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RE: Why would reversing AC leads reduce hum in a component?

Posted by jea48 on February 10, 2017 at 14:17:05:
I would imagine the switch reverses the AC polarity to the primary winding of the power transformer. The correct AC polarity can have an affect on the SQ of piece of audio equipment.

Here is a quote from Charles Hansen:

1) Reversed AC polarity -- All power transformers have an inherent asymmetry to their construction. The primary winding comprises multiple layers, so that one lead is connected to the innermost windings and the other lead is connected to the outermost windings. This means that one lead has a higher coupling capacitance to the core of the transformer. Please remember that the AC supply is also asymmetrical, with the neutral lead essentially being at ground potential (assuming there is not a fault in the house wiring). The result is that one orientation will give a higher AC leakage current to the chassis of the amp (and worse sound) than the other orientation.

Not all transformer manufacturers use consistent markings on their transformers so that the correct orientation can be identified, and not all amp manufacturers pay attention to this even if the transformer is correctly marked. The result is that many audio products have a random chance of being correctly oriented. I would have to assume that the amp was modded to achieve the correct orientation, thereby achieving improved sound quality.

You will also see comments from John Curl.