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Author Topic: Ground loop hum  (Read 1114 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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Re: Ground loop hum
« Reply #9 on: October 16, 2020, 06:54:19 am »


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