When an audio transformer picks up interference, many people's first instinct is to "add shielding." But copper foil, mu-metal cans, and full potting don't solve the same problem. Choose the wrong approach and not only do costs rise, but increased parasitic capacitance or altered grounding paths can make high-frequency noise and hum worse. This article clarifies what each of the three methods targets, how grounding works, and where each is best applied.
First, Identify the Interference Type
| Interference Type | Typical Sources | Effective Measures | Ineffective Measures |
|---|---|---|---|
| Electric field coupling | Power lines, switch-mode power supplies, high-frequency circuits | Copper foil electrostatic shield, grounding | Simply thickening magnetic shielding |
| Magnetic field coupling | Power transformers, motors, high-current traces | Mu-metal magnetic shield, increased distance, orthogonal mounting | Copper foil |
| Ground loops | Multi-point grounding, long shielded cables | Isolation transformer, single-point grounding | Adding a shield can |
| Mechanical vibration | Chassis resonance, fans | Potting, vibration-isolated mounting | Electrical shielding |
This table is the first step in selection: determine where the interference comes from, then decide what to add.
Copper Foil Electrostatic Shielding
Wrapping a layer of copper foil between the primary and secondary windings and grounding it at one end significantly reduces electric field coupling, and is effective against high-frequency noise from switch-mode power supplies and power line crosstalk. Copper foil is non-magnetic and has virtually no effect on low-frequency magnetic fields. Process-wise, note that the copper foil must not form a closed loop—that would act like a shorted turn. The ground connection should be made to the signal ground to avoid injecting interference into the secondary. Adding copper foil increases parasitic capacitance, so wide-bandwidth designs need to re-verify high-frequency response.
Mu-Metal Magnetic Shielding
Mu-metal cans are used to shield against low-frequency magnetic fields, such as leakage flux from power transformers, motors, and high-current traces. The principle is to provide a low-reluctance path for magnetic flux, diverting the interference away. Effectiveness depends on can thickness, how completely it encloses the component, and its distance from the interference source. The can must not have large gaps, or flux will leak through them. Mechanical stress during installation should also be avoided, as it degrades permeability. Magnetic shielding is typically heavier and more expensive, making it suitable for microphones and line transformers that are highly sensitive to hum.
Full Potting
Potting addresses moisture resistance, vibration damping, insulation, and consistency—not electromagnetic shielding. Epoxy or other potting compounds secure the windings, reduce vibration noise, and improve long-term reliability. However, cure shrinkage changes parasitic capacitance and leakage inductance, so frequency response and THD must be re-measured after potting. Potting also affects heat dissipation, so temperature rise must be calculated for high-power applications. For products that may need rework, potting makes repair significantly more difficult.
How to Combine Them
- Start with layout. Shorten signal traces, keep them away from power supplies, and avoid running parallel to power lines—many interference issues can be reduced at the layout stage.
- Then address ground loops. Check whether shield layers are grounded at both ends, and use an isolation transformer if necessary.
- Add copper foil for electric field interference. Ground at one end and watch for changes in parasitic capacitance.
- Add a mu-metal can for magnetic field interference. Ensure complete enclosure and avoid mechanical stress.
- Consider potting last. Use it for moisture resistance, vibration damping, and consistency—not as an electromagnetic shielding method.
Common Mistakes
- Using copper foil to block power transformer leakage flux. Copper foil is non-magnetic and essentially ineffective against low-frequency magnetic fields.
- Grounding the shield can at both ends. This can create a ground loop and actually introduce hum.
- Allowing copper foil to form a closed loop. This acts like a shorted turn, increasing losses and distortion.
- Not re-testing after potting. Changes in distributed parameters can cause high-frequency attenuation or resonance.
FAQ
Why is there still hum after adding a shield can?
The interference may come from a ground loop rather than a magnetic field, or the can may have gaps or an incorrect grounding method. We recommend troubleshooting by interference type.
Can copper foil and mu-metal be used together?
Yes—they target electric and magnetic fields respectively. But this increases size, cost, and parasitic capacitance, so frequency response needs to be re-measured.
Does potting reduce background noise?
Potting mainly suppresses vibration and moisture; it has limited effect on electromagnetic interference. If the noise comes from electric fields or ground loops, prioritize shielding and grounding instead.
How should the shield layer be grounded?
Typically single-point grounding: connect the primary side to signal ground, and leave the secondary side floating or ground it through a small capacitor. The exact approach depends on the overall grounding architecture of the device.
If you're unsure about the interference type, send us your device structure, interference symptoms, and layout photos and we'll help you diagnose: Contact our technical team. You can also check out our audio transformer product page and isolation transformer product page.
