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Subwoofer hum when running on step-down transformer

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  • Subwoofer hum when running on step-down transformer

    Hi, I'm new to the forum - hoping someone here can help with a minor issue I seem to be having:
    I've recently moved to New Zealand from Canada and brought along an Onkyo (SKW-540) subwoofer. I'm running it on a decent step-down power transformer but I noticed a slight hum that I didn't have when running on Canadian voltage. It's definitely coming from the electronics, not the woofer. Different plugs / locations / proximity to other components make no difference. The hum is present as soon as the unit is powered up, regardless of whether line inputs connected. The repair manual lists NPT-1539D transformer. With the exception of the very faint 60hz hum, the unit seems tot work flawlessly, sound is as good as I can remember it being.
    Does anyone know if this hum is related to the transformer now operating at 50 hz rather than 60 hz as intended? Could this be related to something else that I should try to troubleshoot?

  • #2
    Could be that the transformer in the amp has a buzz which is not apparent at 60Hz but is at 50Hz. Sometimes you can fix it by remounting the transformer with some foam or rubber bushings to dampen the vibrations. On toroidal transformers sometimes just loosening the mounting bolt, re-positioning the foam pads and then tightening it back down can stop loose windings from buzzing.

    If i remember correctly, at low loads an isolation/step-down transformer can enter saturation, which may cause some distortion of the output line voltage. That distortion may be heard if the circuitry in the amp buzzes at the frequency of the harmonics. Perhaps try adding additional load to your step-down transformer (e.g. incandescent lights).


    • #3
      If the xformer is rated for both 50 and 60 hz you should expect similar performance from either.

      I was taught that transformers rated for 60 hz may work with higher frequencies but not lower frequencies.


      • #4
        Hi TMM, marvin - thanks for the suggestions.
        marvin, to the best of my knowledge the transformer in the amp is rated for 60hz only, so I'm thinking that running it at 50hz may be producing the hum. I might try to record it to see if it's producing a 60hz or a 50hz sound wave. I too recall reading somewhere that running 50hz through an amp rated for 60hz is problematic whereas running 60hz through an amp rated for 50hz may actually be more efficient. Can't be certain but I seem to recall that heat and fatigue were the concerns, so if heat can be generated then perhaps noise might be another byproduct.
        The load on the stepdown transformer does not seem to have been a factor, although it's a really interesting point. I've run the receiver on it along with the subwoofer and observed same hum as when running the subwoofer alone.
        I may open it up and tweak the mounting to see if it has any effect, just wanted to see first if this could be a common problem when running 50hz through an amp rated for 60hz.


        • #5
          Welcome to kiwiland. Whereabouts are you? I'm in wellington.

          Could this be ground loop hum? that is a potential from a non-grounded component, traveling your audio path to a grounded component?

          In my case, I was runing a 70's power receiver without ground. Rather than lifting ground on my subwoofer - I added a chassis ground to the amplifier (separate plug with earth wire connected to receiver chassis).

          this removed my audio hum.