[Richard Crowley]

Will there be any issues with the presenter wearing a lav while speaking into another mic (feedback)?
Simply, no. Feedback is caused by a microphone picking up its own sound from the speakers. Additional microphones which are not connected to the venue sound system simply cannot generate feedback in any way.

Unless the presenter is in custody and handcuffed to the lectern, putting a wireless lav on him/her is an excellent idea.

I’m concerned about the levels being far to high.
Recording from the venue sound system is also a good idea as a backup to your own (wireless lav) microphone. It is easy enough to deal with the levels and isolation from the house wiring with a simple isolation transformer and attenuator. I am very fond of the Rolls DB25b It combines the function of transformer isolation (to prevent ground-loop hum both in your system, and in the venue sound system) PLUS a variable attenuator so that you can handle ANY kind of level that may be available from the venue sound system.

https://www.amazon.com/Rolls-DB25-Matchbox-Direct-Ground/dp/B00102XXZG

———————————————————————————
Recording audio without metering and monitoring is exactly like framing and focusing without looking at the viewfinder.

[Richard Crowley]

Using a DVI cable with ferrite chokes on the end(s) certainly won’t hurt anything. But it MAY or may NOT solve YOUR issue. You don’t know until you try it.

We don’t know the quality or construction of the cable you are using for the audio signal (and presumably you don’t know, either). Certainly getting a good-quality cable will protect the audio signal from your computer to your speakers. My favorite quote about cables: “A $200 cable sounds better than a $2 cable. But not better than a $5 cable.”

The distance between the speakers is not the issue. The distance between the speakers and the offending monitor IS the issue. While using the same cables, can you move the monitor far to one side, and BOTH of the speakers far to the opposite side? Does that diminish the noise?

———————————————————————————
Recording audio without metering and monitoring is exactly like framing and focusing without looking at the viewfinder.

[Richard Crowley]

There are test procedures for determining whether the interference is being conducted into the speakers through one of the three major routes:

[Richard Crowley]

Please explain what “generally” means here: “Everything is generally plugged into the same circuit ” Either it is or it isn’t. I believe you are on the right track to question the powering scheme. I would be 100.000% sure that everything is plugged into the SAME power circuit (including ground). Even if you have to go out and buy a larger power strip to accommodate everything.

You may need to use an EXTERNAL audio interface, i.e. something where the digital audio comes out through USB and is converted to analog OUTSIDE the computer box.

———————————————————————————
Recording audio without metering and monitoring is exactly like framing and focusing without looking at the viewfinder.

[Richard Crowley]

I went to https://www.sounddogs.com/ and did a search on: scrape -metal
And it returned 4578 samples to choose from.
You could probably come up with a better search term.

———————————————————————————
Recording audio without metering and monitoring is exactly like framing and focusing without looking at the viewfinder.

[Richard Crowley]

Sorry, it is “Sennheiser CL2 1/8 inch to FXLR Cable for Sennheiser EW Series Bodypack Transmitter”

https://www.sweetwater.com/store/detail/CL2Cable?adpos=1t1&creative=156048634519

———————————————————————————
Recording audio without metering and monitoring is exactly like framing and focusing without looking at the viewfinder.

[Richard Crowley]

You should have an adapter cable in your kit which allows you to connect your G3 transmitter to the venue sound system. That way you can transmit the house audio directly to your camera no matter where you are. Sennheiser sells a cable called “G2” which takes XLR line-level output (as from the venue sound system) and plugs into your G3 body-pack transmitter. Appears to be street-price around US$20

If “around a dinner table” actually means a ROUND table, then putting a microphone in the center (perhaps clipped to the centerpiece decoration) may be a viable alternative for picking up casual sound from ad-hoc dinner conversation. Although, unless they are in an isolated banquet room or something, the ambient noise may be too much for such a distant micing technique. Getting good audio coverage from a group of people is always difficult and expensive. Alas, you may not have the resources (gear, budget, time, personnel) to do a really good job.

———————————————————————————
Recording audio without metering and monitoring is exactly like framing and focusing without looking at the viewfinder.

[Richard Crowley]

So if you get the same noise with a different microphone and a different transmitter, that strongly suggests that the receiver is broken.

But that doesn’t correlate very well with your statement “picks up way too much background noise”. Only the microphone itself can “pick up background noise”. And you said that you used a different microphone with the same symptoms. So either you have two broken microphones, or else what you are hearing is not “background noise” but “electronic noise” from the transmitter or receiver.

———————————————————————————
Recording audio without metering and monitoring is exactly like framing and focusing without looking at the viewfinder.

[Richard Crowley]

Have you checked all (ALL!) the transmitter and receiver settings to confirm that they are configured identically. The channel/frequency is only ONE of the many user settings. And assuming you have everything in the audio system “gain staged” properly (since you say you have two kits that are operating properly).

Since you have two working examples, you have the ideal opportunity to swap different parts of the kit. For example use the microphone from one of the good kits on the bad transmitter. Does that change anything?
Or use the receiver from one of the good kits with the bad transmitter. Does that change anything?
Doing that kind of “differential diagnosis” will help you narrow down the exact location of the problem in the chain.

———————————————————————————
Recording audio without metering and monitoring is exactly like framing and focusing without looking at the viewfinder.

[Richard Crowley]

It is unfortunate that neither the Rodelink receiver nor your Canon 6D camera have audio monitoring outputs.

Yes, you could use a “Y-splitter” to take the receiver output and send one branch to the camera, and the other to your headphones/earbuds or whatever. Note, however there is a risk of affecting the signal by connecting your headphones across it. The (unidentified) headphones may have a load impedance low enough that it would affect the audio level or possible introduce distortion.

It would be worth getting a “Y-splitter” and trying it because there is a resonable expectation that it would work OK. But most certainly you should experiment with the lash-up BEFORE actually using it for anything important.
1) Insert the “Y-splitter” between the receiver and the camera with nothing else connected and confirm that the performance is the same as with a simple cable.
2) While recording with your camera, plug in the headphones and note if there is any difference (drop) in audio level.
3) Also note whether there are any other artifacts like noise, distortion, etc.
4) Re-calibrate the system for proper audio levels and note if there if a difference in level using the “Y-splitter” and headphones.

Even worse is the problem that there is no audio monitoring option in the camera. That means that you can’t actually monitor the audio that you are recording. That uncertainty is one of the reasons that recording “double-system” (on a separate audio recorder) is such a popular option.

———————————————————————————
Recording audio without metering and monitoring is exactly like framing and focusing without looking at the viewfinder.