Why an AC Power Cord cannot make a difference

[HaveMeterWillTravel]

So I went around and measured the common mode voltages (CMV) and leakage currents (LC) of several sources and two preamps/amps. A handheld Rohde scope was used to take the voltage and phase measurements, and a Fluke 289 for current.

Sources:
Topping DX3Pro+: 45 Vrms & 100 uA. Type A plug. So the end user may have it in absolute phase or 180 out of phase relative to Line
MiniDSP Flex HTx: 20 Vrms & 100 uA. NEMA 5-15P plug and IEC C13/C14, but it's output is 180 out of phase relative to Line


Preamp/Amps
My vintage McIntosh gear (C28 & MC2105) was by far the best, less than 1 Vrms and 25 uA, Type A-P plugs, and in absolute phase with Line. (*)
Denon PMA-750: 50 Vrms & 50 uA. Type A plug. So the end user may have it in absolute phase or 180 out of phase relative to Line

I then tested the various permutations of connecting a source to the preamp/amp with a standard RCA on the left channel only.

Topping and Mc: With the RCA's fully inserted, there was no discerable noise (casual listening) with the Topping connected in either polarity. When only the center pin was connected, there was a tiny amount when the Topping power was plugged in in absolute-polarity. It was pretty audible when it was not in absolute phase. Note: without a scope or similar device there is no way for the consumer to determine correct polarity of the plug.

Topping and Denon: With the RCA's fully inserted, there was no discerable noise (casual listening) with both were plugged in either in absolute polarity, or both 180 degrees out of polarity. If they were in opposite polarity, the noise was rather audible. With the RCA's only making contact in the possible, it was loud enough I immediately shut it down as to not damage my loudspeaker.

MiniDSP and Denon: Very similar to the Topping and Denon.

MiniDSP and the Mc: With the RCA's fully inserted, there was audible noise. With only the center pin connected, it was again loud enough to cause possible damage.

What's the net result?

To anyone who has basic electrical knowledge, there are no suprises here. Despite it being a well understood phenonemon, people generally are either unware of it's important or forget about it.

When two devices common mode voltages ( or leakage currents ) are in phase, there is no audible effects (at least due to the CMV/LC)
But when they are not, it may or may not make a noise.

I do have to wonder how many times someone thinks they need an additional isolation device (say USB), it's really a CMV issue.

btw: I added the half-connected RCA as a demonstration of one of the possible reasons why it may or may not be an issue with one's own particular equipment.

(*) They are not the original cords, having been replaced many years ago. So the factory phase wiring is indeterminate.

 

[RayDunzl]

Here's a view of the voltage (relative to Earth) of the Hot and Neutral with my gear running a considerable load, a few hundred watts, back when the plasma TV was around, the PC and its monitors are on the same branch, and the Krells were doing their thing, along with all the various small power draws.

The Hot lead (flattened sine) peaks at 170V

The other trace is the Neutral, I don't remember the scale, maybe 1 volt per division, so 3V swings when the gear draws current through their rectifiers.

I have maybe 80 feet of wire in the wall and above the ceiling from this outlet to the breaker box.

Point being, maybe, if there must be one, that Neutral is not gonna be "0 volts" at the gear, it isn't "ground".

 

[SIY]

So I went around and measured the common mode voltages (CMV) and leakage currents (LC) of several sources and two preamps/amps. A handheld Rohde scope was used to take the voltage and phase measurements, and a Fluke 289 for current.

Sources:
Topping DX3Pro+: 45 Vrms & 100 uA. Type A plug. So the end user may have it in absolute phase or 180 out of phase relative to Line
MiniDSP Flex HTx: 20 Vrms & 100 uA. NEMA 5-15P plug and IEC C13/C14, but it's output is 180 out of phase relative to Line


Preamp/Amps
My vintage McIntosh gear (C28 & MC2105) was by far the best, less than 1 Vrms and 25 uA, Type A-P plugs, and in absolute phase with Line. (*)
Denon PMA-750: 50 Vrms & 50 uA. Type A plug. So the end user may have it in absolute phase or 180 out of phase relative to Line

I then tested the various permutations of connecting a source to the preamp/amp with a standard RCA on the left channel only.

Topping and Mc: With the RCA's fully inserted, there was no discerable noise (casual listening) with the Topping connected in either polarity. When only the center pin was connected, there was a tiny amount when the Topping power was plugged in in absolute-polarity. It was pretty audible when it was not in absolute phase. Note: without a scope or similar device there is no way for the consumer to determine correct polarity of the plug.

