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In Reply to: RE: Why is the TI-Shield helpful in this application? (nt) posted by RioTubes on August 26, 2007 at 20:00:37
The R-C networks provide damping for the normal mode ringing on the line. This is the mode where the ringing wave voltage is from the (+) to the (-) wire within the speaker cable.
The common-mode is where both wires act together as a single wire and the wave voltage is from both wires to the outside world, which is close to the AC safety-earth potential. The R-C networks do not provide any damping for this mode, and are carried along with it at the cable ends.
A grounded, isolated material with some resistance is able to provide common-mode damping without adding the circuit complications that would arise if one were to connect R-C networks from the speaker terminals to the AC safety-earth (DO NOT DO THIS!!!).
Another material that might work is carbon fiber cloth. TI Shield is easier to make a solid connection to safety-earth, but be careful of any acoustic resonances in it.
Follow Ups:
In reading some of your other posts on commmon mode ringing, you mention that wrapping the speaker cable in carbon fiber cloth and securing with Teflon tape, particularly near each end (but better for the entire length) addresses common mode issues. But I don't recall mention of grounding the fiber cloth. Is that an "extra step" one could take, or is this entirely different; here we are talking about TI shield or Carbon Fiber betw *only* the RC network and speaker?
TI Shield is conductive, and the reports here from others are that grounding a piece of it wrapped around the R-C networks provides a benefit. I thought about what advantage the grounding could have, and concluded that doing so should improve the common-mode damping. However, I have not yet done something like this myself (too many other projects).
My cables have carbon fiber sleeve on them as you described. Since the sleeve covers both conductors, it is relatively ineffective for normal-mode waves. However, it would be effective for common-mode waves. It would be difficult to make a secure and permanent ground connection to it, as it cannot be soldered. It seems to help just used as an ungrounded wrap. I can't say whether grounding it would make it work better.
Carbon fiber is conductive, but much more resistive than TI Shield material. I use it along most or all of the cable to improve the damping. Using TI Shield in the same way would be like adding a conventional shield, and increase the cable capacitance with little series resistance. This might upset the tonal balance or create problems for the power amp.
Another caution is that I believe the AC safety-earth wiring ("ground") is a carrier of RF noise. Adding another connection to the AC safety-earth may increase the amount of RF noise that gets into the audio system.
Microsorb backing will take care of any ringing concerns.
Alan
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