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philpoole 3rd September 2012 09:51 PM

Home grounding issues, the saga continues: part 176, ethernet
Hi all,

No idea which forum to post this on, I thought solid state might have been appropriate originally, but everything else if in doubt I suppose...

I have had an earthing issue for a while. Initially I thought it was my preamp that was having issues, but since then, I've noticed a lot of RF in my Yamaha home theatre amp (picking up some obscure broadcast).

So in my quest to further my understanding of my issue, I thought I'd try omitting the pre altogether, and moving away from the living room (where the noisy home theatre lives) and feed my power amp (JLH, when feeding the speakers without an input , is VERY quiet) directly from my mac mini (I'd hope they'd think carefully about grounding at least).
Anyway, amp silent. Plug in Mac, muted, and annoying RF!! Pants! And this time on the radio, I can keenly discern Lilly the Pink, and later some bad Tom Jones. So not good. But then, unlike downstairs, after a few seconds, the RF vanishes!

Perplexed, I then decide to just stream some audio via youtube, and lo and behold, the RF is back! After despair, and then some thought, I disconnected the ethernet cable on my Mac, and it vanished.

So, after months (literally, months! The whole summer) I've narrowed down the earthing problem I have in the house down to the ethernet.

Basically, I have two rooms involved, I think they are on separate rings. In the middle (I think on one of the two rings) I have a broadband hub, which I believe is unearthed - just fed by a two pole (plastic earth pin) wallwart type PSU.
This feeds wired ethernet quite simply to the Macmini mentioned on one ring, and a PS3, which shares power with the TV and Yamaha etc, on the other.

If I unplug the PS3 ethernet connection, and leave the Macmini plugged in, there is no issue.
So I think I have a large earth loop here, which is odd, because I thought twisted pair ethernet was galvanically isolated, and so didn't get grounded (or didn't need to be grounded).

Does anyone know how I can fix this (other than wifi!). Is there such a thing as an ethernet isolator?

Help! This low-rent radio station is doing my nut in!!!


multisync 3rd September 2012 10:01 PM

can you dump the wired internet and go wireless from the router to your pc?

ie wireless router to macmini to your JLH amp?

remember ALL wiring is an antenna, grounding may not fix it but shielding and rf bypass might.

philpoole 3rd September 2012 10:28 PM

Thanks for the reply. The Macmini was just a chance to remove the preamp to investigate, but it did reveal the problem. I am sort of stuck with the wired ethernet up there for other reasons.
I think the bigger issue is the PS3 remains a wired connection, and may be feeding hash into the AV setup downstairs. Wifi definitely isn't quick enough for online gaming and stuff.

Good point about all wiring being an antenna. I thought it would be ok as is, being supposedly isolated, but might investigate perhaps shielded cat 6 or something. Seems a bit unnecessary though. Surely this should all just work? O the joys of grounding.

jcx 3rd September 2012 11:32 PM

AM or FM radio? long cable are likely common mode antenna

donuts, clamp-on lossy ferrite RF supressors can help with common mode FM more readily than AM (single pass for FM, AM requires multi-turns, is shorted out at higher RFI freq by the turn-to-turn C)

philpoole 4th September 2012 07:37 AM

Thanks jcx,
I have some small ferrite torroids I might be able to shove an ethernet plug through, I'll give it a try.
I also wondered how on earth are these things being grounded, and wondered if it was the spare POE wires just being grounded (well, the GND anyway). I have a cheap POE injector, which should be able to isolate those pins, so I might try using one of those to see if it breaks anything.
But, I still can't believe its a ground loop, so you might be right about the antenna.

I'm not sure if it's FM or AM. I'd imagine FM, as it's surprisingly clear, and there aren't many AM stations about these days (and it sounds rather local).

I shall investigate further..

Thanks everyone for the help.


marce 4th September 2012 06:18 PM

Ethernet is isolated,and shouldnot pick up radio like taht, think of all the big installatione tec. I would suspect that the HUB may not be up to it. Is the local station near and is it broadcasting over power!Have a look at the EMC club
EMC Information Centre - The EMC Journal (Free in the UK)
the banana skins section of the mag has stories similar to yours, you may find a solution there, this is an EMC problem and this site is one of the best for info. Keith Armstrong is one of the main contributers to the site.

DF96 4th September 2012 06:35 PM

FM breakthrough is unlikely, unless you happen to have something in your system which is resonating near the right frequency so you can get slope detection.

Ethernet isolation is intended mainly to stop power frequencies, including hum loops, not RF. Ferrites should help.

philpoole 4th September 2012 08:51 PM

Hi All,

Listening with my phone radio, it appears that I can probably hear BBC Radio Wiltshire (104.3 MHz FM). Nice. I'm particularly au fait with local radio. I can't listen for much longer to confirm...
From what I can tell from the BBC website, they don't appear to broadcast in AM.
They've gone all modern and started playing Simply Red. Oh help!

I've tried a small ferrite, and it hasn't done anything. I have a bigger sized split ferrite in the post to try.
I shall try moving the PS3 to the same mains ring as everything else to see if its anything to with earth loops, then I guess I might need to try shielded cat6 cable.

Such an odd issue!

philpoole 4th September 2012 09:13 PM

Moved the PS3 over to the same ring as the hub and mac, and same issue - until I unplugged the HDMI connection to the TV (on the other ring). So surely an earth loop - but how can it be?
BTW The PS3 is just plugged in, it's not running. So it's earthed, but that's about it.

philpoole 4th September 2012 09:18 PM

'Seven tears are flowing to the ocean' - OMG!

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