Sub Hum

  • 2 October 2005
  • 3 replies

Hi -- I'm a new Sonos user, but I've been lurking for months trying to anticipate problems, etc.

Finally got Sonos wired into my main system today, sounds great with FLAC off a NAS. However, I'm having significant subwoofer hum when my system is OFF. When the main system is on, no hum, just good music (even when quiet, no hum).

The Sonos is connected via line out to the Video 1 (only option) of a Sony HT mini-system (F100W - very limited space in a tiny cabinet in this room -- big fan of an ampless ZP theory 😃 ), which has a mono line out to a powered sub. The auto power off on the sub doesn't seem to recognize that there's nothing coming across, so the hum continues until I disconnect the line. I've tried turning down the volume on the Sonos, muting Sonos, no effect.

I have other ZP's line out to other systems and subs without this phenomenon.

I'm sure there's some wiring issue at play, obviously. Just not sure where to start and wondering if someone else has experienced this, or any suggestions out there?

Tried searching for "hum", but it's too short for the engine on forum. :)

Thanks for the thoughts.

3 replies

This is not an unusual problem and it is not a uniquely SONOS issue.

When the SONOS switches OFF, it presents an "open circuit" to the input of the subwoofer. Unfortunately, this open circuit allows small amounts of energy to leak into the cable connecting your subwoofer to the SONOS. The subwoofer simply amplifies these signals (probably snickering at your rather boring, single tone musical tastes). In your case it is hum, in other situations it can by a nearby radio station.

Since there is very little energy involved, a resistor connected between the shield and the center conductor of the signal cable will act as a "short circuit" for the leaking energy while not significantly reducing the music signal.

Hopefully, you or an associate can fabricate such a gadget. The exact value of the resistor is not important. Anything in the 5K - 10K Ohm range will be fine. Solder the resistor from the center pin to the shield of an RCA plug then insert the plug into a "Y" cable attached to the subwoofer input and the existing signal cable.
Buzz, thanks for the reply.

Soldering scares me -- I'm a surgeon, not an electrician. 😉 Is there a comercially available product to your knowledge? Is this a ground loop issue?

Thanks for your thoughts.
It could also be a ground loop issue, but the fact that it goes away when the SONOS switches on, adds many points to my leakage hypothesis.

Soldering would be better, but you could always pretend to be suturing and wrap the resistor tightly enough to make contact. If the trick works, worry about soldering the connections later.

Another technique would be to cut a defective RCA patch cable in half (saving the good end) and twist the resistor to the stripped ends of the shield and center conductor.