bluetooth boombox noise problems... help!

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torclausen

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Joined 2012
#1
I've poked around on this site for my problem, my apologies if this is already covered.

My boombox design starts with a cctv li-ion battery with usb and 12v output.

When I connect the bluetooth receiver (using the battery's 5v USB) and the class d amp (using the 12v), I get some major noise with digital clicks and static.

If I use a rca jack input, and only touch the internal prong to the female amp jack, leaving the outer rca shielding free from the amp, it makes the noise. If I shove it all the way on, there is no difference with the noise.

All components work properly on their own. Its only when the amp and the bluetooth are powered from the same battery.

I can only find one affordable usb powered bluetooth receiver, and only one li-ion battery with usb/12v output, so unfortunately I can't vary those. But I've tried 5 different amps, all with the same results.

Can anyone help steer me in the right direction?

parts express Stereo Bluetooth Module
Lepai 2020 amp
ebay cctv usb 12v li-ion battery


Thanks So much, Tor
 

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#6
Its strange, but I haven't seen a single homemade, diy Bluetooth boombox on the internet, yet there are numerous one sold in target, because they are amazing. I think what I am doing is very difficult.

Parts Express had nothing to offer when I called their tech.

I wished I could remedy this easily ~

Tor
 
#7
You can. See post #2 above. Your problem is that you're using 2 separate devices. Each with their own ground plane. When you connect these from the same battery you will have ground loops.

Had it been designed as one single unit it would natually not have the same problems.
 
#9
In Further thinking, how does a bluetooth car deck do it? Obviously both using the common car battery. Or a small bluetooth boombox at retail stores, one battery right?. Or quiet car 12v cigarette powered bluetooth receivers with aux out like the giffin bluetrip? Do they all have expensive dc to dc converters?
 
#11
torclausen said:
In Further thinking, how does a bluetooth car deck do it? Obviously both using the common car battery. Or a small bluetooth boombox at retail stores, one battery right?. Or quiet car 12v cigarette powered bluetooth receivers with aux out like the giffin bluetrip? Do they all have expensive dc to dc converters?
Click to expand...
They almost always use a "ground loop breaker" circuit on their audio inputs - a differential input stage like Eva mentioned. You could breadboard up such a circuit, but I think the transformer method is easier.
 
#13
I am running into this problem too. Works fine when powered by two separate power supplies. The moment the amp and BT share same 12v (BT uses LM317 for 5v) source, buzzes like a car alternator. Sound quality is very good when using separate power. It's a shame really and a total pain. If amp has provision for balanced input, will disconnecting ground from negative line in solve problem?
 
#15
Saturnus said:
Loss of efficiency and considerable extra cost. Probably cheaper to get an extra battery than a fully isolated dc-dc converter.
Click to expand...

For the same cost as a stereo ground loop isolator transformer, you can get a fully isolated DC to DC converter that does both the voltage drop from 12v to 5v and provides isolation in tiny package. The Murata LME1205sc is $10 from Digikey. It is rated at 50mA max which is same as rating for BT-1.
 
#16
xrk971 said:
For the same cost as a stereo ground loop isolator transformer, you can get a fully isolated DC to DC converter that does both the voltage drop from 12v to 5v and provides isolation in tiny package. The Murata LME1205sc is $10 from Digikey. It is rated at 50mA max which is same as rating for BT-1.
Click to expand...

Very tight operating voltage range makes it quite problematic in use, if usable at all. 10.8V to 13.2V is pretty strict when considering operating voltage range for portable audio generally is in the 9V-15V range or 10.2V-14.6V if we narrow it down as much as possible.
 
#19
Murata MEES1205SC works!

xrk971 said:
I think I found the best value isolated DC to DC converter for 12V to 5VDC with 200 mA capability and 1000VDC isolation (83% efficiency). Murata MEE1S1205SC $6.77 ea at Digikey (part number 811-2749-ND).

An externally hosted image should be here but it was not working when we last tested it.


I hope this works.
Click to expand...

This DC to DC converter works great - it is the right part for the job. It is very small and easy to install. Noise is almost all gone.

http://www.digikey.com/product-detail/en/MEE1S1205SC/811-2749-ND/3511371
 
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