Audio Amp Help

Discussion in 'General Electronics Chat' started by Casual_Engineer, Jun 13, 2014.

  1. Casual_Engineer

    Thread Starter New Member

    Jun 13, 2014
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    Hello there! I've been messing around with an audio amp I brought from Sparkfun. Right now, I'm powering the amp using my 150W power supply (30V and 5A). It works great with one JENSEN JRX235 speaker, but when I add another one, I notice thumping sounds coming from them. At first, I thought it could be power supply capacitance issue, so I added a 2200uF cap in parallel with the 1000uf Capacitor. However, the thumping from the speakers was still prevalent. I even added 470uF in series with each speaker.

    I took a couple screen shots to help out. Yellow is the input going into the speaker (which you can see is pretty bad), blue is the voltage on the power supply rail.

    Sorry for the question, but I'm new to audio electronics.
     
  2. wakibaki

    New Member

    Jun 12, 2012
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    Are you sure about this? Either channel is OK on its own with the other channel unloaded, but the symptoms appear when both channels are loaded? Or you only tried it one way round?

    Well, you've got a scope. You can inject a signal and follow it through each channel in turn. Start with a small signal and use a resistive dummy load. You can handily output a test signal from a computer using audacity.

    w
     
  3. Lestraveled

    Well-Known Member

    May 19, 2014
    1,957
    1,215
    Does the thumping occur when you are playing music?
    Have you tried playing one channel at at time with both speaker connected?
    Give us a little more info on how you have things set up.

    Mark
     
  4. Brevor

    Active Member

    Apr 9, 2011
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    The link you supplied to the amp says the maximum supply voltage is 22 Volts. If you are powering it with 30 Volts it may be too much.
     
  5. bertus

    Administrator

    Apr 5, 2008
    15,649
    2,348
    Hello,

    To power the chip with 30 Volts is TOO MUCH.
    In the datasheet is given 24 Volts when idle and 22 Volta when in operate.

    Bertus
     
  6. studiot

    AAC Fanatic!

    Nov 9, 2007
    5,005
    513
    Happily the module has significant self protection, so what you are hearing is likely the self protection switching the module off and on.

    But that will not go on forever without damage.

    Note also that there is a 4 amp cutoff on current stated in the datasheet.
     
  7. Casual_Engineer

    Thread Starter New Member

    Jun 13, 2014
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    To all

    1) I'm powering it with 18V, not 30V (I was simply describing the power supply I'm using)
    2) Like I said earlier, one channel is fine. But when I'm using both channels (i.e. both speakers) thumping occurs
    3) The thumping occurs when I'm doing input power measuring. I tried to get an accurate input power reading by connecting a multimeter in series with the input power.

    Also, yes I'm heatsinking the amplifier chip.
     
    Last edited: Jun 14, 2014
  8. studiot

    AAC Fanatic!

    Nov 9, 2007
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    OK so what is the impedance of your speakers?

    I assume that you have connected one to each channel?

    Does the effect still occur if you swop them over?

    Does the 'thumping' come from the same speaker or from the same channel each time?

    Is the thumping periodic, ie does it occur at fairly regular intervals?

    This could be the result of a faulty capacitor/ joint or connection charging and discharging suddenly.
     
    Last edited: Jun 14, 2014
  9. MrChips

    Moderator

    Oct 2, 2009
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    Does the thumping go away when you remove the ammeter?
     
  10. Casual_Engineer

    Thread Starter New Member

    Jun 13, 2014
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    Yes. Other times, the thumping is non existent.
     
  11. MrChips

    Moderator

    Oct 2, 2009
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    Then remove the ammeter.
     
  12. djsfantasi

    AAC Fanatic!

    Apr 11, 2010
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    "Doctor, it hurts when I do this."
    "Then don't do it!"
     
  13. Casual_Engineer

    Thread Starter New Member

    Jun 13, 2014
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    No educated guess on why it's thumping when it's doing that? I really would like to get an accurate reading of the input power so I can design the power supply more efficiently....
     
  14. studiot

    AAC Fanatic!

    Nov 9, 2007
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    wakibaki told you to use a resistive serial resistor and measure the voltage across it back inpost#2.

    Multimeters are not fast enough to follow audio, and if digital the ADC may lead to pumping, but you have a scope which is. Use it.
     
    Casual_Engineer likes this.
  15. AnalogKid

    Distinguished Member

    Aug 1, 2013
    4,546
    1,252
    Power amplifier chips are notorious for stability problems, and yours is called motorboating. The series ammeter raises the apparent output impedance of the power source, and there is not enough decoupling on the pc board to compensate.

    I can't tell where C3 and C4 are on the pc board, but C6 is too far away from the chip, especially with the thin power traces. Normally you filter power where it enters the board, but for audio PA's, you want a caps big and small within millimeters of the chip. Also, while they have to use thermal relief pads on the GND pins, they should have wider spokes. The reduction in resistance and inductance would be much greater than the additional thermal sink. The data sheet has specific directions about pcb layout and grounding, and I don't think they're all built into this board.

    Try tacking on another 1000 uF on the bottom side of the board from each Vcc pin to pins 7/8.

    ak
     
  16. BR-549

    Well-Known Member

    Sep 22, 2013
    2,004
    394
    If motor boating stops altogether with ammeter removal....solved. If MB continues, make sure speaker wires are same length and twisted. If MB comes from both sides....replace C6. If C6 is leaky....putting another cap across it....it still leaks. It's cheap and fast. If MB is one channel only....trade C7 and C8. Test. Trade C1 and C2. Test. Trade C11 and C12. Test. And as a last resort replace C13. I am not real hip on these audio pwr chips, but I have run in to my share of MB. Mostly it was weak electrolytics in DC path. But it can be in the signal path also. Sometimes.....just reversing a speaker lead can cure it.
     
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