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Old 10th February 2012
Lives for gear
JohnRoberts's Avatar

Originally Posted by Robo View Post
Thanks, I need to do this myself. I just wonder whether Option 3 is still not better than Option 5 assuming you don't use the cap coupling at the floating end technique and/or know which unit would have the most robust chassis ground. If using Option 3, ie leaving the shield unconnected at both ends, the shield would still block some noise wouldn't it, and you also wouldn't be connecting the shield into the audio signal path. But maybe there is an advantage to Option 5 I don't fully understand.
No... If the shield is floating at both ends, any noise will just move the shield like a leaf floating in the breeze. How much wind did the leaf stop? The floating shield will pick up noise and then retransmit it to the internal conductors.
I found another thread in which EveAnna Manley and Monster cable also suggest Option 5 (without cap coupling at one end):

Monster cables unbalanced theory

It's a good read although the OP's posts are a bit redundant.
The cap is a hybrid grounding approach to make the shield more robust at stopping RF, while still NOT providing a low impedance path at mains frequency that could cause loops or unwanted ground contamination.

Note: I didn't make this stuff up. I read it in a book about grounding and shielding a few decades ago.
I checked out a reactance chart (nomograph) and if I'm reading it correct a low impedance cable (75, 100, 150 ohms) even at quite high capacitances doesn't start rolling off until quite high frequency. Like, high enough for anyone recording at 44.1khz not to worry about. I may be wrong though but looks that way from what I can see.

For audio interfaces all (most?) well designed gear will have a finite source impedance (build out resistors), so the dominant cable frequency response effect will be a simple LPF formed by the build out R and the cable C.

Characteristic cable impedance for modest length audio cables is mostly insignificant at audio frequencies. Even DCR is not very significant except for speaker cables where the current can cause IR voltage losses in long speaker runs. For passing RF and Digital signals the wavelengths are short enough wrt cable length, that characteristic cable impedance and proper terminations do matter (reflections inside the cable interfere with signals).