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AKG P170 - 50 hz humming noise

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AKG P170 - 50 hz humming noise

Postby knopf » Thu Aug 13, 2020 9:40 pm

Hi all,

Just so you know, I'm at the very beginning of my journey into audio land, so no previous experiences with microphones whatsoever.

The audio interface is an SSL 2+, the microphone an AKG p170. Both come right out of their boxes, so have not been used at all, and as soon as I connect the akg, I get a 50 hz humming sound through my headphones becoming audible after the gain knob has passed the one o'clock mark. It gets louder or quieter when handling the mic, and it gets the loudest when I put my finger on that hole in the microphone body where the attenuation switch can be found.

What can that be? Can it really be the microphones? I bought two of them and both have the same noise. At first, I thought it is a faulty audio device, so I returned it, got a new one and the hum is still there. Then I thought it might be some sort of ground loop, so instead of connecting the ssl 2+ to the computer, I powered it from a power bank to try to circumvent any potential grounding problem. To no avail. I also tried a different XLR cable but that didn't help either.

Can anybody help? Does anyone have this particular microphone?

Many thanks,
Martin
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Re: AKG P170 - 50 hz humming noise

Postby blinddrew » Thu Aug 13, 2020 10:39 pm

A 50Hz hum suggests some kind of ground issue, can you tell us everything that's plugged into the system (computer, speakers, interface, instruments etc). And are they all powered from the same wall socket?
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Re: AKG P170 - 50 hz humming noise

Postby Hugh Robjohns » Thu Aug 13, 2020 11:58 pm

I suspect we'll find the OP's system is lacking a solid ground altogether.

The interface is bus powered, and the OP is using headphones. If the computer is a laptop there will probably be no ground connection at all, so the mic will essentially be acting as an aerial, picking up radiated mains interference.
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Re: AKG P170 - 50 hz humming noise

Postby zenguitar » Fri Aug 14, 2020 12:02 am

And powering from a power bank confirms the problem. A power supply with no ground connection.

Andy :beamup:
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Re: AKG P170 - 50 hz humming noise

Postby knopf » Fri Aug 14, 2020 8:58 am

Goodness me! Ground loop it is then. Thank you all for making that clear.

The set-up here is microphone > SSL 2+ > Mac Mini > Master plug with 8 sockets > extension lead > battery powered UPS (Uninterruptible Power Supply) > then - at long last - wall socket.

Well, what next? Where to start to resolve this? I'd be immensely grateful for any help.

Cheers,
Martin
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Re: AKG P170 - 50 hz humming noise

Postby Hugh Robjohns » Fri Aug 14, 2020 10:12 am

knopf wrote:Goodness me! Ground loop it is then.

A 'ground-loop' is, as the term implies, a situation where you have two (or more) connections to the mains safety ground forming a loop. This allows noise currents to flow in a loop from the mains ground, into one device, through to the second device via the audio connection cable's screen, and then back into the mains ground via the second device.

It appears that your system has no connections to the mains ground at all. So that's not a ground-loop. It's ground-less...

The set-up here is microphone > SSL 2+ > Mac Mini > Master plug with 8 sockets > extension lead > battery powered UPS (Uninterruptible Power Supply) > then - at long last - wall socket.

The mac mini has a two-core mains lead -- so no mains safety ground. That's perfectly fine for the device itself, but it leaves your audio system with no solid ground reference.

Well, what next? Where to start to resolve this? I'd be immensely grateful for any help.

In short, you need to provide a solid ground reference for the audio system. There are a number of ways this can be achieved.

Typically, the audio system would have active monitor speakers connected, and the majority of those use three-core IEC mains leads with a safety ground connection. So that ground reference would then be passed back to the interface, and thus to the mic, and you'd have a happy and hum-free audio system.

Connecting any other mains-grounded device to the interface would do the same thing.

Hope that helps.
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Re: AKG P170 - 50 hz humming noise

Postby knopf » Fri Aug 14, 2020 8:38 pm

That indeed helped a lot. Thank you so much for that and to make sure I got it right: If I bought some monitors and connected them using the balanced TRS output jack from my SSL 2+, that would then give the SSL - as you said - a solid ground reference, meaning the hum would disappear, right?

What entry level monitors would you recommend?

Many thanks,
Martin
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Re: AKG P170 - 50 hz humming noise

Postby Hugh Robjohns » Fri Aug 14, 2020 9:23 pm

Yes it would -- but do make sure the monitors are class-1 mains grounded types and not class-2 or double-insulated types. If they have the 3-pin IEC mains sockets they'd be fine.

But you don't need to buy monitors. There are other ways of achieving a ground reference. For example you could connect a mains-powered synth to the interface, or even just a grounding wire such as described here:

https://www.soundonsound.com/reviews/groundology-earth-connection-plug
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Re: AKG P170 - 50 hz humming noise

Postby knopf » Wed Aug 19, 2020 2:16 pm

Hello everyone,

Me again and sorry to bother you with this again but it's still not working here as it should. I didn't want to go for the DIY solution Hugh had suggested as I am blind and would need some help with that, but instead, I went for the quicker solution and bought a pair of decent inexpensive monitor speakers (M-Audio BX5 D3). I connected it all and I can definitely say that this kind of tingling sensation when you run your fingers very lightly across the surface of the audio device, that tingling sensation has definitely disappeared but the hum is still there. Holding the microphone in my hand for example, I can make the hum disappear completely by touching the microphone grill with my other hand, or - mic still in hand - I can touch the audio interface where the headphone jack and the housing of the audio interface meet and the hum will also disappear.

I assume this means that the system can temporarily be grounded using my body?

I would be immensely grateful for any help with this.
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Re: AKG P170 - 50 hz humming noise

Postby Hugh Robjohns » Wed Aug 19, 2020 3:20 pm

knopf wrote:I went for the quicker solution and bought a pair of decent inexpensive monitor speakers (M-Audio BX5 D3). I connected it all and I can definitely say that this kind of tingling sensation when you run your fingers very lightly across the surface of the audio device, that tingling sensation has definitely disappeared...

You didn't mention the 'tingling' before, but that's a classic sign of a ground-less system, and that fact that it has now gone indicates that your system is now being properly earthed by the speakers. So that's a very good thing ! :thumbup:

... but the hum is still there. Holding the microphone in my hand for example, I can make the hum disappear completely by touching the microphone grill with my other hand, or - mic still in hand - I can touch the audio interface where the headphone jack and the housing of the audio interface meet and the hum will also disappear.

Hmmm... that's very annoying! It implies the mic itself is lacking an earth. The usual reason for that is a broken mic cable.

Is it safe to assume you're using an XLR-XLR cable between mic and interface? And if so, have you tried a different cable?

Also, have you tried a different mic? Does it behave like this with all mics or just that one?

Can I ask where you're based? It may be possible to arrange a visit to sort this out -- always easier in person than remotely!
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Re: AKG P170 - 50 hz humming noise

Postby knopf » Wed Aug 19, 2020 9:24 pm

Hugh, a thousand thanks. - I found the culprit now. I've got two pairs of microphones here: the AKG P170 which started things off and the AKG 420 and it is one of these that I seem to have picked this morning when I did my testing. I've tried them all now about a hundred times and it is only this particular one, the AKG 420 which has got the hum. So I'll return it and many many thanks for all your help.

Cheers,
Martin

PS: Originally from Germany, I have lived here in the UK for 15 years now and it is near Durham where we are based.
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