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Old 28th August 2001, 10:39 PM   #1
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Electrical noise on the power line is something I've seen in several threads here. I've seen many suggestions for ways to clean up theis noise, many of which also have a bad side. The purpose of this thread is to try to put them all together in one place.

1. It's been suggested that MOVs can be used to clean up some of the noise. The noise removed by an MOV is primarilly high voltage spikes. It does this by shunting the spike to nuetral. By removing these spikes, an MOV should also increase the life of the attached electronics. I know of no negative impact of using MOVs.

2. I've seen lot's of people suggest power filters. A good power filter is probably the best noise remover you can find. The problem is that a GOOD power filter is sometimes difficult to find. Most of the power filters I've seen are 3 stage mirrored PI (inductor on both power lines) filters. Often, these are not adequate. A good power filter should have at least 5 stages, in a mirrored PI configuration. It should also be rated well in excess of the maximum expected current draw, so as to minimize the voltage lost due to resistance. A good example of this is item number EMI-21 from 'http://www.allelectronics.com/'. One important thing to remember is that EACH audio component needs it's own filter. This will keep noise generated by a component from getting into another component.

3. It's also been suggested that an isolation transformer be used. This should remove much of the high frequency noise. It needs to be protected by both MOVs (on both the incomming and the outgoing lines) and fuses. If the transformer has an electrostatic shield, the noise rejection should improve. The negative impacts of using an isolation transformer are weight (they're heavy), cost (they're not cheap) and increased impedance on the power line. The impedance should only be an issue for large power amplifiers, and only if the amp's current draw is a large portion of the rated power of the transformer.

4. I've seen a few people suggest that aftermarket power cords can decrease noise. If you're going from an 18 guage cord to a 14 guage cord, or adding shielding, I might believe that. My suggestion (for what it's worth) would be to use the heaviest power cord that you can find. Anything past 14 guage is usually hard to find.

I use items 1, 2 and 4 (14 guage shielded IEC power cords) in my system. I think that this helps to kill most of the noise that comes from the power line.
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Old 29th August 2001, 01:14 AM   #2
haldor is offline haldor  United States
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Hi Thoth,

MOV's will help reduce voltage spikes, but I don't see how they could have any other positive effect on line noise.

Actually there are two good reasons to not use MOV's. First, they wear out, an MOV is degraded every time it absorbs a voltage spike and will eventually fail, usually catastrophically. Second, MOV's pollute the power for every other piece of equipment on the same power circuit. They take a voltage spike that is on one line and turn it into a common mode voltage surge on both the line and neutral, this is actually harder for other equipment to ignore. In addition the way they are usually used, they dump noise to ground. Most audio engineers won't have anything to do with MOV based line protectors for that reason alone.

The best powerline protection product I know of is called SurgeX. This product uses a big honking inductor and caps to filter noise and spikes from the incoming power line. They aren't cheap, but they do work and don't wear out. I have seen these used to protect $100K mixing consoles.

That was a good point you made about each piece of equipment needing it's own filter. This is very true especially for power amps. Both linear and switching power supplies can feed noise back into the power line (the type of noise generated in each case differs, but that doesn't really matter in this case) and you don't want that noise getting into your preamp.

I have a hard time imagining that anything you do to a line cord is going to make much of a difference. Even if the few feet of cable between the wall outlet and your equipment is perfect, what about all the wiring inside your walls? Using bigger gauge power cords won't make any real difference unless you are using a long extension cord. Your line cord is still in series with the household wiring which last time I checked is not audiophile grade. Even if the line cord impedance is below 0.01 ohm, the wiring in your house typically has several ohms of impedance (I once measured the source impedance of the AC power at my lab bench in my 45 year old house, it was over 8 ohms).

Phil Ouellette
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Old 29th August 2001, 02:04 AM   #3
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Quote:
Originally posted by haldor
MOV's will help reduce voltage spikes, but I don't see how they could have any other positive effect on line noise.
There are places where the PRIMARY noise on the line is high voltage spikes. This would usually be near lots of high power electric motors (an industrial area).
Quote:
Actually there are two good reasons to not use MOV's. First, they wear out, an MOV is degraded every time it absorbs a voltage spike and will eventually fail, usually catastrophically. Second, MOV's pollute the power for every other piece of equipment on the same power circuit. They take a voltage spike that is on one line and turn it into a common mode voltage surge on both the line and neutral, this is actually harder for other equipment to ignore. In addition the way they are usually used, they dump noise to ground. Most audio engineers won't have anything to do with MOV based line protectors for that reason alone.
Good point about the failure mode. As to the pollution, I guess it depends on how they're connected. I've seen them connected across the power leads. I've also seen them connected in a 'triangle', between the power leads, and ground. I think that the second method is the one that throws the noise (spikes) into ground.

