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In Reply to: Re: No, No, No... posted by Triode_Kingdom on May 1, 2007 at 23:41:41:
>>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
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- Re: No, No, No... - kurt s 09:20:31 05/02/07 (0)
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