Grounding audio - simple example

[Music1969]

Hi all,

I understand that ideally you want 1 device in an electrically connected audio system to be grounded - not multiple if possible.

For a simple setup, say I have a USB DAC and headphone amp combo, connected directly to a laptop.

Laptop is connected to AC power via un-earthed typical SMPS power brick.

I have some audible hum going through my headphones but when I earth the USB DAC, hum disappears.

I understand ground loop him is caused by voltage differentials, causing ground currents to travel through my body in this case.

My question : when I earth the USB DAC, instead of ground currents traveling through my body (causing the hum), where are these currents now going ?

I've always wondered - these currents travel in loops, so what is this new loop formed? With only 1 device earthed in this example

cc @solderdude

 

[solderdude]

The problem in this setup is the laptop is connected to mains via an SMPS (power supply) that has to comply to FCC/CE/... rules and has a leakage current between the secondary side (19V DC) and mains (Live wire).
This current will want to go somewhere.
When you connect it to an USB DAC it can travel through USB ground to audio ground.
Still cannot go anywhere but through that device and may find its way via the analog ground.
A small current there will create a very small voltage across the 'wiring'. This includes connectors and PCB traces.
This 'unwanted voltage' adds to the audio signal (SE connections) and when in the audible range can be audible if large enough to peak above the audible threshold of the hearing.

When you ground the DAC the path differs (depending on the DAC construction inside):
laptop PS -> USB ground -> USB input -> ground wire connected to USB DAC. It might not be passing through the analog ground. That is assuming the engineer designing the DAC knew where to ground the enclosure to the circuit. When he used the RCA out for instance current might still pass through the audio path a bit.

 

[Music1969]

Thanks @solderdude but all that you mentioned was already my understanding.

The crux of my question though is:

When you ground the DAC the path differs (depending on the DAC construction inside):
laptop PS -> USB ground -> USB input -> ground wire connected to USB DAC. It might not be passing through the analog ground.

Where are the currents actually going now? If not through my body and headphones

Going to AC earth but then where do they go ?

I've always wondered.

They don't just go to the earth rod in the ground do they?

 

[IAtaman]

My question : when I earth the USB DAC, instead of ground currents traveling through my body (causing the hum), where are these currents now going ?

When we say something outputs 10V, what we actually mean is that there is 10 volts electrical potential differential between its leads. Like distance, voltage is almost always measured in reference to an assumed zero point, which is commonly called the ground. Ground is not the same as Earth, that is, potential of the actual ground might not be the same of the ground in a circuit. Ground in a circuit is just its zero reference. Ground can be 400kV above Earth, as long as there is no current flow, it will all work fine.

In a single circuit, or more often with multiple circuits that are connected together, the need might arise to have physically separate zero reference points. Due to many potential reasons sometimes these two points both of which are supposed to be reference zero points end up having a voltage differential, that is, some grounds are more zero than others. In that case current starts to flow between these points and this is what is called the ground loop.

When you remove a ground loop what you are effectively doing is making sure all reference zeros in a circuit are at the same voltage so that no more ground loop current is flowing.

 

[Speedskater]

Current requires an impedance path between two points of different potential.
So if there is no leakage path, there is no current flow.

 

[Music1969]

When you remove a ground loop what you are effectively doing is making sure all reference zeros in a circuit are at the same voltage so that no more ground loop current is flowing.

Current requires an impedance path between two points of different potential.
So if there is no leakage path, there is no current flow.

Got it - so the way I (and I think a lot of non-electrical people actually) think of it is as diverting or shunting these ground loop currents "to ground", is totally wrong.

USB galvanic isolation should have the same effect then, in my example (assuming nothing is earthed)? Since you block ground currents.

 

[IAtaman]

Got it - so the way I (and I think a lot of non-electrical people actually) think of it is as diverting or shunting these ground loop currents "to ground", is totally wrong.

Not necessarily. One way to bring all grounds in a circuit to the same voltage can be to connect them to the Earth via a low impedance path. It might be that high currents flowing through the ground wire might be causing voltage differential for example, and connecting it to the ground that is also earthed might cause current to take a different path, removing the voltage differential. But if one is not careful, it is also possible to create voltage differentials betwwen different ground points by earthing one of them as well.

