Audio Noise through TB3 connection (!)

Although this is not exactly and audio production blog, I wanted to check with the community here.

I have an Apollo Audio Twin X audio interface, that uses a Thunderbolt 3 connection to connect to the computer, a 2018 Macbook Pro. Turns out that when I connect a pair of iLoud Micro Monitors through RCA cables, I get quite a bit of noise. It’s does not sound like your typical ground loop “hum” at 50Hz, it sounds like “digital noise” --exactly like an old modem, rhythmic bleeps that change depending on what the computer is doing. I can even hear bleeps and clicks when I’m typing on the keyboard, it’s that crazy.

No noise is heard through headphones connected to the Apollo audio interface. The noise is only present in the iLoud Micro Monitor speakers and it is there even if the Apollo is turned off, and dissappears if I disconnect the TB3 cable (although of course the audio interface would not work without connection to the computer)

Now the surprising part: the noise goes away if I disconnect the Display Monitor (a Samsung M7) that is also connected through another TB3 cable. Crystal clear, low latency goodness there, but no external display is no bueno for my tired eyes.

Any suggestions as to what would be solving this? I have tried switching TB3 cables to connect the display and no luck. Another option would be using balanced cables but the iLoud Micro Monitors only have RCA inputs, so I suspect it would not be useful.

Just a guess, but your troubleshooting would point to possibly RFI interference from the Samsung Monitor being picked up on the analog RCA cables.

Can you do anything to shield the RCA cables or the monitor? Or perhaps use longer thunderbolt cable so the resulting connections are further away from the display monitor? (Although longer thunderbolt cables can create more problems of a different kind)

If this is a crucial audio listening setup, studio monitors at the more pro-level always use XLR balanced connections, and might be a worthy investment.

Thanks for your answer! Yes, this is supposed to be the cause. I have found a Gearspace thread where people have similar issues when connected to TB3 docking stations and having power delivery through the TB3 port so a suggested fix is to first connect the regular MBP charger, and then connect everything else. Thank god this Mac has 4 TB3 ports because otherwise I would not be able to connect the display, the audio interface, the power supply and an external TB3 SSD enclosure.

Well I am not running an audio production operation so I can live with the noise or get used to it while I am not using headphones. Just find it quite vexing because I was not trying to cut any expenses here. Perhaps the iLoud Micro Monitors RCA inputs are the weakest link here, but sadly I do not really have real state space in the room to be able to get more pro-level audio monitors.

It is my understanding that TB ports are often configured in pairs such that two share a connection and the other 2 share a separate connection. Have you tried reconfiguring which devices are plugged into which ports? Perhaps if the monitor and audio interface are more isolated, the issue could be eliminated.

Of course, this assumes the noise is actually being transmitted through the TB cable, not from RF (radio frequency) interference as @SpivR suggested. I suspect it is probably the later, but in the event it is not, switching what’s plugged into which port might resolve the issue.

Yes, but to no avail. Tried different TB3 cables, connecting one thing here, other there, multiple times.

But I found the SOLUTION! Using the Apple power brick makes the noise go away! As soon as the Samsung display is providing power through TB3, noise comes back. Baffling!

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I know, obviously a stupid question, BUT what is a “Apple power brick”?

Sorry, the standard Apple charger that came with the MBP

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This is really good to know. PD voltage on the TB cable is probably acting like a little AM radio station. (Just guessing, but I bet the lower voltage and power limits of USB is why most of us have never had to deal with this with USB powered devices.)

If you can avoid using PD on that cable, seeems like the most stable way to fix it. There are third party little dongle boxes that fit inline to existing RCA audio cables, but I’ve never used them and their focus is more typically on removing ground loop power line hum.

In general, the less contraptions and workarounds in the audio chain the better chance for long term stability and not just a short term fix that doesn’t hold up.