DC on the Mains.
Power supplies in Hi-Fi and AV equipment often assume connection to a perfect mains supply. In theory the mains waveform should be pure AC: a perfect, symmetrical sine wave.
Many of the myriad appliances connected to the supply, at home and in industry; from electronic dimmers to computer power supplies, use the AC energy available in a mains cycle unevenly. The net result is the effective addition of DC to the mains network.
This effect is inevitable and also inherently unpredictable. In practice the waveform can arrive at the end-user clipped, distorted and asymmetric. Simplistic analyses of unmatched peak voltages are not the whole story: Any AC waveform which does not have equal energy in both positive and negative phases will contain a DC component; regardless of what the peak voltages are or what the wave shape is.
The AC transformers commonly found in audio equipment cannot, by nature, tolerate significant DC without being compromised.
As all transformers convert power through the medium of magnetism, any DC present will inhibit its capacity to transform AC by partially or completely saturating the magnetic circuit. It is this vulnerability to DC that causes acoustic hum and significantly reduces the transformer's power capacity. Even measured at comparatively low levels we have found that this has a significant negative impact on the following power supply and circuit performance, which in turn can degrade sound quality.
