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In Reply to: Re: No, No, No... posted by kurt s on May 1, 2007 at 22:36:56:
>But now I say a floating differential is a disaster waiting to happen when you try to couple that to a grounded secondary. You need a drain to ground for differential signals everywhere somehow, even if the resistance is large.Where would a high common mode voltage come from? A conductor doesn't acquire a charge merely because it's insulated from ground. Also, consider that common mode impedances in these circuits aren't all that great. They do provide a path to ground that is significant in the context of static buildup.
>I bet the phone company uses center-tapped points to Earth ground along the way. That keeps the static down for the reasons I stated.
I don't know much about telco wiring. If I had to guess, I'd say it's shielded.
>And I still contend you can do well with twisted pairs in unbalanced circuits, most of the time.
I think we frequently do things not because they produce a discernible benefit, but because they aren't demonstrably detrimental. It's likely the joint product of superstition and our love of gambling. In any event, twisted pair works on the theory that if the currents in the two wires are equal and opposite, field cancellation will reduce radiation from (and incursion into) the two conductors. When one of the conductors is grounded, though, that doesn't happen. The grounded conductor might provide a certain degree of shielding, but I don't see how it could be very effective.
Follow Ups:
> > Where would a high common mode voltage come from? A conductor doesn't acquire a charge merely because it's insulated from ground. < <Where? I'm not sure, but I've had it happen to me every time I fail to ground these lines. It makes a crackling sound. If you don't think there's a difference, then don't ground the secondary of the OPT of your amp and let it float to the speakers. Just try it. I've done it and had problems. That's one example. Maybe there's a lot of electrostatic charges building up from the HV supply through the air and it never has anywhere to go. So the number of electrons just go up and up in the conductor until it hits the HV breakdown and discharge point over to the nearest grounded wire. Not a big capacitance of discharge, but a noticeable one.
> > Also, consider that common mode impedances in these circuits aren't all that great. They do provide a path to ground that is significant in the context of static buildup. < <
No they don't really. There's tremendous insulation there and there is no electrostatic discharge path unless you purposely add it in. Now most differential amp inputs have an impedance to ground, but transformer to transformer (or transducer) does not have this.
> > I don't know much about telco wiring. If I had to guess, I'd say it's shielded. < <
Not from my house out to the telephone pole at least. That's as far as I can trace what's happening.
> And I still contend you can do well with twisted pairs in unbalanced circuits, most of the time.
> > twisted pair works on the theory that if the currents in the two wires are equal and opposite, field cancellation will reduce radiation from (and incursion into) the two conductors. When one of the conductors is grounded, though, that doesn't happen. < <
An unbalanced phono cartridge still provides a generator that is differential all the way to where it sees ground in the circuit. It has to have equal and opposite currents up to that point.
Also, if you eliminate ground loops by using two-pronged plugs (or not connecting the ground to the chassis at the plug) and instead opt for double insulation (still a legal choice for ground safety) then you eliminate the secondary ground paths (ground loops) and it will force the twisted pairs to have equal and opposite currents once again. There is no other path of return except along the interconnect. Inside an amp, the same conditions can continue to apply.
> > The grounded conductor might provide a certain degree of shielding, but I don't see how it could be very effective. < <
Well it's not that effective. If you have a really noisy environment you might need triple shielded coax. Noisier still with incredible low current leakage requirements, and you might need triaxial cable with a guard conductor between center and outer. But let's not get ridiculous about it if it's just not necessary.
What I am trying to say is that there's some things that do matter and some things that don't. Floating differential wires should be changed over to balanced wire or at least grounded on one side and changed to single-ended. And since my system is all single-ended, I have to choose the latter. And twisted wires on unbalanced cables is usually good enough. You're free to disagree.
Kurt
'I don't know much about telco wiring. If I had to guess, I'd say it's shielded.'Nope ! All twisted pair for delivery of low speed stuff ; leased lines , PSTN , ISDN ADSL etc . High speed T3/T3/E1/E3/DS3 uses 75 ohm coax , E1 can often be presented as 120 ohm over twisted pairs . Above 34MBPS speed it's fibre optic . Interfaces such as V24/X21/V35 usually have a braid . Lans use UTP , some voice equipment uses the 'balanced asymetrical' I41 interface which is similar to balanced 120 ohm E1 but with an overall foil shield
Hi.Are we talking to a spirit or what? What is your goddamned name?
"All twisted pair for delivery of low speed" ??
UTP (Unshielded Twsited Pairs) can deliver super fast data up to 350MHz at 155Mbps. Surely these data are not "low speed stuff" !
So how come "above 34MBP speed it's fibre optic" ???
c-J
Yes i am sure ,otherwise I wouldn't have written it !!!! I've been working in telecoms for the last 18 years so I can speak from experience . Even though you can probably shove 155mbps down twisted pairs , for 155MBPS in telecoms the industry standard delivery method is STM1 which is fibre optic . LANs typically run at 10 or 100mbps , optical LANs and residential broadband delivery I don't deal with so cannot comment but I still don't know where you get this '350mhz at 155mbps' from . It is nonsense !!! Go and ask your local telco for an T3/E3 and an STM1 and they'll agree with what I wrote previouslyps I will not be responding further to your replies , this is way off topic already . Just read it ok
Hi.So you're an old telco guy. That's why you're still lingering in the voice telco stone age. Get out of your netshell to see the digital world aound you, pal.
Let me update your knowledge in modern data telecom.
Standard UTP can deliver data transfer up to 100MHz at 100Mbps, but what I just quoted is a leading LAN cable manufacturer who supplies
LAN cables, e.g. Cat5e & Cat6, with extended response up to 350Mhz
at superfast speed of 155Mbps, employing its proprietary twisting technology - NO shield.Before you got blown out of the digital scene, may I suggest to click in www.belkin.com to get updated.
Next time don't talk like an idiot, bud.
.
listen pal , you're wasting bandwidth with your ranting about telecoms (which you know nothing about) in an AUDIO forum . Do you really call that a knowledge update ? Well as a telecoms tutor you couldn't teach shit to smell if you ask me (sorry my humor again) . Even since the first morse code was tapped out , it has all been digital , apart from antiquated POTS and FDM systems (which as a telco expert you will know what that means) . There are industrial standards developed so equipment of differing manufacturers is compatible , so show me some equipment that uses this '350mhz' standard .bye
Hi.Do it in some telecom forums. This is about audio, not about "morse code". You sre wasting everybody's time here.
Just click in www.belkin.com to find out from their application engineers which digital equipment their customers are using their superfast LAN cables.
Learn from the digital experts.
I am only correcting misinformation and since when is LAN considered to be telecoms ?ps www.belkin.com sells ipod accessories . I think you mean Belden who manufacture cables
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