Low Pitch Hum When Using PC's AUX 3.5mm

[DW1961]

If I plug a Bluetooth Transmitter into my PC's AUX out jack, I get a low pitched hum. Using my cell phones' 3.5mm jack with the same BT Tx, there is no hum at all. Also, plugged the same BT hardware into into my audio amp in Rx mode, no hum.

Any ideas, and how to fix it?

[magic]

You could try a shorter or better shielded jack cable.
Is it battery powered?

[DW1961]

You could try a shorter or better shielded jack cable.
Is it battery powered?

No it's powered by a small DC converter, like a wall phone charger, but less amps. Also, when I plug it into my phone, I get no hum at all. This is only when it is plugged into my computer AUX sound out jack.

[SilverSolder]

You are getting a ground loop.

An easy fix is an audio isolation transformer, e.g. something like this might work:

https://www.amazon.com/Smof-Isolator-Speaker-Eliminate-Completely/dp/B0171PQLB8

[magic]

Only possible if the charger/PSU is grounded. Is it?

It could also be pickup of mains electric field by the cable. When the device is connected to a phone the whole system and its ground floats together with the electric field and no voltage is induced differentially between conductors.

A more bizarre possibility is flow of current capacitively coupled from mains into the transmitter through the ground conductor of the jack cable and a voltage drop occurring by Ohm's law. Probably too low level to be the problem, though, in any reasonable cable.

At any rate, try different cables. Is the PSU something standard like 5.5/2.1mm or USB? You could experiment with other PSUs maybe, or a power bank if USB...

For science, plug a USB cable into the phone (which doesn't hum) and touch the other end to some grounded metal. I suppose it will start humming too.

[DW1961]

Only possible if the charger/PSU is grounded. Is it?

It could also be pickup of mains electric field by the cable. When the device is connected to a phone the whole system and its ground floats together with the electric field and no voltage is induced differentially between conductors.

A more bizarre possibility is flow of current capacitively coupled from mains into the transmitter through the ground conductor of the jack cable and a voltage drop occurring by Ohm's law. Probably too low level to be the problem, though, in any reasonable cable.

At any rate, try different cables. Is the PSU something standard like 5.5/2.1mm or USB? You could experiment with other PSUs maybe, or a power bank if USB...

For science, plug a USB cable into the phone (which doesn't hum) and touch the other end to some grounded metal. I suppose it will start humming too.

That's why I didn't mention a ground loop. The power plug isn't grounded. I tried several cables. I'm going to buy a 3.5mm cable that is shielded. The power supply is like a phone charger, very small, and is connected to the transmitter using USB A to C.

[magic]

Well, you could try other chargers, but chances are it won't make a difference. Or maybe it will, because an ungrounded charger injects capacitively current into the secondary through its Y capacitors and those caps vary in size between chargers.

Or a USB port on the same computer, although that may result in different noise. Or dunno, maybe don't do that. Hell knows if they use some silly virtual ground setup prone to in presence of a ground loop. Hopefully not, and they surely shouldn't, but better safe than sorry

[SilverSolder]

Capacitively coupled ground loop?

[magic]

Eh, no. For that you would need capacitance from signal ground to mains ground.
All this charger has is capacitance from signal ground to rectified mains. Arguably more interesting

[BrianHG]

Here is a teardown of a charger which does not have any Y capacitor or leakage to the AC mains, IE no ground loop hum or buzz when using this power supply: