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  • Subwoofer Hum - Need Help





    Description of the noise:


    My setup (starting at the source):
    Onkyo Home Theater Receiver (RZ-810),
    o Using the sub preamp output (unbalanced RCA).
    o Note that the receiver has a 2 prong plug, not a 3 prong. So it has no connection to earth ground, right??
    https://www.parts-express.com/rolls-hmb115-2-channel-stereo-analog-audio-balanced-to---from-unbalanced-signal-converter--245-023
    I then run an XLR cable from the Rolls signal converter to a Yamaha pro amp (P2500s).
    o The amp does have a 3 prong plug.
    The amp is connected to the sub using standard 12 gauge speaker wire.
    https://www.parts-express.com/dayton-audio-rss315hfa-8-12-reference-hf-subwoofer-8-ohm--295-445)
    o It is built into a sealed box. Denovo flat pack, 2 cu ft.
    The amp is run in bridged mode, but I have also tried using a single channel, with no difference in the hum.

    What I have tried so far:
    1.
    2.
    3.
    4. I used a 3 prong to 2 prong converter to completely eliminate the earth ground on the amp, no difference. (I would never run it this way because of safety concerns, I was just trying to turn over all stones).
    5. Interestingly, if I disconnect the RCA cable from the input on the Rolls, there is no hum. In other words, the amp is powered up, the Rolls is powered up and is connected to the amp, but there is nothing connected to the input of the Rolls. There is no hum.
    a.

    Where to go from here?


    I believe this is the power adaptor that came with the Rolls https://www.parts-express.com/rolls-ps27s-15-vdc-300ma-power-adapter-with-21mm-x-55mm-plug--245-1228

    Any thoughts or advice would be very much appreciated!

  • #2
    It would be good if you actually determined if your hum noise is 60Hz or something other. Maybe you can use a cell phone app to determine the actual frequency. Typically though, it is 60Hz or maybe a double of that, 120Hz. There are many possibilities. Most hums originate with grounding issues, ground loops and such. You have so much interconnected equipment, that the hum could be anywhere. You have to use a process of elimination. I realize that my reply to your post is of little help, but you have to go about it methodically. Since the hum seems to be because of your RCA cables or RCA inputs, that is probably where you should concentrate your efforts. Have you tried higher quality RCA interconnects? Long RCA cables can act as antennas and they can also be the source of unwanted ground loops. Better RCA audio cables will have two conductors and a separate shield. Cheap generic RCA cables use the shield as one of the conductors.

    Comment


    • #3
      The hum is probably a ground loop. Causes and cures explained here:
      http://www.rane.com/note110.html

      Probably the most common cause in HT systems is the TV cable, which should be transformer isolated.
      www.billfitzmaurice.com
      www.billfitzmaurice.info/forum

      Comment


      • #4
        I had a ground loop with significant hum when I was configuring my TV audio system. Like Bill said, the TV cable was the culprit. Disconnect it temporarily to see if that helps.
        Bill Schneider
        -+-+-+-+-
        www.afterness.com/audio

        Comment


        • #5
          Originally posted by BrianRIC View Post
          5. Interestingly, if I disconnect the RCA cable from the input on the Rolls, there is no hum. In other words, the amp is powered up, the Rolls is powered up and is connected to the amp, but there is nothing connected to the input of the Rolls. There is no hum.
          a. What is even more interesting is that if I then connect an RCA cable to the input on the Rolls, even with the other end of the RCA cable connected to nothing, the hum is there. That has led me to believe it’s not a problem with the receiver.
          That part caught my attention. That sounds like the RCA cable is coupling with a power cable along the run and acting like a big antenna picking up hum. How long is that RCA cable and have you tried a different cable or modifying the run?
          Electronics engineer, woofer enthusiast, and musician.
          Wogg Music
          Published projects: PPA100 Bass Guitar Amp, ISO El-Cheapo Sub, Indy 8 2.1 powered sub, MicroSat, SuperNova Minimus

          Comment


          • #6
            Originally posted by wogg View Post

            That part caught my attention. That sounds like the RCA cable is coupling with a power cable along the run and acting like a big antenna picking up hum. How long is that RCA cable and have you tried a different cable or modifying the run?
            And is the RCA cable shielded ...

            Comment


            • #7
              Everyone - Thank you for the replies so far. I am trying to be as methodical as possible. I also thank you for your patience if I am ignorant on some of these topics. I'm still learning!

              I have tried 3 different RCA cables, all of them sounded the same, all were 6 feet long. I tried getting the cable as far away from other devices as possible, no difference. At least one of the RCA's was shielded, because I just bought it from PE. Part #181-402. It says "Tinned copper braid over Mylar foil provides maximum shielding". Is this cable up to par??

              I should have also noted that I still have my old subwoofer, which is a small Yamaha with a plate amp built in, and it has no hum. It is unbalanced all the way, so no need for a signal converter.

              Bill - when you say the TV cable, you mean the power cord or the cable coaxial? I will try unplugging both to see if it makes a difference when I get home tonight. I can tell you that the hum is there even if the TV is powered off.

              I also read the article that you linked. I think I am running my set up the way they recommend under the heading, "The Next Best Way to Do It". I'm confused on 2 things...
              1. Is the Rolls HMB115 Converter a transformer isolation box as the article says I need?
              2. Do I need to manually ground the Rolls somehow?

