How to reduce the interference of power transformer on audio amplifier circuit

Power transformer It can interfere with the amplifier through magnetic field, electromagnetic induction and circuit, and is the biggest interference source in audio equipment. Therefore, it is necessary to properly handle its working state and application environment to effectively avoid the interference caused by the power transformer and make the amplifier have 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 also reflects the voltage and current changes generated by the amplifier to the power grid. In order to cut off the electrostatic field and capacitive coupling between the windings, isolate and suppress the interference generated by the common mode, and avoid coupling the common mode voltage in the power grid or circuit to the secondary or primary, it is critical to add Faraday electrostatic shielding to the windings of the audio power transformer. This shield can be alternating layers of copper foil or a complete combined structure. In short, the more the winding (especially the primary winding) is surrounded, the better the common mode suppression.

2. The magnetic field interference generated by the power transformer has always been a problem that has plagued the improvement of amplifier quality. 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 magnetic shielding isolation covers (even higher than the transformer itself, which is also the reason why some imported transformers are expensive), general domestic machines rarely use magnetic shielding isolation covers to cut off the magnetic interference of the transformer. Many only use simple iron covers for isolation, or even simply install the transformer naked, so effective magnetic shielding cannot be performed. Foreign high-quality transformers often use a multi-layer structure of alternating manganese alloy and rough copper layers to surround the transformer. On the one hand, the high resistance and high magnetic permeability of manganese alloy are used for magnetic short circuit. On the other hand, the eddy current caused in the copper layer generates a magnetic field opposite to the interference magnetic field to offset the magnetic interference, thereby greatly reducing the magnetic field leakage of the transformer. It is difficult to obtain manganese alloy covers under amateur conditions, but a multi-layer magnetic shielding cover can also be made of 1.5 mm soft iron plates and copper plates.

3. When the transformer primary impedance is equal to the parallel value of the source resistance and the reflected resistance of the load, low-frequency cutoff will occur, increasing the noise from the transformer, so the power transformer must also have sufficient inductance. But 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. 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 the secondary power supply power, which is more appropriate.

4. The iron core of a high-quality transformer has a high magnetic permeability and magnetostrictive effect, and is sensitive to external magnetic fields, pressure, and vibration, which can generate additional voltage and cause interference. For this reason, when assembling or installing the transformer.

The following measures should be taken:

—The core or shield must be demagnetized before assembly.

—Avoid short circuit of the core, which will generate eddy current, reduce magnetic flux and decrease inductance.

—The transformer should be vacuum impregnated so that the laminations cannot move relative to each other.

—The transformer should be installed on a shock-absorbing base, and any magnetic field source should also be installed in a shock-absorbing manner.

—If the installation space permits, the transformer should be acoustically isolated.

5. The form of the transformer is also very important to reduce interference. Generally, ring-type or O-type transformers have high efficiency and small magnetic leakage, but the magnetic flux is easily saturated, 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 iron core can be stabilized. The R type is between the two. Due to the serious pollution of my country's power grid, many "audiophiles" choose EI type transformers as audio power supplies.