The way I've got them connected is across the power leads (4 in parallel), mixed with 4 0.02uF 1KV caps. The power cord is 15 ft. long, 10 guage (plugged into a dryer outlet; 240V/two-phase/30A), with 2 0.005uF 3KV caps in the plug. The caps should pull out the small spikes, leaving the big spikes to be absorbed by the MOVs. I feel that this mix gives the best of both worlds.
Quote:
The best powerline protection product I know of is called SurgeX. This product uses a big honking inductor and caps to filter noise and spikes from the incoming power line. They aren't cheap, but they do work and don't wear out. I have seen these used to protect $100K mixing consoles.
This appears to be an inteligent solid state spike filter (a smart MOV), combined with a hefty capacitor/inductor filter. I've never dealt with them, but there is the potential for a VERY good filter here. From what I can see, they've achieved that.
Quote:

I have a hard time imagining that anything you do to a line cord is going to make much of a difference. Even if the few feet of cable between the wall outlet and your equipment is perfect, what about all the wiring inside your walls? Using bigger gauge power cords won't make any real difference unless you are using a long extension cord. Your line cord is still in series with the household wiring which last time I checked is not audiophile grade. Even if the line cord impedance is below 0.01 ohm, the wiring in your house typically has several ohms of impedance (I once measured the source impedance of the AC power at my lab bench in my 45 year old house, it was over 8 ohms).
I have high power amps (250WPC stereo) that came with an 18 guage (16 guage? I don't recall exactly) IEC power cord. When that was replaced by a 14 guage shielded power cord, the background noise dropped. This was probably more due to the shielding than the wire. On the other hand, I'm sure that the heavier wire cut distortion at high power levels.

[Edited by thoth on 09-04-2001 at 11:27 PM]
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Old 29th August 2001, 03:58 PM   #4
haldor is offline haldor  United States
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[QUOTE]Originally posted by Thoth
Quote:
I have high power amps (250WPC stereo) that came with an 18 guage (16 guage? I don't recall exactly) IEC power cord. When that was replaced by a 14 guage shielded power cord, the background noise dropped. This was probably more due to the shielding than the wire. On the other hand, I'm sure that the heavier wire cut distortion at high power levels.
Sounds like you are picking up noise from other equipment in your system. Interesting, I never have used shielded power cords before. Did you characterise the noise, was it inductively coupled line noise or was it RF? Shielding will attenuate RF, but the only thing that helps inductive coupling is either distance, a tightly twisted pair of wires, steel conduit or some combination of the same.

I wonder if you could get better effect by using a tightly twisted pair (line and neutral twisted at least 3 turns per inch) with a seperate ground wire? It would be interesting to try making some custom power cords like this and see how they compare to your shielded cables.
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Old 29th August 2001, 11:09 PM   #5
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Some of the mega-buck power cables are essentially heavy gauge single strand (i.e. house wiring), which is why they're so stiff. I've never played with them, but they're reputed to be pretty good.
I haven't seen any white papers on this, but my guess is that they're using, say, 14 ga. single strand as a sort of anti-skin effect wire--less surface area for high frequencies to crawl along. This would tie in nicely with Thoth's comment about things being quieter, as it would tend to suppress high frequency garbage, simply by virtue of its larger gauge.
So, in a way, you could say that the wiring in your walls is audiophile grade, after all...

Grey

P.S.: Some people claim that ordinary house wire is the ultimate subwoofer cable. Since all you're asking of the wire is low frequencies, there might be something to that, too. I've done some limited experiments along those lines, but never finished. I'll get back to it one of these days.
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Old 29th August 2001, 11:30 PM   #6
haldor is offline haldor  United States
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Skin effect means that the as the frequency of the current flow goes up the current flow concentrates nearer to the outside of the conductor. The reason why you often see copper tubing used in military high frequency RF equipment is not because a solid conductor won't pass RF just as well, it's that the core of the conductor is not required to carry the current and you can save a lot of weight (very important in airplanes) by using hollow tubes instead of solid conductors.

If what you suggest is actually happening then the noise filtering would be improved by using the smallest diameter solid conductor possible (since the smaller the diamater the less skin region there is, therefore the less high frequency passed). Counter to the usual thinking.

A large diamater wire is going to have virtually the same impedance at RF as it does at DC (ignoring the effects of the wires inductance of the wire which isn't effected by the diameter of the wire). So in my opinion this is not what's going on.

