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Eliminating the ring-shaped transformers noise at the power frequency level
2026-07-07
Selection of power amplifier audio transformer: Strictly control eddy current leakage, completely eliminate the low-frequency noise from the power supply
During the debugging process of Hi-Fi amplifiers, valve amplifiers, professional stage power amplifiers, and active speakers, the most troublesome fault is the continuous 50Hz alternating current hum emitted by the speakers, which is commonly referred to as the power supply background noise in the industry. Many engineers have repeatedly optimized the filter capacitors, adjusted the ground wire layout, and replaced the operational amplifier chips, but still failed to completely eliminate the noise. The root cause is that most of the background noise comes from the leakage magnetic field radiated by the power transformer.
The leakage magnetic field of the transformer can invade the pre-amplifier circuit, signal lines, and PCB traces through spatial coupling. The weak alternating magnetic field is continuously amplified by the high-gain circuit and eventually turns into audible buzzing current sounds. The EI laminated transformer, due to the presence of air gaps in the magnetic circuit, has a large leakage magnetic field and has long been a major area of concern for AC noise; even if a ring-shaped transformer is used, if the winding arrangement is unreasonable or there is no shielding structure, the magnetic leakage problem will still occur. To completely eliminate the background noise of the power amplifier at its source, the core task is to strictly control the leakage level during the selection and customization of the transformer.
- The underlying principle of power supply background noise caused by power frequency leakage magnetic field
1. How leakage magnetic field is transformed into audible AC noise
During the electromagnetic conversion process of the transformer, not all magnetic flux is confined within the iron core. A portion of the magnetic field spreads outward, forming leakage flux. The alternating 50Hz stray magnetic field passes through the wires, copper foil, and amplification circuits inside the chassis, inducing a weak AC electromotive force in the closed circuit. The gain of the preamplifier stage is generally dozens or even hundreds of times, and the millivolt-level induced voltage will be progressively amplified, eventually forming a deep and stable humming sound in the speaker.
During the electromagnetic conversion process of the transformer, not all magnetic flux is confined within the iron core. A portion of the magnetic field spreads outward, forming leakage flux. The alternating 50Hz stray magnetic field passes through the wires, copper foil, and amplification circuits inside the chassis, inducing a weak AC electromotive force in the closed circuit. The gain of the preamplifier stage is generally dozens or even hundreds of times, and the millivolt-level induced voltage will be progressively amplified, eventually forming a deep and stable humming sound in the speaker.
This type of spatial magnetic coupling noise is different from power line conducted interference. It cannot be solved merely by using capacitive filtering or power purification. It can only be completely eradicated by suppressing magnetic leakage radiation. Once the magnetic leakage of the transformer exceeds the limit, even the most sophisticated subsequent circuitry will have difficulty eliminating the background noise completely.
2. The core structure determines the basic level of leakage flux
The EI type transformer is composed of E pieces and I pieces that are inserted and spliced together. There are multiple docking air gaps in the magnetic circuit, and a large amount of magnetic flux will leak out through the gaps. The leakage flux can account for 12% to 18%, and it has a very strong external radiation magnetic field, which is the main source of the AC hum in low-end power amplifiers. The ring-type transformer uses a continuous winding of a whole oriented silicon steel strip, with a completely closed magnetic circuit without any breaks. Naturally, it can lock the magnetic flux inside the core, and the basic leakage flux can be controlled within 3% to 5%. Therefore, most Hi-Fi devices almost exclusively use ring-type cores as the power core. However, a large number of cheap ring-type transformers on the market are loosely wound and have unbalanced primary and secondary coupling, resulting in severe leakage flux and still causing significant background noise.
The EI type transformer is composed of E pieces and I pieces that are inserted and spliced together. There are multiple docking air gaps in the magnetic circuit, and a large amount of magnetic flux will leak out through the gaps. The leakage flux can account for 12% to 18%, and it has a very strong external radiation magnetic field, which is the main source of the AC hum in low-end power amplifiers. The ring-type transformer uses a continuous winding of a whole oriented silicon steel strip, with a completely closed magnetic circuit without any breaks. Naturally, it can lock the magnetic flux inside the core, and the basic leakage flux can be controlled within 3% to 5%. Therefore, most Hi-Fi devices almost exclusively use ring-type cores as the power core. However, a large number of cheap ring-type transformers on the market are loosely wound and have unbalanced primary and secondary coupling, resulting in severe leakage flux and still causing significant background noise.
3. Asymmetry of the windings further amplifies magnetic leakage
The primary and secondary windings are arranged in a scattered manner, with inconsistent tightness, and there is an imbalance in the amperage on both sides, resulting in a large amount of stray magnetic flux. Especially in the common single-layer winding method, one end of the winding is tight while the other is loose, causing an uneven magnetic field distribution and a significant increase in leakage magnetic flux. Even if the core quality is excellent, if the winding process is unqualified, it is still impossible to achieve low magnetic radiation. This is the core reason why the same type of ring transformers, some are quiet and noise-free while others have obvious humming sounds.
The primary and secondary windings are arranged in a scattered manner, with inconsistent tightness, and there is an imbalance in the amperage on both sides, resulting in a large amount of stray magnetic flux. Especially in the common single-layer winding method, one end of the winding is tight while the other is loose, causing an uneven magnetic field distribution and a significant increase in leakage magnetic flux. Even if the core quality is excellent, if the winding process is unqualified, it is still impossible to achieve low magnetic radiation. This is the core reason why the same type of ring transformers, some are quiet and noise-free while others have obvious humming sounds.
- Prioritize low-leakage core structure and reject inherent defects
1. Ranking of mainstream core leakage levels
Ring (ring core) < R-type < C-type strip < ordinary EI laminated. For high-end audio power amplifiers, whether it is the A-type or B-type power stage or the tube power supply, the closed magnetic circuit ring-shaped transformer is the first choice. It has the lowest inherent leakage and is the optimal choice for suppressing background noise. If cost constraints prevent the use of ring core, R-type transformers can be selected. Its circular core joints are fewer, and the leakage is much lower than the EI structure; ordinary EI transformers are only suitable for low-end active speakers and are difficult to achieve no AC hum; it is not recommended to use them in high-fidelity models.
Ring (ring core) < R-type < C-type strip < ordinary EI laminated. For high-end audio power amplifiers, whether it is the A-type or B-type power stage or the tube power supply, the closed magnetic circuit ring-shaped transformer is the first choice. It has the lowest inherent leakage and is the optimal choice for suppressing background noise. If cost constraints prevent the use of ring core, R-type transformers can be selected. Its circular core joints are fewer, and the leakage is much lower than the EI structure; ordinary EI transformers are only suitable for low-end active speakers and are difficult to achieve no AC hum; it is not recommended to use them in high-fidelity models.
2. The core material is strictly selected as high-conductivity oriented silicon steel sheets.
Low-quality non-oriented silicon steel sheets have low magnetic permeability and the magnetic flux is prone to overflow outward during operation, resulting in a large amount of leakage flux. For custom audio-specific transformers, only brand-new grain-oriented silicon steel strips can be used. The core has uniform magnetic density, and the magnetic flux is firmly enclosed within the ring, reducing magnetic leakage from the material source. Even if recycled material cores are made into rings, the stray magnetic field still cannot be controlled and must not be used.
Low-quality non-oriented silicon steel sheets have low magnetic permeability and the magnetic flux is prone to overflow outward during operation, resulting in a large amount of leakage flux. For custom audio-specific transformers, only brand-new grain-oriented silicon steel strips can be used. The core has uniform magnetic density, and the magnetic flux is firmly enclosed within the ring, reducing magnetic leakage from the material source. Even if recycled material cores are made into rings, the stray magnetic field still cannot be controlled and must not be used.
