Hum noise happening when connect the AUX cable with laptop in ADAU1761

Hello,

Does the hum go away when powering your laptop from its battery? If so, the hum is caused by a ground loop between the speaker and laptop. Perhaps you have found the term, "ground loop," in your online research -- if not, go ahead and search for it. What's happening is that the EMI suppression capacitors present in most PSUs couple a small AC current from the mains power to their DC outputs. This current can develop line frequency AC voltages across any wire resistances found in the common connections in both your laptop and your speaker system -- which can be amplified and heard from the speaker. To minimize the hum, you need to make sure that any such current flows harmlessly, not allowed to develop voltages in sensitive areas of your system.

When I connect the headphone output of my (AC powered) laptop to my stereo system made by a famous manufacturer, I can hear ground loop hum -- showing that ground loops can be quite frustrating to deal with. This Wikipedia article might help.

Best regards,

Bob

Hi Bob,

I am really grateful for your help. Yes, you are quite right. The term of “ground loop” I have found when I searched the answer online. I also found the humming disappear when I add a wire connecting the GND between speaker and Earth GND so that is why I ask the question why speaker usually use 2 wires AC cable rather than the common 3 pin AC cable with earth GND pin.

However, I want to thank you again and will try to figure out what is the “ground loop”.

Thank you for your great help.

Hello Chris,

You hit the nail on the head by asking a very good question that has bothered me for years. Before my days at ADI I designed a preamp for the recording studio market. It was a single channel and so many people purchased them for use with audio workstations on a computer. My preamp was properly grounded with the third pin going to the chassis and to pin 1 of the XLR connectors. So many of the USB computer interfaces are either USB powered or powered by a 2-prong AC plug with no connection to ground. So they plug in my preamp and the entire system hums because now it is getting a proper ground through the audio system and so it picks up all this ground current for the USB interface and the computer and all the speakers connected etc. They blame it on my preamp when the issue is ALL the other stuff is not properly grounded!! The solution is to lift the ground on the preamp which is a horrible solution but is easy for the user to do. They usually cannot solder so they cannot lift the shield on the XLR cable.

I do not understand how all the safety organization, UL, CE, etc., can certify a power supply with no ground connection? The company that sells my preamp recently waved the white flag and started shipping units with power supplies that only have two prongs and no ground connection. So ground comes from whatever ground the computer has.

I have also found that USB implementations are all over the map as far as how they deal with ground. So you might find a unit might hum or have HF noise on one computer but not on another. They often have ferrite beads on the ground connection which is also really bad. Anyhow, you touched a nerve with me with your very good question.

Dave T