Dreaded HUM

[Gianluca]

Hello All,

Long time reader, first time poster...Really a wondeful forum.

I have recently finished putting togetter a HT room. The last addition was a Sunfire True Sub and this brought out the dreaded 60HZ hum. I found the issue to be the satalite cable feed comming into my house so I tied all my HT gear into an APC H15 which killed the hum on the spot. I now have had an intermittent problem with hum which seems to kick in only when played loud but not always. I used a cheeter plug that removes the ground pin on the sub but it still hums. Sometimes unpluging the sub gets rid of it and sometimes I need to unplug the RSX-1056 from the mains. I am wondering if this could be due to the subwoofer cable I built myself from canare cable and neutrik RCA connectors. This cable has been tested for solid shorts and resistive shorts with a mutimeter.

Basically I am stomped. Could this gradual hum come from atjacent speaker or mains wire. This would mean it has inducted hum??? I'm lost...Should I buy a cable??


Thank you
Gianluca

[Gianluca]

Hello All,

Any ideas...does the subwoofer RCA need to be kept far from mains and speaker cable. I mean when played loud can the magnetic field from higher power flow within the cable cause this issue, and once the hum starts I need to unplug equipment to make it stop.

I am hopeing this is not related to the RSX-1056 and it's DSP board or something like that. The sunfire sub just got a clean build of health with a revised amp and active sub.

Thanks

Gianluca

[Kevin D]

Hmm.. To answer your thought, the position of the wires will not give you a ground loop, definitely not when playing louder.

Is the sub plugged into a different outlet then the APC unit? Since you made the cable I'm assuming it's far away.

The fact that unplugging the the sub doesn't always kill the hum is odd. Is there anything else that's plugged into the system that's not on the same power outlet?

I'm also wondering if running the sat feed through the H15 fixed it in most instances, but not all. Next time it comes back trying unplugging the sat feed and see if it goes away.

Kevin D.

[Gianluca]

Hi Kevin,

All my gear is plugged to the APC unit including the sub. I don't recall unpluging the sat feed or STB from the apc, I will try that.

What I don't understand is how a ground loop could occur if both the RSX or sub is not grounded? Does the ground loop have to happen between the RSX and sub or any component within the circuit?

Thanks

Gianluca

[Kevin D]

Well they are still grounded. The neutral lead is connected to ground in the electrical panel. Typically making the problem go all the way back to the electrical panel masks the problem, but not always.

It can happen with anything connected to anything. It could be a TV in a different room connected to the same sat dish. It could be something plugged into a different outlet on the same circuit, is the APC on a dedicated outlet?

Kevin D.

[Gianluca]

Hi Kevin,

The APC is not on a dedicated circuit...I will probably do that once I have the time. So basically once I get the HUM I should unplug all devices conected to the APC and anything else I can think of. Looks like loads of FUN :roll:

Thanks for your help

I'm just happy it's not a defective RSX or sub or atleast your not pointing at that.

Gianluca

[Gianluca]

Hi Kevin,

I extensively tried finding what was causing the HUM and found nothing on the APC causing the issue. Discouraged I went in another room and flipped a light switch and low and behold the HUM went away...yes you read right a light switch in another room. I quess a dedicated circuit is next. I already bought all the necessary equipment to add a breaker and got myself a quality NEMA plug.

I would have never thought ground loops in other parts of the house on the same circuit could cause this.

Thanks for opening my eyes to this Kevin.

Gianluca

[Kevin D]

Crazy..

Have you verified that that light is on the same circuit? If you're in the US or somewhere that uses 110/120v and it is on a different circuit, you might try moving the audio breaker to the other phase first.

Every other breaker slot is on the same phase, so moving it up or down one would switch phases. If it's in the middle of other breakers, you'll have to start doing some math.

Kevin D.

[Gianluca]

Hi Kevin,

Actually it's not on the same circuit, thought it was at first, but after so more tinkering found it was not on the same one. So I should try a different phase (i am in Canada), for my dedicated circuit just in case. I hope after installing the dedicated circuit that my issues will be solved.

Thanks

Luca

[Gianluca]

Hi Kevin,

Problem solved...dedicated circuit did the trick. On a side note why do my friends with Bose system not suffer from ground loops or hum from thier subwoofers. 4 of my friends have these lifestyles systems and none of them have had any issues with this. So I was made fun of cause of my big clunky Rotel and B&W 603's. Have to say any decent system killes Bose in music and HT.

Gianluca

Thanks again Kevin

[Gianluca]

Hi again,

I have since added a 2 channel amp to drive my mains, and noticed that the hum came back during a movie. The amp has 3 prong plug and so does the sub, the RSX-1056 is 2 prong. It seems to show up during loud passages and only way I found to get rid of it was unpluging the sub. I also noticed a very low hum from my mains which was not there before. Now have dedicated circuit, and all connected to APC h15 including sat cable.

Again stumped...getting complicated

Thanks

Gianluca

[Kevin D]

Trying getting some 2-3 prong adapters on the new amp or sub and see if that clears it up.

Now that you're on a dedicated circuit, is there anything else connected to the system in some way that might be plugged into a different circuit?

Kevin D.

[Gianluca]

Hi Kevin,

Nope all tied into the same circuit via H15, including cable feeds. But I realized that the sat cable also goes upstairs and thats on another circuit. Could it be that a loop happens from upstairs circuit and downstairs circuit because of the sat feed? The sat feed is also grounded outside via the electrical counter box.

Will try 2 prong plug but it still only a workaround.

Thanks

[Gianluca]

Well it's the tv Monitor that I used for the OSD. Reproduced many times. Hopefully the last of this. Would like to keep it pluged anything out there to fix this.

Thank you Kevin for the helpful hints

Gianluca

[Gianluca]

OK so to conclude I have a bad subwoofer amp plate. Which sucks in itself but happy I found it.

A good solid whack on the side of the sub made the HUM disapear and a second whack made it come back, and then the amp came off and discovered cooked resistors.

Thanks to all