auto noise rejection - Page 1
Forum section: electronics / projects

author: timelessbeing
author_rank: Frequent Contributor
timestamp: March 05, 2018, 11:59:18 pm
content: I bought a wun hung lo USB panel for my car. It's got a little built-in SMPS. It charges devices just fine, but when I use it to power my music player, you can hear noise in the audio output. The nosie varies with engine RPM, so I'm guessing that ripple from the alternator is getting through. Is there a way to eliminate it? Would adding caps to input help?Has anybody purchased a similar device which is better quality, and has known clean output?

author: tautech
author_rank: Super Contributor
timestamp: March 06, 2018, 12:03:34 am
content: Quote from: timelessbeing on March 05, 2018, 11:59:18 pmDon't guess, measure it to verify its source !

author: ammjy
author_rank: Regular Contributor
timestamp: March 06, 2018, 12:06:59 am
content: by the experience, we can use 1 0.1 0.01 uF .. but if it would not give help. we can analyse noise via FFT of scope.and in many case of automotive. ferrite core give lot of help.in my case. i have lot of caps.i just put with many mix. if noise gone then success !!i dont calculate sometimes.. because im lazy......

author: timelessbeing
author_rank: Frequent Contributor
timestamp: March 06, 2018, 02:27:26 am
content: I thought ferrites were for attenuating high freq noise... as in far beyond audible range?

author: tonyh88
author_rank: Regular Contributor
timestamp: March 06, 2018, 03:35:50 am
content: I agree with Tautech. What equipments do you have access to measure it? Scope? Did you check with a multimeter on AC coupling if you see anything at all?You can probably filter it but without knowing the noise frequency it will be a trial and error thing.

author: Dr. Photon
author_rank: Contributor
timestamp: March 08, 2018, 06:43:27 pm
content: While that USB power source is almost certainly cheap and noisy, I doubt that you are getting noise directly from the output USB; though filtering it can't hurt. I'd suspect a ground loop through the car. To fix it, you would need to isolate either the power (either the 12V or the 5V usb) or the audio signal. The audio signal can be isolated fairly cheaply with a ground loop isolator (sometimes called a "noise filter". just make sure it has a stereo audio input and output and no power connections, you don't want the power line noise filter). Sometimes you can even find them with 3.5mm male and female plugs for a plug and play solution. They just contain two (crappy) 1:1 audio isolation transformers.You can try the test of having the MP3 player plugged into the USB power with the car on, but use headphones instead of plugging the audio into the car. If the noise is gone with the headphones, it is a ground loop issue and not dirty power.

author: timelessbeing
author_rank: Frequent Contributor
timestamp: March 10, 2018, 09:49:46 am
content: I think you nailed it Dr. Photon. There is no noise with headphones.I wonder if I can shorten the ground loop by strapping the USB socket negative directly to chassis ground ...

author: timelessbeing
author_rank: Frequent Contributor
timestamp: December 13, 2018, 01:31:41 am
content: I took another stab at this. I tried two more ideas:1) isolating the grounds by putting tape over the "sleeve" portion of the 3.5mm plug. I figured that if the signal is chassis ground referenced on both sides, then this connection isn't needed, and is only introducing ground loops.2) Adding more grounding, by wrapping a chassis grounded wire around the "sleeve". Again, this might stem any electrical current on the common leg.Both ideas made the problemworse. I'd like to avoid having the clutter of those filter dongle things.I did discover something new though. There was still noise, even without the engine running. Not as much, and different, but some was there. So it must be coming from somewhere else, possibly even the cheapie SMPS.I thought about using an isolated 5V supply, but it seems those are extremely expensive and hard to come by, or build.