Low Pitch Hum When Using PC's AUX 3.5mm - Page 1
Forum section: electronics / beginners

author: DW1961
author_rank: Frequent Contributor
timestamp: January 20, 2022, 03:37:48 am
content: 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?

author: magic
author_rank: Super Contributor
timestamp: January 20, 2022, 09:36:22 am
content: You could try a shorter or better shielded jack cable.Is it battery powered?

author: DW1961
author_rank: Frequent Contributor
timestamp: January 20, 2022, 06:49:48 pm
content: Quote from: magic on January 20, 2022, 09:36:22 amNo 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.

author: SilverSolder
author_rank: Super Contributor
timestamp: January 20, 2022, 10:13:41 pm
content: 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

author: magic
author_rank: Super Contributor
timestamp: January 20, 2022, 10:30:29 pm
content: 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 betheproblem, 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.

author: DW1961
author_rank: Frequent Contributor
timestamp: January 20, 2022, 10:52:18 pm
content: Quote from: magic on January 20, 2022, 10:30:29 pmThat'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.

author: magic
author_rank: Super Contributor
timestamp: January 20, 2022, 11:00:41 pm
content: 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 toin presence of a ground loop. Hopefully not, and they surely shouldn't, but better safe than sorry

author: SilverSolder
author_rank: Super Contributor
timestamp: January 21, 2022, 07:14:23 pm
content: Capacitively coupled ground loop?

author: magic
author_rank: Super Contributor
timestamp: January 21, 2022, 08:09:26 pm
content: 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

author: BrianHG
author_rank: Super Contributor
timestamp: January 21, 2022, 10:06:21 pm
content: 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: