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I have a Focusrite Scarlet 2i2 2nd gen audio interface, which is powered through its USB bus. I have a AT2035 condenser mic attached to it, using the onboard phantom power capabilities. The interface occasionally malfunctions and makes static noises until I unplug it and plug it back in. I've since learned that it's probably Focusrite's shoddy drivers causing the issue.

For troubleshooting, I bought a used Rolls PB23 external Phantom Power supply. It didn't resolve the issue. However, I did notice that it was injecting high frequency noise (8-12kHz, well above the rest of the noise floor) that was just barely audible at my normal gain levels. The noise went away when I touched the metal case of the PB23 or the microphone, but not the Focusrite. This noise is not generated when I use the onboard phantom power on the Focusrite.

So two questions:

  1. I've read that external power supplies like the above Rolls are generally inferior to the audio interface's source of phantom power, because the wall warts are a potential source of noise, and they are ungrounded. So what are these things actually useful for?

  2. How is this thing supposed to be grounded? Through the XLR cable back to the audio interface? Is the audio interface grounded through the USB cable?

EDIT: To be clear, question #1 is mostly asking: what's the expected use case of these power supplies when most good mixers or audio interfaces have built-in phantom power? Is there ever a case where I should prefer something like the Rolls over what my audio interface has?

EDIT2: I tested the ground on my desktop; it is good. The desktop (grounded) and the RB23 (ungrounded) are the only things plugged into AC.

Do XLR cables function as grounds between devices? I tested with my multimeter and the casings at either end of the cable do not show continuity. (All three wires do, of course.)

  • This is hard to answer as is stands, for several reasons. 1) wall-warts are notoriously noisy [high frequency switch-mode transformers] & are generally unearthed. 2) your country's earthing policy & 3) whether you have a laptop [with yet another wall-wart] or a desktop correctly earthed through your country's power supply. – Tetsujin Oct 31 at 18:36
  • @Tetsujin Fair. I didn't think to check the desktop ground; I will shortly. I am in the US with a desktop computer, theoretically properly grounded. Will edit with more information soon. – Cody Oct 31 at 19:18
  • @Tetsujin I added more info above. Ground on my desktop is fine. Touching the Focusrite does not get rid of the hum. Touching the RB23 or the mic gets rid of the hum. Does this mean that the XLR cable is not functioning as a ground between the chassis of the RB23 and the Focusrite? Should it? – Cody Nov 1 at 1:17
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Not sure that the rolls is a particularly well regarded brand. Suggest that if you need to use a phantom power adapter, you acquire a battery operated one. Denecke make one that takes a 9v battery and provides phantom across two inputs. Phantom is very low current so a 9v battery would last a while. This will avoid any issues with a power-supply.

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    So it's probably just the design? I got suspicions when I noticed there was no grounding of any sort. They claim their "100kHz switching power supply" eliminates noise, but it looks like just some cheap wall wart to me. I like the battery idea. For now at least, the audio interface is behaving itself. – Cody Nov 1 at 18:09
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1: they're useful for situations where you have an audio input that does not provide phantom power.

  • Fair...I guess I'd rather have it (even with some noise) than not have any input at all, if I was in that situation! – Cody Oct 31 at 19:27

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