Help diagnosing hum in my setup

[tccalvin]

Hey audio peeps.

I'm in need of some education since I can't make sense of what's happening in my setup.

I'm using a couple of DACs at the moment which are an SMSL DS100 and a UA Volt 2 interface, both USB powered, both connected to a pair of Kali LP6.
I was trying to use the DS100 as my PS5/PC "gaming" DAC so I thought powering it with my (video) monitor's USB port would be convenient, and it is, but only if I am using the thing with headphones. As soon as I switch it to RCA output it picks up hum which has a frequency correlated to my monitor's refresh rate (at 60Hz it hums at 60Hz, at 240Hz it hums at 240Hz, which is kinda funny). As soon as I heard this I thought "since it didn't do this with headphones it's likely because the unbalanced cable is picking up EMI from the monitor", but nope! When I plug the thing in my PS5's or my MBP's USB ports, leaving everything else untouched, there's no more hum. Coincidentally (?) both of those devices are not grounded, while the monitor is, which might be a clue (?). But here's the kicker, plugging the Volt, which has balanced outputs (if it matters), into the monitor's USB hub produces no hum whatsoever (?).

All my (non-DAC) devices are plugged in a single power strip that goes into a single power outlet. Monitor and speakers are grounded, PS5 and MBP are not.

I have no Idea what's going on.

Thank you guys for your time.

 

[staticV3]

Computer monitors have notoriously poor USB VBUS supplies and different USB audio accessories can handle that differently.

If you want to get rid of hum entirely, then use a good quality phone charger (e.g. Apple 5W/10W) for power, and Toslink for data.

 

[tccalvin]

Computer monitors have notoriously poor USB VBUS supplies and different USB audio accessories can handle that differently.

If you want to get rid of hum entirely, then use a good quality phone charger (e.g. Apple 10W) for power, and Toslink for data.

Thank you for the reply. The fact that the USB audio signal can carry hum which then disappears once the signal reaches the headphone amp section leaves me very puzzled.

 

[gvl]

Thank you for the reply. The fact that the USB audio signal can carry hum which then disappears once the signal reaches the headphone amp section leaves me very puzzled.

Digital data doesn’t carry the hum, if it did it would be very puzzling indeed. What happens is the ground or power lines get polluted and cheap DACs can’t quite clean that up most of the time and it bleeds to the analog stage.

 

[phofman]

@tccalvin : Please can you make a drawing of your setup, to go along with your complex description?

 

[AnalogSteph]

Not a mystery, this one. IEC Class I monitor, IEC Class I speakers, RCA (unbalanced) audio connection... classic ground loop. Hence why using the UAA Volt presents no obvious issues, the balanced connection provides substantial CMRR as intended. A galvanic isolator either for USB or on the audio side is going to be required.

 

[tccalvin]

@tccalvin : Please can you make a drawing of your setup, to go along with your complex description?

Forgot to write "PC" but I'm sure you can figure it out lol.

So it seems to be a ground loop, but I'm still puzzled as to why the hum doesn't carry over to the headphones...

 

[phofman]

So it seems to be a ground loop, but I'm still puzzled as to why the hum doesn't carry over to the headphones...

Because when playing through speakers, part of the large ground currents which flow in the ground traces in the PC between the point of connection of the ground terminal of the USB output connector and the 0V terminal of PC PSU where the PE wire is also connected to also flows through USB gnd -> monitor -> USB gnd -> SMSL -> audio return channel of SMSL to LP6 -> PE -> back to the PC. And in all these lines the stray currents generate noise voltage. But the audio return channel also carries the analog audio currents to which the noise stray currents are added - hence you can hear them.

If there is no audio-signal channel shared with the stray currents (i.e. the balanced connection, or headphones with no PE loop), the noise stray currents are not added to the audio signal currents.