3. Power Reserve to Avoid Core Saturation
When a transformer is operating under full load for a long time, the core approaches saturation, and the magnetic flux will overflow significantly, causing the leakage flux to rise sharply. The power amplifier is a dynamic load, and the current fluctuates greatly at high volume levels. When selecting the model, a power reserve of more than 30% must be reserved to avoid the core working in the saturation range, ensuring the stability of the entire magnetic circuit and preventing a sudden increase in leakage flux under large dynamic conditions, which would result in fluctuating current noise.
When a transformer is operating under full load for a long time, the core approaches saturation, and the magnetic flux will overflow significantly, causing the leakage flux to rise sharply. The power amplifier is a dynamic load, and the current fluctuates greatly at high volume levels. When selecting the model, a power reserve of more than 30% must be reserved to avoid the core working in the saturation range, ensuring the stability of the entire magnetic circuit and preventing a sudden increase in leakage flux under large dynamic conditions, which would result in fluctuating current noise.
- Symmetrical winding of windings, minimizing internal leakage flux to the lowest level
The core only determines the basic lower limit. The winding structure is the key to controlling the leakage flux. For the vast majority of ring-type transformers, the excessive leakage flux problem lies in the winding process.
1. By using symmetrical segmented winding, the primary and secondary currents are balanced.
Common winding method: First, complete the winding of the primary coil, then wind the secondary coil on the outer side. The magnetic fields of the two sides' windings cannot cancel each other out, resulting in a significant increase in leakage flux. Optimal solution for audio transformers: Use interlayer symmetrical winding. Divide the primary winding evenly into two halves, with one half wound on the inner layer and the other half on the outermost layer. The secondary winding is placed in the middle and sandwiched between the two layers of the primary coils. The primary and secondary magnetic fields cancel each other out, significantly reducing stray flux and reducing the leakage inductance and space radiation magnetic field by more than half. For power amplifier power supplies with dual symmetric output, the number of turns and the tightness of the wire for the two secondary windings must be exactly the same, ensuring magnetic field balance and avoiding single-sided magnetic flux overflow.
2. The entire ring is uniformly and densely wound, eliminating any local unevenness.
The conductors must be evenly spread along the entire circumference of the ring, and cannot be concentratedly wound in a small area. If the local coils are too densely packed, it will cause magnetic field distortion and radiate stray magnetic fields outward. The fully automatic equipment evenly arranges the wires, with each turn closely adhering to the next, ensuring that the overall thickness of the coil is uniform and the magnetic field distribution along the circumference is uniform, without any points where magnetic flux is concentrated and overflows.
3. Primary-secondary tight coupling, reducing winding gap
Only a thin layer of insulation is retained between the primary and secondary, and no large cavities are reserved in the middle. The larger the winding spacing, the more leakage flux. Try to arrange the primary and secondary closely together to enhance magnetic coupling, allowing energy exchange to concentrate within the iron core, and reducing the stray magnetic field that spreads outward. At the same time, strictly control the total thickness of the windings. The thinner the coil, the less magnetic flux leakage.
4. Multiple secondary windings are independently and symmetrically arranged.
Power amplifiers typically use two sets of symmetrical positive and negative power supplies. The two sets of secondary windings must be symmetrically distributed on both sides, with identical number of turns, wire diameter, and winding length. If the number of turns on one side is greater and the current is higher, it will disrupt the magnetic balance, causing unidirectional leakage magnetic field, which directly couples into the preamplifier circuit and forms background noise. It is strictly prohibited to have an asymmetrical design of one thick and one thin, one long and one short.
- Dual Blocking, Completely Locking Out Leakage Magnetic Field
Relying solely on winding optimization can only reduce internal leakage magnetic field. To achieve zero radiation, it is necessary to add an electrostatic shielding and magnetic shielding dual structure. This is the standard design for high-end audio transformers to suppress background noise.
1.Adding a copper foil Faraday electrostatic shielding layer between the primary and secondary windings
Wrap a seamless copper foil around the primary and secondary windings to form an electrostatic shielding layer, and strictly adhere to the single-point connection to the chassis ground. The copper foil can block the capacitive coupling between the primary and secondary, suppress common-mode interference, and simultaneously cancel the induced noise caused by the alternating electric field between the windings. Note that the copper foil should not form a closed loop at the beginning and end to avoid short circuits; reserve a small opening to prevent eddy current heating; the shielding layer should only be grounded at a single point and must not be grounded at multiple points to prevent the formation of a ground loop and the introduction of new noise. Without this layer of shielding, the noise from the primary power grid can easily enter the secondary power supply through distributed capacitance, exacerbating the background noise.
