Hi-Fi power cord polarity (line phase) in the AC mains socket.

Hello and welcome to whatishifi blog; I am confident that most of you will find this post practical, useful and interesting.

Back in the 90's and while living in my parent's home, my stereo system was big enough already. It had 18 components - some of them for 2ch stereo and some of them for home cinema. Back then I was not aware of the power cord polarity issue, so I was plugging my equipment in a random way. But soon I started noticing unexplained hum and noises at the system; as I found out the main cause was the existence of one component with schuko earthed power plug (the rest of the components were with two pin unearthed plugs) combined with my AV equipment that were connected to the central TV antenna.

The weird thing with those noises was that they were not constant - they were changing frequency and volume through the day in a way that back then was appearing random to me. I will explain why later.


Ah, wonderful Luxman! This line phase sensor works like a mains test screwdriver. If by touching it lights, this means that the live and the neutral are swapped so you better flip the mains power plug. 

To make a long story short, if you live in a country where a power plug can be inserted in the wall socket in two ways, it does matter to find which of the pin is the neutral one and plug it to the wall socket accordingly.

At the top, plug types that can be inserted both ways in the socket. At the bottom, plug types that can be inserted only one way in the socket - so no need to worry in this case. (photos from Wikipedia).

So, lets see a practical way to determine which is the "correct" way to plug your equipment at the wall socket.

You will need a mains test screwdriver and a mutlimeter measuring ACV. The important thing is that you will have to measure each one component separately, meaning that it should be connected only to the mains socket and not to other components through any kind of interconnects or aerial / data cables - the only exception here is that you can leave passive speakers connected to the amp, but be careful not to create any boom noises and destroy your speakers. Also, in case the plug of the equipment to be measured has an earthing connection, you have to "earth lift" it, meaning the device should not be earthed during the measurement. You should not touch the device during the measurement, your body can play the role of grounding, something that would alter the measurement.

When I want to earth lift a device, I use this adapter. Sorry for the ugly, non Hi-End duct tape.

First thing is to determine which is the neutral side of the wall socket; this is easy with the use of a mains test screwdriver.

When the mains test screwdriver lights up, it means that it is inserted in the hot part of the socket so in the photo above, the neutral part is on the left side.

Then you need to measure VAC between the wall socket's earth point and a negative (ground) point of an interconnect plug of your device, with your device turned on. As I said, it is absolutely necessary that your device is ground lifted, not connected to other components and not to be touched during the measurement.


Connect one multimeter lead to the earth pin of your wall socket... 

...and the other lead at the negative (ground) side of an interconnect plug. 

Turn on your device and take one measurement with the power plug connected one way; then take another measurement with the power plug reversed. During the measurements your device needs to be on. The correct way to plug your device is the one giving you the lower VAC reading. Mark the plug by writing an N for neutral at the side where the pin that should be plugged at the neutral side of the socket is and continue to the next component.

First measurement gave around 50 VAC.

After flipping 180 degrees the mains plug, we get around 29 VAC. So, this is the right way to plug our device. Knowing that the left part of the socket is the neutral, we can write an N mark at the left side of the device's plug.

It is important to correctly connect all your equipment involved in your Hi-Fi and Home Cinema. This includes the TV, projector, active subwoofer, media player etc. In case your equipment are connected with (wired) LAN (like a smart TV, a computer audio PC or a laptop, a NAS etc.) don't forget to do this including your LAN switch and everything else connected at the LAN switch - even if this includes a network printer!

With some devices it might seem hard to find a negative point of an interconnection, but there is always one; a metal part around the switch LAN network plugs, a screw over a HDMI connector etc. Also, don't get disappointed with devices using external switching power supplies; they might give you similar readings but here is always a lower one, even if it is just a little lower.

Remember at the beginning of this post where I said that the noises I was experiencing where not constant? As it turned out, this happened due to my parent's TV and video. They were connected through a splitter to the same antenna with my video and TV, meaning they where able to electrically communicate with my system through the antenna cable. What I did was to measure and properly connect all the TVs and video players that were in my house! Only then I enjoyed the silence I was seeking. So, think of every equipment communicating with your Hi-Fi / Home Cinema through any kind of cables (except of the power supply cable) and plug each one of them properly.

Why this happens? It is a complicated issue that I have not fully managed to understand. It has to do with ground loops and the interaction between equipment that have a 2 pin unearthed plug and a schuko earthed plug. When an equipment is not plugged in the right way, current irrelevant with the signal will flow through the interconnects deteriorating the sound quality and / or creating hum and noise. Even if all the equipment have only 2 pin unearthed plugs, still the one that is not correctly plugged in will send current through the interconnects to the one that is. What we accomplish with the method described above is to connect all the equipment in a way that minimizes the currents traveling through the interconnects. You can easily hear the difference at devises sensitive to noises like your phono stage - or in the past at your tape deck. I still remember that my tape deck became quitter for two reasons; one reason was because it was simply plugged correctly, the other reason was because when the equipment standing next to the deck was plugged correctly, it stopped affecting (by electromagnetic radiation) the tape deck!

If you found this post useful allow me to suggest some more reading:

- Older Hi-Fi equipments and the 120V, 220V & 240V voltage selector.
- My impressions regarding a Hi-Fi power strip at the end of this post.
- Can power supply cables affect the sound?

Now, let me also add my experience with devices that come with an earthed schuko plug; often, they create noises! Even my laptop comes with an earthed schuko plug at it's power supply and I can't understand why. I earth lifted all but one of these devices and the noises stopped. I am not suggesting you to do the same, it might not be right. Theoretically you can get electrocuted in case of a malfunction. All I am doing is saying what I did in my system, ok?

If you have read any other post you already know that I like to say something funny, but what could be funny with line phase? Well there is. I remember a period I got so much obsessed that I even connected the refrigerator the "right way". I also was installing the garden hoses in a way that the words on them had the same direction with the water flow. My garden immediately became Hi-End and must be very proud of me.

That's all for now, happy listening!

Christos

P.S. If you liked this post, you will probably like most of this blog. Why not having a look at all the things we have wrote about here.