Is there any reason not to use a shielded cord considering the RFI eliminating characteristics of shielded cords?
5 responses Add your response
To address your question: some of the the reasons not to use a shielded PC are:
1.) Tend to be stiffer.
2.) Will cost more.
3.) The shield, if close to the conductors, can increase capacitance, and therefore affect sonics (more of an issue in IC's than PC's IMO)
Shielded PC's are pretty much mandatory for any equipment that can produce digital artifacts (DACs, CD transports, processors, anything with a remote or led readouts, etc.) This will keep rfi from the device from getting out to other cords, cables, TV's, FM tuners, etc -- although shielding helps keep rfi from getting into a cord too.
Shielding can also be effective at keeping the cord from giving or getting EMI (electro magnetic interference) which is often the cause of hum, but wire twist, braiding and other topologies can do the same thing as far as the cord itself radiating EMI. While products like ferrite beads and e.r.s. fabric from Stillpoints are also very effective at absorbing EMI that the cable picks up..
Shielding (both foil and braid) can affect the capacitance characteristics (and therefore the sonics) of a cable which is why some folks don't like to use it (especially in IC's) but it's hard to get the same results any other way.
One thing to remember about shielding configuration: The shield itself should only be connected (along with the ground conductor of course) to the ground terminal of the AC plug that goes into the wall outlet. It should not be connected to anythihng at the component end (it should "float") I bring this up because many OEM power cords are shielded, but in order to get a UL rating, the shield has to be connected at both ends, and then it can become a hum antenna!
1.) Tend to be stiffer.Not my experience. The stiffer cords I have owned have been so because of the tubing the wire is run through, not because of the shielding.
2.) Will cost more.Perhaps. I've never done price comparisons between otherwise identical shielded and unshielded cords. Certainly, ErnieM's shielded cords at less than $50, and SignalCable's MagicAC shielded cords at $60 don't support this hypothesis.
3.) The shield, if close to the conductors, can increase capacitance, and therefore affect sonics (more of an issue in IC's than PC's IMO)By your own definition, this has limited application to power cords, if any.
If you take a cord of a given diameter, wire size, etc., and add a foil shield, another jacket to hold the foil, then a braid shield and some kind of braid or jacket to cover that shield, I think you'll find the shielded version quite a bit stiffer.
Cost? Again, all other things being equal (insulation materials, wire type and size, twisting, stranding, quality plugs, etc) the shielded version will cost more for the raw material (I've seen up to twice as much if the braid is silver plated), and will involve more labor to terminate. So shielded versions of a given size and quality will have to cost more.
Capacitance -- I said IN MY OPINION it affects IC sonics more than PC's. But you can find LOTS of PC manufacturers that would strongly disagree with my opinion; and who have gone to great and costly lengths to minimize the capacitative effects of the shielding in their PC's.
OK, Nsgarch, I guess our experiences with shielded cords differ substantially. I have owned three shielded cords that I could easily wrap around my finger...so stiffness was a non-issue. Again, as far as cost, I will mention specifically the ErnieM power cords (I owned 9awg - 6 feet for less than $50), SignalCable MagicAC (less than $60 for 5 feet) and I just purchased a Volex 17604 (10 feet, 14 gauge, shielded for less than $10).
Stiffness and cost, IMO, are non-issues.
I'm not familiar enough with the issue of capacitance in power cords to argue one way or another, but I would question the significance of capacitance in wire that is not in the signal path.