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Help with power supply hum

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  • Help with power supply hum

    Hi,

    I need some help/insight with regards to hum caused by power supply.
    i designed and built many speakers comprising of

    - minidsp
    - 2 sure amps
    - sure aptX bluetooth unit

    the minidsp and amps run off 19 volts and i had to drop the 19V line to 12V to power the bluetooth unit.
    I used a isolated DC-DC converter, part number PG02S2412A,.. link - https://www.mouser.com/ProductDetail...52BUjKhQ%3D%3D
    And everything worked perfectly. No hum, just clean output.

    The vendor i used for the power supply was Pwr+ 19V 4.8A (laptop power supply). but unfortunately PWR+ at some point changed the model of the power supply to a different type and it introduced a high frequency hum into my design. So i figured, it was just that particular power supply causing it. I then tried several others including the 19V 4.8A power supply available here on PE. - https://www.parts-express.com/19v-48...-plug--120-058, which introduced an even worse hum.

    The unit from PE works fine if disconnect the DC-DC converter.

    But heres the thing, why would some power supplies introduced a hum (the hum alters frequency in a pattern), and others dont. I used an old DELL power brick, 19V 4.8A to test my theory and it was dead silent.

    so, my conclusion is that its the power brick causing it. and maybe i just got lucky with the original PWR+ power supply that i used that worked perfectly.

    arent all laptop power supply switching power supplies? is that what could be introducing the hum maybe, just maybe because the older previous power supply was non switching????

    is there a way to drop the voltage and prevent the hum?

    is there such a thing as non switching power supply (laptop style power brick) even available anymore?

    ive already bought and tried several units and they all hum, the older one i used does not.

    any solution apart from keep buying and trying?


    ive completely run out of ideas.

    hoping to get some help/insight into my hum problem.

    thanking all in advance for any and all help.

    asmd.

  • #2
    Maybe I'm not following. You are using the same power supply to supply both the minidsp and amp boards with [email protected]? Why can't you use a buck-converter off that same power supply to feed 12v. to the bluetooth unit?
    http://techtalk.parts-express.com/fo...khanspires-but
    http://techtalk.parts-express.com/fo...pico-neo-build
    http://techtalk.parts-express.com/fo...ensation-build

    Comment


    • #3
      Originally posted by Kornbread View Post
      Maybe I'm not following. You are using the same power supply to supply both the minidsp and amp boards with [email protected]? Why can't you use a buck-converter off that same power supply to feed 12v. to the bluetooth unit?
      I think he used the Isolated DC to DC converter to bring the 19v down to 12v for the Bluetooth. All of this off the same power supply.

      Could the switching frequency of the two amps (class D I presume) be causing issues between the two of them and the DSP? I THINK I've read of two amps having issues when they were class D before. I have no idea why the isolated DC to DC converter from Mouser would be causing an issue. Did you isolate both sides of the power, + and -?

      I had similar issues with hum and noise on a few previous projects and the isolated converter did the trick for me. I tried 4 or 5 different power supplies, some with a ground plug, others without. They all made noise, some more and some less, but they all had issues. I can't imagine that the Dell PS wasn't a switching unit. Dell does make REALLY good stuff, though, it may have had extra filters or something built in to eliminate extra noise. My wife's school laptop is a Dell... it's over 10 yrs. old and her school is finally switching them out, even though they still work fine.

      I'm sorry I can't really help, but there are a few really knowledgeable guys on here who can probably help figure it out. I'm interested in finding out what the solution is just out of curiosity myself.

      TomZ
      *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 *Cello's Speaker Project Page

      *Building the "Micro-B 2.1 Plate Amplifier -- Part 1 * Part 2 * Part 3 * Part 4 * * Part 5 'Review' * -- Assembly Instructions PDF

      Comment


      • #4
        Hmm ... guess a buck converter is a dc-dc step down converter. I have a bag of them I was going to use for just that purpose but never got around to it. IIRC, the tiny things cost <$2 each off the bay.
        http://techtalk.parts-express.com/fo...khanspires-but
        http://techtalk.parts-express.com/fo...pico-neo-build
        http://techtalk.parts-express.com/fo...ensation-build

        Comment


        • #5
          Originally posted by Kornbread View Post
          Hmm ... guess a buck converter is a dc-dc step down converter. I have a bag of them I was going to use for just that purpose but never got around to it. IIRC, the tiny things cost <$2 each off the bay.
          I was thinking exactly the same thing. AliExpress was where I got mine if my memory is correct... but like you said, so cheap that you pick up half a dozen or so just in case.
          I think if there is no noise that needs to be eliminated, they would be perfect to do the job.

          TomZ
          *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 *Cello's Speaker Project Page

          *Building the "Micro-B 2.1 Plate Amplifier -- Part 1 * Part 2 * Part 3 * Part 4 * * Part 5 'Review' * -- Assembly Instructions PDF

          Comment


          • #6
            to clarify...

