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Author Topic: Ground loop hum  (Read 1117 times)

Curtis McGill

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Ground loop hum
« on: October 15, 2020, 02:03:53 pm »

I'm trying to eliminate a hum in our sound system. I'm fairly certain our projector - up on the ceiling, out of reach - is on the same circuit that the desktop is plugged in to. When I plug the VGA from the projector into the HDMI to VGA adapter and then connect it to the desktop it begins to hum. I've tried removing everything else on the receptacles I can reach and it doesn't help.

The hum is carried through the USB audio interface that is connected to the board. When it is muted the hum goes away.

Other than running another circuit do I need some sort of hum eliminator? Are there other options?
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Rob Spence

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Re: Ground loop hum
« Reply #1 on: October 15, 2020, 04:28:29 pm »

Perhaps a pair of HDMI to Cat cable & back would isolate it?
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Art Welter

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Re: Ground loop hum
« Reply #2 on: October 15, 2020, 05:22:38 pm »

When I plug the VGA from the projector into the HDMI to VGA adapter and then connect it to the desktop it begins to hum.
The hum is carried through the USB audio interface that is connected to the board. When it is muted the hum goes away.

Other than running another circuit do I need some sort of hum eliminator? Are there other options?
Running another circuit won't eliminate a ground loop.

Does the hum level reduce when the USB audio interface level is reduced, rather than muted?
Have you verified that the audio source feeding the USB audio interface is turned up to maximum, or at least a fairly high level?

If not, you may simply have too little signal, requiring lots of make up gain, making the noise (hum) more apparent because the signal to noise ratio is poor.

Art





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Curtis McGill

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Re: Ground loop hum
« Reply #3 on: October 15, 2020, 05:35:57 pm »

Running another circuit won't eliminate a ground loop.

Does the hum level reduce when the USB audio interface level is reduced, rather than muted?
Have you verified that the audio source feeding the USB audio interface is turned up to maximum, or at least a fairly high level?

If not, you may simply have too little signal, requiring lots of make up gain, making the noise (hum) more apparent because the signal to noise ratio is poor.

Art

The audio source is iTunes at full volume. The desktop volume is at 92. The Scarlett 2i2 is at 9:00. The Scarlett is connected to the desktop from USB C to USB 3.0. The X32 board is connected to the Scarlett via XLR to XLR. The desktop has dedicated NVIDIA graphics. The DVI port on the back of the desktop connects to the monitor. The HDMI port is what I am using to connect to the projector's VGA cable via an HDMI to VGA adapter. The HDMI audio is turn off (but, it does want to reset and turn on again regularly). When the adapter is not plugged in to the HDMI port there is no hum.

I will mess with the volume settings on the Scarlett tonight.
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Mike Caldwell

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Re: Ground loop hum
« Reply #4 on: October 15, 2020, 06:58:19 pm »

It sounds like your using the Scarlett for an output interface to feed into the mixer.

The Scarlett has 1/4 inch balanced output jacks, check that the cable from the 1/4 inch to XLR is correctly wired.


Try lifting Pin1 on the XLR connector.
Check that the cable does not have Pin1 tied to the shell of the connector.

Try lifting Pin1 on the  XLR connector at the USB interface end.

You may need transformer isolation between the USB interface and the mixer, such as a direct box.

Curtis McGill

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Re: Ground loop hum
« Reply #5 on: October 15, 2020, 07:15:20 pm »

It sounds like your using the Scarlett for an output interface to feed into the mixer.

The Scarlett has 1/4 inch balanced output jacks, check that the cable from the 1/4 inch to XLR is correctly wired.


Try lifting Pin1 on the XLR connector.
Check that the cable does not have Pin1 tied to the shell of the connector.

Try lifting Pin1 on the  XLR connector at the USB interface end.



You may need transformer isolation between the USB interface and the mixer, such as a direct box.

Sound is coming to the Scarlett from a bus feed. We don't have any problems from the XLR Bus feed.

Sound is going to the X32 from a single 1/4"  TRS (L out) on the Scarlett to dual TRS (Aux 3/4) on the X32. Aux 3/4 are set up for stereo but we don't run our sound in stereo. I have a dual TRS to dual TRS coming soon.

Lifting pins, etc. - I'm outside of my realm for what to do there.

Are you suggesting I put a DI between the Scarlett TRS and the Aux 3/4 on the X32?
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Mike Caldwell

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Re: Ground loop hum
« Reply #6 on: October 15, 2020, 07:37:40 pm »


Are you suggesting I put a DI between the Scarlett TRS and the Aux 3/4 on the X32?

