Solved! Buzzing noise coming from speakers

[composerz]

Okay, I know there are a zillion threads about it, but I ask for your patience once I still couldn't find a match to my specific situation, so I was wondering if anyone could give me a hint of what may be causing these noises. First, my setup:

I have two Macs connected as MASTER- SLAVE, and each one has its own audio system. System A (MASTER) has and external audio interface connected via USB plus a couple peripherals (mouse, keyboard, iPad Pro, external hard drive - all USB). System B (SLAVE) has another external audio interface via USB plus peripherals (keyboard, MIDI controller keyboard, MIDI fader controller, external hard drive - also USB). Both audio interfaces are connected to a Switch which sends the output to a single pair of speakers. This way I can easily change from system A to B and hear what's coming from the MASTER or the SLAVE.

Regarding my energy management, everything is based on a single outlet, like this: Main outlet ---> Online UPS ---> Power conditioner ---> 03 Power Strips (from where everything else is connected). Even though it seems like a safe environment with clean energy, I still get this annoying and quite loud buzz coming out from the speakers. It begins the moment the computers reaches their main screen and get worse when I move the mouse or load the system memory. It happens on both systems, being a bit louder on my Slave's system.

I don't hear any buzz coming directly from the Power Supplys or the motherboards. I also noticed that if I disconnect the TRS cables and plug an earphone into the Scarlett 2i2 (Slave's audio interface) the noise stops and I'm able to hear with no buzz through the earphones. Lowering or raising the volume knob of the individual audio interfaces doesn't change the buzz. The only way I can make it stop is pressing the MUTE button on my switch, once it blocks the audio signal of reaching the speakers. I changed all my TRS cables that are connected to the switch to balanced but it didn't make any difference.

I have no clue what is going on. This unstoppable buzz is driving me nuts. Has anyone experiencied something related?
Thanks ;-)

System A (MASTER): Early 2009 Mac Pro Intel Xeon 2.4ghz Quad Core / 32GB / OS 10.11.6 / External audio interface: Apogee Quartet
System B (SLAVE): Hackintosh OS 10.11.6 / motherboard: ASRock z270 Killer / 32GB / Power supply: Seasonic M12ii 620 Bronze / External audio interface: Focusrite Scarlett 2i2
Monitor Switch: Big Knob Passive by Mackie / Speakers: Neumann kh120

 

[americanaudiophile]

It does sound like you have a ground loop.
Since you have both the master and slave Macs connected to the same AC ground and the audio outputs are grounded to each other through the Big Knob Passive (which probably has a common ground and doesn't isolate the inputs from each other). An active Mackie probably does do that.
If you disconnect one of the Macs from the Knob the buzz should go away. You can also try a 3-2 AC adapter which would lift one computer ground. Not optimal to leave it that way.
Your options would be to isolate the AC grounds or use an isolating audio transformer to isolate one of the audio outputs.
This might help.
http://web.mit.edu/~jhawk/tmp/p/EST016_Ground_Loops_handout.pdf

 

[composerz]

It does sound like you have a ground loop.
Since you have both the master and slave Macs connected to the same AC ground and the audio outputs are grounded to each other through the Big Knob Passive (which probably has a common ground and doesn't isolate the inputs from each other). An active Mackie probably does do that.
If you disconnect one of the Macs from the Knob the buzz should go away. You can also try a 3-2 AC adapter which would lift one computer ground. Not optimal to leave it that way.
Your options would be to isolate the AC grounds or use an isolating audio transformer to isolate one of the audio outputs.
This might help.
http://web.mit.edu/~jhawk/tmp/p/EST016_Ground_Loops_handout.pdf

@americanaudiophile, I can't thank you enough. You have totally nailed it. I tried a 3-2 AC adapter in the Slave computer and it worked like a charm. No buzzing whatsoever ;-) However, you said "not optimal to leave it that way". May I ask why? Both computers are connected to an online UPS, so I guess there is no problem to keep one of them ungrounded. One of your options was to "isolate the AC grounds". How do I do that? Sorry for the silly question, I'm just not so into the subject.
Anyway, thanks for taking the time to read such a long post and also thanks for the great article. Still didn't have the chance to read it but I will definitely do it asap.

 

[americanaudiophile]

Not a silly question.
I myself don't think using the ground lifter is an issue but electrical code requires grounding so there may be a problem in some wiring situations.
The other solutions are more expensive and can affect audio quality adversely.

 

[composerz]

I see. I think I'll just keep the way it is for now and maybe in the future I work properly on the grounding. Cheers