Help me understand a noise in my notebook's mic - Page 1
Forum section: electronics / beginners

author: opablo
author_rank: Regular Contributor
timestamp: December 13, 2013, 03:10:03 pm
content: # If I plug my notebook at home... I have a noise coming from my usb headset# If I unplug the notebook from the outlet... noise dissapears# If I re-plug the notebook and then touch the usb connector shield... noise also dissapearsso... my notebook AC charger is wrong ? (it's a lenovo original one with no earth connector)but... check this new piece of information....# If I plug my notebook at work... I have no noise either...so... my home mains have some kind of noise that my notebook's AC charger is not able to filter out ?When I get home I will start unplugging everything else (1by1) to identity what creates that noise that my AC adapter is unable to filter out....But regardless of that investigation... What's the solution ?(Don't scream at me if this is a stupid question but...)I understand that there are mains-caps that you can plug on your fuse boxes to solve power-factor problems when you have a high amount of non-resistive loads... do you know if installing that kind of caps can also solve high-freq noises on your main's line ?Which brings another question to my mind...(No; I won't do this... I'm just asking out of curiosity... I promise I won't do this)Is there a way to -safelly- plug your osciloscope to see your mains wave ?Will I be seeing a genuine wave if I use an isolation transformer inbetween ? Mains->Scope->Probes->Transf->MainsOr will it be stripped-out of any possible high-freq noises and I would be seeing a smother wave ?

author: opablo
author_rank: Regular Contributor
timestamp: December 13, 2013, 04:29:35 pm
content: good question TheSolderShack....Because at home I plug my computer in an outlet shared with 3 low-power (< 10 W) chineesy switching-mode power supplies for other asorted devices.When I begin my "unplug everything 1 by 1" test... those are my first suspects... I will let you know.-----But as I said earlier.... supose that... one of them is the problem... Which could be a possible solution ?Move to Germany and stop buying from China ? jajaja I hope there's a cheeper solution

author: minime72706
author_rank: Frequent Contributor
timestamp: December 13, 2013, 05:10:27 pm
content: Heh ... Germany; Hameg makes good stuff these days but it's pretty far out of most budgets!One of my little hobbies is to break open any switch-mode brick or wall-wart that I can find which has no purpose anymore to see the quality. I opened one of my Lenovo bricks, either a 65W or a 90W one, I cannot remember. I was rather pleased with what I saw; however, I am not a SMPS designer. I just look for certain features and quality components. There could always be a hidden design issue in the particulars of the filtering or regulation sections. I've also opened an ancient Apple SMPS from the early 90's which may have been associated with a very early apple iBook or whatever they might have called it. I was also pleased with that.Things I don't like seeing include BJTs used for switching mains current, the lack of a fuse (I've seen carbon-film resistors used as fuses), the lack of components like X2 capacitors and common-mode chokes, the use of unsuitable capacitors in place of X or Y capacitors, the utter lack of a controller IC, etc.This is something that is just plain common, unfortunately. Ever hear noise in your headphones, which are probably connected to a desktop and not a laptop, and hear noise in the background that changes with your movements?If you own a decent bench power supply, set it for 19V and current limit it to maybe 2.5A or so (depends on the laptop) and connect that instead. Be careful, of course!

author: SeanB
author_rank: Super Contributor
timestamp: December 13, 2013, 05:50:24 pm
content: Conducted noise. Can you not get a replacement power supply with a 3 pin plug? Otherwise you need an earth, easy if you use a network cable and use a shielded cable and ground the router to mains earth.

author: opablo
author_rank: Regular Contributor
timestamp: December 13, 2013, 06:39:38 pm
content: SeanB: Conducted noise and earthing... yes... that's why I added this note in my original post:(it's a lenovo original one with no earth connector)about that... Do anybody see any safety concern/problem in me adding a third ground wire to the AC adapter wiring (with wire-ties each 20cm) in which... on the computer side it ends on an RJ45 shield (I use only wifi).... and on the other side it enters into de plug adapter that I use to convert USA-plug to ARG-plug (the same that autralia uses but with 220v) and connects to the house earth...

author: Monkeh
author_rank: Super Contributor
timestamp: December 13, 2013, 06:45:23 pm
content: Quote from: opablo on December 13, 2013, 06:39:38 pm...Dude, just buy a proper adapter. And a cable, too.

author: Whales
author_rank: Super Contributor
timestamp: December 13, 2013, 10:53:14 pm
content: I have had a lot of issues with laptops + noise floor before as well. My issues tended to stem from the switching circuitsinsidethe laptops themselves, but a dodgy external adaptor can be just as much of a problem.Try wrapping the power lead a few times through/around a ferrite/equiv. That may help avoid some conducted noise.Otherwise: get a USB microphone. The ADC is done very close to the mic unit and if there are still problems: you can manually add caps to it without having to worry about mains voltages or modifying your laptop.

