Well, I got an Audiobox USB from a friend a couple of weeks ago, when trying to use it would make a "switching" noise, I mean, it is the usual noise that switching power supplies produce in other devices. Now, the box has a switching power supply inside, to give power to the pre-amplifiers and the 48v needed for the phantom power.
Also, the volume controls aren't actually the kind that will cut down to zero, but a gain control that goes from -10 to+30 db.
One of the controls was dirty so I opened and cleaned, but even after that the chanel wasn't working, so search a bit and found a 1000uf x10v that was faulty, This 1000 uf capacitor together with a resistor of 39 ohms plus the variable control in front the the box are the actual gain control circuit.
Meanwhile the switching noise remained, but it was mostly in channel 1, so just in case I also changed the capacitor on that side, but still the noise remained.
So I had the idea of bypassing all the volume control straight with a 100 ohms resistor, then ALL the switching noise WAS GONE!
So, when I saw what was happening, the actual problem is that for cahnnel 2 the front volume control is less than 2 inches from the capacitor mentioned above, whereas for channel 1 it is goes all the way to the other end of the board, gets into a 2 inches long connection wires, and adds another inch in the board where the volume control is attached.
Long story short, since the volume control works to ground, I just cut the part of the board that was going around and put two cables, one from the ground and another from the capacitor straight to the volume control, and now the gain control works as expected, and the switching noise is gone!
I'm planning on adding a youtube video just showing what I did, but just in case someone knows how to get around with a tester and a soldering iron I'm leaving this info here!
Hope it helps someone!