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Low Pitch Hum When Using PC's AUX 3.5mm

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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:

--- Quote from: magic on January 20, 2022, 09:36:22 am ---You could try a shorter or better shielded jack cable.
Is it battery powered?

--- End quote ---

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.

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