Yes but I thought you were going into an XLR input on the mixer.
The DI could still work depending on how much gain you have on the Aux input, the DI
will be outputting high mic level signal. You will need to get a XLR to 1/4 inch TRS cable
to try it.

Curtis McGill

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Re: Ground loop hum
« Reply #7 on: October 15, 2020, 08:03:09 pm »

Yes but I thought you were going into an XLR input on the mixer.
The DI could still work depending on how much gain you have on the Aux input, the DI
will be outputting high mic level signal. You will need to get a XLR to 1/4 inch TRS cable
to try it.

So I took L TRS out of the Scarlett to dual TRS on an Art Dual Transformer/Isolator. On the DTI I took dual TRS out to Aux 3 (single TRS) and it worked. Ugly, but it worked. Now, why/how did it work and what do I need to do to clean up the chain?
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Mike Caldwell

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Re: Ground loop hum
« Reply #8 on: October 15, 2020, 08:33:03 pm »

So I took L TRS out of the Scarlett to dual TRS on an Art Dual Transformer/Isolator. On the DTI I took dual TRS out to Aux 3 (single TRS) and it worked. Ugly, but it worked. Now, why/how did it work and what do I need to do to clean up the chain?

Perfect little box for the job...and you had one laying around!

I guess the Scarlett does not have any isolation on the outputs.
The Scarlett outputs can be set to mono, you only need to use one channel or the other of the DTI.

The transformers in the DTI completely break the physical audio connection between the USB interface and the mixer.
It's not uncommon to need transformer isolation between video systems and sound systems to take care of grounding and noise issues. There can be differences in the grounding paths between the sound system and all the points that components in a video system all getting power from different sources.

Keith Broughton

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Re: Ground loop hum
« Reply #9 on: October 16, 2020, 06:54:19 am »

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Curtis McGill

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Re: Ground loop hum
« Reply #10 on: October 16, 2020, 07:49:17 am »

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Mike Caldwell

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Re: Ground loop hum
« Reply #11 on: October 16, 2020, 08:09:54 am »

What does this do and where would I put it in the chain?

I was going to come back and mention that same piece.

It would replace the Scarlett, USB from the computer, to the Peavey interface, Peavey output to the mixer input.
The outputs are XLR so you would need XLR to 1/4inch TRS cables.

The Peavey has transformer isolated outputs and can be set for mono or stereo operation.

Curtis McGill

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Re: Ground loop hum
« Reply #12 on: October 16, 2020, 08:47:31 am »

I was going to come back and mention that same piece.

It would replace the Scarlett, USB from the computer, to the Peavey interface, Peavey output to the mixer input.
The outputs are XLR so you would need XLR to 1/4inch TRS cables.

The Peavey has transformer isolated outputs and can be set for mono or stereo operation.

Thank you to everyone for helping me and tolerating my questions! I am learning a lot during this season as we prepare our church to best serve the needs of our community.

The Scarlett serves 2 purposes right now. First, it is a feed from the X32 to the computer for our live stream (Bus feed). Second, it is a feed from our computer to an Aux send for audio into the X32 from the computer (music playback and the like). Would a USB device such as the Peavey serve both functions or would it just be a separate and improved way to run audio from the computer?

For our live stream we take audio from the X32 and video from our camera (Vixia HF G50) using an Elgato HD60 S.
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Mike Caldwell

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Re: Ground loop hum
« Reply #13 on: October 16, 2020, 08:52:16 am »

In that case the Scarlett would still be input for the live stream audio and the Peavey would be the output.

Or you could just keep using the ART DTI for the output isolation.

Kevin Maxwell

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Re: Ground loop hum
« Reply #14 on: October 16, 2020, 09:13:44 am »

Thank you to everyone for helping me and tolerating my questions! I am learning a lot during this season as we prepare our church to best serve the needs of our community.

The Scarlett serves 2 purposes right now. First, it is a feed from the X32 to the computer for our live stream (Bus feed). Second, it is a feed from our computer to an Aux send for audio into the X32 from the computer (music playback and the like). Would a USB device such as the Peavey serve both functions or would it just be a separate and improved way to run audio from the computer?

For our live stream we take audio from the X32 and video from our camera (Vixia HF G50) using an Elgato HD60 S.

I am sure that I am missing something in your setup but have you tried just using the USB in/out on the X32 into the USB on the computer?
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Curtis McGill

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Re: Ground loop hum
« Reply #15 on: October 16, 2020, 10:15:11 am »

I am sure that I am missing something in your setup but have you tried just using the USB in/out on the X32 into the USB on the computer?

