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In Reply to: RE: Why would reversing AC leads reduce hum in a component? posted by jedrider on February 10, 2017 at 08:00:37
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.
Follow Ups:
I got a cheater plug and snipped off the expansion tabs (easy to do but newer ones are wider the entire length, so that would be more difficult to snip) and tried reversing AC polarity.
I seem to get a 6-12 dB reduction in noise level. Just guessing, but I wonder how I would measure this with a scope??
So I listen to say Spanish guitar because it's usually on the quiet side and crank the volume louder than I normally listen and then remove the needle from the record and I can hear the hum. It seems lower when polarity is reversed AND the music sounds better. In fact, now it sounds better at the higher volume which is exactly where I want to be.
So I will open the unit up and reverse the primary leads. Note that the power supply is DIY and the primary leads look identical otherwise. I think I will tag the one that is suppose to be hot.
Sounds good!
I seem to get a 6-12 dB reduction in noise level. Just guessing, but I wonder how I would measure this with a scope??
You can easily check for the proper polarity orientation with a multimeter.
1) Disconnect all ICs from the inputs of the piece of equipment to be tested.
Equipment safety ground must also be lifted from the piece of equipment. As you did with the ground cheater.)
2) Set the multimeter to auto AC volts or a range above 125V (USA or Canada)
3) Power up the piece of equipment.
4) Touch one test lead probe of the meter to the metal chassis of the equipment.
Touch the other test lead probe of the meter to the equipment ground contact of the power receptacle. (If the receptacle is an old 2 wire non grounding type receptacle use the neutral contact of the other receptacle of the duplex outlet. The actual reference point for the test is the Grounded Conductor, the neutral conductor.)
Note the voltage reading.
5) Turn off the piece of equipment. Unplug it from the AC power receptacle. Reverse the plug 180 degrees and plug it back in.
6) Power up the equipment. Repeat procedure 4). Note the voltage reading. The lower voltage reading of the two tests is the proper AC polarity orientation.
.
Have Fun and Enjoy the Music
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