Input ground loop hum only when p10 cables or XLR without phantom power are connected

Yep, switching USB cables alone will not help you. There is a known problem with laptop power supplies and ground loops.  I can't answer your questions because I'm pretty confused and don't know all the details. However, the easiest, and least likely to interfere with your sound quality is isolate the USB line. If disconnecting the USB makes your problem vanish, that's what you need to isolate. It's not just a problem for audio. The device I list is actually for medical products, which suffer the same way.

Stereophile does all their USB DAC testing with a laptop  running from battery power. I think it's a boneheaded way to test DAC's, but that's their standard exactly because of this issue.


Best,


Erik

Good luck, and let me know how it works for you.

Best,


Erik

Hey pedro!

You may need to get the extra power supply for it if your laptop does not provide a high-current port. Just FYI.

Best,


Erik

What are "P10 cables" ??

So help me out a little more please. You are going from a Scarlett to your preamp, and those are the cables that cause hum?

Let me know if the ideas on that blog post don't help you. :)

So what is your output? How do you hear the hum? Headphones? Speakers?

So on your inputs.... if you use ONLY XLR to XLR cables,  you get a hum?? Or do they have to be connected to an instrument or mic?

Best,

Erik 

@cleeds I was being a little severe, but I still think it's wrong.

If you are testing a device which is often connected to a source of noise, such as a laptop with charger, you should test with that noise, not do everything you can to eliminate it in the test environment.

In effect you are denying those manufacturers who go out of their way to eliminate ground loops and digital noise the chance to shine, and putting them on a level playing field with DAC's that don't take the extra effort.

One quick thing to check is unplug your USB input and see if the problem goes away. If so, you'll need the USB isolator from my blog page, or one like it.

Best,

Erik

In fact, most of the time, ground loops are independent of cables (unless wired wrong).

A ground loop can't be fixed by noise filters or switching cables, or better shielding. You have to break the metallic contact in the loop.

Best,

Erik