Output hum when locked onto HDMI signal

[cullenhall]

Have a weird one guys with a Shurbourn PT-7030 processor. I went on a DIY subwoofer kick with with Dayton Audio Ultimax 12" subs. So there has been some changes in my theater lately.

I am driving the passive subs via a QSC amplifier with the connection between the amp and processor via balanced XLR.

Situation goes like this; If I turn on the entire setup to play a Blu-ray it is dead silent, however once I start playing a movie and the processor locks onto the signal (via HDMI) and begins decoding I get a VERY slight hum in the subs. I hit pause and the hum goes away - dead silence. The level of the hum is unaffected by the volume control on the processor.

My first thought is ground loop, however, the piece I can not figure out is how a ground loop can come and go based on when the processor is locked onto a stream via HDMI. If I use an analog input it does not seem to hum at all.

I will attempt to lift pin 1 on the processor end of the XLR cable later tonight to see if I eliminate a ground loop path - though I remain skeptical if that will help or not due to the come and go nature of the hum. I have read enough text books to realize that "technically" only one end of a shield should be grounded instead of both like you would get with a traditional XLR cable, however, I have built a number of recording studios and PA systems over the years and really have never had to lift pin 1 on any XLR cables if the facility grounding is good.

What am I missing?

[vcautokid]

Hmmm, The first thing I would look at is the last thing you did. Any new cables or components etc. Ground loops are not loads of fun, but you can trace, and Isolate them. I would for the heck of it, try a different Balanced Cable if you can. You could have a bad ground on that cable. Rare, but it can happen. I know others here will chime in soon, so you are good hands.

[cullenhall]

The only thing that is "new" to the system is the XLR cable, the QSC amp, and the passive subs. I have been using the XLR output from the PT-7030 to feed my, now retired, powered sub for years.

You bring up a great point though I had not considered, the XLR cable, while fresh out of the box, is about 15 years old and I have no idea if it was a good cable or not - I'll have to check that!

[cullenhall]

Well I don't understand the hum coming and going with respect to the processor being locked onto a signal, however, swapping XLR cables did the trick.

Gosh do I feel foolish for not even considering a bad cable!

The new/old cable that gave me issues checked out from a resistance standpoint, however, I suppose its possible that the sheilding was not contiguous around the conductors....

[creimes]

I would have thought more of an HDMI cable issue or an issue with the display it was connected to causing feedback, but glad you found the issue.

[cullenhall]

Well the XLR swap was not the trick, it only went away long enough for me to post that swapping XLR cables did the trick. The hum now is present even with nothing connected to the processor save a single powered sub (XLR or RCA), power to the processor, and an HDMI cable from the BDP-103. Its silent until the processor locks onto a HDMI audio track then I get the hum. Lifting the ground on any of the XLR cables has no effect and the same hum is present in the RCA outs as well. Also I noticed that my first post was incorrect, its not just the sub all the outputs have this hum - I just noticed it more from the sub.

I called tech support to see what they might be able to suggest.

Maybe this will be a good excuse to upgrade to an XMC-1 like I have at the lakehouse! Friends in the know give me a hard time about having the better processor at the house we use relatively little.

[cullenhall]

Mystery solved - mostly. I was eyeing a new processor anyway so I ordered a NOS Marantz AV7702 and had the same issue. Under further investigation I found that I only had the hum when the processor is locked onto a signal via HDMI AND the blower motor in my HVAC air handler was at any speed other than high speed (I have one of the ECM variable volume furnace fans). Sure enough I found a loose, or at least less than ideal ground connection in the air handler.

My home theater and the HVAC are on separate service panels but they do share a common grounding rods - so somehow the noise from the power electronics in the ECM fan motor could be "heard" through both the PT-7030 and the AV7702. Clean and tighten the ground and all is good. still really don't understand why it comes and goes based on a locked on HDMI signal... but both processors did it so there ya go.