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Hello all.I understand that, in the so-called 'quasi-balanced' o 'semi-balanced' cables, the shield and negative line would still be tied together at one end of the cable, so I'm wondering *WHY*, exactly, these cables are so strongly recommended in audio circles and by hi-fi cables manufacturers as offering superior sound quality and rejection noise over normal, high-quality coaxial cable, or how that prevents the stray RF or EMI signal from getting into the audio path.
Any comment will be much appreciated.
Edits: 11/13/11Follow Ups:
I have posted several things that might help you understand what the deal is on this type of construction.
See:
About cable "ground":
http://www.AudioAsylum.com/audio/cables/messages/503.html
DIY Cables and RFI/IGP
http://www.audioasylum.com/audio/cables/messages/80108.html
Cable RF Termination and Cable Loading
http://www.audioasylum.com/audio/cables/messages/80110.html
After reading those, come back here to finish.
ZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZ
Now, perhaps at this point, you can see that a regular coaxial cable used as an audio interconnect has the signal traveling on the shield braid just as much as it is traveling on the center wire.
In order for a shield to work, it must be conductive or a magnetically active material. A shield works by intercepting the interfering signal, and in doing so, currents flow within the shield. A coaxial cable shield is carrying the signal, but now, if an interfering signal is encountered, it induces currents in the shield, and if the shield is connected to "ground" at both ends, those currents can flow into one ground or the other, and corrupt the audio signal by being picked up and/or amplified by the audio gear being connected.
That's just the shielding aspect being involved, if you read the post on "DIY Cables and RFI/IGP", then you would also be aware that different pieces of audio equipment can have their "grounds" at different voltage potentials. This can lead to the classic "ground loop" problem, or the more insidious Interchassis Ground Potential effects, which can involve RFI injection, etc.
Note that when a shield is carrying current, it's effectiveness as a shield is reduced, thus a coaxial cable shield braid has multiple reasons it is less than ideal as a shield and a signal return path.
Now the geometry you show in your post (which more commonly has the outer shield grounded at the source end, not the load), does not have the audio signal currents traveling in the shield, they are in the 'ground wire' of the twisted pair. The currents in the twisted pair are equal but opposite, and thus tend to cancel out the inductance of the wires,
(SEE: Cable inductance, and how it is controlled
http://www.audioasylum.com/audio/cables/messages/17.html) as well as reject outside interference that might get through the shield, due to the twisted pair having the conductors located in nearly the same space, and thus, nearly the same electrical and magnetic fields.
This rejection is not as powerful and deliberate as it is with a true balanced signal set-up and balanced ins and outs, but it still helps nonetheless.
The fact that the "ground" wire is carrying the audio signal just as much as the "hot" wire, means that the nearby insulation will also effect the "ground" wire, as will the conductor purity, etc. So the geometry you show has the advantage of a purer and less contaminated signal path for the signal return wire compared to a coaxial cable shield.
So by separating the work done by the shield and the work done by the signal return path, both duties are handled just a little better than they would be otherwise.
Finally, many commercial and even "high end" twisted pair cables have the shield too close to the twisted pair, this means that the shield is close enough to partially "short out" the electrical field, and curtail and distort the magnetic fields, thus causing the well known sonic problems with a shielded twisted pair compared to the unshielded twisted pair.
Some of my DIY cable designs take this into account, and space the shield out away from the twisted pair, and even take the trouble to keep the shielding material out of the twisted pairs 'crease', so as to minimize the field distorting aspect of the shield, while retaining the benefits of it's interference protection.
I usually strongly recommend to beginner DIYer's that they build a shielded version of an interconnect first, to make sure they are not suffering from any sort of insidious or low-level RFI/EMI interference, and then build a unshielded twisted pair if they are so inclined. At least now they will truly know that the sound of the interconnect is better or worse compared to the shielded version instead of only building an unshielded version and declaring the DIY recipe to "suck".
I have had this happen several times, where the DIYer first built an unshielded interconnect, only to find they had residual or low level interference that make it sound poor; once they added a correctly grounded shield, the sound got better, and usually much better than the cheapo cables they were using before.
I hope this explanation helps in your understanding of the geometry of the shielded twisted pair used unbalanced.
Jon Risch
This is how I make up all my ICs - except I suggest the end which has the shield grounded to the RCA barrel should be the source end (not, as you have shown, the destination end). :-))
Yes, having a shield may introduce some negatives - but so does RFI! :-)) But this is far outweighed by the positives of this construction which are:
* RFI picked up by the shield does not interfere with signal ground.
