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DC offset on the mains causes the normal AC sine wave to become asymmetrical. Here in Europe we talk of getting 240V out of the wall but in fact that's the RMS Root Mean Square voltage. The real peak-to-peak voltage of the AC sine wave should be multiplied by √2 or 1.414. Doing so arrives at 340V. With an asymmetrical load from ‘somewhere’ this peak-to-peak figure will drop by about 1V.
So what you say. This can’t possibly be significant. At first sight that’s true. But now we’re going to look at the average DC voltage. It will be around 300mV. That's what will cause problems in our hifi equipment. This 300mV enters a transformer with a primary DC resistance of 2Ω. There will be a current of 150mA in that primary which is excessive. It will cause the transformer’s core to saturate during the negative half cycle of an AC wave. For toroidal transformers such a situation is far more serious as this type has no air gap to prevent current rises when the inductance drops.
What can one do to prevent DC from entering the power supplies of our hifi equipment? Unfortunately just putting a capacitor in series with the incoming AC won't do. Sure, DC won’t pass a capacitor but a capacitor might get into resonance with the transformer primary we mean to protect. When things get into resonance at the mains frequency we can expect fireworks followed by tears as our precious audio gear gives up its ghost. Therefore such a capacitor needs protection. The best way is an added diode network. Needless to say the value of our capacitor should be well calculated and the capacitor/diode network should not warm up due to ripple currents. It should not be built up with just one electrolytic to establish longevity and it should be suited for the maximum current the transformer will draw.
All that is fine when such a DC filter is tailormade for a specific device whose performance values are known. In the real world audio designers for some unknown reason—!?—do not equip their power supplies with adequate DC filtering. Hence their transformers are prone to buzzing.
