Old 07-10-2024, 06:43 PM   #1
matt_t
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Hi all,

I have a Behringer SB 78A condenser mic. Not expensive by any means, but with very good reviews. I'm using it with an Alesis MultiMix4USB.

With input gain adjusted to record finger-picked acoustic guitar at roughly Kenny's recommended level there's this background noise (with SPAN settings as per Dan Worrall):



I'm wondering about the spike at about (just below?) 50Hz. I have heard tell of a strange beast known as a ground loop, and since this mic needs phantom power and UK electricity is 50Hz AC, I suspect this is one. If it is, is it a problem? And if it's a problem, is there an easy (and cheap) way to get rid of it? (ReaFir in subtract mode? But prevention is better than cure.)

It's only at about -80dB so obviously I can't hear it, and there's a bit of other background noise. But what if I recorded lots of tracks using this mic? Would it eventually build up and start to become audible?

The mic has a low shelf cut switch, and surprisingly with that on it's even worse:



As a comparison, this is what I get with a dirt cheap mic, so cheap it has no brand name on it and practically came free with a mic stand(!):



No phantom power, less 50Hz, but a bit more general background noise (the input gain on the mixer had to be cranked up to get the same level). I can't believe for a second that this mic is "better" than the Behringer.

Please educate/advise me, and for the sake of argument assume that I don't know anything! This *is* the Newbie forum after all, even though I'm an "oldie". Thanks!
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Old 07-10-2024, 08:32 PM   #2
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Ground loop hums, as the name suggests, come from the ground connection of certain devices. I am by no means an electrical engineer, but I would start by making sure all your electrical plugs were not plugged into the same outlets, keeping them plugged in as far away from each other as possible with no crisscrossing of the wires.

When I built my home studio I had my electrician give me 4 separate circuits so that I have plenty of isolated power feeds (computers, amplifiers, FX/EQs, etc.) They all have their own power source. I have never had a problem with ground loops.

Last edited by ggrey; 07-10-2024 at 08:33 PM. Reason: spelling
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Old 07-11-2024, 03:49 AM   #3
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Is it still there with mic unplugged but phantom switched on?
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Old 07-11-2024, 01:35 PM   #4
matt_t
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Thanks for the replies. I only have a couple of 3-pin wall sockets in this room, but need to plug in computer, monitor, USB mixer and speakers, so there's only a limited amount of rearranging I can do. But I'll give it a try. [Edit: Got a garden extension lead, plugged that in in the hallway and the USB mixer into it. Still there.]

Quote:
Originally Posted by Stella645 View Post
Is it still there with mic unplugged but phantom switched on?
Yes, still there with mic unplugged, whether the cable is in or out:



Turning phantom off gets rid of it, apart from the very high frequency stuff.

Last edited by matt_t; 07-11-2024 at 01:58 PM.
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Old 07-11-2024, 02:28 PM   #5
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Have you tried different cables? Your usual XLR ground is pin 1, which could be absent or wired differently, due to legacy standards varying.
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Last edited by bolgwrad; 07-11-2024 at 02:29 PM. Reason: epilog
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Old 07-11-2024, 05:08 PM   #6
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Take a look at your cabling setup and make sure no signal cables are running parallel to any power cables.
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Old 07-12-2024, 05:30 AM   #7
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Quote:
Originally Posted by matt_t View Post

Yes, still there with mic unplugged, whether the cable is in or out:

Turning phantom off gets rid of it, apart from the very high frequency stuff.
So the mixer is probably the issue. Not the mic and not a ground loop.
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Old 07-12-2024, 06:43 AM   #8
mister happy
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Give this a try:

http://www.harmoniccycle.com/hc/musi...tar_Ground.htm
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Old 07-12-2024, 07:36 AM   #9
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Thank you all. I'll experiment a bit and report back.
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Old 07-15-2024, 04:02 PM   #10
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I tried the star ground thing but that didn't work. So I decided to cut to the chase: laptop on battery power, mixer the only thing plugged into the wall in the room. Noise still there. So it looks like it's the mixer.

Thanks for all your help.
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