Ground loop noise 2nd hour

[markstewart]

I live in an older skewing ward and do we zoom 2nd hour meetings. In the cultural hall we use a 2 mic wireless setup for the audience and a wall connection for the teacher.

I’ve shifted to feeding both set of mics into an M-AUDIO usb sound system. The main issue is severe hum when my laptop is plugged into wall power and lesser hum when it is not. Gives me battery anxiety to be unplugged and have had some issues where the laptop wasn’t fully charged when I arrived, forcing me to live with the hum.

Any tips on breaking the loop while still using wall power?

Also as noted still get some hum regardless, assume it comes from the wireless mic receivers wall power, so looking for anything that could vanish it entirely.

[russellhltn]

Not recommended for safety reasons, but if you use a 3 to 2 prong power adapter, that will break the loop.

You're going to have to test things by eliminating one part at a time until you find out where the remaining hum is coming from. There's a possibility it's coming from placing equipment too close to each other such that an audio transformer in one is picking up the magnetic field from the power transformer of another.

[rmrichesjr]

In some situations, an audio isolation transformer can be helpful. They can be found in enclosures with RCA or XLR connectors. For other situations or for reduced cost, you could get a plain component and add an enclosure and connectors. If using a plain component and plan to have audio other than just human voice, make sure to get a transformer with sufficient high frequency capability.

[russellhltn]

If your mixer has a line out, a "crab box" (that should be in the meetinghouse somewhere) can be used to adapt that to xlr mic level and provide isolation. A bit clunky for weekly use, but should be a good tool to tell if you're on the right track.

[markstewart]

Not recommended for safety reasons, but if you use a 3 to 2 prong power adapter, that will break the loop.

You're going to have to test things by eliminating one part at a time until you find out where the remaining hum is coming from. There's a possibility it's coming from placing equipment too close to each other such that an audio transformer in one is picking up the magnetic field from the power transformer of another.

Safety wise, as long as I’m not looking at a thunderstorm, no actual particular danger, correct?

[rmrichesjr]

Safety wise, as long as I’m not looking at a thunderstorm, no actual particular danger, correct?

Not entirely.

If there was an internal conductive (or reactive) path inside any of the pieces of equipment between power and chassis ground, you could have potentially dangerous voltage between the chassis of two pieces of equipment. Over 50 years ago, I was acquainted with a phonograph where the chassis was not isolated from power. Touching the metal parts of the phonograph housing and a metal kitchen counter at the same time would produce an unpleasant shock. Almost 50 years ago, I was acquainted with a piece of consumer audio equipment with a 470k resistor from the antenna input to one side of the power cord. This was before polarized plugs in the US. Depending on which way the power plug was inserted into the outlet, touching the long-wire antenna and a metal window casing could feel like a bee sting.

Another (although unlikely) near-worst-case scenario, _IF_ there were an internal conductive path inside any of the equipment, it could possibly produce substantial AC current in the audio cables that could cause the cables to heat up, possibly dangerously so.

If there were that level of ground fault inside any of the equipment, you'd have more than a noise in the audio. However, Murphy says such a ground fault could spontaneously develop at the worst possible time, etc.

An audio isolation transformer is a much safer idea.

[russellhltn]

Safety wise, as long as I’m not looking at a thunderstorm, no actual particular danger, correct?

rmrichesjr is correct. The danger would be if the laptop's power supply failed in a way to apply power line voltage to the ground connection. For example, if the insulation between the primary and secondary of the laptop's power transformer fails.

You didn't say where the hum was heard. I'm going to guess it was coming out of the speakers in the room. In that case, you'd want transformer isolation on the mic feed to the sound system.

If it was being heard on zoom (and not being picked up by the mics), it would be interesting to find out where the ground loop is.

[markstewart]

Safety wise, as long as I’m not looking at a thunderstorm, no actual particular danger, correct?

rmrichesjr is correct. The danger would be if the laptop's power supply failed in a way to apply power line voltage to the ground connection. For example, if the insulation between the primary and secondary of the laptop's power transformer fails.

You didn't say where the hum was heard. I'm going to guess it was coming out of the speakers in the room. In that case, you'd want transformer isolation on the mic feed to the sound system.

If it was being heard on zoom (and not being picked up by the mics), it would be interesting to find out where the ground loop is.

I picked up a Pyle PHE400 hum eliminator, hopefully it will kill the hum if both mics are plugged into it before my usb box.

Right now you are correct the noise is over the room speakers and not heard on the on cast.

[russellhltn]

I picked up a Pyle PHE400 hum eliminator, hopefully it will kill the hum if both mics are plugged into it before my usb box.

I doubt if that will help. I think the loop is between your laptop's ground and the sound system ground. You need something between the mixer and the sound system.

[drepouille]

A year or two ago, the FM group gave our ward a wireless receiver with two handheld microphones. It was connected to the pulpit using a mono audio cable, not the XLR connection. From Day One, this caused a low level 60Hz hum, so we always unplugged it from the pulpit when it was not in use.

Last month, a contractor replaced the entire sound system. Now when I connect this wireless receiver into the mono audio on the pulpit, there is a very loud 60Hz hum, making it unusable.

I will try using the crab box to isolate the receiver. I will also try using an XLR cable instead of the mono audio cable.

Thanks for the ideas.