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

February 23, 2022

The Power Transformer can interfere with the amplifier through magnetic fields, electromagnetic induction and circuits, and is the largest source of interference in audio machines. Therefore, in order to properly handle its working state and application environment, the interference generated by the power transformer can be effectively avoided, and 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 to 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 into the power grid. In order to cut off the electrostatic field and capacitive coupling between the windings, the isolation and common mode suppress the resulting interference, avoid coupling the common mode voltage in the grid or circuit to the secondary or primary, and add windings to the audio power transformer. Faraday electrostatic shielding is critical. This shielding can be an alternating copper foil between layers, or it can be a complete combined structure. In general, the more the windings (especially the primary windings) are surrounded, the better the common mode rejection.

2. The magnetic field interference generated by the power transformer has always been a problem that plagues the quality of the amplifier. Even with a pure power supply, the magnetic field induction from it can cause a serious drop in the quality of the amplifier. Due to the high price of the magnetic shield isolation cover (even higher than the transformer itself, which is also the reason for the high price of some imported transformers), the general domestic machine rarely uses the magnetic shield isolation cover to cut off the magnetic interference of the transformer, many of which use simple iron sheets. The cover is isolated and even the transformer is barely mounted, so effective magnetic shielding is not possible. Foreign high-quality transformers often use a multi-layer manganese alloy and a thick copper layer structure to surround the transformer. On the one hand, the magnetic resistance of the manganese alloy is high resistance and high magnetic permeability, and on the other hand, it is caused by the copper layer. The eddy current produces a magnetic field opposite to the disturbing magnetic field to cancel the magnetic interference, thus greatly reducing the magnetic field leakage of the transformer. Under the amateur conditions, it is difficult to obtain a manganese alloy cover, but a 1.5 mm soft iron plate and a copper plate can also be used to form 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 cutoff will occur, increasing the noise originating from the transformer, so the power transformer must also have sufficient inductance. But this does not justify blindly increasing the output power of the transformer. Because the primary inductance of the transformer changes with the core flux density, when the secondary load power is small, the core flux density will also decrease, causing the inductance to drop. Generally, the power of the power transformer can be selected between 1.4 and 2 times the secondary power supply, which is more appropriate.
4. The core of high-quality transformer has high magnetic permeability and high magnetostrictive effect. It is sensitive to the influence of external magnetic field, pressure and vibration, and can generate additional voltage and cause interference. For this purpose, when assembling or installing a transformer.

To take the following measures:
- The core or shield must be demagnetized before assembly.
- Avoid short-circuiting the core, generate eddy currents, reduce the magnetic flux, and reduce the inductance.
- The transformer should be vacuum impregnated so that the laminations cannot move relative to each other.
- The transformer should be mounted on a shock absorbing base, and any magnetic field source should be shock absorbing.
- The transformer should be acoustically isolated if the installation space permits.
5. The form of the 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 be saturated, which is not conducive to resisting the interference of 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 between the two. Because China's power grid pollution is more serious, many "fans" have chosen EI-type transformers as the sound power source.

Author:

Mr. James

E-mail:

intl@qre.com.cn

Phone/WhatsApp:

+86 13588429173