USB galvanic isolation should have the same effect then, in my example (assuming nothing is earthed)? Since you block ground currents.

I am not an expert on how those devices work however I imagine they might be attempting to remove noise by creating a new, cleaner ground reference isolated from the potentially noisy ground of fast digital circuits.

 

[solderdude]

Where are the currents actually going now? If not through my body and headphones

Going to AC earth but then where do they go ?

I've always wondered.

They don't just go to the earth rod in the ground do they?

The currents go through the ground wire in the outlet to where the mains enters the house.
There, or very close-by, the neutral is connected to a ground rod.
So Neutral and safety ground are the same basically as they are connected.

The current leakage thus goes from live (through the power supply which has the leakage) back to neutral via the ground wire in the mains socket (or waterpiping or central heating whatever you grounded the device with)

 

[Speedskater]

There, or very close-by, the neutral is connected to a ground rod.

Yep, it's the Neutral at the main breaker box, that is connected to the ground rod that is connected to Planet Earth.
But the ground rod has nothing to do with the day-to-day operation of the AC system inside the house.
Our concern is the Safety Ground that is also connected to the Neutral at the main breaker box.
* * * * * * * * * *
Many computer power supplies only have Hot & Neutral AC power connections. There is leakage from the PC supply to their floating metal chassis.
So when you connect the PC to the audio system, this leakage travels thru your interconnects to your components, then thru the Safety Ground back to the main breaker box.

 

[Music1969]

The current leakage thus goes from live (through the power supply which has the leakage) back to neutral via the ground wire in the mains socket (or waterpiping or central heating whatever you grounded the device with)

Thanks ! The picture is much clearer.

So when these currents go back to neutral, do they come back again?

My basic understanding is currents always travel in loops?

Or once they go back to neutral thats the end of the road?

 

[Speedskater]

All current wants to go back to it's source.
Think of the current source as that big power company transformer down the street. (or your main breaker box)
So the currents (both useful & leakage) travel down the Hot wire to the PC power supply. The useful current is transformed into DC and travels back to it's source down the Neutral wire. The leakage currents use whatever paths are available back to the main breaker box. (one of those paths might be a component's Safety Ground.

 

[Groundrod]

Hi all,

I understand that ideally you want 1 device in an electrically connected audio system to be grounded - not multiple if possible.

For a simple setup, say I have a USB DAC and headphone amp combo, connected directly to a laptop.

Laptop is connected to AC power via un-earthed typical SMPS power brick.

I have some audible hum going through my headphones but when I earth the USB DAC, hum disappears.

I understand ground loop him is caused by voltage differentials, causing ground currents to travel through my body in this case.

My question : when I earth the USB DAC, instead of ground currents traveling through my body (causing the hum), where are these currents now going ?

I've always wondered - these currents travel in loops, so what is this new loop formed? With only 1 device earthed in this example

cc @solderdude

When an ungrounded device is grounded by another devices connection to it, it is referred to as a "Vampire Connection", meaning it is not a good thing!

The rules for grounding a system are as follows:

1. Never connect a 2 pronged AC powered device to a 3 pronged AC powered device.
2. All devices in a system must be grounded!
3. AC outlets used must be wired correctly, so get a tester and make sure your outlets have the proper polarity and are grounded.
4. All power bricks must have a 3 prong grounded AC input, so any device with a 2 prong power supply needs a new brick that is grounded.
5 All devices must be on the same AC circuit, so plug all the devices into a power bar so they are all connected to the same outlet.


No device should be getting its ground through a USB , HDMI, or Audio cable shield connection, it needs its own ground connection.

Cheers!

 

[Music1969]

4. All power bricks must have a 3 prong grounded AC input, so any device with a 2 prong power supply needs a new brick that is grounded.

Most SMPS power bricks have a 3rd pin/prong that does nothing - not continuous with its DC output plug . Exception is medical grade.

So all powerbricks need to be replaced with medical grade?

1. Never connect a 2 pronged AC powered device to a 3 pronged AC powered device.

Why?

I have a 2 pronged laptop (VERY common) connected to a 3 pronged headphone amplifier (with DAC integrated).

When I previously had a 2 pronged headphone amplifier, I would hear buzzing in headphones until I touched metal case.

So without 3 pronged headamp, I would have leakage currents passing through my headphones/body.