Topping and Denon: With the RCA's fully inserted, there was no discerable noise (casual listening) with both were plugged in either in absolute polarity, or both 180 degrees out of polarity. If they were in opposite polarity, the noise was rather audible. With the RCA's only making contact in the possible, it was loud enough I immediately shut it down as to not damage my loudspeaker.

MiniDSP and Denon: Very similar to the Topping and Denon.

MiniDSP and the Mc: With the RCA's fully inserted, there was audible noise. With only the center pin connected, it was again loud enough to cause possible damage.

What's the net result?

To anyone who has basic electrical knowledge, there are no suprises here. Despite it being a well understood phenonemon, people generally are either unware of it's important or forget about it.

When two devices common mode voltages ( or leakage currents ) are in phase, there is no audible effects (at least due to the CMV/LC)
But when they are not, it may or may not make a noise.

I do have to wonder how many times someone thinks they need an additional isolation device (say USB), it's really a CMV issue.

btw: I added the half-connected RCA as a demonstration of one of the possible reasons why it may or may not be an issue with one's own particular equipment.

(*) They are not the original cords, having been replaced many years ago. So the factory phase wiring is indeterminate.

This seems to be more about house wiring and weirdly half-disconnecting RCA plugs than about power cords. At least it seems that way, but I admit I have trouble following what you're trying to say.

 

[JustJones]

Just plug one of these quantum QRT dodads from Nordost in a receptacle to cure your electricity problems.
(Best used with Nordost power cords and AC distribution unit)

 

[HaveMeterWillTravel]

Here's a view of the voltage (relative to Earth) of the Hot and Neutral with my gear running a considerable load, a few hundred watts, back when the plasma TV was around, the PC and its monitors are on the same branch, and the Krells were doing their thing, along with all the various small power draws.

The Hot lead (flattened sine) peaks at 170V

The other trace is the Neutral, I don't remember the scale, maybe 1 volt per division, so 3V swings when the gear draws current through their rectifiers.

I have maybe 80 feet of wire in the wall and above the ceiling from this outlet to the breaker box.

Point being, maybe, if there must be one, that Neutral is not gonna be "0 volts" at the gear, it isn't "ground".

View attachment 420123

I guess I forgot to specify how I took the common mode voltage and phase measurements.
They were from the RCA return (outer barrel) to earth ground.

When appreciable common mode voltages are present, the differential magnitude between two devices will be minimized when they are both in-phase, and at a maximum if they are out of phase for each other. The result is a minimum or maximum of current flow between the two devices. Generally, this current will flow between the two devices through BOTH the center pin (positive) and outer barrel (RTN or Shield). Unfortunately, most RCA inputs are single-ended or local-chassis references (they inherently need not be, one can actually have differential RCA connections but I've only seen it a few times) and the input stage only has a limited signal CMRR, certainly not for a combined 100 to 120 Vrms!
The primary reason why this effect is far less on typical XLR connections is less that there are two differential pins or that it is shielded, but that the induced current equalizes over that 3rd pin and is kept out of the signal path.

Unfortunately, there is not really a standardized that manufactures need to adhere to. It definitely seems to be far more common with the plethora of cheap "wall warts" and "laptop style" power supplies.

BTW: this effect is not limited to only Type A (two-pin) plugs. It's just easier to test.

 

[HaveMeterWillTravel]

This seems to be more about house wiring and weirdly half-disconnecting RCA plugs than about power cords. At least it seems that way, but I admit I have trouble following what you're trying to say.

It has little to do with the house wiring, at least when everything is plugged into the same out electrical panel phase.
I suppose the CMV may well be doubled if one device is plugged into a Phase A outlet and the other is Phase B.
I can't remember if having Phase A and Phase B sourced outlets in the same room allowed by NEC/NPFA code. However, I could easily see it occuring if you run your signal cables from one room to another.

Maybe it confused the issue and I should not have includeed the half-disconnected RCA plugs. However, I offered it as an audible effect commonly assumed to be audible which is (predominately) caused by the same issue we're discussion.

Again, this is an example of what can cause audible differences and which most people seem to be unaware.

I'm now curious how many who 'needed' something like a USB isolator or had group loop issues could have solved it with just experimenting with the absolute power phase of all their equipment.

