Hum without ground loop

Question

I have an audio measurement system. The system is connected to and powered by a laptop. The system has no other connection to the AC grid. Now, if the laptop is connected to the AC grid and i measure an audio channel, the resulting digital signal contains a lot of humming. However, if I disconnect the laptop from the AC grid, i.e. let the whole system run on the laptop battery, there is usually no humming, at least if the whole setup is not next to a grid component like a multi socket.

This confuses me. Often, 50 Hz hum is said to be caused by ground loop. However, connecting the system at one point to the AC grid, cannot create any loop. A ground loop as I understand it can only occur if two devices are connected to different grounds. Additionally, the laptop has a euro connector, thus, it is not even connected to the earth / ground line, but only the two AC lines. Why seems there by 50 Hz induced into the signal line, but only if the system is connected to the grid? Or is it just remaining ripple in the power supply? That humming also occurs if the system is disconnected from the grid is even more strange, though I suppose both error cases could be unrelated in the sense that in the first case, the ripple of the power supply is the baddie, and in the second case sth is just induced into the audio lines directly.

Answer 1

Many laptop PSUs, especially if they have unearthed power cables (figure of 8 connector), will have capacitive coupling from the mains onto the DC output, via filter and startup capacitors. This coupled voltage is very low current, but if you measure from the dc output to true ground with a multimeter, you will often see the full mains voltage.

This voltage can couple through to audio circuits which have relative high impedance, but are capacitively coupling to earth.

To check if this is the problem, connect the negative pole of the laptop DC supply to earth. This is normally enough to eliminate the AC hum, and sort the audio issue.

I've seen this problem several times with sensitive ADC devices connected to laptops, and grounding the dc negative has solved the problem.

Answer 2

It might be ripple from the PSU and all the internal stuff in the laptop. Have you tried using an external, high quality ADC (like a scarlett audio interface) to capture the audio and see if the problem persists ? I get much cleaner audio recordings from an external ADC than from the builtin line-in port.