2. Add a high-magnetic-conductivity alloy magnetic shielding cover to the outer layer
For high-gain preamplifiers and valve preamplifier power supplies, even with optimized windings, there will still be a small amount of magnetic leakage. At this point, a palladium alloy or low-carbon soft iron shielding layer needs to be wrapped around the entire outer layer of the coil to confine the residual leakage magnetic flux within the shielding shell, completely blocking the magnetic field from radiating outward. The iron shielding cover will redirect the scattered magnetic flux back into the magnetic circuit, reducing the external magnetic field strength by another order of magnitude, completely preventing the space magnetic field from coupling into the signal line. The multi-layer shielding structure is as follows: iron core → primary winding → copper foil shielding → secondary winding → outer layer ferromagnetic shielding cover. This provides triple protection, achieving almost zero magnetic leakage.
3. Overall vacuum impregnation curing
The coil undergoes vacuum pressure impregnation treatment, and the winding becomes a solid whole. The conductors do not vibrate or shift, avoiding the deformation of the winding during operation, which would cause imbalance in the magnetic field. When loose coils are in operation, the electromagnetic force pulls the conductors, and the magnetic field constantly changes, resulting in fluctuations in leakage flux, forming alternating noise that varies in intensity. After impregnation curing, the winding structure is permanently stable, and the magnetic field distribution remains consistent over a long period.
The coil undergoes vacuum pressure impregnation treatment, and the winding becomes a solid whole. The conductors do not vibrate or shift, avoiding the deformation of the winding during operation, which would cause imbalance in the magnetic field. When loose coils are in operation, the electromagnetic force pulls the conductors, and the magnetic field constantly changes, resulting in fluctuations in leakage flux, forming alternating noise that varies in intensity. After impregnation curing, the winding structure is permanently stable, and the magnetic field distribution remains consistent over a long period.
- Avoid spatial coupling and prevent secondary magnetic leakage interference
Even if the magnetic leakage of the transformer is well controlled, if the cabinet layout is unreasonable, the residual magnetic field will still enter the circuit, resulting in failure.
1. Increase distance and stagger positions The transformer must be far away from the pre-amplifier board and the audio input signal line. The minimum spacing should be no less than 5 centimeters. The magnetic field of the toroidal transformer radiates outward along the radial direction. Adjust the placement angle to make the radial direction with the strongest magnetic field avoid the PCB traces and high-gain circuits. It is best to place the transformer at the rear output end of the cabinet, separated from the front-stage amplification circuit at both ends of the cabinet, and maintain a physical distance.
1. Increase distance and stagger positions The transformer must be far away from the pre-amplifier board and the audio input signal line. The minimum spacing should be no less than 5 centimeters. The magnetic field of the toroidal transformer radiates outward along the radial direction. Adjust the placement angle to make the radial direction with the strongest magnetic field avoid the PCB traces and high-gain circuits. It is best to place the transformer at the rear output end of the cabinet, separated from the front-stage amplification circuit at both ends of the cabinet, and maintain a physical distance.
2. Separate the strong and weak circuits for wiring. The AC input leads and secondary power lines should not be placed parallel to the audio signal lines. This is to prevent the alternating magnetic field from cutting through the signal lines and generating induced noise. The power lines should be as short as possible and closely attached to the bottom plate of the chassis to reduce the loop area and lower the induced electromotive force.
3. Shock absorption and isolation to prevent magnetic field distortion caused by chassis vibration. Silicone shock-absorbing pads are installed at the bottom of the transformer to prevent the vibration of the iron core from being transmitted to the chassis. Vibration can cause slight displacement of the windings， resulting in continuous magnetic field disturbance. This not only causes mechanical buzzing but also intensifies magnetic leakage and amplifies the power supply noise.
