Silence Your Gear!!! Power Cord Polarity...

...OK, maybe not Total Silence, as that would imply just turning it off

Pretty much all of our old gear have non-polarized AC cords. Not a big problem since the transformer in our stuff is isolated from the frame. So....its not supposed to make a difference which way it's plugged in, right??

.......not necessarily. Transformer construction on many amps will allow a high-impedence 115V (in the US) AC signal to appear on the chassis, even if the transformer is electrically isolated from the chassis (they rarely are). This is called 'chassis to ground potential' and some transformers do a much better job of keeping this low than others. There are alot of reasons why the voltage is there that I'm not going to get into now (more boring tech stuff), but suffice it to say that keeping this potential low is expensive for manufacturers. This AC voltage, being on the chassis and surrounding it on every side, has the potential to stir up quite a bit of hum and noise in high-gain amp stages, such as phono amps. Test you say? Alrighty...

Unplug the gear from AC, and disconnect any cables running to or from other gear...we want the UUT (unit under test) to be isolated. Now, get out your digital meter. Set to a proper scale to read 115VAC if US, and 220V if UK. Connect the red lead of the meter to the chassis of the amp/preamp/tuner/whatever. Connect the other end to AC ground.

I want to mention a word about connecting to ground. The most convienent ground is the third pin on your AC receptical. This is fine to connect to, but anytime you are fooling around with or near big voltages, do not do anything foolish!! The easiest way to connect to ground is to buy a 'ground cheater'. Get one that has some way to connect your meter to the ground pin...a small wire pigtail or a metal tab on the cheater is usually supplied. Needless to say, this is going to work only is the ground connection on the AC outlet is actually functional...if in doubt, buy a $5 AC outlet tester that tells you if the ground is connected, and if hot and neutral are reversed (I consider this an essential item for any household).

Once you have a connection to ground and the other to the chassis of the equipment, take the power cord of the unit and somehow scratch/scribe/or note one side of the plug so you can tell it apart from the other. Plug it in, and power the unit up. Read the voltage, write it down (it could be anything from a few volts all the way up to your line voltage). This voltage that appears on the chassis cannot hurt you unless there is something seriously wrong with the gear. OK, now power down and unplug the unit from the AC outlet. If you are checking a power amplifier here, I recommend you wait a few minutes before continuing to allow the amp caps to discharge a bit. OK, now plug the AC cord of the unit in again, but with the plug reversed from where it was a minute ago. Power up, and read the voltage.

The orientation that shows the lowest voltage on the chassis of the gear is what you want. Mark your plug somehow, and see that it is oriented correctly whenever you plug it in.

Incidentally, when I get a piece of gear in that needs a new power cord, I always replace with a polarized cord. I use the procedure above to determine which way to orient the plug before soldering it up...with the polarized plug I know the unit will always be plugged in in a way that minimises the chassis potential.

Some of you all may have polarized plugs already. If you would like to see if the manufacturer connected the AC in the best fashion possible, get a cheater plug, clip the ground off of it, and file/grind the polarized side of the cheater plug so that you can reverse it at the outlet if you like. This will save you from having to modify your unit's AC cord, and now you can check for chassis potential.

All my cords vintage or not have some writing or ribbing or sumthing to distinguish one side from the other, I plug them all in according to the way the stripe/writing appears on the newer polarized plugs, is this what you mean?

Well this should be an interesting topic. Several thoughts come to mind:

Usually we are dealing with changes and tweaks that bring on subjective 'Improvements' without the any measurable proof of an improvement. Here we have one based on measurement but of unknown benefit to the sound.

I suspect that this procedure may help some lessor units here or there but be unnoticable in many quality units. Turntables would be one device I might consider checking out via your procedure because of the super low levels involved (MC cartridges work in the microvolt level!) and the need for a grounding wire for turntable to preamp.

What about units that came with 3 conductor power cords as original design. Do you suppose the the manufacture knew the correct 'phase' that best isolated the transformer primary winding? Or would it not matter as the chassis is truly at ground potential via the power cable?

Me, I'll stick with my headphone tests. If I can't hear any hum or hiss with no source selected but with the volumn cranked then I am happy with the system/components. That's why I gave up on equilizers. I tried about 4-5 different brands that I had found at thrift stores and everyone of them would fail my headphone test when switched on, they all added hiss. Luckly equilizers seem to sell well on E-bay and I had no problem unloading them, even though I do miss the bouncing lights at times..:

Lefty

Bouncy lights are kewl

Just remember: Black to brass will save your ass

No more than 500ua leakage current..... Oh you were talking about Stereo gear..... Sorry for being a smart ass. :uzi: :uzi:

I ought to put a grounded plug set up on one of the recievers and bring home a Safety Analyzer and see what shows up.

Or just set your Multimeter to read ua and go from chassis to ground. Any more than 800ua and I would not get damp and grab the monster. You just might discover how well the human body conducts.....

Sorry for going off topic but safety is one of my jobs and I lost a friend years ago on 80ua. It was at 100kvp. Just remember that ohms law.

Wow jerry...80µA? I guess at 100KV (explain the 'p' to me...) that's 8 watts. Prolly more than the human body is ready to deal with.

