Earthing problem - but not what you expect

[Owen]

I have a new Macbook which will not support 2 x external monitors. I like having 2 monitors becuase it helps me keep a handle on information. I bought a Plugable USB-C 4K Display Link port which does the twin monitor thing really well. I cried a bit when I bought it because it was expense on top of buying a new Macbook which was expensive in itself. Don't cry for me though, the Macbook goes like a stabbed rat.

My problem is that after a period of at  best a couple of hours I begin to get a static-y scratching or rustling sound when I move the mouse, load new pages or drag pages around . This is there if my volume is up or down on my SSL 2+ interface. Once I reach up to my amp (1981 Mission Currys 1 - these days it would be called vintage, but we all know that vintage is only 1965 and earlier) and toggle between inputs the rustling goes. But it will be back later. I am using a Bluetooth Keyboard and Mouse.

All the kit is coming from the same dual point wall socket. I actually plugged the amp into the same 6 block as the hub/laptop/monitors and it got worse.

This is a genuine first world problem. But does anyone have any ideas?

 

 

[TwoTimesBass]

1 hour ago, owen said:

This is a genuine first world problem.

And yet not an uncommon one. I'd had this issue lots of times on different Mac laptops, you're correct in diagnosing an earth issue. It's basically a sort of ground loop i think that builds in intensity if you like (like mic feedback) which is probably why reselecting the amp input (or disconnecting the power cable on the Mac?) fixes the issue for a while. I'd give everything a good old clean for a start, particularly the Mac power cable and charging socket. Likewise earth pins in the plugs and audio connections. Beyond that try removing bits of kit apart from the Mac/Soundcard/Amp and see if it makes a difference. Any metal touching the case?

There are a couple of audio isolation transformers around which I've used successfully in the past, Radial Engineering make a number of options.

Good luck!

[Owen]

I have isolated it to the box itself. The set up is fine when I am using a passive hub to attach to another screen and peripherals. Using just the Plugable unit with no peripherals and Plugable audio generates the same rustling. I have cleaned the earth prong on the mains plug and plugged it into a different main socket away from the extensino lead I use for the rest of the desktop. Initially that was fine but after five hours the gently rustling was back. I guess the next step is to plug it directly into the wall with no mains extension. That is a bit of a mission given how my desk is set up and proximity of plugs but I will have to try.

I guess I could run the jack outs from my interface into a couple of DI boxes and hit the earth lift on them but then I would need XLR to phono leads to plug into my hi-fi amp which I use for my audio. And the DI boxes would need a constant power source from somewhere.

Any other suggestions? TIA

[Owen]

The dedicated mains plug in the wall made no difference. Ho hum

[TwoTimesBass]

You're trying all the right things, and you're correct that it points to the active hub. It sounds like it's more complex than a simple audio ground loop, more likely a ground loop somewhere in the USB system that is manifesting itself on the audio. Is the soundcard plugged into the Pluggable box too? If so can you try it directly into the other USB/Thunderbadger socket on the Mac? Or vice versa if it's not?

Rather than a DI box i'd be looking at an isolation transformer something like this:

https://www.gak.co.uk/en/art-dti/781?gclid=CjwKCAiAjeSABhAPEiwAqfxURceWCxxo3LumXwArloolWZv0RVUgjLkKEVkxlfPWzSyFyLEFNCOa7BoCzzcQAvD_BwE&gclsrc=aw.ds

which allows you to go in and out via phono. No guarantees though, I'd want to make sure you can return it if it doesn't solve your problem.

One other thing, are there any Energy Saving features active on the Mac? Sometimes odd things can happen if the Mac is putting things to sleep in the background after a few hours.

[Owen]

The USB system makes sense. It does NOT like the USB webcam - it makes the signal to noise ratio appreciably worse.

Interestingly (!) the audio interface plugged directly into the mac actually make it worse. The interference is still there taking audio directly out of the Plugable box.

Taking audio out of the Macbook headphone output and unplugging the Audio interface solves the problem. That means I can get audio out cleanly but doing online meetings is not great

Thanks for the link to the isolation transformer. I will probably try it, even though it is yet another purchase on top of a significant outlay

No energy saving going on. And it happens as soon as I open the system now. My life is so hard!

[Owen]

Ooh! Ooh! USB C out of the interface into USB C on the Display Link port doobrie seems to be behaving. I will give it a couple of hours to misbehave.

[Owen]

I know you are all desperate to know the outcome. It seems to be behaving at the moment. I have been using it all afternoon.

[Owen]

The problem is back. Much less than it was and Plugable are going to send me a new unit. I shall report back.

[Owen]

I solved it by moving all the audio to the left side of my desk and the Plugable video to the right. My guess is that running balanced lines would have much the same effect, but I am not set up to do that. Happy ending. I know you were all waiting. 2 x screens on an M1 Macbook and nice quiet audio.

[Killed_by_Death]

 

Trouble is that modern lappies aren't actually grounded.

Had a noise issue with my Lenovo ThinkPad when trying to use it with RockSmith, essentially sending my instrument output to two inputs, my amp & my TV soundbar via the PC.

The amp & TV are grounded, but the PC is not, so therein lies the rub.

My solution was a ground-lift AND an isolation-transformer, which are both available in this handy little device: