Home Theatre Hum - carried by projector hdmi cable

I’ve a combined music and home theatre system, mostly powered by my P20.
The audio components, amps, blu-ray players, set top box and processor are all plugged into my P20.
My projector is at the other end of the room and plugged into a normal wall socket.

Something has happened recently that is causing a ground loop hum through my main stereo speakers. I’ve determined that the projector is causing the hum and further, it’s the hdmi cable that is ‘carrying’ the earth discrepancy from the P20 end.
I know ground loops are caused when there are two different earth points so in my case it’s the projector and the P20 are plugged into different sockets.
By the way, I’m in the UK and houses here use a ‘ring mains’ circuits.

Points to note.

  1. the audio part, (7.2.4 setup), on the P20 is silent. Confirmed by using the TV as display.
  2. when the projector mains plug is removed from the wall the hum disappears.
  3. when the projector is plugged into the wall and the hdmi cable is removed from the projector the hum disappears.

I’ve tested the hdmi cables going to the blu ray players and set top boxes by removing them from the processor, hum remains.
I’ve also tested the long hdmi cable going to projector and it’s ok.
I removed the last two ‘upgrades’ from my processor, hum remains.

I know this will take a lot of patience and it’s mostly a case of removing cables, plugs etc to identify what’s causing the hum but any pointers would be appreciated.

(I suppose a long extension cable powered from my P20 with the projector plugged into this would work as they would both be sharing the same earth point? This sounds messy and not sure whether I would lose video quality with a cheap extension cable?)

As I suspected, running a garden extension cable from my P20 and plugging the projector into the extension lead cures my earth loop hum.
Not an ideal solution.
I don’t want to have to go down the road of getting some half decent power cable and running it around my room.
I’ll see where my investigations lead me but it’s possible solution, if all else fails.
Strange how this has appeared after so many years of running my combined music and home theatre system.

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I had similar issue with EVERYTHING plugged into a P10 in the theater. HUM. My anthem avm60 did not get it or it was very very low and needed ear on speaker to hear, but when I tried a Marantz (switching xfrm) big time hum. Used the 99 Cent solution to fix and put projector and I think the Oppo plugs on the 3 to 2 plugs that removes the ground plug. hum gone. Went back to Anthem and got the 70 instead of the Marantz 7706 I think was the model and removed. They bugged me that much and I found out Anthem does do DSD via hdmi just does not advertise it since they did not pay for sticker, but the chip does it. thought that was interesting.

As long as the extension cord is of sufficient wire gauge to carry the projector current needs, and that’s rarely a problem in a 240v country, you will be fine. Your projector is also much better protected from mains spikes and other power issues when plugged into the power plant. A standard hardware store extension of sufficient wire gauge will cause no video degradation.

Just make sure you adhere to local building codes if you attach the extension cord permanently to the walls.

Maybe one of the Audioquest active optical cables could be a solution?

Sure, an optical HDMI link would break the ground loop.
But why would you not want to plug your projector into your PS Power Plant? Not so much for better pictures as I don’t believe that would happen, but the Power Pant does offer spike and surge protection.

I solved my hum issue, may be helpful for other UK owners.
In the UK the vast majority of equipment comes with live neutral and earth connections and we use a mains plug that has all three of these cables.
For some reason, which I don’t ever remember reading about, most av processers only come with live and neutral connections in their plugs.

After removing a screw from the back of the processor I added a metal washer to it and screwed it back in again.
I had already done this with my P20 but wasn’t currently using it.
I attached an earth wire to both the P20 and the processor chassis. The washers give much better contact for the earth wire than just using the back panel screws.
I presume this is giving the processor an earth via the P20?
Anyway it worked a treat, hum gone and my home theatre seems to be as silent as my music system now, which is a first.

I used to use RCA cables for my subs and was always plagued by hum of some sort, eventually getting it down to manageable levels but I’ve recently gone to XLR cables for the subs. I still had a little hum which I think was this projector ground loop issue, which for some reason really reared its head last weekend.
Now with the ground loop solved I’m hearing how effective the XLR cables are for the subs.
I’ll get to listen and confirm tonight and post a final update tomorrow.

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Well FWEIW, most ground loop problems in professional environments are solved by trial and error as well.

As for XLR cables, yes they do work very well. BUT, you need to have a balanced output on your preamp/processor and input on your sub woofer as well. Just changing an RCA cable with an XLR cable via passive adapters does not make for a true balanced link. If fact it’s no better than the RCA cable.

They do make conversion boxes to and from balanced XLR to RCA. But they are rather expensive. Look at pro sound vendors for such products.

This would also be a great product for PS audio to offer. Hum on long subwoofer line level cables is a common problem. The only good way to cure this is with true balanced interconnects.

Oops, problem not solved.

When testing my ‘earth wire’ fix between my processor and P20 I had forgotten I finished the previous night by listening to a new cd that had arrived in the post that day.
So the reason why my HT sound was as quiet as my music system side was because it was the music system still. When I tried it last night and realised I was still on music mode, changing to HT brought the hum back. (I was at home only a short time yesterday to do the fix and test briefly).

I know it’s the hdmi cable going from the processor to the projector that is carrying the ground loop issue so I need to break that connection. This leads me to think hdmi wifi extenders might resolve so I’ll investigate this angle.
(hdmi cable was checked with another one, same result.)

I have the fallback option of running a long power cable from the P20 around my room so that the processor and projector have the same earth but I’m trying not to have an unsightly power cord running around the room as it would be pretty difficult to hide.

Glimmie comments above are helpful, my Arcam AV40 processor, although having balanced outputs, which I use, are not true balanced outputs and I’m sure are contributing to the ground loop problem.

Not sure why this issue has arisen now as I’ve used a projector in my system for years without hum problems?

Resolved my issue. I had two options.

  1. Ebbtech Hum Buster … inserted between processor and power amp. It changed the hum to a much quieter ‘thistle’. So still there and wasn’t noticeable when content was playing.
    But being an audiophile I went for the next option.
  2. Use the P20 to power the projector. Sytem now pretty silent.
    I had initially veered away from this option due to the aesthetics involved, cable visible, running round the room, running alongside atmos speaker cables, but with a bit of diy work I can run it down the other side of the room under the door frame, which will be a much neater and more discreet solution.
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