GND loop noise - Page 1
Forum section: electronics / projects

author: Saimoun
author_rank: Frequent Contributor
timestamp: February 13, 2023, 02:38:40 pm
content: Hey thereI have a noise issue on my device, and I would love to hear your thoughtsThe device is 5V USB powered, and the noise comes from an LED display (which is PWM'ed).I think the issue is coming from a GND loop, or more exactly ground currents not going where they are supposed to. It's my analysis but I am not 100% sure.Basically the display creates voltage noise on the 5V supply, which is fully sorted, so that's not an issue. But it makes sense that the return currents (i.e. GND) have the same noise, which is not a problem either when these currents go through the USB connector's GND.Problems start to arise when I have another connector on the board, which gives another path for these return currents. See image:(note: I guess the red arrow on the top right is either very small or not existent, since most currents will go back to the source which is the laptop / wall plug on the left)There is a big difference from one audio equipment to another, whether this noise on GND is hearable or not. On some you cannot hear a single thing, on others it is very obvious.The solutions I found so far are:Telling the users to avoid this type of wiring (i.e. use a separate wall plug for the device)Removing the noise by not PWM'ing the display -> a big change on PCB/electronics, and it would require another displayTaming the noise by putting a 470uF cap in front of it -> difficult to do because of the USB in-rush current limitation, also that does not get rid of the noise completelySomehow avoiding connecting the ground on the Jacks -> unfortunately this is not possible on one of the jacks since the signal is single-ended and needs a referenceAs usual I am using this forum both to hear your opinions and ideas, but also as a self-reflexion board(also this could be useful to people in the future having similar issues)Thanks!Simon

author: Saimoun
author_rank: Frequent Contributor
timestamp: February 13, 2023, 03:12:05 pm
content: Just answering myself here, I think I have another solution:5. adding a small resistor between GND and the gnd connection of the Jack connector. I think just 10ohm already would reduce the noise by quite a bit.

author: jonpaul
author_rank: Super Contributor
timestamp: February 13, 2023, 03:20:44 pm
content: Suggest to do a bit of research on grounding, and audio noise reductionSee SYN AUD COM and many AES papers on this issue.https://www.prosoundtraining.com/Best refs is Henry OTT Noise reduction techniques in electronic systemshttps://www.amazon.com/Noise-reduction-techniques-electronic-systems/dp/0471657263/See my AES papers on digital audio transmission and transformers.Use transformers in digital audio linksbreak ground loops with audio transformer for baseband audioNeutrik and any others make inline adapters.Jon

author: pqass
author_rank: Super Contributor
timestamp: February 13, 2023, 03:40:26 pm
content: Do you hear the same noise if you plug the laptop into the same wall receptacle as the audio amp?

author: Saimoun
author_rank: Frequent Contributor
timestamp: February 13, 2023, 05:44:38 pm
content: Thank you jonpaul I will definitely look into all of thesepqass -> on my example the audio equipment is also USB powered. I think there is no noise at all if it has its own power supply, especially if the laptop is in a different wall plug.I actually just tested now adding a 10 ohm resistor from Sleeve to GND on the Jack and most of the noise disappears. Do you guys see any issues with the 10 ohm resistor? The Jack has a quite high output impendance (about 1k).

author: pqass
author_rank: Super Contributor
timestamp: February 13, 2023, 06:23:42 pm
content: "...10 ohm resistor from Sleeve to GND on the Jack..." bad connecton? I'm not sure I understand.Is there normally a physical connection between the display and the jack? like a USB powered audio player (with stereo outputs)? Or, could the jack wiring just be picking up stray EMI? If the jack and the display aren't connected, what happens if you move the jack and it's wire away from the display?

