No preamp, just a Scarlett connected via USB cables into my notebook. The cables causing the problems are the one on the Scarlett inputs, that I use whem I connect my guitar or bass with TRS cables or a mic with XLR ones...I've read that post, but what intrigues me is that the interference/hum only appears when I'm recording, when there's something connected on the inputs. When there's nothing there, the output sound/playback is perfect
Actually, pro-audio equipment (such as the Scarlett) typically have floating ground on their internal circuits. This requires special cable wiring on certain conditions such as a TRS-to-RCA cable where you actually have to disconnect the ground/shield wire from the RCA cable itself. See this link:
Many pre-made cables are not wired correctly here. I actually had to modify a Mogami RCA-to-TRS cable as well as a custom XLR-to-TRS cable to remove a ground loop hum issue when I used a Rane MLM82a stereo line mixer between my computer/dac and some powered monitors.
Just re-read through your configuration description. The inputs on the Scarlett 2i4 are balanced, but you might try switching the input switch to "INST" below the input. Otherwise, you might try building the Cable #14 (make sure the mono plug is connected to the guitar/bass and the balanced 3-pin plug is connected to Scarlett -- you essentially have to disconnect the groud/shield wire on the balanced plug).
One quick thing you could try here is to disconnect the power cord from your laptop and try running a recording session using the laptop battery power only. If this removes the ground-loop hum, you might need a usb galvanic isolation adapter between your laptop and the Scarlett.
Yeah, I've tried that, when I disconnect the power cord the hum vanishes...But if the problem is in the USB cable, why does the output sound perfect when there's nothing plugged into the Scarlett inputs? (I've tried switching the USB cable from the interface, and problem persists...haven't tried the galvanic isolation adapter)
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.
Thanks, Erik, I'll try the isolator then...I can't really disconnect the USB because the Scarlett 2i4 has no external power source, it's powered solely through USB, so I need it connected in order to hear anything. The hum goes away when I disconnect my notebook power cord, using the battery...But anyhow, I had not heard about these USB galvanic isolators, I'll give them a try.
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 loopThat's not exactly correct. What's needed is to get all of the grounds at the same potential. Breaking "metalic contact" is one solution.
@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.
Nothing is connected to any of the outputs? Correct?
The guitar and mike plug directly into the inputs of the Scarlett 2i4? Correct?
Are you sure what you are hearing is a hum and not a buzz?
Scarlett 2i4 User Guide
Phantom power? Do you mean engaging the 48V Phantom power button on the Scarlett 2i4 unit? Is the microphone you are using a capacitor (condenser) type? If yes the Phantom Power button should be engaged according to the User Guide of 2i4 unit.