bluetooth boombox noise problems... help!

torclausen · 2012-03-25 2:23 am

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

Saturnus · 2012-03-25 3:25 am

It's a ground loop problem. Only 2 solutions (you'll not like either)

1) run 2 separate batteries

2) add a fully insolated DC-DC for one or the other part, probably the USB device.

torclausen · 2012-03-25 4:15 am

Why won't I like the dc to dc thing? What does that entail?

Saturnus · 2012-03-25 10:38 am

torclausen said:
Why won't I like the dc to dc thing? What does that entail?

Loss of efficiency and considerable extra cost. Probably cheaper to get an extra battery than a fully isolated dc-dc converter.

torclausen · 2012-03-26 8:10 pm

I'm being told I can filter it out. (no luck yet though, its very loud). I have a hard time believing that every bluetooth boombox on the market has two batteries running the amp and the bluetooth. I must be able to do it.

http://musicalfurnishings.com/diagrammm.jpg

Noise - YouTube

Tor

torclausen · 2012-03-27 2:14 am

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

Saturnus · 2012-03-27 6:52 am

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.

gmarsh · 2012-03-27 6:05 pm

How about something like this:

http://www.amazon.com/Stinger-SGN20-Ground-Loop-Isolator/dp/B002ZRQ3PW/ref=dp_cp_ob_e_title_2

A lot cheaper than an isolated DC/DC or extra battery.

torclausen · 2012-03-27 9:54 pm

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?

Eva · 2012-03-27 11:13 pm

Use either one audio 1:1 transformer or one balanced input op-amp stage per channel to break the ground loop.

gmarsh · 2012-03-28 12:10 am

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?

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.

Oddpixel · 2012-09-15 5:09 pm

Ground loop breaker can be found on eBay for about £5...that should solve the problem...at least it did for me when building my little tripath / lithium ion / Bluetooth micro boombox...

There's a picture of the one I bought in this video, just ff a min or so:
Hacker Radio Getting Hacked.... on Vimeo

xrk971 · 2013-09-29 4:44 am

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?

xrk971 · 2013-09-29 5:25 am

I just soldered two ground straps (copper solder wick braid) between the ground planes of the BT and amp boards and it cut the noise by 95%. Tremendous improvement but still slightly audible.

xrk971 · 2013-09-29 1:33 pm

Saturnus said:
Loss of efficiency and considerable extra cost. Probably cheaper to get an extra battery than a fully isolated dc-dc converter.

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.

Saturnus · 2013-09-29 5:00 pm

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.

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.

xrk971 · 2013-09-30 1:21 am

I was going to use 12v SMPS to power the whole thing so stable voltage supply is not the issue. The sharing common power source is what I am trying to get around without having to run 120vac mains into the speaker. Just want to run 5.5 mm power supply jack from wall wart SMPS.

xrk971 · 2013-09-30 3:13 pm

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.

xrk971 · 2013-10-03 3:36 am

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.

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

bluetooth boombox noise problems... help!

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