              Comment


              • #8
                Originally posted by Millstonemike View Post
                And is the RCA cable shielded ...
                ​I would think that all RCA cable are shielded, they come that way
                I however have rolled my own, dual symmetrical conductors surrounded by a completely separate shield. I connect the shield to only one end of the cable, so that it isn't a signal conductor and I also slip over a ferrite bead to suppress any EMI/RFI.

                Comment


                • #9
                  Originally posted by BrianRIC View Post
                  Everyone - Thank you for the replies so far. I am trying to be as methodical as possible. I also thank you for your patience if I am ignorant on some of these topics. I'm still learning!

                  I have tried 3 different RCA cables, all of them sounded the same, all were 6 feet long. I tried getting the cable as far away from other devices as possible, no difference. At least one of the RCA's was shielded, because I just bought it from PE. Part #181-402. It says "Tinned copper braid over Mylar foil provides maximum shielding". Is this cable up to par??

                  I should have also noted that I still have my old subwoofer, which is a small Yamaha with a plate amp built in, and it has no hum. It is unbalanced all the way, so no need for a signal converter.

                  Bill - when you say the TV cable, you mean the power cord or the cable coaxial? I will try unplugging both to see if it makes a difference when I get home tonight. I can tell you that the hum is there even if the TV is powered off.

                  I also read the article that you linked. I think I am running my set up the way they recommend under the heading, "The Next Best Way to Do It". I'm confused on 2 things...
                  1. Is the Rolls HMB115 Converter a transformer isolation box as the article says I need?
                  2. Do I need to manually ground the Rolls somehow?
                  ​He certainly means the coaxial antenna/signal cable, not the power cord.
                  RCA cables come shielded, so I wouldn't worry too much about that.
                  You could temporarily connect a separate grounding wire to the Rolls and try that, shouldn't hurt anything.
                  A combination of balanced and unbalanced connections is never a good thing.

                  ​One other thing, try "flipping" over some of your AC power cords. Even though 120VAC is theoretically not polarized, most power cords are mechanically polarized. That is they can only be inserted into the wall receptacle one way, not the other. Flipping over an AC cord is not always easy to do, because most modern AC cords have one narrow prong and one wide prong, which again is designed to be inserted into the wall receptacle in only one direction. You either have to modify the AC cord or use a modified ground cheater.

                  If nothing else works, then you'll have to take it all outside and shoot it.

                  Comment


                  • #10
                    Originally posted by BrianRIC View Post

                    Bill - when you say the TV cable, you mean the power cord or the cable coaxial? ?
                    The coax. Many assume that ground loops are caused by lack of grounding or shielding. They're not. They're caused by too many pathways to ground. TV coax is a prime offender, because the level of ground loop noise is proportional to the distance of multiple ground pathways [i]squared.[/quote]When you have the AC outlet grounds and TV coax grounds both tying to the building ground back at the service panel that can make for a lot of wire. TV cable boxes should be transformer isolated as a matter of standard practice, but they aren't. The Rolls is a power transformer, not an isolation transformer. That would be these:
                    https://www.amazon.com/TII-220-Groun...re-bullets-btf
                    https://www.parts-express.com/art-dt...lator--245-875
                    www.billfitzmaurice.com
                    www.billfitzmaurice.info/forum

                    Comment


                    • #11
                      I have also experienced speaker hum problems years ago. It was coming from my rooftop over the air antenna connection--even when the TV was off!?! Disconnected the coax from the wall and the hum disappeared. I also learned that cable boxes often cause a similar hum. Try disconnecting all your antenna or cable box connections. If it clears up, reconnect one at a time to find the culprit.

                      I was able to use coax isolation transformers from PE, but PE no longer sells the ones I used. Bill linked to one above that looks similar.
                      https://www.amazon.com/TII-220-Groun...re-bullets-btf

                      Good luck!
                      Marv

                      Comment


                      • #12
                        Originally posted by marvin View Post
                        even when the TV was off!?
                        When the TV is off the wire is still there. That would have been a monster loop, with the second ground path going from the ground up to the antenna, from there back down to the TV.

                        www.billfitzmaurice.com
                        www.billfitzmaurice.info/forum

                        Comment


                        • #13
                          Originally posted by billfitzmaurice View Post
                          When the TV is off the wire is still there. That would have been a monster loop, with the second ground path going from the ground up to the antenna, from there back down to the TV.

                          That actually sounds scary or even dangerous!

                          Comment


                          • #14
                            Not at all, the antenna would have been grounded like a lightning rod, for the same reason. The problem with ground loops is that what causes them is perfectly OK as far as code is concerned. But code doesn't concern itself with how your gear sounds, only if it's safe.
                            www.billfitzmaurice.com
                            www.billfitzmaurice.info/forum

                            Comment


                            • #15
                              So here's what I've discovered...

                              ​Unscrewing the Cable/modem coax from the wall definitely reduces the hum, but does not eliminate it.
                              ​Unplugging the power adaptor for the cable box reduces it a little further.
                              ​Unplugging the Bluray player reduces it a little further.
                              ​Unplugging the TV reduces it a little further.
                              Unplugging the receiver reduces it a little further.
                              ​Unplugging the second subwoofer reduces it a little further, to the point that it is eliminated.

                              ​I will certainly be buying one of those ground loop isolators for the coax. Thank you for suggesting that. I would have never guessed the coax.

                              But what do I do about the other stuff? It seems the only way I can eliminate it is to unplug everything except the sub amp and the signal converter, which is not gonna do me much good. As I plugged things back in, I tried reversing the polarity for the ones with two prongs, but it did not help.

                              What do you all suggest that I do next? Thanks for your help so far!

                              Comment

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