Mains hum in USB-C microphone - Page 1
Forum section: electronics / repair

author: tony359
author_rank: Frequent Contributor
timestamp: March 11, 2025, 05:30:49 pm
content: Hi all,This is really not a repair but I think this is the best section for my question.I've purchased a USB-C microphone for my iPad and a USB-C hub which allows to plug PD - so that the iPad gets charged WHILE using the microphone. Unfortunately when the PSU is plugged in, I have 50Hz hum in the recordings.I blamed the cheap "AmazonBasics" microphone and bought another one - same issue. I tried several different PSUs, the less offending is my Lenovo laptop PSU which I believe has the least amount of "ghost voltage" when I use it with one of my USB-C soldering iron.What puzzles me is where is this noise coming from: it's clearly mains and sounds like ground loop - though now I am thinking about those PSUs are not grounded - so I am not sure where the 50hz is flowing to.Without having to spend £50 on a USB-C isolator, is there some simple fixes I can try to implement?Thanks!

author: themadhippy
author_rank: Super Contributor
timestamp: March 11, 2025, 06:15:26 pm
content: Quotedont charge whilst recording,

author: rteodor
author_rank: Frequent Contributor
timestamp: March 11, 2025, 06:54:33 pm
content: PSU most likely. Measure the AC voltage between GND and PE (protective earth). In UK I think you will get about 80V AC. If you only measure a few volts AC you might be lucky enough to found either a very good PSU or a very bad PSU.You might have better luck with PSU's that have PE wire.Try to connect another USB device that has a PSU that leads GND to PE (i.e. a USB from a monitor). It could work but it could make things worse some other way.Or, you might not have PE in the plug in your house, check somewhere else.Simple fix: spend £25 on a power bank.

author: tony359
author_rank: Frequent Contributor
timestamp: March 11, 2025, 07:16:47 pm
content: Quote from: themadhippy on March 11, 2025, 06:15:26 pmI'm trying to think if this might qualify for the most useless comment I've ever received!QuoteThanks.yes, I do have 90V there. With the Lenovo PSU I have only millivolts.Is there a grounded USB-C charger somewhere? Or can I ground the USB-C somehow? Are there some adaptors which allow to ground them?Power bank is an idea though it's another thing to charge.Thanks.Edit - wiring the shell to ground via a 1MOhm resistor greatly reduces the hum. I'll get a USB-C adaptor to wire ground into one of the ports of the hub (the shells are all connected together) and will experiment with resistors, that should help.

author: rteodor
author_rank: Frequent Contributor
timestamp: March 12, 2025, 06:58:22 am
content: Quote from: tony359 on March 11, 2025, 07:16:47 pmTha Lenovo PSU might be already connected to PE if it has 3 wires. Check for continuity between ground (and shield) in one side and PE on the other.Quote from: tony359I suppose by shell you mean shield or chassis. Where exactly did you put that 1M resistor ?LE: I might be wrong blaming it all on the PSU. USB cabling and devices are not exactly an example when it comes to shielding/EMC.It could be a current loop carrying that about 1uA AC from PSU all the way thru the USB cabling and back to PE. (Shield cabling must not carry any current !). With that resistor you might have reduced that loop depending on where you placed it.

author: tony359
author_rank: Frequent Contributor
timestamp: March 14, 2025, 05:15:21 pm
content: The Lenovo PSU has ground but I am confident that the USB-C shell (the metal plug) is not connected to ground.The 1MOhm resistor is in my antistatic wrist band. I touched the USB-C plug at the hub - I basically stuck a metal pin in between the socket and the plug.On cables, you might be right. The cable coming with one of the microphones introduces a huge amount of 50hz. Others are better.