You don't have any earthed outlets at all? Now that sucks. Slightly old electrical installation in a country that was slow to adopt them, huh? Any higher-power switch-mode power supply including the one in your PC relies on having a PE connection for its mains filtering. Your PC case is now floating at half the mains voltage - granted, just via a few nF of coupling capacitance tops, but still, probably quite enough to light a phase tester and maybe give you a tingly feeling when touching it.
Now should your amplifier also be connected to some other source that is actually grounded e.g. via an antenna connection, your ground loop troubles would not surprise me in the least - then the mains filter leakage current would take the most convenient path to earth, which happens to be via your audio cable's shield.
Worse yet, any safety earthed (IEC Class I) device relies on the presence of a PE connection in case of a fault condition. You couldn't even legally use a toaster or a microwave or stuff like that - and when looking around here, essentially all my PC peripherals (monitors, laser printer, scanner) are Class I as well, as is the majority of pro audio equipment. Should any such device ever develop a fault that has its case become live and then proceed to shock and kill someone, I rather doubt your insurance would cover that (or the landlord's if it's a rented apartment).
Back in the olden days, it was common practice to wire up PE in outlets to neutral if all you had was 2-wire cabling, but of course any accidental miswiring can make something like this highly lethal, not to mention that sharing the power return and protective earth tends to make ground loop problems a lot worse.
The magic is in the input, not the output. Turning an unbalanced output into a balanced one is dead easy, comparatively speaking. All you really need is balanced impedances.I've tested a balanced output of my mackie sound card and it is perfectly silent. It seems to be the right solution but balanced output professional cards are painfully expensive
So what do you get on an unbalanced output?
Signal: Typically, via several tens to hundreds of ohm of output resistance, and there might be a coupling capacitor in series as well.
Ground: A very-low impedance connection.
What do you need for a balanced input?
Hot and Cold: The signal pair, inverting and noninverting input.
Shield: Just that.
So what do you do? Assuming you are no stranger to the hot end of a soldering iron and some heatshrink and stuff (which might just happen on this very forum), the solution could be quite simple.
Hot: Let's hook that up to Signal.
Cold: This should go to Ground somehow, but we'll need to balance out the impedances. So instead of a direct connection, you replicate whatever is found in the Signal output as closely as you feel motivated to.
Shield: You could attach this to Ground, but it's arguably better connected to the slot bracket instead, or even a nearby chassis part. I think the slot bracket is not a bad idea, assuming it makes good electrical contact with the case (PC case makers are so stupid at times, but that's another rant for another day). The whole contraption will be quite device-specific anyway. You could temporarily remove the slot bracket from the card to drill a hole into it for a screw or something.
I would terminate the whole adapter thingy in male XLR connectors so you can use ordinary XLR cabling, assuming that's what your amplifier uses.
Plan B, employ a transformer for the unbal --> bal conversion, like the trusty Behringer HD400. Run a 3.5 mm stereo to 2x 6.35 mm mono unbalanced cable into it (depending on what kind of output jack(s) your card has), continue with two balanced 6.35 mm TRS to XLR cables. Works like a charm here, and you don't even need to warm up the soldering iron if you don't want to.
PS:
I forgot to mention that if in doubt, the balanced cable should be the shorter of the two - maybe no longer than 1.5 m. There's only so much capacitance a transformer can drive easily, even if it's a 600 ohm job. Higher-impedance ones can be a lot more critical still.
I've find one grounded outlet in the room, and wired all on it
And after few tests i've decided to use balanced connexions.
(but the ADUM4160 works well by suppressing the noises of my old USB unbalanced interface)
There are 47µF caps on each pin of the balanced outputs of my interface...
Sure, but being coupling caps, those are in series with the signal and do not load the output.There are 47µF caps on each pin of the balanced outputs of my interface...
Here it seems to decouple.
The capacitor is used to isolate the Opamp output from the DC.
(But as the circuit had been designed with that capacitor, removing it should be catastrophic and useless, Modify the value or the characteristics as well)
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I think we should make sure we agree on terminology. Decoupling caps are those you find on the power pins going to ground. Coupling caps are those used to pass the signal and maybe jump across some DC potential difference. I am aware not all languages handle these terms in the same way.
If you draw up a schematic and compute impedance, you should find that the coupling caps are essentially shorts at higher frequencies, making no difference when it comes to the effect of capacitive loading on the output.
If you draw up a schematic and compute impedance, you should find that the coupling caps are essentially shorts at higher frequencies, making no difference when it comes to the effect of capacitive loading on the output.
You are right !
I've performed technical translations many times...
And (perhaps) you can't immagine how many significations one word can have... for mechanicians, electricians, Computer scientists and technically unskilled persons.
Sometimes one word on a drawing can have two or many more different significations depending of the context and the people culture.
If you have know only few 2000 words to survive in the life... you must learn 10000 different significations for thoses words.
We should add 10000 new words to the language IMHO.
I've performed technical translations many times...
And (perhaps) you can't immagine how many significations one word can have... for mechanicians, electricians, Computer scientists and technically unskilled persons.
Sometimes one word on a drawing can have two or many more different significations depending of the context and the people culture.
If you have know only few 2000 words to survive in the life... you must learn 10000 different significations for thoses words.
We should add 10000 new words to the language IMHO.
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