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Ground-loop issue with MacBook Pro 2019

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Re: Ground-loop issue with MacBook Pro 2019

PostPosted: Wed Apr 07, 2021 6:11 pm
by Sam Spoons
dDog45 wrote:
Sam Spoons wrote:edit:- I've just checked and the 1k ohm resistance in the earth path is within the PSU itself.

When you say PSU are you referring to the 96W Plug or the internals of the computer?

Also, that product you mentioned before.. I will also have at least one jack socket available on the interface but I'm not very confident I entirely understand what you mean in your explanation of connecting it as such? Do you mean that product just gives out a ground and you feed that ground to the interface through a TS cable?

The 'wall wart/line rat' PSU not the computer internals.

You need to somehow make a connection between the earth of the system and the mains earth which your system doesn't have at present. Your old desktop PC will have had a good earth through it's mains lead, your new MBP doesn't and that is what is causing the problems. As James said if you can get a suitable connection between the GroundBox and your mains earth that will work fine, or you could even make your own with a length of wire, a TS jack plug and a 13A plug top (in most cases you could just remove the Line and Neutral pins leaving just the Earth pin in the plug) and use the spare jack socket in your interface.

Re: Ground-loop issue with MacBook Pro 2019

PostPosted: Wed Apr 07, 2021 10:02 pm
by Dave Rowles
I've got a 2017 MBP. I have the same issue....when I use the plug that connects directly to the PSU.

What you can get is an extender that connects to the PSU, adding an additional cable to the PSU between it and the main socket. This did NOT come with the MBP, but I had it from my previous MBP. So when I encountered that problem one first plugin I switched out the cable and it sorted it.

This is the thing I'm talking about: ... a1644ecd63

Of course, if you've already got that, then I'd suggest the grounding options already above!

Re: Ground-loop issue with MacBook Pro 2019

PostPosted: Thu Apr 08, 2021 9:53 am
by Sam Spoons
Easy way to check, have you got a multimeter? just set to low ohms and measure the resistance between the chassis of the MBP (use one of the tiny screws on the back to avoid the anodising) and the earth pin on the plug. It should read open circuit, then remove the plug adapter and test between the MBP and the metal locating peg on the PSU, my 202 65 watt USB C PSU measures 995 ohms, my 2011 Magsafe PSU measures 975 ohms, both with the extension cable in place.

The adapter plug does not have a connection between the earth pin and the locating lug so the extension cable may solve the problem for you. But it may not as the earth path through the MBP's power supply is not low resistance.

The first step though is to find a suitable earth point and try the 'guitar lead' test to see if adding a good earth solves the problem. Central heating pipework should be earth bonded (so scrape a little paint of the back of a pipe to clean metal).