10 metres audio cable going into PC = too long?



Very true.
Not to mention that "no hum at all" is only in the perception of the customer, whereas telco people tend to actually measure it.
Granted though, a telephone installer just uses a very simple test set that gives a "good/bad" indication, not a specific number. And that would be the most that a customer would likely ever see. But when a cable is installed the pairs are very specifically measured and compared against design specifications, which were calculated very closely prior to construction. Nobody wants to invest in new cable plant and end up with a cable that can't be used...
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They seem to do things differently over there Floyd, my friend who works for a Telco here reckons that if they get 80 working pairs out of a newly installed 100 pair their doing well;!.
All due to employing subbies who sub out and then sub some more;(.
He said they didn't measure things like signal to noise ratios and such anymore as they don't need to its all going digital anyway and digital is perceived as "perfect" so no need!........
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Over here when a cable is 30 years old we might accept having only 80% of the pairs usable.
And you know what? Given the cost of copper, I don't think it is any different over there. Whoever is telling you these things is either not speaking clearly, is pulling your leg, or isn't too bright.

You friend is neither a technician nor an engineer. He sounds like a lineman or an installer. Which is to say that he clearly doesn't understand either how it works, or what the test equipment he uses is actually doing.
Trust me, they measure signal to noise... in *some* way. But installers generally have a little idiot proof box that gets plugged in and does all the tests automatically, and then turns on a red, a yellow, or a green light. He has no idea what it did.
Keep in mind that the original Bell System here in the US was reputed to have been "Designed by geniuses, to be operated by idiots". Particularly in the outside plant area that has not changed at all...
I've always joked that because the Bell System hired all the genius engineers, here in Alaska the telephone system was instead designed by idiots, and required geniuses to operate it! And in fact, due to the remote locations and distances involved, that actually was true to a great degree. :-)
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On Sun, 30 Apr 2006 04:41:55 -0800, snipped-for-privacy@apaflo.com (Floyd L. Davidson) wrote:

Oh dear, he's further gone than we thought.................
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Errmm... No he's not an engineer. They don't have "them" anymore;-(. You may not know it but most UK industry is run by accountants these days. Once the mighty BBC had an engineering director.. they don't nowadays. They have bullshit speak mongers. Engineers are shat on from a great height, and are treated like dirt, thats why a lot went over there, brain drain we called it;(.
No self respecting UK parent who wanted their son or daughter to do well would have them do engineering. Law, Accountancy, medicine or the civil service are all the "Professions". Engineers?, no way!..
No the one I'm referring to is a technician who does know their system inside out, and is very good at what he does but he won't be there next round of redundancies and job cuts, as the management don't think they need him!.
You haven't seen what some of the subbies get up to. Suppose over there you have your own in house people.
They don't do that here;((

Yes they do have such go-nogo devices...

Humm... Over there?.. Over here?.....
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Do they have engineers anymore?, the accountants that run the industry say they don't need 'em!..
That above example was for a small transmitter that is in a remote location that is fed by a long overhead copper pair, well two of them for stereo, and that goes into line trannies and equalisers and it didn't have any discernible humm on it. However thats about to change, a digital microwave link is to be installed as soon as, copper is on its way out it seems!...
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On Fri, 21 Apr 2006 02:01:10 -0800, snipped-for-privacy@apaflo.com (Floyd L. Davidson) wrote:

My circuit showed what kind of poor installation causes ground-loop induced hum. You get a ground loop in a domestic Hi Fi (and yes, that is what we are talking about here -forget your telephone cables).when you join to bits of kit together with coax cables, and also have them both connected to mains ground.

No better. You need to understand that the load is on the far end of the cable, not the same end as the source.

We are trying to work out how to prevent a domestic system suffering hum from a ground loop. So no, the exact mechanism by which the coupling takes place doesn't matter. What matters is how you connect your kit up.

Wrong. If you connect two bits of kit with a single coax cable, and there are no other connections, you will not have a loop that causes hum.

No there isn't, there is a connection to a bit of wire. Here in the UK that is a piece of thinnish bare copper contained within the twin-and-earth 2.5 squ mm cable that connects all domestic sockets to each other. At some distant point where the power enters the house there is an earth connection.

We are talking about a 10 metre cable connecting a PC to a Hi Fi.

Good for you! And the connection with the current question is?

Learn some terminology. In electronic engineering generally as well as audio, "signal" refers only to the wanted stuff. Hum, noise and distortion are *not* signal.

Yes they do. You will find that there is a left signal and a right signal. That, I'm afraid, is your lot.
I'm always open to learn something new of course. If you have other signals in your stereo do tell me all about them. Tell us all, in fact - I'm sure we would all be glad of the new knowledge.

I'm still perfectly happy to let my words stand for judgment.
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snipped-for-privacy@pearce.uk.com (Don Pearce) wrote:

So please do comment on the above. If what you say is true, nobody would install telephone cables like that, because it would cause *huge* ground loop currents every time! It fits *all* of your criteria perfectly. But it not only doesn't cause a ground loop, it is done specifically to *reduce* noise in the signal wires.
Incidentally, the exact same technique would work if the above were applied to a coax cable with one single ended audio channel on it. For example, a 10 meter connection between stereo components!

You "circuit" was identical to the above. It clearly does *not* necessarily cause ground loop induced hum.

You get hum in domestic HiFi for *exactly* the reasons I've stated.
If you ran separate ground cables all the way to the earth ground (or to some point which represents a low impedance connection to an earth ground), you would *not* have the common path that I've repeated demonstrated is the culprit. But instead people connect all of the equipment grounds to the ground wire in a power outlet, and because that is a very long *common* path, *all* signals on it are induced into each circuit that is connect to it. That includes 60 Hz power and any number of audio (e.g., mono or stereo), video, RF, or whatever signals.
In your case, it is a stereo hifi, but the same applies to any other signal too.

You apparently have never worked with equivalent circuits, eh? The load is *not* at the far end of the cable. All that cable is doing is providing a connection to *two*, count them, signal sources.
The point in the above diagram is to emphasize the voltage divider made up of the two impedances, Rload and Rgrnd. The cable is shown *only* because it was convenient to draw it that way and label it so that readers would understand what it was.
If I weren't dealing with someone like you, it would have looked more like this:
Signal Sources
Desired Cable Induction Ground Differential o o o | | | +--------+ +--------+ +--------+ | Rload1 | | Rload2 | | Rload3 | +--------+ +--------+ +--------+ | | | o---------------------o---------------------o | +-------+ | Rgrnd | <=== common path the causes "ground loop hum" +-------+ | | ----- Earth --- Ground -
But obviously that is a more difficult to relate to the problem described, and hence while it might be technically correct, it doesn't suit the pedagogical requirements.

You have adequately demonstrated that understanding the mechanism is a requirement to *fixing* the problem! You've incorrectly described what happens, why it happens, and what can be done to correct it.
Knowing how to avoid ground loops is what matters in how you "connect your kit up".

You also can't turn it on, as there is no AC connection.
The point is that people *do* want to turn it on, the do want various other components connected, and they *do* want to have various components grounded... both to avoid AC hum and to avoid electrical shock.
Hence there *are* going to be other connections to active components.
And regardless of that... an easy proof that what you say is not correct is known to almost anyone who has ever tried cabling microphone systems. Just hook up a microphone through several feet (10m would do!) of shielded cable and connect the shield at both ends... No other connections. No ground. No AC either. No loop. But just see what a nice ground loop that makes as soon as someone picks up the microphone! Bzzzzzzzzzzzzzz.

And of course *that* is what is shown in both the diagram I drew, and equally well in the one you drew!
The points at which I place the ground symbol are the points where the system is connected, via whatever means, to a ground. The impedance of that connection is represented in the equivalent circuit by Rgrnd.
You seem to have the mistaken idea that there has to be some sort of "kit", or equipment, connected on both sides of that cable to cause ground loop hum! Wrong!
You do realize that in your diagram there were *two* ground loops, right? Not one, two. See if you can find them both.

It works exactly the same, whether 10 meters or 10 miles.

You brought it up, what *was* the point? (None. You didn't make a valid point.)

Signal in one circuit is noise in the other. Your audio of course does not bother the AC power lines, but regardless it is nothing but noise when coupled into the power line. The AC power is the desired signal when on the power line, but noise when on the audio line.
The problem, once again, is having those two signals on a *common* *path* (where *both* are the desired signal). In that way each of them becomes a noise for the other's circuit, even though they are in fact both the *desired* signal on the common path.

Thanks for the laughs. That is hilarious.

You are confusing cause and effect. A stereo has two signals, but two signals does not make a stereo. There are *many* things that have two signals that are never described as "stereo". The most obvious at this point is a single channel of audio and a single channel of AC power. Two signals... no stereo.

They are!
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On Fri, 21 Apr 2006 05:52:27 -0800, snipped-for-privacy@apaflo.com (Floyd L. Davidson) wrote:

OK I will comment on the above. There are ground loops, and they induce hum. The hum doesn't reach the signal because it is common mode, while the signal is differential - the hum is thus cancelled.

Yes it does.

Try and understand. There is only one signal - that is the wanted signal. The rest is interference.

You now appear to have three separate load resistors. This just gets more and more bizarre.

I have not at any point addressed what should be done to correct it, but I will now. One must ensure that there is but a single ground connection between two pieces of equipment. In the UK these days that is simple. Amplifiers have mains plugs in which the ground is connected. All ancillary kit - tuners, CD players etc are safety-protected by double insulation, and have a mains plug with no ground connection. The only ground connection they have is via the coax cable, thus making sure that ground loops don't happen, and there is no hum.

There are these things called batteries, you see....

See my paragraph above.

Connect the shield to what at both ends? Try and write a little more clearly, please. And yes, I work regularly with microphones, and guess what? The shield is connected to things at both ends.
And now you can explain how somebody picking a microphone up could create a ground loop. Or is that simply your name for anything that goes buzz?

I suggest you look at your diagrams again. They show no such thing.

No - you have placed final earth connections there - not the same thing at all.

No, the kit is there to reveal it. The hum will be generated whatever.

There is one ground loop in my diagram. It comprises the coax screen in one direction, and the return via the mains ground cable. I challenge you to find a second.

No, I pointed out the errors in your diagrams. You turned it into this nonsense in a desperate attempt to deflect the criticism.

In audio the signal is the music. All else is interference.
AC power is not a signal - signal is information, not power.

What two signals are you talking about now?

I'm glad you are amused.

I know a little girl with an iPod. I suspect even should would call you an idiot.

They are? What is that supposed to mean? What are what?
Get a grip, please.
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snipped-for-privacy@pearce.uk.com (Don Pearce) wrote:

If that were true, why would anyone in their right mind ever install a telephone cable that way? Common mode never cancels all noise, because the balance is never that perfect, so if what you say is true every telco cable in the country is installed *wrong* and the noise in the cable could be reduced simply by going around and cutting that ground connect.
Has it yet occurred to you that you *can't* be right?

And nobody in the entire telecom industry knows as much as you do about it, so they *all* do it wrong???
Or, just perhaps... you don't understand it?

Actually 4 load resistors. The Rgrnd is also part of the load on each signal source. We could label them as Rl1, Rl2, Rl3 and Rlg just as well.

Wrong. A single low impedance connection to ground for each equipment is the key.

And that is *exactly* where you get a common path for ground returns, which is what causes a ground loop.

Which works fine... as long as all of these cables are very short. And by the same token, it does not work so well when there are 10 meter cables or when there is more than one amplifier or other grounded device.

So now everything is going to have to be battery powered.
You're grasping...

Connect it to the equipment! One end to the base of the microphone, and the other end to whatever it is plugged into.

Remember... two signals and a common path...
Those of use who actually have worked with mic cables have a different experience than you.

There is a ground loop at each end. The ground lead from the box carries signals both from the box and from the cable shield.
That is true at both ends, therefore there are two ground loops.

AC power and audio.

Just as you do? Of course that is simply because you understand just about as much of this as a little girl with an iPod would be expected to understand, so the comparison does seem apt. Thank you for point it out.
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On Fri, 21 Apr 2006 08:02:00 -0800, snipped-for-privacy@apaflo.com (Floyd L. Davidson) wrote:

No. The way telephones are connected cancels sufficient common mode noise to work. All of telephony is the business of doing just enough - expensive is the last thing you want. To quote a saying - "Don't let the best be an enemy to the good".

Er - no. It would just be you that has this wrong.

This is one load resistor. It is the input impedance of the amplifier.

That's right. And the way it is done here in the UK is exactly as I described. But at last you have admitted that to avoid hum you do need a single ground connection - I call that progress, of a sort.

No. That is a single point of connection to ground for the entire system. You can't have a loop with just one connection - think about it.

It works perfectly with any domestic length of cable you like. The worst problem you would find with longer cables is capacitive losses.

Follow the logic! I wasn't saying everything had to be battery powered, I was countering your claim that everything had to have an AC power connection.

OK. My microphones have a screen in the cable. At the equipment end it connects to the ground terminal. At the microphone end it connects to the casing. With my condenser microphones it also connects to the lower end of the amplifier circuit.

First - no buzz. None of my microphones buzzes when I pick it up. If yours do, you have a problem. And of course it isn't a ground loop, unless you are also grabbing something grounded.

No. Those who have worked with broken mic cables have a different experience. Fix them!

What on earth (pun) are you talking about? A single connection is not a loop - it is a connection. Do you even know what a loop is? I'm serious - you appear to have no concept.

You need to learn how to keep AC power signals out of your audio leads. Your system must be a disaster area.

Your grammar is sinking as fast as your logic at this point. Take a break.
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On Fri, 21 Apr 2006 17:18:33 GMT, snipped-for-privacy@pearce.uk.com (Don Pearce) Gave us:

Tell us again how a person that is NOT connected to anything else at can touch a microphone and cause "hum" then. Then tell us how many SIGNAL PATHS there are while said person is causing said noise.
Don't duck this one, Chucko. You cannot avoid the righteousness of the Floyd.
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On Sat, 22 Apr 2006 05:20:46 GMT, Roy L. Fuchs

OK, I've had it with you pair of incompetents, and this is going to be my last word on the subject.
I have made a recording and put it on my web site
http://81.174.169.10 /
It is of me speaking into the most insensitive microphone I have - a high impedance ribbon.
Halfway through I take hold of the microphone body. I am sure you will be amazed to hear that the result is absolutely no hum whatever. Do you have any idea why this surprise result should come about? No? Well, I'll tell you. It is because I know what I am doing, I understand ground loops and I don't allow them in my system.
That, as far as I am concerned, is it.
Have a nice day, the pair of you, and remember - if you don't know the words, you can always hum.
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snipped-for-privacy@pearce.uk.com (Don Pearce) wrote:

Hilarious. Regardless, virtually *any* engineering text on cable systems will explain that what you say is not true, and in fact the very reason cable sheaths are grounded is to reduce noise and *improve* on common mode rejection, simply because otherwise there would often be too much power line influence on the cable pairs.
Regardless... I ran across something that explains it in terms closer to what you claim to understand: audio sound systems (granted, not at the home user level you deal with, but...).
64.70.157.146/pdf/Bondingcableshields.pdf
"Bonding Cable Shields at Both Ends to Reduce Noise", by Tony Waldron and Keith Armstrong. To be quite honest, I was surprised at how much the effect was when applied to short cable runs. I had thought it was only that significant when the cable lengths were in thousands of feet, but they demonstrate very effectively that even when used with cables only a few meters in length the effect is significant.
Here is the part you need to know:
12 Conclusions
We have shown that concerns about 50/60Hz noise generated by ground loops when bonding both ends of the shields of balanced audio cables are without basis. Ground loop currents are not a real problem for correctly-designed pro-audio equipment, in fact they are a real benefit. Ground loop currents flowing in the shield of a balanced cable cannot give rise to significant DM noise due to inductive coupling between the shield and the signal conductors within a cable, because this mode of coupling only has a small effect and is intrinsically very well balanced. (With attention to the practical details of the shield-chassis connection at each end of the cable, no significant inductive coupling need occur there either.)
As one addition, since it is not mentioned in that article, I'll comment on the use of a PEC (Parallel Earth Conductor). There is one included within the sheathing of virtually all shielded cables intended for long (more than 10-20 feet) cable runs. Generally the "shield" is a metal foil. The PEC is a single copper conductor.

The above cited document demonstrates, *in* *detail*, with measurements and the required theory, exactly why that is not true.

If you would spend less time on petty personal attacks and more on technical accuracy, you might actually be able to learn enough to understand what I've shown you.
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On Fri, 21 Apr 2006 14:26:10 GMT, snipped-for-privacy@pearce.uk.com (Don Pearce) Gave us:

The elements of his circuit diagram are 100% accurate.
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On Fri, 21 Apr 2006 14:26:10 GMT, snipped-for-privacy@pearce.uk.com (Don Pearce) Gave us:

The person's body creates an additional SIGNAL PATH.
Maybe one day, you will get it.
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Whatever is going on, on your planet?.. This doesn't happen here!....
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wrote:

They have both passed Magna Cum Laude in advanced incompetence. I hope they aren't allowed sharp objects - you may have heard that a knife will cut a few millimetres off your fingers every time you pick it up.
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snipped-for-privacy@pearce.uk.com (Don Pearce) wrote:

It *does* happen, under the circumstances described. If all you do is extend the shield to surround the microphone element, and do *not* connect to it, then you get no noise. Try hooking that shield to the microphone element and see what you get!

For a fellow who has posted exactly *zero* evidence that he understands anything at all about electricity, that is a very telling bit of ad hominem. It says you are so insecure that gratuitous insults are the closest thing you have to an argument...
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On Sat, 22 Apr 2006 02:49:48 -0800, snipped-for-privacy@apaflo.com (Floyd L. Davidson) wrote:

OK, now I'm actually laughing.

So you haven't listened to that piece of evidence I posted on my web site? Thought you might want to avoid that.
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