 

[Salt]

Just plug one of these quantum QRT dodads from Nordost in a receptacle to cure your electricity problems.
(Best used with Nordost power cords and AC distribution unit)
View attachment 420126

Does not fit into my power plug, would need a hammer

 

[JustJones]

Does not fit into my power plug, would need a hammer

EU ? No problemo

 

[SIY]

It has little to do with the house wiring, at least when everything is plugged into the same out electrical panel phase.
I suppose the CMV may well be doubled if one device is plugged into a Phase A outlet and the other is Phase B.
I can't remember if having Phase A and Phase B sourced outlets in the same room allowed by NEC/NPFA code. However, I could easily see it occuring if you run your signal cables from one room to another.

Maybe it confused the issue and I should not have includeed the half-disconnected RCA plugs. However, I offered it as an audible effect commonly assumed to be audible which is (predominately) caused by the same issue we're discussion.

Again, this is an example of what can cause audible differences and which most people seem to be unaware.

I'm now curious how many who 'needed' something like a USB isolator or had group loop issues could have solved it with just experimenting with the absolute power phase of all their equipment.

Again, this has NOTHING to do with power cords.

 

[HaveMeterWillTravel]

Again, this has NOTHING to do with power cords.

Actually it does. Or rather, it is part of it.

Unless you wish to modify your equipment internally, changing the common mode phase alignment is easiest done with the power cords.

And I havn't even brought the interplay of this effect with the potential effects of the placement location of EMI capacitors (those where typically "Y" caps are required).

https://audiosciencereview.com/foru...ttributed-to-bruno-putzeys.57057/post-2086678

 

[Speedskater]

They were from the RCA return (outer barrel) to earth ground.

But the RCA barrel should be attached to the metal chassis (at it's connector), as should the Safety Ground/Protective Earth (near where the power cord enters the chassis).

 

[HaveMeterWillTravel]

But the RCA barrel should be attached to the metal chassis (at it's connector), as should the Safety Ground/Protective Earth (near where the power cord enters the chassis).

Neither Type A plugs nor do Type II power supplies have that connection present.

 

[Punter]

Common mode noise reduction is a feature of balanced audio circuits connected with twisted pair cable. RCA started life as an RF connector and is an inherently unbalanced standard having only two conductors rather than three in a balanced circuit. Grounding and hum loop problems are usually not subtle and have known, simple remedies. As far as I can determine, no mains power system incorporates common mode rejection effects as part of its specification as it would be a completely redundant feature. Where AC power is concerned, the reactive components are much more important (capacitive and inductive). Ultimately, your equipment would care less as its all running off DC!

 

[Doodski]

Isn't this last run of regular "non-audiophile" wire inside the amp's chassis subject to the same problems cable proponents ascribe to standard AC cords? Or does the cable crowd say this somehow doesn't matter?

Apparently the AC cable crowd thinks somehow magically that the 6' cable has magical properties delivering more pure power and they disregard the fact that there is plain old wire from the power pole transformer to the house and hundreds of feet of plain old wire in the walls and as you point out plain old wire inside the components and a fuse with tiny conductor inside. It's a dream fantasy but they know what's good and bad and oftentimes there's no educating them or trying to change them because they are set in their ways and not ready to accept facts.

 

[HaveMeterWillTravel]

Common mode noise reduction is a feature of balanced audio circuits connected with twisted pair cable. RCA started life as an RF connector and is an inherently unbalanced standard having only two conductors rather than three in a balanced circuit. Grounding and hum loop problems are usually not subtle and have known, simple remedies. As far as I can determine, no mains power system incorporates common mode rejection effects as part of its specification as it would be a completely redundant feature. Where AC power is concerned, the reactive components are much more important (capacitive and inductive). Ultimately, your equipment would care less as its all running off DC!

Simple questions from someone ignorant of the engineering, but highly skeptical of the claim that a power cord could make any audible difference in a hi-fi component's sound quality...

Here's what the wiring looks like behind the plug receptacle of a highly regarded amplifier:

View attachment 420186

Oh my, that wire going into that 5-pin connector looks pretty thin!

I would imagine that an awful lot of these "audiophile" AC cords are plugged into amps just like the one above where there's a couple-o-three inches (or more) of plain 'ol wire running from the back-panel receptacle to the amp's power supply circuitry.

Isn't this last run of regular "non-audiophile" wire inside the amp's chassis subject to the same problems cable proponents ascribe to standard AC cords? Or does the cable crowd say this somehow doesn't matter?

Who said anything about "audiophile" ac cords? This is basic electrical theory.

If you have a ground loop, hum, etc. which is caused by mains phase inversion induced common mode noise between two devices; say between a streamer/daq and your amp \ ask yourself. Would you prefer to correct it with using a different power cord, or opening up your equipment and rewiring it?

