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Huming sound in homemade bluetooth speaker

Discussion in 'Radio and Wireless' started by Hans Avlund, Feb 14, 2017.

  1. Hans Avlund

    Hans Avlund

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    Feb 14, 2017
    The question:

    How do I make this 200Khz huming sound go away?

    More info:

    I could use some help. Saw a YouTube video of a guy making a independant bluetooth speaker/bombox. It has a 3000mah battery at 12v. It uses a cheap chineese USB bluetooth reciver. I purchaced a AMP that supportes 2x15Watts.

    In the video he makes the stereo signal into mono using resistors, but I did this using two capasators, works fine.

    He used this dubble amplifying trick, where he runs the mono signal through the AMP twice. I did this, but found out that a ground loop isolater was needed for this to work.

    About the problem:

    When the signal goes out of the amplifier it gets a humming noice. My teacher used the ocyloscope to find out that the huming/buzz had a frequenzy of 200KHz. I tried using a ground loop isolater between the speaker and amplifier, but then it just mad this loud banging sound.
    So I want to remove this huming.

    So again:

    How do I make this 200Khz huming sound go away?
    (It is at 2v if it matters)
     
  2. Hans Avlund

    Hans Avlund

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    Feb 14, 2017
    Thank you for any replies, and sorry for bad english.
    -17y/o Norwegian
     
  3. duke37

    duke37

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    Jan 9, 2011
    You will not be able to hear 200kHz (not 200KHZ). It has nothing to do with degrees Kelvin.

    Using a power amplifier as a preamp gives a large voltage and current gain which must be controlled with very careful ground connections. Why do you put the amplifiers in series?
    The buzz is probably ripple in the power supply.

    Show us a circuit and layout and give the specification of the amplifier.
     
    Hans Avlund and davenn like this.
  4. Hans Avlund

    Hans Avlund

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    Feb 14, 2017
    Amplifier:
    http://www.ebay.com/itm/DC-12V-TA20...272505?hash=item1eb16606b9:g:aQIAAOSwR5dXQ~tA

    Scematic:
    http://imgur.com/a/eKf8o

    Resistor R1 is replaced with condensators, dont remeber the value. And I have no treble speakers as it was out of budget.

    More IRL pictures
    http://imgur.com/a/aNcH5

    Thanks for fast response
     
    Last edited: Feb 14, 2017
  5. duke37

    duke37

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    726
    Jan 9, 2011
    It seems as if this is a class T amplifier which is a version of a class D amplifier. These use high frequency switches in the output stage instead of linear devices. To make the waveform you want, the switching varies in frequency or pulse width or both. The output is filtered to reduce the high frequency spurious but there will always be some there. Speakers will not respond to this frequency.

    The 200kHz you see on a scope is just the normal operation of the amplifier.
     
    Hans Avlund likes this.
  6. Audioguru

    Audioguru

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    593
    Sep 24, 2016
    Your amplifier shows a fairly old IC made by Tripath. Nobody knows who makes the cheap Chinese amplifier.
    Tripath went bankrupt some years ago, maybe because their amplifier ICs did not work properly.
     
    Hans Avlund likes this.
  7. Hans Avlund

    Hans Avlund

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    Feb 14, 2017



    So you think there is no fix to this problem? Like maby making a sort of filter?
     
  8. duke37

    duke37

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    726
    Jan 9, 2011
    Why do you use two amplifiers in series? You are amplifying the high frequency leak from the first amplifier. If you need more gain, then make a simple analog amplifier with two transistors or one operational amplifier. This will be much simpler than trying to filter out the high frequency.

    Amplifiers in series will not give more power. You can get more by using the amplifiers in anti-phase and connecting the speaker between the two outputs.

    If you have an amplifier spare, then ground the input to stop it going wild.
     
    davenn likes this.
  9. Audioguru

    Audioguru

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    Sep 24, 2016
    The Chinese amplifier already has bridged amplifiers for both its stereo channels. That is how it produces 15 Whats (10 Watts) into 4 ohms per channel when powered by only 12V.

    The hum comes from the inputs (maybe unshielded wires or a ground loop) or from the power supply. Does the hum go away when the inputs are disconnected?
     
  10. Hans Avlund

    Hans Avlund

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    Feb 14, 2017
    I have not tried, I can look at it next week since we have this only once a week. Tuesday.



     
  11. Audioguru

    Audioguru

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    593
    Sep 24, 2016
    Your photo of the wiring shows ordinary wires all over the place.
    Your schematic shows two wires for each speaker but only a single wire at the amplifier input that might have its shield disconnected so this wire is an antenna that is picking up 50Hz mains hum that is all around you. The shield blocks hum when it is connected properly.

    The bridged speaker outputs do not have a ground wire since both wires are driven for almost 4 times the power of having only one wire driven with a ground wire.
     
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