Building on the causes, the handbook prescribes targeted mitigation strategies at the design, manufacturing, and application stages.

  1. At the Core – Reduce Excitation Source:

    • Use Low-Magnetostriction Material: Specify core steel with a low magnetostriction constant.

    • Lower Operating Flux Density ($B_m$): A lower $B_m$ significantly reduces magnetostrictive strain. This is a key design trade-off for low-noise transformers (e.g., in audio).

    • Ensure Core is Stress-Free and Well-Processed: For toroids, proper annealing is non-negotiable. For laminated cores, avoid rough handling that introduces burrs or bends.

  2. Dampen and Constrain Core Vibration:

    • Secure Core Clamping: Apply correct torque to core clamping bolts and use resilient pads if necessary to keep laminations tight under varying thermal cycles.

    • Core Coating/Impregnation: For some high-end applications, the core itself can be coated or impregnated with a damping compound (e.g., a special varnish) to reduce high-frequency vibration components.

  3. Secure the Windings:

    • Tight Winding Under Tension: Wind coils with consistent, appropriate tension.

    • Thorough Vacuum Impregnation (VPI): This is the single most effective step for winding noise. It completely locks the turns, layers, and lead wires into a solid, vibration-damped mass.

  4. Isolate and Dampen at the Mounting:

    • Use Resilient Mounts: Install the transformer using rubber grommets, silicone pads, or other vibration-damping mounts. This decouples the transformer’s vibration from the chassis/panel, preventing amplification.

    • Avoid Resonance: Design the mounting system so its natural frequency is far from 100/120 Hz. Adding mass or stiffness can shift the resonance point.

    • Anti-Vibration Baseplate: Mounting a toroidal transformer on a heavy, rigid metal plate with a damping layer between the core and plate can be very effective.

  5. Application Consideration:

    • DC Bias Blocking: As mentioned, a DC blocking circuit in series with the primary can eliminate the hum caused by mains DC offset.

Handbook’s Summary: A low-noise transformer is the result of a systematic approach: 1) Minimize the excitation (low $B_m$, good material), 2) Dampen the vibrating elements (impregnation, coating), and 3) Isolate the final assembly from the environment (resilient mounts).