my ht, and my problem (1 Viewer)

[jhndflpp]

this is really a question about an audio issue i'm having in my ht, but i figured i'd need to describe my theater first, so i hope this is the right place to put this thread...

first the room - my theater is in a room about 10' x 30', the last 5' of length being a walk in closet. for video, i have a mitsubishi 1080p projector, hung about in the center of the main part of the room. for speakers, i have bic venturi speakers - towers in the front and bookshelfs for the mid and back - with an infinity center channel and a 300w wharfedale sub. all speakers are layed out in a normal way in the room with the sub in the back near the left rear speaker. all cables are run along the ceiling or floor of the room to the rear left, through the wall and into the closet where i have my receiver, an onkyo tx-sr705. my only component is a ps3 (feel free to mock me, but it does everything i want in a single unit).

okay, on to my problem. it's relatively simple - when i have both the projector, with its 25' hdmi cable, and the subwoofer, with its 3' rca cable, attached to the receiver, i get a fairly loud humming over all my speakers. if i unplug the sub, it stops; if i unplug the projector, it stops. if i rig it so the projector is plugged into the ps3 and i use the optical out on the ps3 to the receiver, it also stops. i was using this last option until sony upgraded the ps3 to use 7.1, but only through hdmi (thanks, sony). presently i am simply going without my sub because my front towers are nice enough, but this obviously isn't ideal.

other info that may or may not be helpful - the whole room is on the same electrical breaker, and it is grounded. none of the cables are cable-tied together in any way, and i even made the front speakers go along the floor and the mids and rears go along the ceiling. the projector hdmi cable runs near some speaker cables, but i have tried separating them without affect. all cables are shielded.

does anyone have any suggestions? i will test anything you suggest and let you know asap.

thanks!

 

[Brent_S]

First of all, don't blame Sony for requiring HDMI for 7.1. The legacy optical/coax digital pathways simply don't have the bandwidth for 5/7.1 LPCM, which you need to get TrueHD/DTS-HD from the PS3. You're limited to lossY DD/DTS via the optical connection you were using.

Assuming "unplugged" means you disconnected said device from the output jack on the 705, not unplugged from the wall.

That said...seems like there have been several reports in AVS's 705 thread about humming problems with the 705 with HDMI connected, but that doesn't quite sound like you since your hum goes away with the sub disconnected. Some have said the hum goes away with the Direct/Pure modes, which also disables BM/Audyssey/etc.

In your case, it definitely sounds like a ground loop problem between the sub and projector. Have you tried a different video connection from 705 to the Mitsu like component or s-vid? If that removes the hum, it should squarely point at the HDMI pathway, although it could still be the Mitsu somehow. You'd need an alternate HDMI display to confirm this path.

Have you tried a different HDMI cable or sub interconnect? The sub cable could have a bad ground connection, not sure about HDMI.

Could you borrow a different sub from someone? Does your sub have a 3 prong connection or two? I can't remember if the 705/805 have grounds or not. What about the Mitsu? If only one of your devices has a three prong or one doesn't, that would also be a likely culprit.

Another workaround. Does your sub have speaker level inputs? You would lose the BM and delay processing available via the preout, but it might enable you to use the sub and HDMI together.

-Brent

 

[jhndflpp]

thanks for your response, brent. you spurred me on to try a bunch of different things, and though i'd be happy to answer your questions, after my findings, most of them are no longer necessary.

i tried a bunch of things with the audio cables, like using a rca cable instead of the hdmi to the projector, and that was all without luck. i eventually tried unplugging the projector from the ceiling outlet - instant silence. i tried plugging it into the wall in the room, buzzing came back; i tried plugging it into an outlet in a different room, buzzing came back; and i tried plugging it into the outlet in the closet right below where the sub is plugged in - beautiful silence.

i checked my breaker box and found that the closet was on a different circuit as the main room, which was on the same circuit as the rest of that floor. so apparently it works okay as long as the subwoofer is on the same circuit as the projector. "okay," i though, "then i'll keep the projector plugged into its ceiling outlet and plug the sub into the main room" (i was just plugging it into the closet out of convenience). with that option, i have a faint, but audible buzzing; much better than before, but not ideal. so now i'm back to plugging the projector into the closet, which will be some weird looking wiring, considering there'll be an empty outlet on the ceiling next to the projector and the power will be run back along the wall.

i assume i must resign myself to that setup, unless there are any other suggestions?

thanks again brent!

 

[Brent_S]

It's scary sometimes to find out how illogically circuits were arranged by the original electrician.

We have a half-bath off the foyer. There are two outlets, one at the sink which is regular and a GFI on the opposite wall. Recently, the sink outlet, as well as all the oulets in the two full baths went dead, but that powder room GFI stayed hot. Hmm, must have failed open. Killed power, replaced with a new device...it *was* wired incorrectly (in parallel to downstream instead of series). No joy, all other outlets still dead. Figured one must have developed a bad connection so just replaced all with new parts. Still dead. Huh? Eventually, some random act lead me to discover I had dead outlets in the garage too...hmmm, this is gettin' weird. The garage GFI outlet had power even when tripped. Ruh-roh. Turns out the garage GFI controlled the sink outlet in the powder room and both full baths. The GFI in the powder room was on a completely different circuit. Why???? They're the only 2 outlets in the room and they're only 5' apart? If you've kept up with my confusing narrative this long, the scary part is I replaced 9 outlets on a live circuit b/c I turned off the circuit feeding the powder room GFI thinking the powder room GFI was protecting the other two baths.