Now that you say it... no, to be honest, it didn't cross my mind because the desktop is a new addition to stream the sound from the X32. Before that it was a borrowed laptop. Then I got the wild hare of having ProPresenter ready to run on the desktop "in case." That's when the sound issue was introduced.

For some reason, routing on the X32 isn't intuitive to me yet. Can you point me towards a resource for how to route the X32 to the desktop for audio in? USB in/out might actually be the best idea because my next step is to set up a virtual soundcheck so I can train myself and others on sound.

I need a course that I can learn from online for how to do all of this stuff! Recommendations?
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Keith Broughton

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Re: Ground loop hum
« Reply #16 on: October 16, 2020, 11:33:56 am »

What does this do and where would I put it in the chain?
This would go on the USB of the computer and give you an XLR isolated (no hum) audio out to go to the console.
If you want to go into a 1/4 TRS on the console, you would need XLR to TRS adapter cables.
Clean and simple.
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Curtis McGill

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Re: Ground loop hum
« Reply #17 on: October 16, 2020, 01:21:00 pm »

This would go on the USB of the computer and give you an XLR isolated (no hum) audio out to go to the console.
If you want to go into a 1/4 TRS on the console, you would need XLR to TRS adapter cables.
Clean and simple.

How do I route the USB in on the X32? Does it already go to a specific channel? Do I pick an open channel? This is where I get lost.
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Keith Broughton

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Re: Ground loop hum
« Reply #18 on: October 16, 2020, 01:42:13 pm »

How do I route the USB in on the X32? Does it already go to a specific channel? Do I pick an open channel? This is where I get lost.
You are using the USB out on the computer to the USB in on the Peavey box and converting to analogue audio to go out into the X32 via the XLR out.
This replaces what you have and solves the hum.

The USB on the X32 can be used with a thumb drive to record or play music, connect to a computer for multi track recording or load and save setup files, but that's another thing completely.
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Curtis McGill

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Re: Ground loop hum
« Reply #19 on: October 17, 2020, 06:45:15 am »

I am sure that I am missing something in your setup but have you tried just using the USB in/out on the X32 into the USB on the computer?

Is there a way to set the USB in/out where it just uses a single Aux or do I automatically use 2 when I route through the Aux remap?

Is there a better way to route it to a channel on the board?

I havenít messed with it enough to understand or be comfortable with the routing process.
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Kevin Maxwell

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Re: Ground loop hum
« Reply #20 on: October 17, 2020, 09:55:59 am »

Is there a way to set the USB in/out where it just uses a single Aux or do I automatically use 2 when I route through the Aux remap?

Is there a better way to route it to a channel on the board?

I havenít messed with it enough to understand or be comfortable with the routing process.

I was hoping that someone with more experience using the USB hooked up to a computer with the X32 would answer.

A couple of years ago we were doing Mamma Mia and the pit involved playback from a computer, besides some other instruments. I had 2 Midas M32 mixers at Front of House mix position (FOH) and one X32 in the pit. We used the X32 as a stage box and a Monitor mixer. We also let the audio for video person (someone I have known for years) take 2 Mix Bus sends and control the mix of them using a Mac running X32 edit. I also had a network switch hooked up to the X32 with one Cat5 going to the audio for video and one Cat5 going to a PC in front of me. I think I used the same dumb switch that the wireless racks in the pit were hooked up to so I could monitor them at FOH. This way if I had to make any adjustments to the pits hard wired IEMs or anything else on the X32 I could. I am leaving out a lot of other details that donít directly relate to the question.       

We were given a bunch of things in the pit (given to the conductor) to get the multi-track playback out of the computer. The pit also consisted of 3 keyboards each playing thru computers with so many different patches that I could never tell by what I was hearing what keyboard was playing what part at what time. The stuff we were given included a USB interface (from the computer to analog out to us) that a very experienced person with the Mac recording and playback could never get to work properly with the computer. One of the problems seemed to be the DAW software on the Mac wasnít the Pro version as it was when this setup was used before. This setup supposedly had been used for a different production of Mamma Mia before with the same conductor/keyboard player given to her and all setup to work.

We then tried to just hook up the USB from the Mac directly into the X32 and on the X32 I think it is in the SETUP - Card tab, when hooked up together we had the in/out choice. We selected 8x8 and setup what tracks we wanted to go where and it worked.

I just searched and there are all sorts of videos on how to interface the USB on a computer to the USB card in the X32. That would probably help you more than I can. Itís not that hard and I could probably figure it out very quickly if I had it all in front of me. But I donít have that at the moment.           
       
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Re: Ground loop hum
¬ę Reply #20 on: October 17, 2020, 09:55:59 am ¬Ľ


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