Don't get me started about the audibility of wire. I am blessed with the inability to hear much difference between cables as long as they are appropriate for the purpose and well constructed. This means I have that much more money to blow on electronics. ;^)

Phil
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Old 30th August 2001, 12:52 AM   #7
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Phil,
You are correct that skin effect concentrates current on the surface as frequency increases, but your conclusions might need rethinking.
As you decrease the diameter of the wire strand, the wire diameter eventually reaches the point where the high frequencies can travel throughout the diameter of the strand. This leads to something approaching 100% utilization of the wire diameter for both low and high frequencies.
Conversely, a larger diameter wire carries high frequencies only on the surface, but low frequencies can permeate the entire wire cross section, leading to high frequencies only being able to use a comparatively small percentage of the cross sectional area compared to low frequencies, which are able to use the entire area. The result is a decrease in carrying efficiency at higher frequencies. Hence, a large wire doesn't have the same Z at RF as it does at DC, which is exactly why they are able to use tubes instead of wires.
In other words, the usual thinking is correct.

Grey
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Old 30th August 2001, 03:48 AM   #8
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Default Skin effect?

Has anyone actually punched the numbers into the skin effect formula to see the difference between, say 60 Hz and 16 KHz?

Cheers,

Pete
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Old 30th August 2001, 04:58 AM   #9
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Pete,
Actually, if you approach it from a purely numbers viewpoint, skin effect isn't really a serious factor at audio frequencies. But, like some other theoretical vs. what-you-hear debates, it does seem to matter--at least on good systems. For low to mid-fi systems, I'd say it's a waste of time to worry about it.
Now, from Thoth's side of the table (mine too, I guess), I imagine the question devolves to this: What frequencies are riding in on top of the AC? RF? Supersonic? Audio range? Subsonic?
And: How do these effect the perceived 'silence between the notes'? What mechanisms are involved?
N.B.: If we start talking spikes, then Fourier analysis will give us a spray of frequencies from DC to light--the only difference being that the problem is of short (let's say some fraction of a second) duration. This isn't really a separate problem unless the pure magnitude of the voltage overwhelms the ratings of some device downstream from the power supply. Elseways, it's simply business as usual, trying to keep non-60Hz AC out of the power supply.

Grey

I'm thinking out loud, here...
Supposing something rides in on the ground wire, not the hot. All assumptions are that ground is ground, and that ground remains stable. (After all, ground is where the "quiet" comes from, right?) But if ground *isn't* stable, it's going to raise hell with all assumptions that you can 'shunt the incoming trash to ground.' Ergo, you need some way to "ground" ground. I've got a half-formed idea about isolation transformers, but I'm going to sit on it until I've had time to think it through. Note that, at least here in the US, the implication is that anything picked up on the ground wire enters between the breaker box and the power supply, i.e. local interference such as RF, hash coming back out of digital equipment, etc. Shielded AC wiring might be in order, after all.
A friend of mine discovered that the outlet he was running his stereo off of wasn't grounded properly (old house with old wiring). He solved the problem by ramming a ground spike into the ground right underneath his window, then ran a purely local ground wire to that. I'm beginning to think he may have been onto something.
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Old 30th August 2001, 05:38 AM   #10
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Default Are we believing the marketing hype?

G'day Grey,

I guess my point is, is skin effect simply a convenient reason, essentially invented in this application, to explain something we don't understand? Skin effect exists, I believe we can all agree with that, however so do many other effects. Every day the earth's magnetic field shifts slightly, nevertheless I feel quite confident that this effect can be discounted as affecting the operation of my system. Nevertheless the effect is a known scientific fact, just as is skin effect. There are virtually unlimited effects on the different components within the system, why the emphasis on skin effect?

It seems ironic that audiophiles as a whole latch on to simple things with dubious objective merit simply because they are easy to understand, yet ignore factors very much know to have a marked affect on the quality of the end product. I have easily lost count how many threads I’ve read about cables, and yet have barely heard mention (with the exception of “off-the-shelf” op-amps) of the affect of different semiconductors, other than saying “errr, match them the best you can”. Indeed, and correct me if my source is wrong, I believe it was Nelson Pass himself who makes this very same point. Why don’t we see “after market transistors”, “upgrade MOSFETS”, etc. on the market. Well perhaps it’s the cynic within me, but it seems a lot easier to market a couple of wires that can easily be plugged in and are universal, than an upgrade that possibly requires extensive modification to specific models. As if to prove this point, look at the vacuum tube arena; they are easy to replace by anyone, and “upgrade” versions are marketed at high prices.

Notice I’m not saying that cables don’t make a difference, I used to be in that camp until I set up an experiment with speaker cables and was proved wrong. However I will confess that I have been unable to hear any difference between different interconnect cables I have tried.

So are we taking the marketing hype to be fact?

Just stirring the pot in the hope somebody may like to come up with some answers. Have taken cover and await the flak.

Cheers,

Pete
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