MBP aux audio quality degrades when external monitor connected

[aaa]

I'm very surprised that plugging in an external display affects the quality of the audio coming from my mac's headphone port, and I'm not sure how to fix it.

I'm using a 2020 M1 macbook pro. It is connected to my KRK Rokit 5s with an AUX to RCA cable plugged into the headphone port on my mac. It sounds pretty great (there's some very subtle white noise but that's negligible). However the background noise becomes much louder if I plug in an external display—a Dell 2720Q—via a USB-C power cable. The noise is bad in no matter which of the two USB-C ports on my mac I have it plugged into, but it's slightly more mild in the top port than the second one, although in practice they're both basically unlistenable. I have no idea why this happens. At the moment I don't have any other USB-C cables to test to see if that improves the audio quality. Any ideas?

 

[staticV3]

I have no idea why this happens.

Your Dell monitor probably does PD passthrough to the Mac via Type-C, and its power supply is so dirty it bleeds into the audio circuit of your Mac

At the moment I don't have any other USB-C cables to test to see if that improves the audio quality.

Wouldn't help.

Any ideas?

Disable PD passthrough or use HDMI instead of USB Type-C to connect to the monitor.

 

[antcollinet]

Almost certainly the new display has introduced a ground loop. This is nothing to do with the quality of the mac audio or of any of your other devices.

the display is earthed. That earth is connected to your mac via USB, then to the speakers via the unbalanced audio connection.The circulating earth currents from earth, through the display/mac/speakers and back to earth induce interference in the analogue connection to the speakers.

you can try to minimise it by connecting the speakers to the same electrical socket as PC and Display, running mains lead and audio lead close together, and away from display hardware if possible.

However, you are unlikely to eliminate it completely unless you can find a way to make the analogue connection balanced, or introduce a galvanically isolated connection somewhere into the loop to break the earth.

A couple of ways to do that: -
use a DAC with balanced outputs.
Use a low cost wireless streaming device, such as the Wiim mini, or Bluetooth receiver.
Use an isolation transformer between PC and speakers (likely more expensive for quality than the other options)

 

[aaa]

Thanks for the help, @tonycollinet and @staticV3! I ordered a DisplayPort cable to replace the powered USB-C cable running between my macbook and my external display. Subjectively it reduced much of the ground noise. With a quick test (recording the speaker noise using a microphone) I found that the powered USB-C cable introduced roughly -30 dB of noise. DisplayPort brought it down to -43 dB. And as a baseline, I get -46 dB noise just from the stock Apple charging cable with no second display connected. So DisplayPort is a major improvement, adding only about 2.9 dB RMS, compared to the 15.6 dB RMS added by powered USB-C!