external sound card, RCA in out USB

[Katia]

Unsuprisingly so, but it's always good to know that theory and practice line up.

I'd generally prefer the headphone output. It should be driving the HD400 with ease which may not be quite the same with the line-out, and its output impedance is lower which is always a good thing when driving transformers.

Please explain where exactly this hum occurs. Is it in the PC recording or the monitoring setup or headphones or what? (You might have yet another ground loop going outside of what you just fixed, or a 3.5 mm cable with a bad ground connection.) Is there any change when unplugging the USB?

Even the common-mode rejection of a HD400 has its limits should one side have a ground potential that's at like half the mains voltage. I don't see how that would be the case here though. I assume the PC is a regular old desktop / tower PC that takes the same IEC power cords as the mixer?

You wouldn't happen to have a multimeter around...? Doesn't have to be anything fancy, you can get a plenty decent unit for 20-30€ or something and pretty much the most basic DMM would do here.

Hi @AnalogSteph
first of all, sorry for the delay in reading and responding to you but my job makes me live with a suitcase always ready to go and when I'm away on business I can't keep up with anything else.

Unfortunately, I can't use the headphone output of the sound card because with the audio cables you know, the connector above the Line-In is too big and doesn't allow the insertion of another connector to the first output below the sound card which is the Headphone Port.

While for the hum, it occurs in the headphone connected to the mixer although just now I found that after installing the 2 HD400s, although there should be no correlation, however the hum is almost gone! The workstation/PC uses a regular 220v power cord which I will soon connect to a UPS.

So far, it's doing just fine.

I'm only back for a day and will leave again tonight.
When I have more time, I will test more thoroughly and in the event of any problems I will bother you again. Thank you for everything.

 

[AnalogSteph]

Unfortunately, I can't use the headphone output of the sound card because with the audio cables you know, the connector above the Line-In is too big and doesn't allow the insertion of another connector to the first output below the sound card which is the Headphone Port.

So the 3.5 mm connectors are getting in each others' way? Bummer, but I guess it happens. I have memories of things getting tight around the back of a soundcard, too, though it tended to involve 6.3mm adapters and such. You could try using a 3.5 mm extension cable with slimmer connectors if need be.

While for the hum, it occurs in the headphone connected to the mixer although just now I found that after installing the 2 HD400s, although there should be no correlation, however the hum is almost gone!

You may have had ground loop currents travelling alongside the audio ground path inside the mixer before, in fact this is quite likely since you obviously had them going between the mixer and the PC and they had to have gotten from the mixer's IEC mains connection to the RCA jacks somehow to begin with, and not everyone constructs their stuff like Bruno Putzeys would (the famous "G word" whitepaper). Is there anything else left connected now except power and USB, and does unplugging even the USB make a difference?