Lefty,

I suspect that this procedure may help some lessor units here or there but be unnoticable in many quality units.

Click to expand...

You are probably correct, but seeing as how we all have vintage gear, it's not an uncommon occurance to find it necessary to replace a line cord. If one found themselves in that position, and briefly wondered if it made any difference which lead should go to neutral, and which to hot (especially since most all replacement cords are polarized), now they know.

What about units that came with 3 conductor power cords as original design. Do you suppose the the manufacture knew the correct 'phase' that best isolated the transformer primary winding? Or would it not matter as the chassis is truly at ground potential via the power cable?

Click to expand...

If the transformer leakage is significant, then ground loops can be induced easily. But again, on quality units where leakage is small, the current is shunted to ground with little ill effects.

...i don't have one piece of gear here with a three-prong plug. Adding a ground to existing equipment is begging for a ground loop, which is why you generally see three-prong plugs only on pro equipment where balanced lines are the norm.

Been through this years ago...

Here in NZ we run 230/240V 50hz power.
Most equipment is fitted with a three pin (P/E/N) plug so usually there's no switching around, hopefully the local distributor will have sorted out the power polarity otherwise it's a PIA.

Most of the gear I have purchased locally has had the plug wired correctly but I have come across a couple of amps where P and N pins were reversed

With gear imported from the US and Japan, these are often hardwired with a NZ three pin plug (with luck it will be wired correctly) with the earth pin unconnected or left with a simple two pin | | or / \ plug.

Easy to use an adaptor to sort out which way 'round sounds best.
Then mark the plug for future reference.

The only snag is sometimes the orientation that sounds the best doesn't always give the lowest reading on the voltmeter.

Why is that so ?

In the 80s an alternative to using a meter was to give your gear the finger. :

A sensitive finger run along the back where most power supplies reside could be used to detect changes in vibrations when the power plug was reversed. Less vibration was best.


cheerio

Echo Wars. Sorry I did not throw the whole equation in there.

Factor RF as in X-ray freq and yes 80ma @ 100kvp will kill.
Sorry to mislead you.

Once you get bit by a X-Ray machine you never forget. They like to reach out and touch you.

Great post, EchoWars!

I happen to sell a couple of different brands of polarity-reversible power cords, and reversing AC polarity can make a HUGE difference in sound quality.

Happy listening,
Brian

Posts like these are why I check AK everyday, M-F (and occasionaly on weekends). I have a new "hum" on my Yamaha CR 1020 that was not evident when it was set up elsewhere. Tonight, I will be playing with polarity.

Some of the old guitar amps actually have the toggle switch to flip the polarity to reduce the noise or hum. I was always amazed at the people who never knew what the switch was for.

These days I just wonder who invented the wider prong blade on cords? Ralph Nader? I have a 50% chance of getting the plug turned right when attempting to connect the plug to an outlet. Unbelievably, I am 99% wrong most of the time especially when in a bind or cannot see completely. Hope I never meet that guy, he'll be punched squarely in the mouth... :yes:

Okay...rant over...

Thanks for the post EchoWars, I'll have to test this again to your procedure. I tried flipping the plug around and measuring voltage from chassis to Neutral with everything connected. I noticed in my tuner a 2.2M ohm resistor from one AC line to chassis/ground, can anyone explain this? J

Should I consider re-fitting equipment with 3-wire plugs?

Thanks for the neat post. I'm going to check my receiver out.

Cheers

I've had a few pieces of gear where you could you could feel the voltage on the edges of the metal faceplate. Not so much with fingers...but if you touched it to your forearm you could feel it. Reversing the plug made it go away.

Maybe the old 9-volt battery on the tongue test would also work on old gear?

willyrover said:

Maybe the old 9-volt battery on the tongue test would also work on old gear?

Click to expand...

I reckon that EVERY AK member has this "test" in common.

Ken

EW,
Timely reminder - I consider this "standard procedure" any time I go thru a tuner and if it's being modded, raising the transformer. I guess I'm going to make it a standard test procedure if for no other reason than someone down the line might actually run a "tongue Test" on it!
It would be nice if there would be a good source of quality polarized cords. I'll have to look see......

For the 3 pin plug adaption, can be done but can also cause more trouble than you bargin for ground loop wise. Not that it can't be done but I don't think the benefit is all that great over just polarizing the 2-blade plug.

thanks for the post EW, i look forward to you're helpful hints. :thmbsp:

So far all of the Yammies I have serviced have the side of the line cord with a rib, white tracer, or writing, connected to the side that has the 2.2 megohn resistor run to the chassis. Put that prong in the wide blade opening of the receptacle and you are good to go. When I have had the occasion to replace a line cord I follow their practice. This happens mainly with the multi-voltage models and the standard seemss to be brown wire = hot, blue wire = ground.

A very good reason for following this rule is that not all test equipment has isolated ground and you may rest assured that ALL of my test equipment has been verified as having the chassis at ground potential. Do not want to let out the magic smoke.

Rob

Ah, yes. Thanks for the info merrylander, resistor to nuetral. On my tuner (Akai At-V04) the plug is not polarized but does have a ridge on one side of the cord. I'll get in there eventually to finish a recap job and check that too. J