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I have a AKG Pro Audio C214 Condenser Microphone powered by a 48V phantom power device (Neewer NW-100). There is a very loud, low buzzing, the frequency of which would indicate that its AC interference.

When I turn off the phantom power device, the buzzing goes away and the mic is very clear for the short period of time before the capacitors drain. Would this indicate that the phantom power device is bad or poorly constructed? Is there anything else it could be?

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    Looks like an un-earthed, switch-mode PSU, so yes, quite likely. Can't say for certain & you can't earth it to test. – Tetsujin Jul 29 '18 at 20:11
  • When you say “unearthed” is that similar to saying ungrounded? Is there a particular PSU you would recommend? – w.brian Jul 29 '18 at 20:13
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    It's hard to tell from the photo, but it looks like a US or Japanese un-earthed plug [US calls that ground, which is technically wrong, mains earth & signal ground are not the same thing]. The only thing you could do to test is buy a new PSU with a real earth... then hope you have sockets with one, too. [I live in a country where un-earthed sockets are illegal] – Tetsujin Jul 29 '18 at 20:16
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    If you're recording to a regular computer, I'd be tempted to follow this advice, even though I expect the AKG to be a considerably better mic, using USB with built-in phantom might solve your ground loop/lift issues - sound.stackexchange.com/questions/44184/buzzing-hissing-issues/… – Tetsujin Jul 29 '18 at 20:22
  • Thank you for your help. I'm looking at another PSU that has a DC power adapter, which seems like it may improve the issue since AC->DC conversion is occurring well away from the PSU itself. – w.brian Jul 29 '18 at 20:26
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To get from 18VAC to 48VDC you need to rectify and smooth and then use DC/DC conversion for doubling the voltage. That gives you the inefficiency and weight of a transformer (I saw one review claiming to have it melt out of its casing) coupled with the noise problems of a DC/DC converter.

Now the NW-100 is more or less "designed" for use with Neewer "condenser" mics that are electret (and can also work at 5V). Those have significantly higher signal levels, being suitable for computer mic inputs, than a good balanced microphone. Being built to complement a high-gain low-quality mic, using it with a low-gain high-quality mic will showcase its qualities.

It also talks about its "robust plastic casing" which will not help with shielding. Be sure not to place the phantom power box not anywhere near AC power including its own power brick.

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