage of the transformer. It is difficult to obtain a manganese alloy cover under amateur conditions, but a 1.5 mm soft iron plate and a copper plate can also be used to make a multi-layer magnetic shield.
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, a low frequency cut-off will occur, increasing the noise from the transformer, so the power transformer must also have sufficient inductance. But this cannot be the reason for blindly increasing the output power of the transformer. Because the primary inductance of the transformer changes with the magnetic flux density of the iron core, when the secondary load power is small, the magnetic flux density of the iron core will also decrease, causing the inductance to decrease. Generally, the power of the power transformer can be selected between 1.4 and 2 times of the secondary power supply, which is more appropriate.
4. The core permeability of high-quality transformers is very high, and the magnetostrictive effect is also very high. It is sensitive to the influence of external magnetic fields, pressures, and vibrations, which can generate additional voltages and cause interference. For this reason, when assembling or installing transformers.
The following measures should be taken:
-Demagnetization must be done before the iron core or shield is assembled.
—Avoid short circuit of the iron core, generate eddy current, reduce magnetic flux, and reduce inductance.
—The transformer should be vacuum impregnated so that the laminations cannot move with each other.
—The transformer should be installed on the shock-absorbing base, and any magnetic field source should also be shock-absorbing.
—If the installation space allows, the transformer should be acoustically isolated.
5. The form of the transformer is also important to reduce interference. Generally, toroidal or O-type transformers have high efficiency and low magnetic leakage, but the magnetic flux is easily saturated, which is not conducive to resisting interference from the power grid. The EI type is the opposite, and because of the existence of a certain air gap, the magnetic permeability of the iron core can be stabilized. The R type is somewhere in between. Due to the serious pollution of China's power grid, many "audiophiles" have chosen EI-type transformers as audio power sources.
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