Unusual Question - DC Thump from Audio Out on XTrend ET10000

[flintdk]

Hi folks,

OK - this is an odd question so completely accept if it's outside the expertise of users here. But just thought I'd chance asking... My XTrend ET10000 has a stereo audio out on the back. The box is sitting beside an old stereo amplifier that only has twin rca inputs (the old red and white stereo leads). I love the amp (and the radio that's attached to it) so that's staying. I did think 'why not connect the XTrend to the old amp' for matches and the like to pump up the volume in the room a bit.

So I did. And it works great - when connected. But I do get a big 'DC thump' from the speakers when I select the XTrend input on the amp. I had a look and my XTrend only has a two-core power supply cable. So the chassis of the XTrend box is clearly not grounded - and I would think that's the most likely cause of the crack/bang when I switch to it/select that input on the amp.

So... has anyone grounded the case of their linux receiver? Is it safe to do so? Would love to hear some feedback on the idea. I tried using my google-fu to find the answer but haven't hit gold yet.

[Rob van der Does]

First of all: I have my ET8000 (as test box) also connected to an amp via RCA, and I don't experience that issue. I say this to illustrate that thiis is not a general problem, but will be related to local circumstances.

Grounding a STB can be a problem; especially when the distance between dish/antenna and STB is large, there is the danger of getting a 'ground loop'.
Please see the attached document (hoping you understand German, but otherwise I'm sure you'll find some information yourself): 51112

[Joe_90]

Rob - Im just pointing out that in a domestic situation the dish/antenna would not normally be grounded, so you shouldn't encounter a ground loop scenario. In shared apartments or some other communal feed the dish/LNB/antenna/ distribution switches should be earth-bonded (grounded) to prevent fault voltages affecting other users. It's likely that the STB's phono connections have a floating voltage present and possibly a wire link from a earthed point on the amplifier to the ground side of the phono connectors may solve the issue. One for experimentation. I've had no such issues with more modern AV amps which are ungrounded.

[Rob van der Does]

Rob - Im just pointing out that in a domestic situation the dish/antenna would not normally be grounded, so you shouldn't encounter a ground loop scenario.
Well, that's actually causing a ground loop. Unless all are earthed on the same (very thick) lead. Especially when the antenna/dish is a bit away from the house (at the end of the garden, attached to a shed/garage), earthing bot antenna and STB will create the ground loop scenario.

[Joe_90]

No - I'm saying the antenna or dish are NOT earthed (grounded).

[flintdk]

Rob, Tony - thanks both for your input.

My dish is not grounded. I did see some mention on here about grounding the dish to allow for static discharge - but I didn't act on it. The dish is bolted to the house. End of. And unless I messed up there's no contact between the dish and the cables running down to the box. So the double isolated linux box and the three 20m'ish runs of cable attached to it float probably at some non-zero voltage, relative to the ground of my amp.

The amp in question is quite old (valve driven) - it's quite possible a more recent av amp would have some better sort of protection against this. So I'm not worried about a ground loop (but do appreciate the warning). I think I'll just make up a grounding lead, ground the case on the receiver and see what happens. I only hear the 'thump' when choosing inputs on my amp - will be pretty easy to tell if grounding the case works or not! Will post back sure when I get a chance to test it later.

Thank you both for taking the time to reply.

[Rob van der Does]

No - I'm saying the antenna or dish are NOT earthed (grounded).
Sorry, I overlooked the 'NOT' :o