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Ground loop problem?

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HobGoblyn:
Hi all, please bear with me while I try and explain my problem.

Due to medical problems, I currently need to be able to use my music software while sitting in my armchair, hence need a computer and speakers on an over the chair/bed  type table on wheels , and I have a 37 note midi controller keyboard that comfortably sits across my armchair.

I have a MacBook Pro M1 with powered Presonus  Eris 3.5 monitors attached.

I’ve only had the Mac for two weeks, initially I had a mains powered soundbar plugged into it, and when  using this, I didn’t notice the problem,  unfortunately it sounded awful hence returned it  and got the Presonus Eris 3.5’s instead.

The left  Presonus Eris speaker has all the controls/inputs/power socket on it, (right  speaker simply has speaker cable socket on it with the speaker cable coming from the left speaker).  It’s powered by one of those  figure 8 plugs (C7) which can plug in either way round and  obviously doesn’t use the earth pin in my UK plug or wall socket.



First, the problem. 

Initially, I had a stereo 3.5mm  jack to  twin RCA,  with the 3.5mm jack plugged into the MacBooks headphone port and the two RCA’s plugged into the speakers RCA ports.

The MacBook wasn’t connected to the mains (MagSafe cable from mains charger not connected to MacBook)

When using my MacBook Pro, after about 5 odd minutes,  when I move my hand along the laptop case (when typing/using trackpad), I can feel a tingle where my hand touches it.  If I hold my hand  completely still on the case, I can’t feel anything , but as soon as I move it, tingling is there.

Very hard to explain the sensation, almost kind of feels like some weird haptic feedback and sort of feels like there’s a slight resistance between the case and my moving hand.  The slower I move my hand the greater the sensation.

If I remove my hand completely, sometimes the sensation is still there when I put it back, other times it’s fine for a few mins before the problem returns.

The second I unplug  the mains plug for my new Eris 3.5 speakers, the problem goes away.   When I plug it back in, it’s fine at first, then it comes back.

I’ve got my wife to unplug and re plug in the Eris mains cable so I could see/feel with 100% certainty that unplugging them stopped the sensation immediately.




What  I’ve tried.

1:   Tried a stereo 3.5mm to 3.5mm cable from MacBook headphone port to the Aux in on my Eris speakers.


2:  I purchased a “Presonus Audiobox Go” audio interface. 

The  Audiobox is connected to the speakers by two balanced 6.35mm TRS  cables

This came with a "USB C"  to “USB  A” connector to connect to the laptop,  but as my MacBook  only has USB C/ thunderbolt sockets, I tried with a spare USB C to USB C.


3: Audiobox still connected to the speakers by two balanced  6.35mm TRS cables

Used the USB C to USB A cable that came with the Audiobox, the USB C plugging into the Audiobox,  the USB A plugging into Anker USB hub, the hub plugging  into my  MacBook


With any of the above (tried in all MacBook ports) the problem is still there.

Before buying the Audiobox, I tried different wall sockets and a different fig 8 (c7) mains cable.  Most tests were done without the MacBook being attached to the mains but I did a few with it charging, nothing made a difference.


So currently the speakers are connected to my Audiobox by balanced cables, the Audiobox is connected to my Anker USB hub by USB C to USB A   and My Anker USB hub is connected to one of the MacBooks USB C/Thunderbolt ports. The problems just as bad as as my original try and still, the second my speakers are unplugged, the problems gone.

Any suggestions please

Thanks

Links below to speakers etc



Eris 3.5 Speakers
https://www.amazon.co.uk/PreSonus-Eris-3-5-Gen-Studio-Quality/dp/B0CC36XVLM/ref=asc_df_B0CC36XVLM/?


Audiobox Go Audio interface
https://www.amazon.co.uk/PreSonus-AudioBox-GO-production-Instruments/dp/B09N557CYK?


Anker Hub
https://www.amazon.co.uk/Anker-Upgraded-Delivery-Pixelbook-A83460A2-Gray/dp/B07ZVKTP53/ref=asc_df_B07ZVKTP53/?



tooki:
The tingle is from the interference suppression capacitors on an ungrounded switching power supply, a question that has come up many, many times here on the forums. It means your housing is floating at 1/2 mains voltage, but at an absolutely minuscule current, which is why this is not only allowed by code, but required, in order to meet EMI standards. This is normal and expected from a properly filtered supply. The fix is to actually have a ground, e.g. by tying the speakers’ case to ground (if it’s metal), or using your MacBook with the grounded mains cord (not the snap-on plug) on its power supply.

themadhippy:
passive DI box on the audio lines should clean things up

tooki:

--- Quote from: themadhippy on June 05, 2024, 11:37:19 pm ---passive DI box on the audio lines should clean things up

--- End quote ---
This isn’t a ground loop problem, nor an impedance mismatch. So how exactly do you think the DI box would solve this? (Bear in mind that I already gave the solution above.)

themadhippy:
it will break the physical connection between 2 items that when joined together are causing the tingle,which id guess is going from the speakers ground to the metal work of the un plugged mac.Being the speakers are using a 2 pole mains connector id bet its double insulated ,and adding an earth is a no no ,at least in the uk's  electrical rules

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