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Why do people put capacitors on the GND/PWR tabs...

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  • Why do people put capacitors on the GND/PWR tabs...

    Why do people put capacitors on the GND/PWR tabs of small BT amplifiers? On some of my 5-6v setups there was a suggestion to put a capacitor on the pos/neg terminals of the small amp itself, but i have never questioned it as it appears to work better with no brown outs/restarts.

    For example on using one of those CSR BT amp modules, it was suggested you put a 10v 2200uf capacitor soldered directly to the lead/terminals, yet on a recent build using similar parts it is now 10v 1000uf variant. Without it if i push the amp it will restart the BT component, but with it, it just pushes through no problem. So of course my mind starts to think what magic is this and how are they sized? These Chips also allow one to modify the on board EQ and filters, which made me wonder "could i size this capacitor and alter the EQ/Bass shelf without leading to saturation when pushed"

    Its one of those items i've never questioned as it works, but currently looking for ways to refine the process, build and see what else i can do.

    I seem to be having issues uploading an image to demonstrate this better.

  • #2
    It still wont let me post an image, says path is empty.

    Just incase there in any confustion, i'm talking about the power leads for the BT amp and not talking about why they are used in a crossover.

    Comment


    • djg
      djg commented
      Editing a comment
      My path is also empty, also before that error code, I got a picture too many MBs, and my camera was set as usual for all the pics previously posted. I reduced resolution and then got the empty path code. I smell a rat.

  • #3
    Hey man,

    I don't know the answer to your question for sure, but I wonder if it isn't for a similar reason as to why they used to put huge caps on the input voltage side of car stereo amp setups -- I think they called them stiffening capacitors... to store a little 'instant' power for the big bass hits.

    Maybe those caps are doing the same thing -- storing just enough power so that the voltage doesn't drop below the threshold, and it doesn't shut down?

    I really shouldn't even be replying as I have no actual idea, but the thought just occurred to me, so I thought I'd throw it out there.

    TomZ

    P.S. I can't attach any photos either.
    Zarbo Audio Projects Youtube Channel: https://www.youtube.com/channel/UCEZ...aFQSTl6NdOwgxQ * 320-641 Amp Review Youtube: https://youtu.be/ugjfcI5p6m0 *Veneering curves, seams, using heat-lock iron on method *Trimming veneer & tips *Curved Sides glue-up video
    *Part 2 *Gluing multiple curved laminations of HDF

    Comment


    • #4
      Originally posted by tomzarbo View Post
      Hey man,

      I don't know the answer to your question for sure, but I wonder if it isn't for a similar reason as to why they used to put huge caps on the input voltage side of car stereo amp setups -- I think they called them stiffening capacitors... to store a little 'instant' power for the big bass hits.

      Maybe those caps are doing the same thing -- storing just enough power so that the voltage doesn't drop below the threshold, and it doesn't shut down?

      I really shouldn't even be replying as I have no actual idea, but the thought just occurred to me, so I thought I'd throw it out there.

      TomZ

      P.S. I can't attach any photos either.
      i was thinking something similar, just via running the same setup without it and watching another buck converters digital voltage display freak out once a bass shelf filter was added. No science, just me adding it back on and not asking any questions.

      Comment


      • #5
        It gives you some more robustness to voltage drop from the power supply. Most switch mode DC power supplies will happily deliver their rated max power continuously, but voltage will drop quickly if more current in drawn. A Class D amp doesn't draw a continuous load, the current draw follows the output waveform. Consider playing a constant sine wave, the amplifier needs to draw enough instantaneous power for the peak of the sine wave. If you put enough capacitance on the input to the amplifier, it absorbs the peaks, and smooths out the load on the power supply. If you have sufficient capacitance to smooth the lowest signal frequencies, the power supply only needs to deliver the RMS power, not the peak power, so you effectively get a 40% boost in usable power output from the same power supply.

        Comment


        • 3rutu5
          3rutu5 commented
          Editing a comment
          oh right, thats cool. That explains why i get push my little 5-6.5v bt chips harder (well to max volume) with bass heavy tunes and i dont get any dips. But what i hear it potentially comes at a cost with more power draw. Might be why the guy went from a 2200uf to a 1000uf.

          I was thinking about trying to saturate the signal by putting a bass shelf onto a DSP/EQ of a board and seeing what i needed to do to not have it cut out. Maybe this is a way to get that little bit more low end out of the drivers by putting on a cap to cover the peaks. what size that cap is i dont know (or if it would even work the way i think it would).

      • #6
        Originally posted by zx82net View Post
        It gives you some more robustness to voltage drop from the power supply. Most switch mode DC power supplies will happily deliver their rated max power continuously, but voltage will drop quickly if more current in drawn. A Class D amp doesn't draw a continuous load, the current draw follows the output waveform. Consider playing a constant sine wave, the amplifier needs to draw enough instantaneous power for the peak of the sine wave. If you put enough capacitance on the input to the amplifier, it absorbs the peaks, and smooths out the load on the power supply. If you have sufficient capacitance to smooth the lowest signal frequencies, the power supply only needs to deliver the RMS power, not the peak power, so you effectively get a 40% boost in usable power output from the same power supply.
        The capacitor only needs to cover the peaks of the lowest tone you want to play. So say it is 20Hz, it needs to cover the peak of that waveform without significant sag, which is 25ms. What I'm getting at is a capacitor can be big enough, and beyond that you are solving a different problem. If you go to beyond that you are trying to cover significant peaks in music, past the continuous capability of the power supply and sustain the current for 100s of ms or more. The thing is, putting a big capacitor directly on the output of a power supply is not a very kind thing to do. Beyond a certain point the supply might detect a short circuit and shut off, or blow a fuse, or catch fire... 🔥

        Comment


        • #7
          Originally posted by zx82net View Post

          The capacitor only needs to cover the peaks of the lowest tone you want to play. So say it is 20Hz, it needs to cover the peak of that waveform without significant sag, which is 25ms. What I'm getting at is a capacitor can be big enough, and beyond that you are solving a different problem. If you go to beyond that you are trying to cover significant peaks in music, past the continuous capability of the power supply and sustain the current for 100s of ms or more. The thing is, putting a big capacitor directly on the output of a power supply is not a very kind thing to do. Beyond a certain point the supply might detect a short circuit and shut off, or blow a fuse, or catch fire... 🔥
          oh ok, thats not cool, would you deem a 10v 2200uf cap "too big"? im assuming the guy was putting one on this to cover the low end as thats all that triggers an issue.

          im not sure where to start but i feel like i need to know more, might be going down a rabbit hole again

          Comment


          • #8
            Originally posted by 3rutu5 View Post

            oh ok, thats not cool, would you deem a 10v 2200uf cap "too big"? im assuming the guy was putting one on this to cover the low end as thats all that triggers an issue.

            im not sure where to start but i feel like i need to know more, might be going down a rabbit hole again
            In principle, the calculation is simple, but you need to know quite a few variables, it depends how quickly the power supply voltage starts to drop when excess current is drawn, the sensitivity of the amplifier to voltage drop, (see the data sheet), that will tell you how much distortion is being produced.

            I'd not get into that rabbit hole if I were you :-D However,
            I believe massive capacitor banks are a thing in car audio, you might find some recommendations or calculators in that field.

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