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Tweaks for systems, rooms and Do It Yourself (DIY) help. FAQ.

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

I did answer the OP's question. Your posts are the reason for our back and forth discussions. I suggest you should reread the OP's posted message again.

My response to the OP:

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.



Ryelands said:
* The use of unearthed but insulated devices long predates current definitions and standards which was why I wrote "except for what we now call 'double insulated' devices".

We have already beat that horse to death. Audio equipment manufactured in the 1970s, and earlier, sold in the USA and Canada was manufactures with a captive 2 wire power cord and a 2 wire non polarized plug. The internal AC power wiring back then was not doubled insulated. You are just trying to change, or maybe clarify, what you said in previous posts on the subject.

Ryelands said:
* The socket pictured in your link doesn't show shutters though the text describes them.

* It's inaccurate to suggest that "Type G wall sockets almost always include switches for extra safety". The switch is for convenience, not safety.

What? All I wanted to know is the picture in the Link I provided look like the 3 wire receptacle and 3 wire plug used in a residential dwelling unit in the UK. You were the one that brought up the subject of the UK 3 wire receptacle and the use of equipment/appliances that use a 2 wire power cord using a 3 wire plug on the 2 wire cord.

Ryelands said:
* Also inaccurate is "The lack of such an earth pin on a type C plug makes it impossible to connect it to a type G receptacle". Fused adapters (usually 1 or 2.5 amp) that make type C plugs perfectly safe in UK sockets are readily available.

Type C plug? Where in any of your responses did you ever mention any plug other than a 3 wire plug?


Here is the bottom line. IN the USA or Canada anyone that has a piece of vintage audio equipment with a captive 2 wire power cord with a non polarized plug you have a 50/50 chance, when plugging the unit into the AC power receptacle, having the proper AC polarity orientation correct feeding the primary winding of the power transformer of the piece of equipment.

You can easily check for the proper polarity plug orientation with a multimeter.

1) Disconnect all ICs from the inputs of the piece of equipment to be tested.

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.
*If the equipment is a power amp you might want to allow a few minutes to pass for the electrolytic caps in the power supply to bleed off before turning the equipment back on.

6) Power up the equipment. Repeat procedure 4). Note the voltage reading. The lower voltage of the two tests is the proper AC polarity plug orientation. Mark, identify, one side of the plug. Best place is on the neutral blade/contact side.

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Follow Ups Full Thread
Follow Ups
  • RE: Why would reversing AC leads reduce hum in a component? - jea48 10:26:16 02/12/17 (0)


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