Confused about ground loops

Hi, Sorry if this seems obvious, but I've been doing quite a bit of reading about ground loops. One article recommended on this forum was this one:

http://www.rane.com/note110.html

If you look at the first diagram under the title "The Absolute Best Right Way To Do It" it shows two devices connected by a balanced cable.
Now my question is what if both of these devices are mains powered, this means that they both will be plugged into the mains, the grounds of the mains sockets are connected in the wall , then it follows that we will have created a loop through the ground cable in the wall, through the first device, then the interconnecting balanced cable which is grounded to both devices and back into the wall socket??? Won't this cause a ground loop???

Yours groundloopedly

Albert

[quote author="mkoijn"]Hi, Sorry if this seems obvious, but I've been doing quite a bit of reading about ground loops. One article recommended on this forum was this one:

http://www.rane.com/note110.html

If you look at the first diagram under the title "The Absolute Best Right Way To Do It" it shows two devices connected by a balanced cable.
Now my question is what if both of these devices are mains powered, this means that they both will be plugged into the mains, the grounds of the mains sockets are connected in the wall , then it follows that we will have created a loop through the ground cable in the wall, through the first device, then the interconnecting balanced cable which is grounded to both devices and back into the wall socket??? Won't this cause a ground loop???[/quote]

Yes, but ...

The "but" being that the shield is really an extension of the chassis and does not carry any signal. When the shield is connected directly to the chassis, then noise picked up by the shield, as well as currents caused by mains ground differences, are drained to the enclosure, where they can't do any harm.

"Ground loop" problems occur when the cable shield is connected directly to a circuit analog ground. This is the classic "Pin 1 Problem." In this case, those ground currents are injected into your circuit reference, and the shield noise is mixed in with your audio.

-a

Thanks, Okay I see what you mean. My problem now is, I have built a preamplifier with balanced input and unbalanced output, because my computor soundcard is unbalanced. So, I see that I should connect pin 1 of the inputs directly to the case, and star ground the audio ground to the case. But what about the unbalanced outputs? should the shield of the outputs also be connected directly to the case aswell?

Regards

Albert

I think you could connect the unbalanced output ground to the ground of the preamplifier output stage, what is your unbalanced output connector type ? (it needs to be isolated from chassis)

Or you could use a balanced output and unbalance it at the audio soundcard input

Ground loops are unavoidable with safety ground chassis connections. The loops can be ignored by differential inputs used in combination with differential (3 circuit) wiring. The audio signal can be completely described by the difference between the + and - leads which will not be corrupted by ground currents if they only touch ground at one point (say the send end of an unbalanced output). Only when a signal lead is connected to ground at two different places can it be corrupted by flowing ground currents or voltage induced by magnetic fields.

JR

Thank you everybody.
Keefaz, the output connectors are cinch connectors, but they are not isolated from the chassis. I will isolate them and see what that does.

Hi
The advice is correct, the output connector ground should be disconnected from chassis.

Also I recommend to ground lift the circuit inside the box. To do this insert a 10 ohm resistor in parallel with a 10nF ceramic cap between audio ground star node and chassis. This will prevent any ground loops between unbalanced equipment.

Not to be crass, but a good bit of those Rane notes are crap in the real world..... there will always be some tie to ground that makes it to the audio path. Single point sheild termination, done at the patchbay, and proper power grounding to technical earth is the only practical way to elimanate ground loops. This does require ground isolated equipment mounting, a place where a chassis ground lift can actually come in handy.
The only exception to this is extremely high RF feilds, and you are better off moving your studio that attempting to sole that one!

Thank you all for the advice, I have started rewiring the preamp incorporating the above suggestions.

Thanks again

Albert