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I have 2 brand new Lewitt 550 LCT microphones plugged into the RME Fireface UC audio interface using XLR cables. The interface is plugged into a MacBook via USB cable. There is a constant humming sound in both playback and during recording. I've tried the following tests:

  • I plugged in 2 Neumann mics into the interface with no humming issues on any laptop.
  • When I plug the Lewitt mics into a Zoom handheld recorder there is no hum.
  • When I touch the Lewitt mics while plugged into the interface, the hum quiets down slightly.
  • I've tried using the Lewitt mics and the interface with the following laptops: 2 Lenovos (it produces no hum), 2 ASUS (it produces hum), and the MacBook (it produces hum).

Here is a short clip of the humming sound: http://vocaroo.com/i/s0FPE1nDasAJ

EDIT: The above clip is when the MacBook is on battery power. When the MacBook is plugged into the wall, the hum is greatly reduced but it is still there.

I would greatly appreciate any tips that will help me get back to recording. Thank you very much.

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  • You used the same cables for the Neumanns, eliminating the cable as the cause? Does it do the same whether the Mac is on battery or mains power? – Tetsujin Mar 26 '17 at 11:09
  • I did use the same cables for both the Neumanns and the Lewitts. The humming is really loud when the Mac is on battery power. When I plug in the Mac the humming is greatly reduced, but it's still there if you listen closely. Thank you for any help! – Aelin Mar 26 '17 at 11:36
  • I found out that when I plug the interface directly into the wall and plug the laptop into a power strip that's also plugged into the wall but turned off, there is no hum... so long as the strip stays off... – Aelin Mar 28 '17 at 14:11
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    Earthing issue then, by the sound of it – Tetsujin Mar 28 '17 at 14:49

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