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How to reduce the electromagnetic interference of transformers on amplifiers
Release time: 2018-03-14 Click times: 996 times
Power transformers can interfere with amplifiers through magnetic fields, electromagnetic induction, and circuits, and are the largest source of interference in audio equipment. Therefore, it is necessary to properly deal with its working state and application environment, so as to effectively avoid the interference caused by the power transformer, so that the amplifier can obtain excellent sound effects. I will discuss this with you below.
1. In addition to powering 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 to the power grid. In order to cut off the electrostatic field and capacitive coupling between the windings, isolate and suppress the interference caused by the common mode, and avoid coupling the common mode voltage in the power grid or circuit to the secondary or primary. Faraday electrostatic shielding is critical. This shielding can be a copper foil alternating between layers, or a complete composite structure. In short, the more the winding (especially the primary winding) is surrounded, the better the common mode rejection.
2. The magnetic field interference generated by the power transformer has always been a problem that has disturbed the quality of the amplifier. Even if there is a pure power source, the magnetic field induction from it can cause the amplifier quality to be seriously degraded. Due to the high price of magnetic shields (even higher than the transformer itself, which is also the reason for the high prices of some imported transformers), general domestic machines rarely use magnetic shields to cut off the magnetic interference of the transformer. Many simply use simple iron sheets. The cover is isolated, and the transformer is even installed barely, so effective magnetic shielding cannot be performed. Foreign high-quality transformers often use a multilayer structure of manganese alloys and coarse copper layers to surround the transformer. On the one hand, the high resistance and high permeability of manganese alloys are used for magnetic short circuits, and on the other hand, they are caused by copper layers. The eddy current generates a magnetic field opposite to the interference magnetic field to cancel the magnetic interference, thus greatly reducing the magnetic field leakage of the transformer. In amateur conditions, it is difficult to obtain a manganese alloy alloy cover, but a 1.5 mm soft iron plate and a copper plate can also be used to make a magnetic shield with a multilayer structure.
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. 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 core's magnetic flux density, when the secondary load power is small, the core's magnetic flux density will also decrease, which will reduce 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 of high-quality transformers has high magnetic permeability and high magnetostrictive effect. It is sensitive to the influence of external magnetic fields, pressure, and vibration, which can generate additional voltage and cause interference. For this purpose, when assembling or installing the transformer.
Take the following measures:
The core or shield must be demagnetized before assembly.
Avoid short-circuit of the iron core, generate eddy current, reduce magnetic flux, and reduce inductance.
Transformers should be vacuum impregnated so that the laminations cannot move with each other.
The transformer should be mounted on a shock-absorbing base, and any magnetic field source should also be shock-mounted.
If installation space permits, the transformer should be acoustically isolated.
5. The form of the transformer is also important to reduce interference. Generally, ring or O-type transformers have high efficiency and low magnetic leakage.
However, the magnetic flux is easy to saturate, 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 core can be stabilized. The R type is somewhere in between. Due to the serious pollution of the power grid in our country, many people choose EI transformers as audio power sources.
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