author: minime72706
author_rank: Frequent Contributor
timestamp: December 14, 2013, 12:27:25 am
content: The Lenovo adapters are good quality, but I don't think they bother with a grounded cable until you approach a certain power level. I remember my old (2006) Dell having a grounded switching module. I suppose Y capacitors don't do jack shit if they don't have a proper place (earth ground rather than neutral) to throw the high frequency noise. Some ferrous medicine may help the situation, as Whales said.I want a pair of TOSLINK headphones ... heh.

author: mohf
author_rank: Contributor
timestamp: December 14, 2013, 12:52:53 am
content: I have a similar problem, but not with a mic.Anyway. The problem is as follow: my PC, speakers, monitor, and lots of other devices are all connected together on a power brick which has earth. Therefore, the earth pins of all the devices are connected together. However, the Earth itself is NOT connected to the outlet (it does not have earth pin). Furthermore, some AC adapters are leaking a very small (uA ~ mA) current to the AC, which then enters a different power supply. This sometimes causes some anoying issues.Not sure if you have the same problem, but who knows.Solution would be to try a different plug where no current is leaking into the earth pin of your laptop, or to just connect earth.

author: Stonent
author_rank: Super Contributor
timestamp: December 14, 2013, 01:48:50 am
content: This is not directly the same thing but I have found that laptops tend to be noisy when you mute the audio. I can actually hear the cursor being moved through the headphones on many laptops.

author: minime72706
author_rank: Frequent Contributor
timestamp: December 14, 2013, 01:54:42 am
content: Quote from: Stonent on December 14, 2013, 01:48:50 amThis goes for most computers. I think what you might be hearing is actually noise from a video card.

author: Mitchell Andrews
author_rank: Contributor
timestamp: December 14, 2013, 08:27:43 am
content: Appears like your problem is noise in your AC current present in the home wiring. Noise always exists, many factors can cause it to be worse. Like the electric motors in refrigerators and other appliances. Electric Motors going bad can cause an increase also. Wiring problems in outlets can cause "more than normal" noise. However, it is common for AC to DC adapters, like for your computer, to lack the standard noise filtering capability as it should.As a solution;(1).I would buy a AC Power Noise Filter (adapter). Some power strips and surge protectors also include this filter. It must say "Noise Filter" for it to be included. You can purchase a Noise Filter alone to plug into the receptacle, then you plug your computer power adapter into it.(2). Replace your computer's power adapter. Unfortunately a "cheap" generic may not be any better.Since you do not have the noise problem at work, I would start with the wall receptacle noise filter adapter. If the noise is really bad, it is possible to need two filters in series. This is probably easier than trying to find the noise source in the home's wiring..Don't mess with the grounding, you don't know if there are any grounding issues in the wall....

author: amyk
author_rank: Super Contributor
timestamp: December 14, 2013, 11:25:04 am
content: Quote from: minime72706 on December 14, 2013, 01:54:42 amIt's more likely to be the buck converter coils for the CPU core voltage: when you move the trackpad/mouse it causes an interrupt, waking up the CPU so it can update the cursor position and do whatever other processing needed. Assuming the system is idle, it then goes back to sleep until the next event. Sleep power consumption is something like 1/10th full power or even greater, so the buck converter is dealing with very short spikes of very high current (tens of amps). The reason why it's audible is the mouse interrupt rate is usually some low audio frequency like 100 ~ 200Hz, so the magnetostriction from the inductors is audible. There are many other interrupt sources but they usually go at such high frequencies or are not regular enough that they don't cause audible noise, but along with this one can combine to cause a "hiss" or "whine". Any ceramic decoupling caps on the vcore can also "sing".

author: ivan747
author_rank: Super Contributor
timestamp: December 15, 2013, 02:15:41 am
content: I would at least try connecting the laptop to earth. You can plug an earthed device if you are unsure about earthing the laptop yourself. Something like an office printer with earth or a monitor will work. On a MacBook, when I swap the grounded lead in the charger for the ungrounded lead (they include a long earthed cable and a small non earthed adapter) I get more noise in the audio output.

author: minime72706
author_rank: Frequent Contributor
timestamp: December 16, 2013, 03:02:55 am
content: Quote from: amyk on December 14, 2013, 11:25:04 amI guess I forgot that mouse movement fires an ass-load of interrupts. That makes more sense than my video card idea.