Ground loop hum with Class I/II connections

Question

I've recently revived a pair of speakers by replacing the class D amp with a new separate module (plus a simple opamp active EQ) and keeping the stock SMPS which seems to be working fine. The unit is powered by a figure 8 mains cable hence it being a Class II earth equipment (enclosure is wood/plastic).

The original amp had a 3.5mm aux input socket other than the Bluetooth (now disabled), my plan was to use this as main stereo audio in. Everything works well when playing music from the phone but connecting to any Class I equipment would generate a pretty gnarly buzz.

Also connecting to my laptop would generate a hum if the charger is plugged in (the charger has an earth pin but can't actually measure continuity from earth to any of the pins).

My assumption was that since the amp's ground reference would be floating, then connecting to a Class I device would align its ground plane to the Class I device earth without any ground loops. But it seems that there is somewhere an exchange of current which creates this buzz.

Interestingly if I connect only the ground "sleeve" of the aux input to earth I have no noise at all or a reduction in noise due to shielding, and when I connect the "tip" to a class I device output (and so lowering the input impedance of the amp's input) I start to hear the hum, which continues even if I short the input to ground.

I could see in the original design of the amp the ground/shield from the aux in was connected directly to the ground, so if they've made it work there must be a way I'd assume. I fancy the idea of using inductors but not sure where.

Added block diagram and schematic of EQ section.

Answer 1

Yes this is ground loop hum, even though there is no classical DC-connected loop here. The "isolated" SMPS is allowed to have up to a few mA 50Hz leakage to the DC side.

When you connect to floating devices, this 50Hz current cannot flow. The whole circuit will merely float around at a large 50 Hz common-mode voltage of ~100VAC without much differential signal voltage.

When you connect a device with Earth ground to the Aux jack, the SMPS leakage current will now flow back to earth through the AUX cable shield, it will produce a 50 Hz signal voltage because of the shield resistance. AUX connection is a failed design that uses the shield also as signal return, making itself sensitive to ground loops (which are agood and useful thing when properly signal transmission systems).

You can validate this the following way: Take multimeter and set to VAC. When Aux is not plugged it, you should measure ~100 VAC between Earth and any node on your amp (doesn't matter which). When you connect Aux to floating device, nothing changes..When you connect Aux to a Class I device, the large VAC on your amp will vanish because it is converted to the buzz-causing current.