A/C power line polarity on new phono preamp

You can do this for all electronic components. This is the best way to achieve the lowest noise floor for the whole system. Listening tests are not accurate, so get out that volt meter!

Get a volt meter, move the phono stage over near to the wall socket, plug it in, measure from the chassis to the center screw of the AC cover, note the reading. Reverse the plug and measure again. Compare the two readings on the volt meter. The lower reading is the correct orientation of the AC plug. If the phono stage plug is polarized (one blade wider than the other) you will need to use a two-prong cheater plug to make the voltage readings. This is the proper way to determine the plug orientation!

Since AC plugs are almost always polarized you will have to leave the cheater plug in place for the proper orientation.

You are measuring eddy currents on the chassis. The center screw on the AC outlet cover is ground. Less measured voltage means less eddy currents and a lower noise floor.

@gs5556 : I think you could be right! That switch could be for signal polarity!

I have never bothered with checking the AC plug orientation. The difference is so small that it is inaudible to me.

The difference in eddy currents from AC plug orientation is usually too small to worry about.