There's an interesting article by Hans Polak about this back in Volume 10 of the late Linear Audio series from Jan Didden. I think you can get back issues through Amazon or download individual articles from Jan's website.
It works as a Faraday cage - although openings in the screen at either end of the cable may compromise the integrity of the cage and allow for some penetration by outside electromagnetic (EM) fields.
The penetration of EM radiation is limited to oscillations that have wavelengths shorter than 2x the diameter of the opening, so a 1cm opening would admit 2cm and shorter wavelengths, which correspond to 150+ GHz noise.
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I once tried a twin core coax a bit like that drawing. It had a dc potential on it and acted as a microphone, picking up a light touch to the sheath to produce a scraping sound. Don’t let dc on your cable.
Why not tie the open end to the chassis via a small cap (say 5-10 nF). This then makes the sending equipment chassis, the screen and the receiving equipment one chassis to RF. A similar principle is used with XLR connectors where pin 1 is connected to the chassis both ends (no cap of course).
I don’t think leaving one end floating is a good idea. You really need to bond the screen both ends (directly or with a cap or cap + resistor network) but of course take care not to introduce and loop currents In the signal return wire.
The screen wire should do one of two things here:-
1. bond both ends to the equipment chassis
or
bond one end to the sending equipment chassis and bond the other end to the receiving chassis with a 10 nF capacitor.
Either of these two approaches will provide you with some worthwhile RF immunity.
See slide 49 to 52 here:-
Ground Loops - how to wire up an amplifier
(1nF caps mentioned in the slides - that should read 5-10 nF)
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Leaving the destination end floating works just fine for normal hi-fi interconnection purposes.
Heck, I've even left the screen floating at the source end with no ill effect!
Bottom line is, what is the advantage of a 'directional' cable? Why not just wire up the RCA cable as per usual using single screened coaxial cable?
P.S. I may have an answer to the first question!
Heck, I've even left the screen floating at the source end with no ill effect!
Bottom line is, what is the advantage of a 'directional' cable? Why not just wire up the RCA cable as per usual using single screened coaxial cable?
P.S. I may have an answer to the first question!
Consider connecting two items of equipment with a normal single core screened RCA interconnect.
Consider also that each item is separately plugged into the mains power supply.
Any ground voltage difference developed in the house wiring will appear across the ends of the grounded cable screen.
A noise current will then flow in the screen, resulting in a hum issue.
The purpose of a directional cable is to avoid current flow in the screen and hence eliminate hum.
Consider also that each item is separately plugged into the mains power supply.
Any ground voltage difference developed in the house wiring will appear across the ends of the grounded cable screen.
A noise current will then flow in the screen, resulting in a hum issue.
The purpose of a directional cable is to avoid current flow in the screen and hence eliminate hum.
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Normally a standard screened cable is what I’d use for single ended and then tie the screens to the chassis either end with the 10nF caps. However, if you want to use a screened cable and use the inner two cables in single ended mode, then you really need to tie both ends to the chassis either director with caps. Earth loops in single ended systems arise when loop currents flow in the signal return connection. You should be able to avoid that with good wiring practices.
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I get that, Bonsai. You obviously have your preamp design hat on, and know way more than I do about grounding.
Now, let's assume that all hi-fi equipment manufacturers employ good wiring practices. 🤔
Think about the average punter who simply wants to connect his shop bought CD player to his shop bought preamp.
Would using a directional RCA cable, such as qguy2000 illustrates, offer any advantage over a using a 'normal' RCA cable?
Actually the exact opposite will happen!
Because an interconnect system is a voltage amplifier (almost no current flows), it will amplify the chassis voltage differences. The best plan is to use a coax with a heavy braided shield to reduce this voltage difference.
So, if a ground-loop current flows in the screen, it will generate a noise voltage across the resistance of the screen that adds directly to the signal voltage.
That's why you recommend a heavy braided screen which will reduce the resistance and hence the noise voltage.
Have I got that correct?
Thanks Bonsai, I kinda thought that at the outset but, inquisitive as I am, wanted to explore the differences.
I always use standard RCA interconnects. The only time I've left one end of the screen floating was when I made two balanced TRS jack to unbalanced RCA plug cables. Rightly or wrongly, I followed the advice I could garner at the time. The cables did the job well - no noise problems.