Not likely that this would be the direct cause of the hum. High mains conditions can cause higher levels of transformer "hum" due to the abrupt increase in excitation current that occurs as the magnetic material nears saturation. The most likely cause of the "hum" is a high noise floor in your equipment. As for a method of diagnosing the specific component, I can offer the following trouble shooting tips. If the level of the "hum" is affected by either the source that you select (tuner, CD, phono, etc) or by the setting of the level control, then your problem is either the preamplifier or the specific source. If it is a constant level, regardless of the state of the above mentioned controls, then it is likely either pre or power amplifier self noise (with power amplifier being more typical). Other trouble shooting steps can be taken, but more specific details as to your system and its configuration would be required. Kevin Halverson
