Hi there

A bit of an electronics novice here, forgive the lack of knowledge. But I'm trying to record my voice to my laptop via a USB-powered audio interface (it has USB cable for both power and data).

A low hum has appeared which gets worse if I touch my mic body, and almost goes away completely when I unplug my laptop power cable. But it's still there, and I'd like to eliminate it. Ideally I'd like to be able to plug my laptop back in so I don't run out of battery while recording.

Briefly, my setup is as follows:

I have a Sennheiser e614 pencil sdc mic, connected via XLR cable to a Behringer UMC202HD audio interface. This connects via USB to my MacBook Pro. I've turned on 48v phantom power as it's a condenser mic.

The same hum happens with a different laptop, and the same hum appears with a different mic (rode nt1a condenser), just slightly less so.

The first thing I want to try is buying a powered USB hub. Apparently if I connect my interface to it, then to my laptop, it might isolate the electrical signal and eliminate the hum.

Is this worth a try or should I try something else first?

Thanks a million!

 

Is the computer on a three wire connection for AC power? (is ground connected)
What other things are connected? printer, powered speakers, ..... I am looking for something that plugs into the wall.
Have you replaced the mic cable? The shielding might be broken.

USB hub is low cost and might work. Can you barrow one from a friend. I have some but the drive is too long. lol I find them at the secondhand store for very little money.

 

Thanks for your response! Yes I'm in the UK so 3-prong plugs - I've disconnected everything else but the audio interface so no other peripherals.

Generally what might it mean when the hum gets louder when I touch the body of the mic?

 

What are you powering the module with to get the 48 volt phantom power? AND, given that the microphone should have a differential output that is not tied to common, how is the input arranged?? Where is the shield on the mic cable connected??
Is it at all tied to any common anywhere??
I see lots of places that hum could be coming from.

 

Hi,

Thanks for replying.

It all comes from one plug socket: my laptop is plugged into that mains socket, then the laptop connects via USB to my audio interface. The interface is powered by that cable, and thus provides phantom power to my mic.

I just have one more cable in the setup which is a balanced 3 pin XLR cable going from mic to audio interface.

 

Does the USB audio interface really have a balanced XLR input? In my experience these are only found on professional equipment.

Link to the interface?

 

Ground Loops
Ground loop noise (50 or 60 Hz hum) is, unfortunately, a common issue in audio systems, particularly in older buildings. A ground loop happens when various audio equipment are plugged into different AC outlets

Diagnosing hum and hiss

I doubt that this is actually a ground loop. The article above covers various causes of hum.

 

Does the USB audio interface really have a balanced XLR input? In my experience these are only found on professional equipment.

Link to the interface?

https://www.behringer.com/product.html?modelCode=0805-AAR

 

In a single plug in assembled system like this, HUM can also be added in any connection carrying an audio signal, or be added by a distorted mains power feed, such as solid state light dimmers. There may even be a hum current in the "green wire" connection to the mains. I have observed that first hand, with a 60 Hz buzz in the sound system that was solved by using an adapter to lift the green wire "ground" connection at the plug.

 

Thanks for your help so far everyone, I'll try a few things and report back. Hopefully this can help others who come across this thread later, so it's much appreciated.

 

https://www.behringer.com/product.html?modelCode=0805-AAR

Yep, that would qualify as what I called professional equipment. It does indeed have XLR inputs. I would susoect either a faulty mic or cable, where the ground has disconnected.

 

If the microphone uses phantom power, then the shield connection needs to be present. BUT inside the XLR connectors, usually, "case ground" is separate from the shield connection. I did have a problem in one installation where the XLR connector shared a double electrical box with a grounded AC outlet. Separating the shield pin from the connector shell connection on both the box side and the cable side solved that hum issue. Evidently there is a bit of mains voltage present on the "green-wire" that is so solidly tied to "ground". Of course, in low-level audio systems all it takes is a very few millivolts.

 

Just getting back in touch about this, I never got to the root of the issue so I returned both the mic and the interface.

I tried 3 laptops with both devices and the hum was there. But tried the mic individually on a friend's desktop computer and no hum. I should have tested that mic PLUS the interface on that same desktop, but I didn't have it with me.

Was it just a faulty interface? Possibly. But then why did I get a hum connecting this mic to my spare interface too?

So was it just a faulty mic? Possibly, but the mic gave zero hum on my friend's desktop computer with his own interface.

Macbook wiring issue? Don't think so: as I mentioned, I tried an older interface and mic with these macbooks and got no hum.

I tried different cables and that didn't change anything.

Very odd.

It just seemed that this specific combination of a) that particular mic with b) that particular interface, to c) any macbook (2015/2017/2022) ...was creating the hum.

So yes, I returned the items and will be trying something else!

Thanks again for your help everyone.

 

One very simple thing to try, which did totally solve the hum issue in a church sound system, is to use a ground connection adapter to break the mains outlet ground connection. That connection creates a path for the ambient mains power electric field to induce a hum voltage relative to any internal common that is somehow connected to that safety ground circuit. The exact path is often complex and hard to follow, but using the adapter solves the problem.
An option would be to use a different computer power module that has only a 2 prong mains plug.

 

One very simple thing to try, which did totally solve the hum issue in a church sound system, is to use a ground connection adapter to break the mains outlet ground connection. That connection creates a path for the ambient mains power electric field to induce a hum voltage relative to any internal common that is somehow connected to that safety ground circuit. The exact path is often complex and hard to follow, but using the adapter solves the problem.
An option would be to use a different computer power module that has only a 2 prong mains plug.

Something like this?

https://amzn.eu/d/0hgJpff2

 

NO, that item at $49.17 is not at all what I was describing. The device I suggested should not cost over $2 (USD) and be available locally, NOT AMAZON.
It is simply a device that fits onto the 3-pin mains plug and has only the two flat blades to pug into the electrical outlet.
IT DOES NOT IN ANY WAY ATTACH TO A USB CONNECTION.
I see the problem as being in that third wire mains connection, as I see that it is bypassing the isolation provided by the external power supply for the computer. I have seen this same problem before.

 

Ah I see.

Isn't that effectively removing the earth entirely though, and therefore the safety?

 

Ah I see.

Isn't that effectively removing the earth entirely though, and therefore the safety?

Certainly it is effectively removing the wired thru the electrical distribution system "ground" connection. That is the whole purpose.
NOW, consider that in the entire setup described there are NO DEVICES WITH MAINS CONNECTIONS, except the separate power supply for the computer, WHICH IS AN ISOLATED output device. Then consider that to be a shock hazard there must be the following conditions: First, the mains voltage must be present within the device, second, thru some internal failure, that internal mains voltage must become connected to an outside conductive surface of that device, and THIRD, there must, for a shock hazard to exist relative to the use of that system, there must be access to some grounded conductive surface close enough so that an individual could make contact with but surfaces at the same time.

This means that if you are using the system within reach of grounded plumbing hardware ( This includes stainless steel sinks,) you could possibly get shocked, or if you are standing barefoot on a bare cement floor resting on moist earth. Or if you are outside doing a gig on a grounded metal stage.

In the vast majority of locations normally used by humans there is no ground connection available to complete the circuit to deliver a shock!!!

But if you are doing this project in a basement barefoot with a wet floor, and if the isolation in the isolated computer power supply fails, and if somehow the mains hot side gets connected to a conductive surface on some part of the system that you are grasping firmly, then there is a possibility that you might receive a shock.
Your odds of winning several million in a big lottery are much greater than that.