How to eliminate noise from switching power supply

To get long story short.
I am making Android head unit using Lenovo android tablet, class D 4ch amplifier, arduino with a relay (reads acc voltage and locks/unlocks the screen according to ACC status), usb hub, DAB+ usb, 3 step down converters (LM2596S).

I hardwired everything to the tablet (no original connectors for usb, headphones, battery at all), I`m using step down converter instead of tablet`s battery, adjusted to 4,20v.

While troubleshooting I removed everything except amplifier and tablet with it`s step down converter from the circuit.
There is that awful ground loop sound (digital kind and hissing) that stops when I cut the signal from tablet to the amp.

I made it quieter for like 90% by installing ground loop isolator (Caliber LT 1S), but that 10% is still there.
The sound I`m getting from the speakers is the same sound that comes from the step down converter used to power the tablet (coil whine).

Brez naslova.jpg

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What else can I try to eliminate this sound? Could this be due to cheap ebay step down converter?

Thank you for help
 

anatech

Administrator
Paid Member
2004-06-06 8:31 pm
Georgetown, On
Hi diy1995,
Something you can try right now is to put a linear regulator between the switching supply and the tablet. Adjust the switching supply to have enough voltage for the linear regulator, say approximately 2 VDC above the output voltage from the linear regulator. Use something like an LM317T. Quick and dirty. Remember to decouple the linear regulator on both sides, and also place a reverse biased diode across the linear regulator (1N4002 or better, or one from the 5400 series). You will need this diode as the switcher will probably die faster than the voltage on the output of the linear regulator. Failure to include that diode will probably cause the linear regulator top fail when you turn it off.

Of course, I am assuming you have already used an oscilloscope to look at the voltage waveforms, both B+ and the common.

-Chris
 
Hi diy1995,
Something you can try right now is to put a linear regulator between the switching supply and the tablet. Adjust the switching supply to have enough voltage for the linear regulator, say approximately 2 VDC above the output voltage from the linear regulator. Use something like an LM317T. Quick and dirty. Remember to decouple the linear regulator on both sides, and also place a reverse biased diode across the linear regulator (1N4002 or better, or one from the 5400 series). You will need this diode as the switcher will probably die faster than the voltage on the output of the linear regulator. Failure to include that diode will probably cause the linear regulator top fail when you turn it off.

Of course, I am assuming you have already used an oscilloscope to look at the voltage waveforms, both B+ and the common.

-Chris

Hy,
I would not like to use linear regulator as the tablet will be running all the time. I even managed to make arduino to turn off itself after locking the tablet to lower the consumption from 120mA to 60mA and I would not want to have higher standby consumption especialy because this car will not be my daily driver.

I do not have an osciloscope, but I did try to use laptop 12v power adapter and also the car battery (just the battery, not in the car) to confirm that power source is not the issue.

The switcher only starts to produce this sound when under load (when CPU needs to work, when there is alot activity on the screen, etc..), if I leave the tablet alone playing music or music paused I don't get that sound, so thinking out loud could the simple capacitor across + and - solve this issue?

Thank you for help
 
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Are all of the voltages needed less than 12V? If so then I don't see the need for a switching supply at all, and just the linear reg to drop to the voltage required should be sufficient? 4.2V is it?

Tony.

Hy,
Yes I only need 5v and 4,2v, but I need 4,2v to run the tablet all the time.
Standby consumption (ACC off) is 60mA right now and I would not like to go higher, so I realy can't afford to transfer my battery power into heat when car turned off.
Thank you for help
 

wintermute

Administrator
Paid Member
2003-08-03 11:43 am
Sydney
OK I was thinking it would only be powered when the key was in the accessory or ignition position. Could you use a relay to switch the power source depending on whether the key was in the off position or the accessory/ignition position?

ie when you switch off it cuts over to the switcher, but when in operation uses the linear regulated supply, and the switcher is off/isolated?

Tony.
 
The switcher only starts to produce this sound when under load (when CPU needs to work, when there is alot activity on the screen, etc..),


That's not switching noise, that's noise from the tablet motherboard I think. Digital computers when running code tend to produce interference at a wide range of frequencies all across the audio band, very much dependent on the pattern of activity. Some of this is imposed on the local ground which will sail directly into your amp given the ground loop. Some of it may directly modulate the analog output of the tablet's audio output, that's not going to be fixable (cheap electronics often compromizes on such performance).
 
That's not switching noise, that's noise from the tablet motherboard I think. Digital computers when running code tend to produce interference at a wide range of frequencies all across the audio band, very much dependent on the pattern of activity.
It sounds like that. I know what you mean, it`s kinda similar to old dial tone modem, but there is also the hissing sound with it.
However the voltage regulator is producing the same noise at a same time.
I also discovered that if I power amplifier from one power source and the regulator for the tablet from isolated power source the regulator is still making that noise, but speakers are silent.
 
OK I was thinking it would only be powered when the key was in the accessory or ignition position. Could you use a relay to switch the power source depending on whether the key was in the off position or the accessory/ignition position?

ie when you switch off it cuts over to the switcher, but when in operation uses the linear regulated supply, and the switcher is off/isolated?

Tony.

Yes, this would definitely be the solution. A relay with NC and NO pins, coil powered by ACC 12v.
I actualy did similar setup with arduino in order to power off itself 5 seconds after it`s power supply went out.

I just need to do a little search on adjustable linear regulator to figure the circuit out and test if a relay will switch from NO to NC fast enough to keep the tablet running and put everything together.

I will post the result when done.
EDIT:
I will try isolated switcher first.
Thank you
 
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Use a quality isolated DC-DC converter to break the ground loop at the supply side too? The cheap eBay units are not isolated, nor shielded, nor filtered.


A good common-mode choke on the power wires from the LM2596 unit to the tablet might help.
This would also be the solution.
Will try a choke first what value should I use? Should I use it on positive and negative lead or just positive?
I will do a search for better regulator. Is there any brand/model you would recommend?

thank you
 
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I tried a few chokes, I desoldered from old computer PSUs and it didn`t help (my guess values weren`t right either).
Then I did a bit of a research on LM2596S step down converters and discovered that I should add capacitor to the output of the converter to minimize the voltage ripple.
This silenced noise coming from the converter and a bit on the audio side :)
But I`m still not there yet...