Audio Static in Cultural Hall

[Hilohi]

We tried to watch a movie in our cultural hall but there was constant audio static coming through the cultural hall speakers. Our setup was a MacBook Pro with an Anker USB hub connected to it. Connected to the USB hub was an external DVD drive, a USB C power cable and an HDMI cable, which was connected to a projector. For audio, I connected a 3.5mm to XLR adapter to my computer's headphone jack, which was connected to an XLR cable that connected to one of the XLR inputs in the cultural hall. All connections were secure so I don't think it was a loose connection issue with any of the cables/adapter. I did some trouble shooting and it appears the issue was caused by the HDMI cable - it was only when I connected the HDMI cable to the USB hub that there would be audio static. To reiterate, we were using the HDMI cable only for video - to connect my computer to the projector - so I don't know why that would affect the audio (we were using my computer's headphone jack for audio). I tried a different USB hub, but had the same audio static issue. I tried connecting a mic to the XLR inputs in the cultural hall to see if the XLR inputs were faulty, but there was no static when I connected the mic. At that point, I thought my setup was to blame, but I went to another room in the building that has an XLR input, connected my setup in that room, and I had no issues with audio static in that room. Also, I tested this setup in the cultural hall a couple of weeks ago and there was audio static then. Any suggestions on how I can fix this audio static issue in the cultural hall when using the aformentioned setup?

[russellhltn]

How are you connecting the headphone jack to the XLR? Headphone outputs are close to line level and the building inputs are are mic level - making them very sensitive.

I'm suspecting the problem isn't the HDMI cable, but connecting the projector. The third pin ground on the projector is creating a ground loop with the XLR cable. If you use the crab box, there's a switch that allows you lift the ground as well as properly attenuating the line level to mic level.

Use the black cased crab box as it has a better sound. If you have a gray one, it can be modified to make it like the black one.

[Hilohi]

How are you connecting the headphone jack to the XLR? Headphone outputs are close to line level and the building inputs are are mic level - making them very sensitive.

I'm suspecting the problem isn't the HDMI cable, but connecting the projector. The third pin ground on the projector is creating a ground loop with the XLR cable. If you use the crab box, there's a switch that allows you lift the ground as well as properly attenuating the line level to mic level.

Use the black cased crab box as it has a better sound. If you have a gray one, it can be modified to make it like the black one.

I'm using a 3.5mm to XLR adapter to connect the headphone jack to the XLR (like this one: https://www.amazon.com/tisino-3-5mm-Unb ... 318bd&th=1). Using this setup, I'm able to control the volume of the cultural hall speakers with my Mac. The funny thing is, this setup has worked in the cultural hall before without any audio static issues, but then sometimes it doesn't work and we get audio static. It's really hit or miss, and I'm hoping I can find a way to always make it hit.

Thanks for sharing your thoughts on this !

[russellhltn]

I'm thinking the issue is that the laptop is connecting your system ground to the third pin power ground. While your laptop may also have a third pin ground, the power brick doesn't always connect the laptop itself to ground.

If I'm right, you'll have problems anytime you connect the projector - even if it's powered off and the cable is not in use. And you'll have the same problem anytime you connect to a system that has one non-laptop part with a three-pin ground.

Here's a test you can do: Set up your system. You'll have noise. Power off the projector. Then unplug the projector from power while still leaving it connected to the HDMI cable. If I'm right, only when you disconnect it from power will the noise go away.

The nice thing about the crab box is that it provides transformer isolation to prevent ground loops.

[Hilohi]

I'm thinking the issue is that the laptop is connecting your system ground to the third pin power ground. While your laptop may also have a third pin ground, the power brick doesn't always connect the laptop itself to ground.

If I'm right, you'll have problems anytime you connect the projector - even if it's powered off and the cable is not in use. And you'll have the same problem anytime you connect to a system that has one non-laptop part with a three-pin ground.

Here's a test you can do: Set up your system. You'll have noise. Power off the projector. Then unplug the projector from power while still leaving it connected to the HDMI cable. If I'm right, only when you disconnect it from power will the noise go away.

The nice thing about the crab box is that it provides transformer isolation to prevent ground loops.

This is very helpful. Thank you! I'll try that.

[rmrichesjr]

Have you tried a "phantom blocker" in the path between the PA system and the 3.5mm-to-XLR adapter? Last I was informed, the PA systems provide phantom power to the microphones. Some unbalanced-to-balanced adapters do a poor job of handling phantom power on the XLR side.

Some months ago, my ward bought a wireless microphone for use during 2nd hour meetings. There was a very loud pop whenever the receiver was plugged into the PA system or unplugged. There was also a fair amount of AC hum/buzz and occasional scratchy noise. A phantom blocker solved all of those symptoms.

[adamberkey]

I believe the trs to xlr adapter you are using is not a transformer but rather a straight adapter. You should be uaing a transformer that adapts the unbalanced impedance to balanced. Curious to me is that you state that the setup works in a different room, when it should not work. Your adapter should look like this:

https://a.co/d/aUojVAS

How are you connecting the headphone jack to the XLR? Headphone outputs are close to line level and the building inputs are are mic level - making them very sensitive.

I'm suspecting the problem isn't the HDMI cable, but connecting the projector. The third pin ground on the projector is creating a ground loop with the XLR cable. If you use the crab box, there's a switch that allows you lift the ground as well as properly attenuating the line level to mic level.

Use the black cased crab box as it has a better sound. If you have a gray one, it can be modified to make it like the black one.

I'm using a 3.5mm to XLR adapter to connect the headphone jack to the XLR (like this one: https://www.amazon.com/tisino-3-5mm-Unb ... 318bd&th=1). Using this setup, I'm able to control the volume of the cultural hall speakers with my Mac. The funny thing is, this setup has worked in the cultural hall before without any audio static issues, but then sometimes it doesn't work and we get audio static. It's really hit or miss, and I'm hoping I can find a way to always make it hit.

Thanks for sharing your thoughts on this !

[russellhltn]

I believe the trs to xlr adapter you are using is not a transformer but rather a straight adapter. You should be uaing a transformer that adapts the unbalanced impedance to balanced. Curious to me is that you state that the setup works in a different room, when it should not work. Your adapter should look like this:

https://a.co/d/aUojVAS

Most inputs are not balanced (think a center tapped transformer) but floating/differential (think isolated transformer winding). As such, you can use a resistor network. However, it doesn't provide any ground isolation. It would work fine for things plugged into the same power strip, but a problem for things being powered from different electrical panels.

Note that the device you linked to won't attenuate the line/headphone output down to mic level so an attenuator will be needed.

I suspect the reason it works in another room is it's being plugged into a different outlet (which might be on a different panel) and the sound system may be in a different rack. Either one changes the dynamics of the suspected ground loop.

[Hilohi]

Thanks, all! I appreciate your input on this topic. I'll try the crab box and then go from there.

Looking back, my computer was on a metal projector stand when I got the audio static, which I believe can cause grounding issues. When I didn't have audio static issues, my computer was either on a wooden or plastic table or on the carpet. I'm not sure if having my computer on a metal surface caused the grounding issues, but that's the only variable that I can think of between when I had audio static issues and when I didn't.

[rmrichesjr]

Another factor I didn't think of until now is phone plugs/jacks, whether 1/4" or 3.5mm, are notorious for getting oxidized and causing crackling sounds with the slightest motion, even motion that is not detectable by ordinary means. In theory, gold-plated should help, but YMMV. One suggestion that might help is to rotate the plug in the jack a little to try to wipe off the oxide and make a better connection.