The Laptop sounds suspicious...

Have you tried removing the laptop power supply and running the laptop using only its internal battery?

- Yes, no difference.

Have you tried removing the laptop from the system?

- Then we don't have a system. It's on the laptop that the noise is evident.

 

- Then we don't have a system. It's on the laptop that the noise is evident.

What do you mean?
Can you not substitute a different audio source in place of the laptop?

You might need a "hum/noise eliminator" in the audio line from the laptop to the rest of the system.
You can buy these. But I wanted to be sure the noise/hum source is through the laptop because its the only device that doesn't have ground connected the same as the other devices, yet its connections are common with other devices.

 

Probably a correctly wired "bridge box" transformer would olve the problem. That would have a well isolated tranformer with the secondary feeding a balanced input, and the center tap of that secondary connected to the hield of that lead. The sound signal from the laptop would be fed to the transformer primary terminals.

 

What do you mean?
Can you not substitute a different audio source in place of the laptop?

The laptop isn't the audio source - on the contrary, it takes input from the sound system for streaming via Zoom.

 

The laptop isn't the audio source - on the contrary, it takes input from the sound system for streaming via Zoom.

Are you saying that the splitter is an audio input source to the laptop?

 

Are you saying that the splitter is an audio input source to the laptop?

It's both input and output, as indicated by the arrows. The laptop has a combined headset socket rather than separate headphone and mic sockets. The splitter splits them out. That way, not only can people on Zoom hear everything coming through the church mics, but we can un-mute people on Zoom and their voices can be heard by the local congregation and their images seen if the Zoom window is moved across to the projector. So the home and in-church congregations can be completly integrated and can participate on equal terms.

 

OK, it sounds a bit like more isolation transformer between both laptop input and system as well a laptop output to the system. And still, lifting that green-wire AC ground completely, at least for an experiment, with nobody else touching anything. That way you can gain knowledge without any safety hazard to anybody. THAT will provide more information.
The big question is about the nature of the noise: is it at mains frequency, or at the video frequency? And it might be that you will need a wireless video link to the projector. Some times that is what it takes.

 

"The big question is about the nature of the noise: is it at mains frequency, or at the video frequency?"
Good question. AND/OR is it also a buzzing type sound.
There are also noise eliminators ( I think Berhinger sells them) that have audio I/O filtering.

 

If the video projector is found to be a source of the noise, there are USB plugin sticks to allow isolation with a wireless link.

 

You can also buy a USB isolator. It does not have to be wireless.

 

In the xlr cable is ground actually connected to earth on one side?

 

You can also buy a USB isolator. It does not have to be wireless.

USB isolator won't isolate a VGA cable to my knowledge.

 

In the xlr cable is ground actually connected to earth on one side?

I did once experience a hum introduction caused by exactly that. The XLR shield tied to the connector body mounted in a steel box grounded to the electrical system by a round-about path. It was quite a subtle connection.

AND a USB isolator is not in any way, shape, or form, going to affect a VGA cable connection. If it is a long VGA cable it is even less likely to even be ableto do it with a wired USB connection, which does have length limitations.

 

In the xlr cable is ground actually connected to earth on one side?

Yes. Checked early on.

 

That earth ground connection of the XLR connector was a hum source at one point in one wide-spread instalation that I had a problem with. Removing that ground connection removed the hum.

 

The acid test will come with the Sunday service tomorrow, but it appears to have been an earthing problem. (Please make sure you've understood my schematic posted on Thur 1st Jul before comenting on this post.)

The clue came when I disconnected the VGA cable from the laptop and measured 0.25V DC between the shells of the plug and the socket. That implied a current (probably noisy) was flowing from the VGA/CAT5 converters to ground via the mic input to the laptop, which comes from the sound system. It also appears that the sound system and the projector are fed through different fuseboxes, possibly even from different phases of the mains - though that would be worrying!

I fitted a solid earth between the VGA shield on the converter at the laptop end and the mains ground at a mains socket feeding the sound system in order to divert this current by the shortest possible path to ground. The noise is gone. (Multiple fingers, toes, and anything else crossable, crossed.)

 

GOOD! and fortunate that you were able to diagnose the problem. A wireless USB video link woud also solve the problem but adequate grounding connections, in this case, also fixed them.
Voltage differences in potentials in that green-wire ground can cause al sorts of noise problems, that is certain. And it is still all within the safety limits, not a shock hazard at all.

 

The laptop probably does not have an earth ground. I do better with a power brick that is 3-wire (ground) but for other reasons like static, I haven't checked to see if the laptop is actually grounded and can't now.

I had like what appears to be lockups due to static and a chair.

For some dumb reason, the pins in the power cord socket pushed into the adapter. A never used spare adapter did the same thing immediately - this time, just the ground pin.

Ground loops are not fun. I designed an industrial system and I missed one.

I converted a six-device, in-house made process readout box so it could be computer controlled and messed up.

The first thing I did was made it easy to calibrate. You flip a switch and you put +5V or full scale to the meter. Second, I put a panel mounted 10T screwdriver adjust trimmer in the front and third, DIP swithes to set the decimal place. That only made the thing easier to use.

The box consisted of 6 voltmeters (measure a 0-5V sgnal) and convert to engineering units, a +-15V power supply.
Anyway, there were six mass flow controllers (MFC) , https://www.digchip.com/datasheets/parts/datasheet/798/FC260-pdf.phpand about 15 feet away from the readout/power supply, connected to the power supply and the setpoint was done with a potentiometer. The potentiometer was then local to each MFC, so no ground loops in local mode.

When I then had six 0-5V signals, they each needed to be referenced to each mass flow controller. I did not take that into account. I should have added 6 difference amplifiers to the system.

The OEM version is a 4 unit model with only 1 display that can display one of the 4 MFC's, They were not easily calibrated. We used strange gasses, and we calibrated each one. The OEM had a jumper that could be tied to the system reference.

I didn't want to use cards in a computer anyway and it came back and bit management, That part of the design was FORCED on me. I only had voltage out. I would have liked 0-20mA current out or better yet 0-20mA current out.

Anyway, 6-slot nu-bus Macintosh computers (MACIIx) series were no more and 3 slots were the max, so they had to use used computers.