Eliminating hiss - how to find ground loop?

[devinbost]

We're having an issue with getting a hiss from our podium mic. We've put in multiple FIRs, and the solution we've been given is to use a "crab box," an EmTech EJ-2 Plus adapter on our mics. (Side note: What's obnoxious about this device, aside from being expensive, is it's an XLR to RCA device, so we always need to adapt it. Additionally, it's a source of operator error because if the knob is turned up too much, it creates other problems.)
My main concern is that it seems like there's a root problem that's not being solved.

I read that a hiss on the line is typically caused by a ground loop in the system. If that's the case, wouldn't there be a more fundamental solution to preventing/eliminating a ground loop? It seems like the offending device needs to be identified, and a ground isolation transformer should be added to the appropriate place.

[Wattsuk]

Although general principles of advice could be shared with you, if you share your particular setup then users may be able to give more specific advice.

There are lots of sound systems in use depending on the age of the system and also the location in the world.

Lack of or a breakdown in communication with FM is typically where these things get stuck.
If occasion requires, you may need to have the Stake President experience the issue and then have him personally escalate it with FM.

For us, we get hiss, feedback and other random issues when the sennheiser wireless microphone receiver is connected. After multiple attempts with FM to resolve our issue (new cables, new receiver, new power transformer) it has been left in limbo and we have to disconnect the power to the wireless receiver unless we really have a need for the wireless mics.

If you are able to, disconnect anything unnecessary in your system that could be causing ground loop or other interference issues.

Do you have handheld dynamic microphones? Do they also hiss when plugged into another XLR socket?
Have you tried a known good podium microphone?

[russellhltn]

I read that a hiss on the line is typically caused by a ground loop in the system.

I disagree. Ground loops cause hum or buzz, not hiss. You're being sent the wrong direction.

Hiss comes from weak signal electronics, such as microphone inputs/preamps or receivers. Normally, it's unnoticeable in the background because the desired signal is strong enough. Much like how you don't notice the chapel's A/C noise because you can hear the speaker.

In my experience, his is caused by:

• poor "gain staging" where something has to turned up too much to make up for a weak signal
• too much electronics in the audio chain, especially microphone inputs/preamps
• A failing device causing excessive noise

Any kind of wireless would be suspect.

Something similar to hiss could be cause by a digital signal leaking into the audio, either though poor shielding or stacking devices too close to each other.

[dave.swartz]

A couple of comments in the hope that they may prove helpful. It's hard to troubleshoot stuff from afar so this is just some general info related to your thread.
1. The crab box inline on the podium mic is the wrong approach. This is an attenuator designed to reduce the levels of line level audio to microphone level audio for injection into a microphone jack. Since the podium mic is a microphone there is no need to attenuate the signal. It also can eliminate ground loops because I believe it uses an audio transformer. With such a transformer there isn't an electrical path between the microphone and the audio processor.
2. The podium microphone is an electrolytic microphone. It needs +48 volt DC phantom power down the microphone cable. The audio processor typically will provide this power to the microphone. Inserting filters such as transformers may block the flow of this DC power. Without this power the podium microphone will not work.
3. I have experienced ground loops on the podium microphone before simply by having the outside of the connector resting against metal conduit inside the podium enclosure. This provided an alternative path down the audio cable for grounding. Please ensure that any metal connectors are not touching any metal "stuff" inside the podium. What you are looking for is a microphone that is isolated from any grounds at the end of a long microphone cable.
4. While I have only seen a few church systems so the population of this informal survey is small in general the audio systems are professionally installed using the proper cable. Ask yourself the following: What has changed from when it did work?
5. This Tech Forum has an entire section related to meetinghouse audio. See if what you are experiencing has already been discussed.

Reach out via the path Stake Technology Specialist -> High Council rep over facilities -> FM group manager. Have the HC rep put in a FIR request. If you can have him confirm what you are hearing so much the better. What you are doing is building a case that what you are experiencing is real and that you have exhausted all first level steps that the STS is authorized to perform.

I really like the EMTech crab box. It's is a great piece of equipment. I am grateful that the church provides this device to all meeting houses. I have used it for many a presentation in the cultural center where we need to take laptop audio and insert it into the cultural hall sound system.

[russellhltn]

The crab box inline on the podium mic is the wrong approach. This is an attenuator designed to reduce the levels of line level audio to microphone level audio for injection into a microphone jack.

However, it would work for connecting the line-level "record out" from the sound system down to mic level. However, it is a XLR output which may create issues depending on what the next piece of equipment is. Laptop "mic inputs" are, uh, "interesting". You have to understand what's being connected and the pinouts.

The crab box has a rocker switch for separating or connecting the grounds.

[devinbost]

I discovered something quite interesting. We disconnected the podium mic, and I used a Behringer UMC 404HD to get a spectrum on the noise coming from the podium mic line. Even when the podium mic is disconnected, we're getting quite a lot of noise.
I attached a screenshot of the spectrum.
We also get noise like this from the organ when the organ is turned on.
However, if I connect a mic directly to the UMC 404HD, there's no noise.

I tried eliminating the noise with the crab box, and I was unable to do so.

Additionally, we do get a buzz in one of the rooms when the volume is turned up all the way in that room. We're not getting that specific noise in the room that's closer to the chapel.

[russellhltn]

I discovered something quite interesting. We disconnected the podium mic, and I used a Behringer UMC 404HD to get a spectrum on the noise coming from the podium mic line. Even when the podium mic is disconnected, we're getting quite a lot of noise.

As a test of your test setup, I'd suggest running an equally long mic line with no mic connected and plug it into the Behringer. Maybe even see if running an external mic line from the podium to the sound rack solves the issue.