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Old 12th May 2014, 05:25 PM   #1
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Default earthed PCB and USB chips?

Iíve noticed that for most laptops, USB hubs, and other peripherals with an external PSU, the PSU is galvanically isolated, only fed by Live & Neutral, and that measuring the voltage potential between the PSUís DC Neg and AC Earth measures 85-130 VAC. But the PSU on my ASUS laptop is fed by L-N & Earth and its PSU DC Neg has a voltage potential of 0-1 VAC to AC Earth.

So what happens when I connect the powered USB hub to the ASUS laptop? It immediately drops to 0 VAC -- same as when I touch the USB out from the powered hub to earth. On an audio electronics forum last year, a couple people were conjecturing whether having a large difference in voltage potential between a USB peripheralís plug and a laptopís USB socket might result in a surge that could fry the USB chip. Does this seem possible? Is it likely that the ASUS with AC L-N&E going into its PSU might have protection circuitry for such an event, while a laptop that is galvanically isolated might be vulnerable in this scenario?

Many consider the quality of the power supply to be all important to the quality of the sound. So one person was positing that because most electronics with external PSUs have no connection to Earth, one might improve the supply of clean power by Earthing the PCB. So he held that attaching an Earth wire to the laptop via a USB plug sometimes yields good results.

I am wondering about the voltage potential differences mentioned above as a general issue, and then specifically in this last audio laptop application.
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Old 13th May 2014, 12:50 AM   #2
kimbo is offline kimbo  Australia
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Quote:
Originally Posted by brucew268 View Post
....a couple people were conjecturing whether having a large difference in voltage potential between a USB peripheralís plug and a laptopís USB socket might result in a surge that could fry the USB chip. Does this seem possible? .
Sounds plausible to me. But I have limited knowledge in this area so I will watch this thread with considerable interest.....particularly as I have just purchased an Asus S551LA. I'll have a look at the power supply and check the earthing when I get home from work tonight.
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Old 16th May 2014, 09:28 AM   #3
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No one has relevant information or experience on this?
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Old 16th May 2014, 12:38 PM   #4
phofman is offline phofman  Czech Republic
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How did you measure the voltage between galvanically separated circuits? A high-impedance voltmeter will always measure some induced voltage. It is similar to testing the live line with a neon-lamp tester. An free/unconnected wire running alongside the live wire will light the lamp too, making the test unusable. That is the induced voltage feeding the high-impedance neon light.

The ground line hooking your laptop with your DAC will null the voltage, I would not worry about that.

A diffferent story is when the ground line hooks distinct devices where e.g. atmosferic electricity is induced. But that is not the case here.


PS: IME newer notebooks use 3 wire power connection, i.e. they are not galvanically separated from the power network.
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Old 16th May 2014, 01:02 PM   #5
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MM is set to VAC. Black Com lead is connected to AC Earth at a socket on the same house circuit as the PSU's are plugged into. Red voltage lead is connected to PSU's DC Neg out or to the USB cable's Earth collar.

Voltages are different for each PSU and have consistent values when retested over multiple days and plugged into different AC sockets.

DAC is powered by the USB from the laptop. I'm unsure whether you are saying that hooking an Earth line from the laptop's USB to AC earth has a potential risk or not, when the AC voltage potential as measured above is 80-120 VAC.
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Old 19th May 2014, 09:05 PM   #6
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The root cause of power supply ground floating at half mains voltage is capacitors in the AC power filter (we're talking several nF here). In a safety earthed supply, these go to earth. It is not uncommon for the secondary-side ground to be connected to safety earth as well. Now remove the mains-side safety earth connection, et voilŗ. Certainly one reason why notebook often require audio isolation transformers. I can actually hear slight hum in very sensitive earphones on my notebook when used on mains (L+N only).

It usually isn't really dangerous, but may pose a problem when hot-plugging audio connections (and in the past, some printers were good for nasty sparks) and could kill electronic input selector ICs.

The best solution from an audio POV is a power supply with a safety earth connection whose secondary-side ground is loosely tied to safety earth to break ground loops (about a kOhm has been found in some model, though I don't remember the specifics).
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Old 20th May 2014, 07:58 AM   #7
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Thank you. After more than a week and asking on several forums, yours is the first post to give an answer that makes some sense to me and relates to technical knowledge and real world experience.

Quote:
Originally Posted by sgrossklass View Post
The root cause of power supply ground floating at half mains voltage is capacitors in the AC power filter (we're talking several nF here). In a safety earthed supply, these go to earth. It is not uncommon for the secondary-side ground to be connected to safety earth as well. Now remove the mains-side safety earth connection, et voilŗ. Certainly one reason why notebook often require audio isolation transformers. I can actually hear slight hum in very sensitive earphones on my notebook when used on mains (L+N only).

It usually isn't really dangerous, but may pose a problem when hot-plugging audio connections (and in the past, some printers were good for nasty sparks) and could kill electronic input selector ICs.

The best solution from an audio POV is a power supply with a safety earth connection whose secondary-side ground is loosely tied to safety earth to break ground loops (about a kOhm has been found in some model, though I don't remember the specifics).
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