Root cause of Magnetic Humming due to Transformer

In Audio Design in cassettes, magnetic head picks up magnetic stray fields and cause irritating humming background noise. The 3rd harmonic of 50 Hz gets amplified when the speaker resonance coincides. This R&D report validates the root cause. The solution was to have a physcial distance, while the current produced units had a wire loop to create a 150Hz pickup coil to phase cancellation manually tuned at PCB.Read less