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Powered Speaker Hum When Plugged In

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    Powered Speaker Hum When Plugged In

    M-Audio Studiopro 3 Relatively inexpensive powered speaker set.

    Produces 60 HZ hum. Thought it was only when turned on regardless of volume, but it can be heard through headphone jack and when you put the speaker next to your ear as soon as you plug it in. There is one large cap which I replaced. Checked solder joints on bottom of PCB. Re-soldered a few (which actually looked OK).

    Hum still there.

    https://usermanual.wiki/M-Audio/MAud...433470494/html

    Any ideas?

    Thanks,
    John
    Last edited by craftech; 04-03-2021, 10:10 AM.

    #2
    Re: Powered Speaker Hum When Plugged In

    need a foto of the insides.
    it sounds like a cap though.

    Comment


      #3
      Re: Powered Speaker Hum When Plugged In

      Originally posted by stj View Post
      need a foto of the insides.
      it sounds like a cap though.
      Here is the PCB (Note: the red wire is a mod the guy who wrote the article did to bypass the amp to make then passive}

      https://blog.jonandtina.net/uploads/IMG_0301.png

      External view:
      https://media.guitarcenter.com/is/im...00-500x500.jpg

      My photos: (NOTE: The only large cap is the one I replaced that wasn't blown) None of the caps look like they are bulged or leaking. The brown smears are phenolic glue.
      Attached Files
      Last edited by craftech; 04-03-2021, 10:53 AM.

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        #4
        Re: Powered Speaker Hum When Plugged In

        that's terrible, it could even be radiating between the nest of wires!!

        Comment


          #5
          Re: Powered Speaker Hum When Plugged In

          Originally posted by stj View Post
          that's terrible, it could even be radiating between the nest of wires!!
          Actually, when I was experimenting to find the cause I disconnected the wires and separated the chassis from the speakers, then ran them to their jacks and the hum was still there. I kind of thought it was the transformer or the transistor (IC) mounted on the heatsink.

          John
          Last edited by craftech; 04-03-2021, 12:07 PM.

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            #6
            Re: Powered Speaker Hum When Plugged In

            check the rectifier

            Comment


              #7
              Re: Powered Speaker Hum When Plugged In

              Originally posted by petehall347 View Post
              check the rectifier
              DMM set to Diode:

              Blk lead of DMM to + of Bridge Rectifier
              Red lead to each AC leg of Bridge Rectifier

              Both read .486 V

              Red lead of DMM to - of Bridge Rectifier
              Blk lead to each AC leg of Bridge Rectifier

              Both read .488 V

              My understanding is that between .4v and .6v are OK

              John
              Last edited by craftech; 04-03-2021, 02:39 PM.

              Comment


                #8
                Re: Powered Speaker Hum When Plugged In

                you got a scope there ?

                Comment


                  #9
                  Re: Powered Speaker Hum When Plugged In

                  do you still get audio playing ?

                  Comment


                    #10
                    Re: Powered Speaker Hum When Plugged In

                    Originally posted by petehall347 View Post
                    do you still get audio playing ?
                    I should have indicated that in the OP. (I'll add it in) Sorry can't edit the OP any longer.

                    The powered speaker set plays perfectly. It's just the hum you can hear when there is no music or quiet music and even when it is off but plugged in.

                    But they sound great.

                    Thanks,

                    John
                    Last edited by craftech; 04-03-2021, 03:25 PM.

                    Comment


                      #11
                      Re: Powered Speaker Hum When Plugged In

                      Originally posted by petehall347 View Post
                      you got a scope there ?
                      No

                      Comment


                        #12
                        Re: Powered Speaker Hum When Plugged In

                        short the inputs to ground and see if hum goes away

                        Comment


                          #13
                          Re: Powered Speaker Hum When Plugged In

                          Originally posted by petehall347 View Post
                          short the inputs to ground and see if hum goes away
                          The positive terminal? I tried grounding all the negative terminals yesterday.
                          Last edited by craftech; 04-03-2021, 04:29 PM.

                          Comment


                            #14
                            Re: Powered Speaker Hum When Plugged In

                            yep just short to ground .

                            Comment


                              #15
                              Re: Powered Speaker Hum When Plugged In

                              Originally posted by petehall347 View Post
                              yep just short to ground .
                              No change. Both RCA inputs and the 1/8" input as well.


                              In diagram 4 that I posted the terminal on the right is the 115V / 230V selector slide switch. I don't see any soldering issues. ??
                              Last edited by craftech; 04-03-2021, 04:55 PM.

                              Comment


                                #16
                                Re: Powered Speaker Hum When Plugged In

                                I am pretty sure the hum is coming from the transformer.

                                Is there a reasonable supplier for a replacement?

                                I don't know the specs. The sticker says BDT-48A002(SP3)
                                Shenzhen Xingyaoda Electronics Not sure if that helps though?

                                Output 15V 1A
                                Two red wires out

                                John
                                Last edited by craftech; 04-09-2021, 07:54 PM.

                                Comment


                                  #17
                                  Re: Powered Speaker Hum When Plugged In

                                  so it's a physical hum?
                                  if it's the windings or frame that's loose then epoxy or laquer will fix it.

                                  Comment


                                    #18
                                    Re: Powered Speaker Hum When Plugged In

                                    Originally posted by stj View Post
                                    so it's a physical hum?
                                    if it's the windings or frame that's loose then epoxy or laquer will fix it.
                                    I don't think it is rattling. It is mounted on a piece of gasket material and the screws are covered with a resin. It's not old so I don't think the laminations are loose.

                                    I can hear it with the unit plugged in (but turned off) if I listen close and louder when I turn it on.

                                    John

                                    Comment


                                      #19
                                      Re: Powered Speaker Hum When Plugged In

                                      it does sound like the plastic coil-frame may be vibrating against the iron.

                                      Comment


                                        #20
                                        Re: Powered Speaker Hum When Plugged In

                                        The first post indicated that the sound can be hear at the headphones. This makes it less likely that the transformer is cause a vibration that is producing the electric hum. You need to see if you can follow the input area towards some capacitors and test those. In my limit experience with a "humming" guitar amp, there was a bad capacitor. It did not visually look bad but tested bad.

                                        Comment

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