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Microphone hum

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Microphone hum

PostPosted: Wed Aug 05, 2020 1:43 pm
by Scouser
I have a Rode NT3 which seems to have a hum, which goes away when I touch the mic ?

Any Ideas ?

Re: Microphone hum

PostPosted: Wed Aug 05, 2020 1:46 pm
by blinddrew
Sounds like an earthing issue. What's it plugged into and what else is plugged into the rest of the system?

Re: Microphone hum

PostPosted: Wed Aug 05, 2020 2:24 pm
by James Perrett
Have you tried a different mic cable? As Drew says, it sounds like an earthing problem and the mic cable would be the first thing to check.

Re: Microphone hum

PostPosted: Wed Aug 05, 2020 2:43 pm
by Hugh Robjohns
Agreed. Definitely an earthing problem.

It is worth checking the cable, although if you're running the NT3 on phantom power, the cable screen must be intact since it provides the return path for the phantom supply.

I suspect this will turn out to be another of those earth-less systems -- battery-powered laptop, bus-powered interface... etc etc... nothing properly grounded in the system.

So please let us know exactly what is connected to what in the system, and how they are powered.

Re: Microphone hum

PostPosted: Thu Aug 06, 2020 10:19 am
by Scouser
I am plugged in to a Mix Pre 3M which is being powered by the mains. I tried it on my KM184 and noise goes away ?

Re: Microphone hum

PostPosted: Thu Aug 06, 2020 10:46 am
by Hugh Robjohns
I can't remember, but I suspect the mains power unit for the MixPre is a class-2 (double insulated type -- look for the box-within-a-box symbol on the line lump power unit). If that's the case, then you have a floating system -- by design.

Well designed equipment using balanced connections shouldn't have a problem working in a ground-less system. Planes manage it, after all... :lol:

But not all equipment is designed as well as it should be and I suspect the NT3 has a bit of an internal grounding problem -- to be fair, it is very easy to engineer a situation where the connection between mic's outer metal shell and the XLR pin 1 acts as an aerial to RF and I am aware of a couple of other Rode mics that have had this problem. In a fully grounded system -- which is most of them -- it's not an issue, but in a ground-less system external RFI can get into the audio electronics of the mic and thus becomes audible.

This is confirmed by swapping to the Neumann mic and the problem going away -- Neumann's long history serving European broadcasters means they know very well how to engineer to exlude RFI!

Re: Microphone hum

PostPosted: Sun Aug 09, 2020 4:34 am
by Guest
cool so all you gotta do is go to a magic store, buy a finger and tape the finger to the mic, problem solved.

Re: Microphone hum

PostPosted: Thu Sep 17, 2020 9:39 am
by Scouser
Ok, so it's been a while since I last posted,

The noise is actually there on the KM184 after all. So I'm guessing this is some kind of grounding issue, can anyone give me some possible solutions so I might eliminate this problem.

Re: Microphone hum

PostPosted: Thu Sep 17, 2020 11:33 am
by Hugh Robjohns
Scouser wrote:The noise is actually there on the KM184 after all. So I'm guessing this is some kind of grounding issue, can anyone give me some possible solutions so I might eliminate this problem.

It's always 'some kind of grounding issue'! :lol:

In general, systems need to be grounded, but grounded in only one place... and the challenge is to work through the equipment and wiring to establish a coherent and practical grounding solution.

I can't give you solutions because I don't know what's in your system or how it's wired up... but i can give you some suggestions and techniques to find and solve any problems.

The first is to make sure everything is powered from a single wall socket via a star arrangement of mains distribution boards. And make sure the most power-hungry devices are plugged into the sockets closest to the wall socket. That minimises the potential ground-loop area.

Also, make a note of which items of equipment are class-1 (with a mains safety earth connection) and which are class-2 (double insulated, and ground-less), as you'll need t o know when it comes to solving problems later.

Ground-loop noises can only exist between Class-1 devices. Ground-less noises (which can appear very similar) can only exist in systems comprising all class-2 equipment!

Next disconnect everything and then work through methodically reconnecting each item, listening for unwanted hims/buzzes as you go -- and if you find any, resolve them before connecting anything else.

Use balanced connections everywhere if possible, and treat any unbalanced connections with suspicion as they will almost always require some special attention!

I'd start with the core system of computer, interface and monitor speakers. Make sure that's quiet and hum-free first. Then add other items, checking as you go until you have the full system again.

Re: Microphone hum

PostPosted: Thu Sep 17, 2020 12:20 pm
by blinddrew
Is it worth copying this ^^^ post into a new thread on the useful info forum?
Then we can just point people at it as a starter in the future.

Re: Microphone hum

PostPosted: Thu Sep 17, 2020 12:33 pm
by Hugh Robjohns
blinddrew wrote:Is it worth copying this ^^^ post into a new thread on the useful info forum?

Good thinking ... done!

Re: Microphone hum

PostPosted: Fri Sep 18, 2020 11:34 am
by Scouser
Thanks for all your help with this Hugh ( and everyone else )

Whilst I am not very technically minded, I am doing my best to try and understand all your advice.

Essentially, All I am using is the mix pre 3m into a wall socket and a microphone, so It is a very simple set up and therefore shouldn't pose to many problems, to troubleshoot.

So I have started again, from square one. The Mix Pre is plugged into a wall socket as mentioned above, the headphone amp on these things has lots of gain, and if I turn it up to 100% there is some white noise, but I am guessing that is normal ? This obviously changes as soon as you plug a mic in.

Oddly enough I am back to where I was when I first posted, ie noise only from the Rode NT3, I can't get any noise whatsoever from the KM184 ? Which is a little odd, as a while back I was getting a little noise from it, I can only put this down to a dodgy lead, which is now binned, and it is working fine with all other leads. I also have a SM7B which again is not producing any earthy noise. Could it just be the NT3 ?

Have just edited this post, as I just tried the Mix Pre on battery power and noise is gone !

I have also tried plugging unit into all different sockets around the house and get hum wherever I plug in. So is there anything can I do, given I am not an electrician ?

Re: Microphone hum

PostPosted: Fri Sep 18, 2020 3:59 pm
by Mike Stranks
A qualified response until Hugh arrives again... :lol:

I suspect that you have a floating-earth problem as Hugh suggested a few posts back.

If that is the case then one of these: https://www.soundonsound.com/reviews/groundology-earth-connection-plug should probably solve it.

I've had various funnies like you're experiencing in the past with some kit. Having read Hugh's review of the item referenced above I bought one and it did the trick nicely.

Probably, in your situation I'd use the ring of a 3.5mm plug and attach it to an appropraite socket on the MixPre.

Re: Microphone hum

PostPosted: Fri Sep 18, 2020 4:55 pm
by Scouser
Thanks Mike,

So are you saying I should use one of these to power the mix pre ? It is powered via usb c so would I need a usb c connector coming from the groundology plug ?

Re: Microphone hum

PostPosted: Fri Sep 18, 2020 5:19 pm
by Sam Spoons
You need to power the mix pre in your normal manner, the Groundology plug doesn't provide power, it is a 'dummy' plug with just an earth connection (but none to the line or neutral) so that you can earth Class 2 or battery powered equipment, via a spare jack socket, to the mains earth.