            - yes, all the minidsp and both sure amps are powered off the same 19V power supply. and there is no hum whatsoever.
            -once i tap into the 19V with the D-DC converter to bring down the voltage to 12V there is this alternating high frequency hum,
            - this never happened in when i originally designed it using the old no longer available power supply (PWR+ 19V 5A laptop power supply)
            - it also doesnt happen i tested with an older DELL 19V 4,6A power supply.
            - it hums with the 19V power supply that i linked to, from parts express. It also hums other power supplies that ive recently procured, looking for one that doesnt.
            - i built many copies of my design with the older 19V 5A PWR+ power adapter with no problems whatsoever. therefore, im thinking, the only difference is the power supply.


            hope that clarifies my problem, .. in the hope of finding a solution.

            the newer power supplies, such as the one from PE are causing this, which was never there before.... but why?

            and what could i do to fix it.

            Comment


            • #7
              Why overly complicate things. Use a 7812 voltage regulator, good for an amp and half, and a small filter cap, 47uF, to feed the bluetooth module. 2 Parts 2 bucks + shipping. Not a lot to go wrong.

              Comment


              • #8
                And I assume this brings up filtering. How would one go about filtering the output of a switching power supply?
                http://techtalk.parts-express.com/fo...khanspires-but
                http://techtalk.parts-express.com/fo...pico-neo-build
                http://techtalk.parts-express.com/fo...ensation-build

                Comment


                • #9
                  Originally posted by Kornbread View Post
                  And I assume this brings up filtering. How would one go about filtering the output of a switching power supply?
                  Very simple, LC circuit.
                  craigk

                  " Voicing is often the term used for band aids to cover for initial design/planning errors " - Pallas

                  Comment


                  • #10
                    Originally posted by craigk View Post

                    Very simple, LC circuit.
                    Yup. Rule of thumb down a decade from the switching frequency of the supply and a simple 2nd order filter as a starting point. +- 20% is usually close enough if you're using components you have on hand.

                    Comment


                    • #11
                      ok, thank you all for replies.
                      this is going a little over my head.
                      some links to the parts would be helpful.
                      LC circuit, what do i need to make one. components list, a schematic would be helpful.

                      still doesnt explain why some power supplies hum and some dont.....

                      cant i use the filter on the DC-DC converter i already have? and it would have to filter more than one frequency. as the hum pitches up and down. (im guessing the adapters that cause this dont have a filtered output?)

                      if so, where would i put the filter?

                      im ok with basic electronics putting off the shelf parts together. and i thought i did great with my original design. but i guess i got lucky with the old PWR+ power supply. should have bought a bunch more and kept for later. i just assumed they all worked the same and any 90W power adapter would fucntion the same way. FML. another learning curve for me, lol.



                      Comment


                      • #12
                        also, had a look on ebay. the smaller ones are rated for 3A. adjustable output DC-DC converter.
                        input voltage is 19V 4.6A. there are 5A units, they are bigger. and space is a premium.
                        do these automatically filter noise out?

                        the single component DC-DC component converter that i got from mouser seemed like an elegant solution at the time and everything just worked,

                        Comment


                        • #13
                          also to get a more technical understanding..

                          - why does the power supply from PE nut hum when just using it with 2 amps boards and the minidsp (source input, line in to the minidsp)?
                          - but does when past the output of the DC-DC converter that i used for the bluetooth module (used as source)?
                          - is the problem fundamentally the DC-DC converter causing the noise, to where there was none? why would it add noise if there was no noise to begin with? it never did before with the previous power supply or the DELL power supply.


                          appreciate the explanation.

                          Comment


                          • #14
                            Stupid question probably, but does the proximity of any of the items cause the noise to be more or less?

                            Does the Bluetooth module generate noise if powered independently of the main 19v PS? (as in with another power source not using the DC/DC isolated converter)

                            I don't know much about this stuff, but I'm puzzled. The isolated converter should have taken care of the issue I would have guessed; I think you made a great choice using that part personally.

                            I'm actually suspicious of the Sure Bluetooth unit. I know they seem to be really nice units, but I've used a few and they have had issues for me... Connection issues as well as 'staying on' issues in my case, but even cheap amazon/Aliexpress units seem to perform better in my opinion. Like I said, I know basically nothing about this stuff, and I hate to poo-poo a PE product and I think Sure is basically a good company, but it might be worth looking into another Bluetooth board if you can't find another fix.

                            TomZ
                            *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 *Cello's Speaker Project Page

                            *Building the "Micro-B 2.1 Plate Amplifier -- Part 1 * Part 2 * Part 3 * Part 4 * * Part 5 'Review' * -- Assembly Instructions PDF

                            Comment


                            • #15
                              If I'm understanding this correctly the hum starts when you hook up the DC-DC convertor without the bluetooth board. It's possible that the hum is caused because the DC-DC convertor isn't under load.

                              Does it still hum if you hook up the bluetooth board without the audio cables?
                              Does it still hum if you hook up the bluetooth board and the audio cables?

                              See post #7 for a quick fix. In fact just skip the cap and use a 7812 on the output of the power supply to drive the bluetooth board. Just pull one from an old PC power supply to try it.

                              As an aside after looking at the specs for the DC-DC convertor and the bluetooth board the convertor doesn't meet the spec for the minimum current Sure specs for the bluetooth board

                              Comment

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