4. The shielding shell is reliably grounded at a single point. The outer magnetic shielding cover is only connected to the chassis of the machine, not to the signal ground wire. This prevents the ground current from introducing interference into the audio circuit, and strictly separates the chassis ground from the signal ground, eliminating ground loop hum.
- Common Misunderstandings in Model Selection and How to Avoid Repeated Pitfalls
Misconception 1: Just because it is a toroidal transformer, it definitely has no leakage magnetic field. Cheap toroidal windings are arranged chaotically, the primary and secondary are asymmetrical, and there is no copper foil shielding. The leakage magnetic field is still very high, and AC background noise will still occur. One should not only focus on the shape of the iron core.
Misconception 2: Only control electrical shielding, ignoring magnetic shielding. Copper foil can only suppress electrical field interference, but it cannot block alternating magnetic fields. High-gain models only perform electrostatic shielding, and residual leakage magnetic field will still induce humming. Misconception 3: Transformer operates at full load for a long time. Once the iron core enters magnetic saturation, a large amount of magnetic flux leaks out, and the leakage noise will suddenly increase. The background noise will significantly increase during dynamic high-volume operation. Misconception 4: The wiring of the chassis is not divided into areas, and the transformer is close to the front circuit board. Even the best low-leakage transformer, if placed too close to the signal lines, the magnetic field will still be picked up by the circuit, and the layout defect will cancel out all the noise reduction designs of the transformer.
The power supply's power frequency background noise in amplifiers is mostly caused by the leakage magnetic field of the transformer, which is transmitted through spatial coupling. Simply optimizing the subsequent circuit will only address the symptoms and not the root cause. To completely eliminate the AC hum, the noise reduction work must be carried out earlier, at the stage of transformer selection and customization.
The entire low leakage magnetic design can be summarized as a five-step plan:
First, prefer to use a closed magnetic circuit ring-shaped iron core, use high-conductivity oriented silicon steel strips, reserve sufficient power margin to avoid magnetic saturation leading to magnetic flux leakage;
Second, adopt a primary-secondary-primary sandwich symmetrical winding method, evenly arrange the entire ring, ensure that the primary and secondary magnetic fields cancel each other out, and reduce leakage magnetic flux from the inside;
Third, install a single-point grounded copper foil electrostatic shield between the primary and secondary, and wrap the outer layer with an iron magnetic shielding cover, locking the residual stray magnetic field with two layers;
Fourth, implement vacuum impregnation process, solidify the winding structure, ensure the magnetic field is stable and without distortion for a long time;
Fifth, optimize the chassis layout, increase the distance between the transformer and the pre-stage circuit, stagger the magnetic field direction, and block the spatial magnetic coupling path.
First, prefer to use a closed magnetic circuit ring-shaped iron core, use high-conductivity oriented silicon steel strips, reserve sufficient power margin to avoid magnetic saturation leading to magnetic flux leakage;
Second, adopt a primary-secondary-primary sandwich symmetrical winding method, evenly arrange the entire ring, ensure that the primary and secondary magnetic fields cancel each other out, and reduce leakage magnetic flux from the inside;
Third, install a single-point grounded copper foil electrostatic shield between the primary and secondary, and wrap the outer layer with an iron magnetic shielding cover, locking the residual stray magnetic field with two layers;
Fourth, implement vacuum impregnation process, solidify the winding structure, ensure the magnetic field is stable and without distortion for a long time;
Fifth, optimize the chassis layout, increase the distance between the transformer and the pre-stage circuit, stagger the magnetic field direction, and block the spatial magnetic coupling path.
If these standards are strictly followed, the leakage magnetic field of the transformer can be suppressed to an extremely weak level. The power supply output will be clean without any noise, and the power amplifier speaker will be quiet without any AC background noise. The pure musical foundation can be fully restored. Whether it is a high-power power amplifier or a high-fidelity tube amplifier power supply, only by strictly controlling the leakage magnetic field can a high-quality power supply be established as the foundation.
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