author: Saimoun
author_rank: Frequent Contributor
timestamp: February 13, 2023, 06:51:14 pm
content: Quote from: pqass on February 13, 2023, 06:23:42 pmI mean if I add that 10 ohm series resistor from Sleeve pin (i.e. ground pin) on the connector, then the problem disappears.Quote from: pqass on February 13, 2023, 06:23:42 pmWell the display is on the same PCB as the Jack. But the only Physical connection is Ground. Moving the wires makes no difference.The way I understand it is that1) the ground current makes the reference voltage move a bit2) if somewhere inside the Audio equipment there is a single ended signal that uses this "moving" reference voltage as a... well, reference, then it "looks" like the audio voltage is moving, i.e. the amplified audio signal (sent to the speaker) has the noise.Same as 2) if instead of having 1 audio equipment you have 2 connected together with a single ended audio cable, where that red GND arrow is going through. Then the cable picks up a buzz sound even tho the audio voltage is 0V (because the GND is "moving"). That case creates the most noise especially with a long cable.Sorry if this is not clear it's difficult to explainAnd feel free to disagree

author: jonpaul
author_rank: Super Contributor
timestamp: February 13, 2023, 06:55:55 pm
content: currents flow in a closed loop.Sneak/stray paths develop eg mains PSU/transformer capacitances, or flow of current in ground systemsThere is no such thing as a "ground"!! every conductor has L and R however small.This the stray CM and DM currents induce a voltage drop which may be summed with the desired signal.The key isa/ Breaking unwanted current loopsb/ Use a transformer preferable a shielded to break a loopc/ Avoid copper TX eg Bluetooth, or TOSlink Opticald/ Use shielded transformerKind RegardsJon

author: wasedadoc
author_rank: Super Contributor
timestamp: February 13, 2023, 07:04:15 pm
content: Reducing the noise source could also help. Improve the decoupling at the display electronics.

author: pqass
author_rank: Super Contributor
timestamp: February 13, 2023, 07:42:01 pm
content: Quote from: simonlasnier on February 13, 2023, 06:51:14 pmAre you sure the tip or ring pins of the jack aren't connected to something on the PCB?I sounds like you've got a missing GND until you've added the 10 ohm resistor to the jack sleeve pin.If so, as Dave would say, the inputs to the amp are "flapping around in the breeze."

author: Saimoun
author_rank: Frequent Contributor
timestamp: February 13, 2023, 07:52:51 pm
content: @Jon: yes I understand. In my example (diagram), my "unwanted current loop" goes:wall plug -> laptop -> USB cable -> USB plug on PCB -> display -> Jack plug on PCB -> Audio equipment -> Laptop -> wall plugwhile it should gowall plug -> laptop -> USB cable -> USB plug on PCB -> display -> USB plug on PCB -> USB cable -> laptop -> wall plugAdding that 10 ohm resistor does not break the loop but it increases its resistance so forces most of the current to take the other path.Yes a transformer would be even better you're right@wasedadoc: agreed! I put a LC filter which cuts at 3.3kHz in front of the display. That removed the high frequency stuff (probably good for EMC), but did not really make any difference sound-wise. Which makes sense since the display's PWM frequency is at about 120Hz.@pqass: good point! But no unfortunately, everything is connected to where it should be, the tip/ring are connected to the signal lines and the sleeve is connected to ground. The board is a proper PCB and the connectors are soldered onto it. (to test adding that 10 ohm resistor I soldered it in a cable, so I can try with a "normal" cable and then with my "10 ohm on sleeve" cable to compare)

author: magic
author_rank: Super Contributor
timestamp: February 13, 2023, 08:30:00 pm
content: Quote from: simonlasnier on February 13, 2023, 07:52:51 pmThat was my first thought too. Strictly, your problem is voltage drop developed across the laptop-audio ground conductor when the display's current flows there.Increasing resistance of the display-audio ground connection reduces this problem.OTOH, the difference between display and audio grounds increases and now this link will suffer more noise.The ideal solution is for the display to have its own PSU or at least some internal separation of USB and audio grounds.So that's your X problem, and what is the Y actually?

author: wasedadoc
author_rank: Super Contributor
timestamp: February 13, 2023, 10:26:27 pm
content: Quote from: simonlasnier on February 13, 2023, 07:52:51 pmTo deal with 120Hz you need large capacitance electrolytic(s).

author: Saimoun
author_rank: Frequent Contributor
timestamp: February 14, 2023, 07:31:07 am
content: @magic: oh yes very good point - if the buzz noise comes from that Jack cable then yes the 10ohm resistor makes no difference => there is less noisy current going through it but the voltage noise is the same (less current going through more resistance = same voltage like you said).But in most use cases, the buzz audio noise is coming from further, usually somewhere inside the audio equipment. And reducing the noisy current going that way helps a lot.What do you mean with the X problem Y thing? is that an expression?@wasedadoc: yep I tried that - even 470uF does not remove it completely (but it tames it quite a bit). Then I would also need to handle the inrush current.

author: JohanH
author_rank: Frequent Contributor
timestamp: February 14, 2023, 08:13:32 am
content: This is the issue with single ended audio signals. A similar issue exists with audio gear that have grounded chassis. I've overcome this issue by not connecting amplifier signal ground directly to chassis. A usual configuration is to connect the incoming audio signal ground to amplifier chassis at the incoming RCA jacks. This caused 50 Hz noise in speaker output. By lifting the audio signal ground from chassis with a 100 ohm resistor and 100 nF capacitor in parallel, the noise disappeared.

author: Saimoun
author_rank: Frequent Contributor
timestamp: February 14, 2023, 09:20:01 am
content: Yep that makes sense

author: jonpaul
author_rank: Super Contributor
timestamp: February 14, 2023, 10:17:08 am
content: 120 Hz buzz:1/ Show the noise on a scope to see wave shape.2/ Audio signal level noise/buzz vs peak/avg/min signal?3/ PSU is SMPS or linear?Buzz is a 120 Hz pulse created by the mains rectifiers. Sometimes a RC snubber across the rectifier can reduce.4/ Identify is the a CM or DM mode noise?Good luckJon

author: wasedadoc
author_rank: Super Contributor
timestamp: February 14, 2023, 11:31:08 am
content: Quote from: jonpaul on February 14, 2023, 10:17:08 amThe OP appears to be in Denmark so rather unlikely the 120Hz is created by mains rectifiers.

author: Saimoun
author_rank: Frequent Contributor
timestamp: February 14, 2023, 04:55:25 pm
content: @Jon: the noise is from the display, not from anything else. The display is basically a bunch of LEDs which are PWM'ed, and the return current of these LEDs goes through the "wrong" GND path instead of going back the USB path. If there is any single-ended audio on that path which will use this GND as reference, then you can hear the noise.If I turn the display off the noise completely disappears.As for whether that is CM or DM mode I am not sureI solved the problem adding those 10ohm resistors between Jack's sleeve and GND. I understand this is not ideal, especially for EMC and I also understand that it means any external noise (i.e. not the noise from the display but any other noise coming from somewhere else) going through the Jack will be amplified because of this. Thank you magic for pointing that outBut the device can handle quite a bit of noise on its inputs without being affected.Maybe I'll make a new version with a huge cap in front of the display with an in-rush current limiter in the future.Thank you everybody for helping me with this in any case

author: jonpaul
author_rank: Super Contributor
timestamp: February 14, 2023, 05:00:50 pm
content: Look at the DC bus, see if any pulses at the video rate. If yes, try à large ELCO cap on the LED display DC bus.Better than the resistance 10R0, an inductor, perhaps 100 mhy 1A 1 ohm or less.Jon

author: Saimoun
author_rank: Frequent Contributor
timestamp: February 15, 2023, 08:13:02 am
content: yes a large cap on the display DC bus - what I said in my last post(I'll just have to manage the in-rush current).As for the resistor, are you saying I should put a 1 or 0.1 ohm instead of 10 ohm? And why 1A?

author: jonpaul
author_rank: Super Contributor
timestamp: February 15, 2023, 11:54:31 am
content: resistance same impedance regardless of frequencyinductor impedance proportion to frequency120 Hz buzz implies a pulse at 120 rate, thus many harmonics.Suggested replace the 10 ohm R with a,carefully selected inductorJon

author: Saimoun
author_rank: Frequent Contributor
timestamp: February 15, 2023, 12:55:20 pm
content: Ha ha ok, I just had to understand your "100 mhy 1A 1 ohm", you mean an inductor of 100mH 1A 1ohm?It's an interesting point! 100mH sounds like a LOT though.

author: jonpaul
author_rank: Super Contributor
timestamp: February 15, 2023, 04:56:28 pm
content: at audio frequency and mains frequency inductors of 100s mhy..henries are common.We have small,axial 1 HyOld valve PS series chokes were 10s of HyReactance is Xl= 2 pi F L ( Hz, Hy)jon