* you can use solid-core wire for both signal 'hot' & signal 'ground' and avoid the negative effect of stranded wire.
Regards,
Andy
Thanks for your input, andy.
As it has been said previously, in the 'pseudo-balanced' configuration, the shield and negative line are still tied together at one end of the cable.
How, then, that prevents the stray RF or EMI signal from getting into the audio path and from interfering with signal ground?
I have had coaxial ics, twisted pairs, passive shielded, unshielded, solid wire, multi-wire, ribbon, actively shielded, copper, silver, palladium, carbon, gold, shield connected at the source and at the load, etc. Presently, I am using long linear crystal silver, charged, non-coaxial interconnects in both balanced and single-ended formats. These are the best cables, I have heard thus far. Obviously there are many ways to skin the cat. I suspect that the amount of RFI and EMI is far greater today than in the past, which suggests to me that we all need a measuring instrument that would tell us the level of it at our systems.
Hi MM - you asked " how does that prevent stray RFI/EMI signals from getting into the audio path and interfering with signal ground ?".
A guru like Duster will probably give you quite a different explanation but mine is as follows:
* if you just had the twisted pair (with no shield) then RFI would certainly impact on the signal wires. Whether the twisted pair was sufficiently effective for your requirements (as a TP does reject some RFI) would depend on the level of RFI at your location.
* if you added a shield connected to the signal ground at both ends then the shield would just become a second ground path. RFI which is picked up by the outside of the cable (ie. the shield) would be able to pass into the signal ground wire.
* if you add a shield which is connected to signal ground (the RCA barrel) at just one end, then it cannot be part of the signal chain; all it is doing is shielding the 2 wires inside from RFI. (It is shielding, because it is grounded at one end - if it were not grounded at all then it wouldn't be very effective as a "shield".)
Regards,
Andy
I'm certainly not a guru, but Roger Skoff the founder of XLO Electric (who may easily be considered one of the original old-school cable gurus) advocated unshielded interconnect cables to such a degree that both shielded and unshielded versions of the same cable design were offered, with the unshielded versions greatly preferred due to subjectively superior performance levels unless RFI was an obvious problem within an audio system. That said, like all cable synergy "to shield or not to shield" is essentially a component/system/listener dependent issue, IMHO. In the end, custom system tuning rules supreme in my audiophile book ;-) my 2 pennies
I wonder how long ago he formed his ideas? Back when amps might be lucky to have a 30-50KHz roll-off point, unshielded cables might've been fine. However, many modern amps have bandwidths up to 150KHz or more and I do not want RFI getting into my amps! :-))
AIUI, the more you physically space the shield away from the signal wires, the less the negative effect of the shielding. I typically use 2 cotton bootlaces between the signal wires and the shield.
Regards,
Andy
All XLO cables (except speaker cables)were either shielded as delivered or available shielded as an option. Andyr is certainly correct about keeping the shield as far from the signal leads as possible. At XLO, starting all the way back in 1991, we always spaced our accessory shielding a lot more than just a couple of bootlace thicknesses from the conductors. If you're reading this, Andyr, I'm sorry to disappoint you, but apparently I'm a lot smarter, more experienced, or better educated than you seem to think I am: we were doing spaced shielding a full 20 years before the date of your post, you probably just didn't notice.
Roger Skoff
From what I've recently perused, the current XLO product lines still feature unshielded interconnect cables as primary options, unless for use in extra long runs or in environments with unusually high levels of hum, noise and EMI/RFI interference. See link:
A coaxial cable implements the shielding as the return conductor so the geometry of the cable is asymmetrical to the signal conductor. A shielded twisted pair features symmetrical signal and return conductors that are twisted for the purpose of noise cancellation, with additional shielding that does not function directly in the signal pathway, although the shielding may adversely affect the signal (such as additional cable capacitance). An unshielded tightly twisted pair geometry can offer more than adequate noise rejection without any need of shielding, which is an option that many audiophiles tend to prefer for line-level applications.
What is pictured is a standard, garden variety, shielded, single ended interconnect. Nothing special. Many manufacturers have gone to unshielded cables since geometry of the wire is can also reject EMI/RFI. In many cases, the existence of a floating shield (as pictured - floating on on end and connected to return on the other) can have negative consequences. It's all a delicate balancing act.
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