Schematic for USB Bluetooth speaker

Discussion in 'The Projects Forum' started by tjohnson, Oct 21, 2016.

  1. tjohnson

    Thread Starter Active Member

    Dec 23, 2014
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    I'm obtaining the parts to build a USB Bluetooth speaker from this Instructable:
    [​IMG]
    I'm having some trouble figuring out the schematic for this project, since there isn't a complete one posted in the instructions. I think I've figured everything out at this point except I'm confused by the way that the Bluetooth chip is wired. As far as I can tell, the Bluetooth chip in the photo is identical to this one: http://www.dx.com/p/bluetooth-board-module-4-pin-121326. It appears to me that this chip doesn't support audio, and I can't tell from the photo how it's wired.

    The Bluetooth module that is mentioned in the parts list is a different one that I have already ordered: http://www.dx.com/p/bluetooth-v2-0-...w-3-5mm-audio-male-to-male-cable-black-163694. I think it might work if I hack it by removing the headphone socket and rewiring it as an input in parallel, but I'm not really sure. What Bluetooth module do I need to build this speaker?
    [​IMG]
     
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  2. wayneh

    Expert

    Sep 9, 2010
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    It's not clear where the digital information that passes over Bluetooth is converted to analog audio. The Bluetooth module does not appear to do that, so it must be the "amplifier" (and DAC). But it's hard to tell.

    I think if you can get the datasheets for the modules, it will become obvious how to hook it all up.
     
  3. tjohnson

    Thread Starter Active Member

    Dec 23, 2014
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    I figured out how to wire everything (except for the charging LED), and the speaker works now but there is a faint clicking noise in the background. I think the cause is a ground loop because of the way that the Bluetooth module is connected to the same battery as the audio amp.

    I read here that someone else fixed this issue by isolating the audio circuits with an NTE1 audio transformer:
    [​IMG]
    I wasn't able to find much documentation on this transformer, but I believe that it should work if I connect the red and black wires to the audio channels on the Bluetooth module, and the yellow and white wires to the audio channels on the amp board. (I think red corresponds to yellow and black to white, but I tried wiring it both ways and it didn't make a difference.)

    I soldered the transformer into my circuit and expected it to improve the quality of the sound, but instead when I turned my speaker on, all I heard was a lot of static noise and not even the "Bluetooth connected" sound. I bypassed the transformer and now everything is working again, but with the faint clicking sound still noticeable. Why does the transformer result in static noise and no audio rather than clearer audio?
     
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  4. wayneh

    Expert

    Sep 9, 2010
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    Use an ohmmeter to measure the resistance of the two sides of the transformer.

    Did you use separate power sources for your experiment?
     
  5. tjohnson

    Thread Starter Active Member

    Dec 23, 2014
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    The resistance between the yellow and white wires (on the same side of the transformer) is around 42.5 ohms, and since it's a 1:1 transformer I don't think it should matter which direction it is wired in the circuit. There isn't any resistance between the wires on opposite sides (red to yellow, black to white).

    I read this about the transformer:
    so I thought I should connect it the same way.

    No, I didn't use separate power sources for my experiment, since my purpose for using the transformer is to be able to power the Bluetooth module and audio amp from the same power source. I guess I'm not sure I understand your question.
     
  6. wayneh

    Expert

    Sep 9, 2010
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    How about the red-to-black resistance?
     
  7. wayneh

    Expert

    Sep 9, 2010
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    If the problem is caused by noise placed onto the power supply by the bluetooth circuit ending up in the audio, I'm not sure adding a coupling transformer into the audio signal would make any difference. (It shouldn't kill the signal, as you are seeing.) The noise on the power supply will still be there.

    But one thing at a time. The isolation transformer shouldn't kill the audio signal unless the impedance of the transformer input is so low that the source cannot drive it. The impedance will be quite a bit higher than the DC resistance that you measured, and 42.5Ω doesn't sound too bad for that.
     
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  8. tjohnson

    Thread Starter Active Member

    Dec 23, 2014
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    I had to desolder the red and black wires to measure their resistance accurately. I got a reading of about 56 ohms.

    If adding a transformer into the audio signal won't fix the problem, then what would I need I to do instead? Would a voltage regulator for the Bluetooth module work?
     
  9. wayneh

    Expert

    Sep 9, 2010
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    Hmmm, that's more-or-less what you'd expect. Hard to say why that didn't work.
    Just for the sake of experiment, can you use a separate battery for the two modules? This would test your hypothesis about the source of the problem. If this fails to fix the clicking, then obviously even a regulator for the BT module won't make the clicking go away. If the isolated batteries work, then you can work on a strategy for a fix.
     
  10. tjohnson

    Thread Starter Active Member

    Dec 23, 2014
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    Sorry for taking so long to reply, I've been busy with school and kept meaning to get around to testing this but didn't until tonight.

    I desoldered the connections from the Bluetooth module to the rechargeable battery, and instead connected three AA batteries in series (~4.5V). Using this separate power supply, the clicking sound went away as I expected.

    Therefore, since isolated batteries work, it appears that my hypothesis was correct. What can I do now to fix this issue?
     
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