2. All devices in a system must be grounded!

Isn't this a recipe for ground loops, especially with RCA interconnects ?

And none of your points said best to use balanced XLR interconnects?


cc @Speedskater @solderdude @IAtaman

 

[TRDK]

I do have a power strip where it is possible to ground it. But I am sure there to ground it?
(Living in Germany)

THX

Torben

 

[dorakeg]

I do have a power strip where it is possible to ground it. But I am sure there to ground it?
(Living in Germany)

THX


View attachment 273001

Torben

This is needed only if your wall socket is strictly 2 pin without any earth. its not needed in most situations.

https://diy.stackexchange.com/questions/136809/properly-grounding-equipment-with-a-ground-screw

 

[solderdude]

Most SMPS power bricks have a 3rd pin/prong that does nothing - not continuous with its DC output plug . Exception is medical grade.

So all powerbricks need to be replaced with medical grade?


Why?

I have a 2 pronged laptop (VERY common) connected to a 3 pronged headphone amplifier (with DAC integrated).

When I previously had a 2 pronged headphone amplifier, I would hear buzzing in headphones until I touched metal case.

So without 3 pronged headamp, I would have leakage currents passing through my headphones/body.


Isn't this a recipe for ground loops, especially with RCA interconnects ?

And none of your points said best to use balanced XLR interconnects?


cc @Speedskater @solderdude @IAtaman

From a safety standpoint and EMC standpoint (as far as that 'works' ) the 5 rules make a lot of sense (except maybe rule 4 as one can safely mix 3 and 2 prong devices when designed to work that way)
Rule 5 is something that one should take at heart when using different 3-prong outlets (because it is handy) as it could (doesn't have to be) have a different voltage between mains outlets on the ground pin which then like to flow through shielding of interconnected audio devices.

In practice, I live in an old house with safety ground only in 'wet' rooms and 2-prong in all other rooms, I use 3 and 2 prong systems and SE cabling + USB + video all connected to 2 prong mains but all connected to 1 mains socket, and are not plagued by hum or weird noises in any of the systems nor do not feel a tingle when touching metal parts.
Been doing this for over 40 years now (always lived in old houses) and can work fine.

However, from a safety standpoint

 

[Music1969]

the 5 rules make a lot of sense (except maybe rule 4 as one can safely mix 3 and 2 prong devices when designed to work that way)

But also rule 1 doesn't make a sense ? "Never connect a 2 pronged AC powered device to a 3 pronged AC powered device."

So 2 of 5 rules don't make sense.

I gave very simple example - what other way around it would I have? Ask Apple to make a grounded power adapter for Macbok , with MagSafe connector?

"I have a 2 pronged laptop (VERY common) connected to a 3 pronged headphone amplifier (with DAC integrated).
When I previously had a 2 pronged headphone amplifier, I would hear buzzing in headphones until I touched metal case.
So without 3 pronged headamp, I would have leakage currents passing through my headphones/body."

 

[solderdude]

Well... the 'never' part is a bit exaggerated.
In fact almost all audio systems might well have a mix of 2 and 3 prong devices that should be connected to a 3-prong power socket but is for safety reasons, not 'unwanted hum' reasons.
However, I think that's not what he meant with the rule.
When one has the nickname 'Groundrod' it is somewhat safe to assume one works in that field and looks at this from a safety standpoint.

Rule 1 basically should have said: Never connect a 3-prong device to a 2-prong outlet. (again for safety reasons, the safety ground connection is there for a reason)

1. Never connect a 2 pronged AC powered device to a 3 pronged AC powered device.

It just is written down a bit confusing as the [blue] '2 pronged AC powered device' probably should have stated '2 pronged outlet'.

 

[Music1969]

Never connect a 3-prong device to a 2-prong outlet. (again for safety reasons, the safety ground connection is there for a reason)

Ah yes, your correction makes total sense.

And (fortunately) nobody in this thread has brought up doing such.

This thread is really about hum and leakage current noise etc

 

[solderdude]

Yep safety, groundloops, mixed (prong) systems, SE interlinks and 'hum issues' sometimes, when not done correctly, can bite each other.
Fortunately there can be solutions that maintain safety regulations yet can break ground loops.
These just aren't easily recognized when one is not versed in these matters.