-Brent

 

[Brent_S]

Oh, yeah. Since the ceiling outlet was most likely a later add-on. Any chance you could access where it ties into the room circuit and simply reroute it to the closet circuit? Electrically, that should be the same as plugging them into the same outlet.

You still get the buzzing with both plugged in to the room b/c there's still a ground loop somewhere between the room and closet circuits, it's just not as pronounced when the sub/projector are on the same circuit. You could try the usual suspects...CATV/Sat antenna feed. There are inexpensive devices that will break the ground loop on these lines. My powercenter even seems to do it...had rolling noise lines on the projector until CATV was routed through power center before VCR. But the simplest method, as you discovered, is plugging all gear into the same circuit.

-Brent

 

[jhndflpp]

that's some wicked wiring you've got there! the main part of my house was built in the 30s, so other than the stove and the dryer, most of that part of the house is on one circuit, but on the newer part of the house, it appears they used the random wiring method you've been privy to.

it's interesting that it might have something to do with the cable. we don't have cable tv, but we do have cable internet, and though it's on the completely opposite side of the house, it definitely might be on the same circuit. when i get home from work today, i'll have to try unplugging the modem and see what happens. it may be easier to adjust the circuit that that is plugged into than messing with the outlets in the theater, but who knows. =)

i found a site (apparently i can't post a link to it until i've made ten posts) at audioholics called "Ground Loops - Eliminating System Hum and Buzz" discussing more about ground loops, but the only device they seem to recommend is a $60 cable ground isolator, and while compared to pretty much everything else in the theater, that is cheap, it doesn't seem particularly cheap when the work around is simply using a different (albeit inconvenient) outlet. do you know of some cheaper options?

 

[Brent_S]

I thought Radio Shack and Parts Express carried something in the $20 & under range. I'll check later. My Monster Powercenter and Scientific Atlanta 8300HD-DVR seem to have this type of circuit built in.

BTW, when I mentioned CATV/Sat feeds. I was referring to the RG59/RG6 coaxial lead that connects via F-type connector to the back of a VCR/Sat Box/Cable box. Often the CATV system or Sat dish is grounded to a different location than your house electrical feed...or worse, not grounded at all.

-Brent

 

[jhndflpp]

yep, i knew you were talking about the coax cable, but i assume what you're saying is that the coax input is probably not well grounded, and since the modem is plugged in to the same circuit as the projector for its power, that's what's causing the issue in my theater, correct?

looks like radio shack has a ground loop isolator for $16.99. it's stereo rca, but i can probably just use one for my subwoofer - that's definitely cheap enough to experiment with. but i suppose it would make more sense to find an isolator for the coax, assuming that's the issue. it looks like those are slightly harder to find, but available online.

thanks again for your advice, brent!

 

[jhndflpp]

well, unplugging the coaxial cable from the cable modem did not have any affect on the hum, so i'm not exactly sure what is causing the ground loop. so i guess i'll probably pick up an rca ground loop isolator and try it on the subwoofer. if that fails, then i'll have to run the projector's electricity to the closet.

thanks again for all the advice!

 

[jhndflpp]

well, just to bring this thread to a close, i picked up a ground loop isolator from best buy for about $18, used it on the subwoofer line, and my humming issue is gone.

 

[chuckg]

These ground loops can be a pain. In nearly every case, a simple three-prong to two-prong AC adaptor will fix it. There are no metal parts on the outside of a subwoofer, and many of them use only two-prong plugs to begin with. That is where I would first try the adaptor. Since your projector is most likely out of reach, you could try putting the adaptor there, too.

If you have an irrational fear of ungrounded devices, you could rewire the projector's ceiling outlet to the outlet in the closet - seeing as how that worked in temp mode!

If manufacturers would keep signal ground and safety ground separate, we would never have this trouble.

 

[Allan Jayne]

I have often suggested running a 14 or 16 gauge bare wire, daisy chaining from one component frame to another, including in your case the subwoofer and the projector. For each component you would need to find a screw that goes all the way to the chassis, or disassemble that component (subwoofer?) enough to find exposed metal frame (amp chassis, not speaker basket) to tie the bare ground wire to. The far end of the exposed ground wire is connected to the house electrical ground for example at a grounded receptacle. Do not tie to black binding posts or RCA jack shells.

Home theater hints: Constructing a Basement Home Theater

 

[Vin_G]

As we all know the subwoofer has a 3-prong and other components typically have two so the interconnections cause the ground loop. Here is how I remedy the problem of a subwoofer ground loop:

PAC SNI-1 RCA Ground Loop Noise Isolator at Crutchfield.com