I have a headphone amplifier that is afflicted by mains hum 50 Hz and upper harmonics, below is an FFT of the spectrum with nothing connected to its inputs.
In headphones you can clearly hear a buzz-drone sound, volume pot doesn't change its level either. Is this to do with the rectifier, single ground planes or something else? I tried two different transformers one R-core, and the other a novatel sealed one. No effect, changed out caps, still with this noise in the spectrum present.