Well, that is assuming you realize what the problem is. I bet most people will reach for an Isolator of some kind. More $$ wasted.

 

[HaveMeterWillTravel]

Apparently the AC cable crowd thinks somehow magically that the 6' cable has magical properties delivering more pure power and they disregard the fact that there is plain old wire from the power pole transformer to the house and hundreds of feet of plain old wire in the walls and as you point out plain old wire inside the components and a fuse with tiny conductor inside. It's a dream fantasy but they know what's good and bad and oftentimes there's no educating them or trying to change them because they are set in their ways and not ready to accept facts.

The straw is strong with this man.
Care to respond on topic?
I've already linked to another well respected poster who posted frequency spectrum showing the effect can be very audible.

IF one has the issue, changing the power cord to invert the absolute phase may be the easiest and most effective method of mitigation.
Attempting to filter mains frequency common mode currents/voltages tends to require herculean efforts.

 

[SIY]

But hey, I did learn that "mains phase inversion induced common mode noise" falls under the category of "basic electrical theory".

Back in the old days, we'd just reverse the plug to reduce noise. Harder to do now because of polarized plugs (one blade is larger than the other), so the easiest way is to take the plug off the power cable, then rewire swapping black for white.

With IEC cords, this is not easily done, but it's also almost never necessary.

 

[solderdude]

I feel a new audiophool product being born.... L-N flipper.
There won't be a switch on it for a simple reason... one might not hear any effect.

Of course it will be expensive, have an extremely thick and sleeved power cord (with fancy names and a nice brochure explaining it with fancy words) and expensive plugs and sockets...
Might also be a budget version (say around $ 100.-) in the future if it sold really well and then the demand for the 'high-end' one plummeted a little.

 

[HaveMeterWillTravel]

Yeah, I wasn't replying to anything you posted, since I don't understand anything you wrote.

But after reading through this entire thread - and taking particular note of this picture posted by Punter:

View attachment 420334

...I was curious how people rationalize using an "audiophile" ac cord a product like this, where there is not only inferior wire behind the wall receptacle all the way back to the power plant, but also inferior wire after the product inside the amp.

I'm guessing the above contraption is due to the structural deficient of his outlet. aka, the dang cord is too heavy and it kept falling out!

(I also would have asked if those cable lifters were Tibetan singing bowls, but that would have been off-topic)

But hey, I did learn that "mains phase inversion induced common mode noise" falls under the category of "basic electrical theory". I honestly had no idea how little I knew about basic electrical theory, so thanks for that!

Most people and even most tests only concern themselves with differential noise, even thou line frequency common mode on many power supplies are easily 50-70 Vrms.

Some additional questions for you, if I may. Say I was experiencing the problem you describe above. What kind of different power cord would I use to correct the problem? How would the cord be different? Where would I purchase such a cord? Thanks in advance for your reply!

From an engineering perspective, one can use a scope to determine it. From a practical sense if it's not bad enough to be audible, it probably doesn't matter.
But if it's loud enough to be audible it seems like the first thought of many are to try isolation transformer or similar device.

For the US market, IF the power cord has a non-polarized NEMA 1-15-P type A plug or it it has a detachable connector using a non-polarized IEC 320 C7/C8 then it's simple: rotating the plug 180 degrees. If it is using a polarized C8, a non-polarized C7 also fits. However, most other combinations will require some sort of adapter or custom cable.
There then is the complication of standard/code requirements of for CE, UL-listing etc. One won't generally find a 3-conductor power cable with Line/Neutral reversed on the market.

This all stems from the use of non-polarized plugs and/or equipment where line phase polarity has been inadvertently flipped. In an ideal world, all audio equipment would have polarized plugs and be internally wired in the same absolute phase and where the power supply's transformer induced leakage current was kept to a minimum.

 

[HaveMeterWillTravel]

Back in the old days, we'd just reverse the plug to reduce noise. Harder to do now because of polarized plugs (one blade is larger than the other), so the easiest way is to take the plug off the power cable, then rewire swapping black for white.

With a single device plugged in, the absolute phase typically does not really matter. However, when you have two or more devices connected, then it may matter.
If they are all in the same phase, the relative common mode voltages will also be in phase so the differential voltage between them will be minimized. if they are in opposite phases, they will be maximized.

With IEC cords, this is not easily done, but it's also almost never necessary.

With correctly designed equipment with polarized equipment, I will agree. I would also say that RCA's are an abomination and should have been abandoned 50+ years ago.

However, there are seemingly no standard with all the various "wall wart" or "laptop" style power supplies that are being used more and more.