Why does especially the toroid transformer hum, sometimes hard?
The iron core is very efficient and the magnetic flux is high, very near magnetic saturation of the core. This gives very little room for "extra" flux which DC can cause. DC on AC mains is simply unsymmetrical waveform of the sinus wave. What happens if you have a hair drier with half speed? This is accomplished with a diode and halfway rectification, causing the positive peak voltage become lower than the negative. This creates a small DC level which some apparatus don't like, especially toroids. An another reason for DC on the mains is switching transients in large networks.
List of dangerous source of HUM in domestic house:
• Alogen lamp with intensity variable potentiometer
• Personal computer switching power supply
• DVD switching power supply
• Domestic Boiler without good AC mains insulation
• Satellite receiver as switching power supply
• General switching power supply
• Loop ground come from ac mains distributors ( all ground togheter connected)
• Magnetics flux come from another power supply
The DC trap makes sure that no DC slips through. The primary winding is AC coupled with help from two big electrolytic capacitors. The inrush current will go through the diodes when the capacitors have been charged over 1 or 3 volts and the normal current will only go through the capacitors.