UPDATE
I brought the other monitor upstairs.
The problem is this: AS LONG AS I CONNECT BOTH THE SPEAKERS, the hum starts.

How is this possible? Is the monitor damaged? If so, why it does output NO noise and NO hum when it's not connected to another monitor?

Bad Monitor alone = Works fine
Bad Monitor with the other monitor = Works bad

I guess the audio interface with the mono cables (one per speaker) should be the solution here.

Any other inputs / tests I should do?

EDIT: The noise is ON even if the other speaker is turned off. It's just that, as soon as I put the cable in and turn the bad monitor on, the hum starts happening. Everything leads to believe it's the cables.

At this point it's most likely NOT a ground issue, right? I don't really wanna mess with that stuff and I hope so. I'd rather spend 50 € than dealing with that crap. For anyone wondering:
Electrical outlet => Power strip --> Power strip <-->
Pc, monitor, speakers and everything else go through both the power strips.

Ok I'm still talking to myself. It has to be the cables and the fact that I'm using a cable that ends straight into one end instead of two mono cables. Tomorrow I will order from Amazon the interface + cables.

 

Last edited: Oct 7, 2016

You have a grounding problem on one of your power cables. The humming speaker earths through the common on your audio cable to the computer.

Plug it intor phone alone, you have no earth loop so nho hum.

Plug both speakers into your phone it earths from one speaker to the other through your phone.

Check the earth connections in your power cables.

 

The hum occurs, weirdly enough, even on this setup:

Bad speaker connected to source
Good speaker connected to same source (I even tried on a different outlet)
The final output, the end of the cable, is not connected to anything at this point. Not even to the phone. Nothing.
Bad speaker has no hum AS LONG AS I don't plug the other end of the cable into the other monitor.
If the stereo cable (I think it's called Y-cable or something) has both the monitor on, it doesn't matter anything else: there's hum. Monitor off? Hum. No pc/mobile connected? Hum. I tried every combination of this, changing power strips and even changing the power cable (I found a similar one from the printer): nothing.

I could try more cables, I'm pretty sure I have some more lying around, but at this point I doubt that's it. Unless I'm missing something, in that case . . .

 

No matter what the solution that you find for this mysterious hum of your's, do yourself a favor and buy yourself a dedicated sound card. You are a member of this audio forum which tips me off that you like to create music or work with sound in general. With even a basic entry level unit like an inexpensive Focustite Scarlett or that Behringer box previously mentioned, you will be taking a wonderful step up in quality. Go for it and you won't regret it. (This message has been provided by the over 1,000 members of the AS forum as a community service!)

 

Yes crappy on board sound card is not good. Your computer make a lot of noise ( electrical noise ) that is. It's most likely getting transferred to the audio out. Get a Focus right 2i2 they are really cheap now and way better than the on board sound card you are using. I do not know much about the Behringe,r but I am in total agreement to what others had said, you can only go up by upgrading your sound card.

 

A long time ago my 19 inch CRT Displays used to make all kinds of noise that my old alesis monitors picked up. The solution was in how I routed my cables.

But it does sound like a faulty capacitor, like what @LV4-26 said.

I know its your house setup, but bad capacitors are almost always of the cause of car engine hum in your car stereo or line in. Cheap or old noisy caps in the alternator making noise on the power. A ground lifter/isolator will help a lot if it is, but not get rid of it completely

 

Last edited: Oct 8, 2016

Wait? a Y cable? unbalanced? Is it stereo to L/R or mono to mono 1/2. Could be an impedance mismatch (Ohm) problem if its the latter. You would be dropping the impedance by half and could possibly fry your amp/audio device if it isn't rated for it. Like when you wire your subs in parallel to get a 2ohm load, but only if your amp is rated for it. Im talking car audio now but it is the same principle. Are the speakers powered? If yes, then maybe not fry, but it could be the source of the noise.

 

Last edited: Oct 8, 2016

Just ordered these:
www.amazon.it/dp/B003ZLOB9W
www.amazon.it/dp/B0023BYDHK/

@subGENRE: the cable I'm using right now is similar to this one (just another brand)

The speakers are active speakers, each one of them requires the power cable.
If the interface and cable change won't solve the issue, I'm afraid I'll have to mess with ground and stuff. And in worst case, it has to do with the speaker itself (which although proved to fully function while not connected to the other).

Thanks everybody for all the answers

 

Yes that's the right cable that you should be using

 

Hard to tell without investigating more.
One thing is for sure: As soon as you connect both monitors to the audio cable (without even attaching it to a computer or phone), you connect their grounds, and if your monitor power plugs have a third ground connector, you automatically have a ground loop.
That's not necessarily a problem per se, but could be ...

First thing I would try: Use different audio cables and power the computer and both monitors from one multi-out power extension. Also try to turn the power plugs 180 degrees if you're in a region where plugs are designed to allow that.
Then, swap power cables.
Second thing:
It could also be bad ground isolation in the humming monitor's power supply.
If all else fails, swapping the power supplies between monitors will tell you if that's the case.
That is if you really want to go that mile

I always have at least one free Behringer HD400 fur such cases when I'm too lazy

 

Last edited: Oct 8, 2016

Thanks for your input
I just noticed that in the manual there's no mention of using a Y-cable, maybe they are not meant to be used this way.
If the interface fails, I shall try those suggestions, thanks fiction

 

One more thing.As @subGENRE mentioned, you need a balanced cable.As i see, your monitors have xlr inputs as well.iI would go on that one.Also it matters if you have other devices on the same phase that may add noise in your signal by the ground cable.A fridge,a fan,anything.Lifting the earth is an option as mentioned before.You may try it on each monitor.Also someone mentioned capacitor problem.t might be, but not on the monitor itself.It might be a noise protection capacitor on another device.If i was in your place i would first try lifting the ground, change the cable to a balanced one AND try connecting them to an iPod, a phone , a radio, something that runs on batteries(and shut down the PC which i suppose runs on the same phase with monitors).If this solves the problem,then your PC is likely the source.

 

Here is one really cheap.
http://www.adorama.com/beumc22.html?sdtid=9162115&utm_medium=Affiliate&utm_source=rflaid62905
39.99 I do not know how good this is Behringer U-Phoria UMC 22 but hey $40.00 SD you can not go wrong..

 

The hum is not gone
I'm trying to keep calm but I'm getting really nervous now
I hope at least it was worth for the quality (which I still need to test)

Time to fuck around with the ground and cables

EDIT
I ended up asking on GS
But I already know I'll get the same answers so, time to call some electricians I guess?
Raise your volume: here's the pulsating/hum sound (I'm moving the volume knob, it's not fading or anything)
Another one here:

 

Last edited: Oct 10, 2016

Do you know anyone local to you who has a mixer set up. If so, try taking the monitors there and connecting to the mixer via a ts/ts cable, but take your rca/ts cables too.

 

Last edited: Oct 10, 2016

I think I know, and I was going to speak with him this week (although he isn't really into monitors and stuff, he just DJs though so he should knows something).
I'm afraid it's something inside the speaker at this point

 

You just need to try them out with his setup and cables. See if he has power cables that will fit too, just in case it's being caused by one of the mains cables.