hum from diy amp

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  • Coconutout
    Senior Member
    • Oct 2006
    • 329

    hum from diy amp

    first of all i didn't build the amp but being a diy kit, i am willing to modify it with your help to fix the humming problem. im getting 60hz hum from both channels thats rather noticable on idle. however when i disconnect the rca cable from the preamp while leaving it still attached to the amp's input, the hum goes away. with both preamp and the dac's powercord unplugged at their chassis and rcas still connected the hum is present. with only dac connecting to the amp it still hums. with the cables swapped to different ones it still hums! so the amp by itself or with any rcas connected from its inputs there is no hum but otherwise it's reacting to my other components like they're bad case of allergies. what do you think could be the problem? im baffled
  • Johnloudb
    Super Senior Member
    • May 2007
    • 1877

    #2
    It sounds like a ground loop, unless you're other components are actually generating the hum. So, you could try lifting the ground on you're amp first ... do this at you're own risk, as any live wire that may touch the chassis may not blow the fuse and if you touch you're amp in this case, you could well be electrocuted!!!!!
    John unk:

    "Why can't we all just, get along?" ~ Jack Nicholson (Mars Attacks)

    My Website (hyperacusis, tinnitus, my story)

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    • technodanvan
      Senior Member
      • Nov 2009
      • 986

      #3
      Is everything plugged into the same outlet?

      Edit:

      I just reread your post and you mentioned having the power cords unplugged for everything else. Interesting...was anything else plugged into the preamp while it was unplugged from the wall?
      - Danny

      Comment

      • JonMarsh
        Mad Max Moderator
        • Aug 2000
        • 15209

        #4
        Originally posted by Coconutout
        first of all i didn't build the amp but being a diy kit, i am willing to modify it with your help to fix the humming problem. im getting 60hz hum from both channels thats rather noticable on idle. however when i disconnect the rca cable from the preamp while leaving it still attached to the amp's input, the hum goes away. with both preamp and the dac's powercord unplugged at their chassis and rcas still connected the hum is present. with only dac connecting to the amp it still hums. with the cables swapped to different ones it still hums! so the amp by itself or with any rcas connected from its inputs there is no hum but otherwise it's reacting to my other components like they're bad case of allergies. what do you think could be the problem? im baffled
        This could be several different problems, but the most likely one is a problem with the star grounding wiring for the amplifier; i.e., how ground is connected internally, and the input ground for the RCA's is being returned to a ground point for which there is rectifier ripple current voltage drop between it and the actual small signal input/feedback reference ground; by connecting the input, the ground noise potential at the input connector is coupled into the input circuitry by the source impedance of the input.

        I've seen this sort of thing frequently with DIY amps, first time I debugged one with this problem was in the mid 70's.
        the AudioWorx
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        Modula Neo DCC
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        Resistance is not futile, it is Volts divided by Amperes...
        Just ask Mr. Ohm....

        Comment

        • kingpin
          Senior Member
          • Jun 2006
          • 958

          #5
          Originally posted by JonMarsh
          This could be several different problems, but the most likely one is a problem with the star grounding wiring for the amplifier; i.e., how ground is connected internally, and the input ground for the RCA's is being returned to a ground point for which there is rectifier ripple current voltage drop between it and the actual small signal input/feedback reference ground; by connecting the input, the ground noise potential at the input connector is coupled into the input circuitry by the source impedance of the input.



          8O I think I have just added english as a second language. LOL

          Mike
          Call me "MIKE"
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          Comment

          • Not2Evil
            Member
            • Dec 2008
            • 99

            #6
            Most audio components have an external ground on the back of the chassis. If not, loosen a chasis screw and scratch away some paint. I'd try connecting the biggest wire you have from unit to unit and then pick a good ground on your amp. Heavy ground braid would be better, but harder to come by.

            If that doesn't work, you could try making a custom RCA cable using twisted pairs.

            You know sooner or later.....It's got to happen.

            It hums cause it doesn't know the words. :P

            Comment

            • Johnloudb
              Super Senior Member
              • May 2007
              • 1877

              #7
              It hums cause it doesn't know the words. :P
              Words of wisdom my friend! ;x(
              John unk:

              "Why can't we all just, get along?" ~ Jack Nicholson (Mars Attacks)

              My Website (hyperacusis, tinnitus, my story)

              Comment

              • Brian Kingsbury
                Member
                • Sep 2009
                • 44

                #8
                My diy amp had some hum issues when I built it using a single star-wired grounding scheme. I had all the grounds (power supply, signal input, negative speaker) all star wired together to a single point and from that point to the chassis. The power supply was causing a 60hz hum to be transfered into the signal input. To fix it, I made one star wired ground point where the power supply ground came together and attached from there to the chassis. I then made another star-wired point where the signal/negative speaker grounds came together and attached that to the chassis at same place where the power supply ground connected. Problem solved. YMMV.
                ;x( We're not worthy! ;x(

                Comment

                • Coconutout
                  Senior Member
                  • Oct 2006
                  • 329

                  #9
                  thanks for help guys and sorry i was mistaken in my experiment- the amp does not hum with just the dac connected. the problem was in the dying pairs of 12ax7 in my preamp. and i'm guessing the reason how it could make the amp hum without the powercord attached to the preamp is because of residual power in the transformer. two years of continuous use of the sovtek 12ax7s had finally made them microphonic. i think the new pair of 12ax7 i'm buying is the first 'necessary' purchase i'm making in this hobby. of course they don't necessarily have to be mullard nos xD. once again, thank you for your help, gentlemen.

                  Comment

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