Print

Please login or register.
1 Hour 1 Day 1 Week 1 Month Forever
Login with username, password and session length

[Mike Sokol]

I gotta say, as rudimentary as my knowledge is about electricity, this section of the forum continues to fascinate me even though 90% of the terminology flies over my head. This is one area of simple ignorance I have a growing interest to rectify.

This stuff ain't easy, which is confirmed by the fact that so many big-buck consultants have been confused by it for decades. I've battled all sorts of audio and video hum for 40+ years, but I'm just now getting into the math and physics behind it all. Once we get the causes boiled down to some basic concepts, then figuring out how to stop it is possible. But I do know that simply throwing more "grounds" at the problem doesn't work well. Nor does "lifting everything" fix the problem most of the time.

My head is spinning with possibilities, but this is a great area to study.

[Jerome Malsack]

With home theater the powered subs and cable company feeds tend to be the problem. 

Some times adding the video isolation transformer is the answer. 

http://www.mcmelectronics.com/product/DISTRIBUTED-BY-MCM-4170-/33-8700

I have one available to test with. 

as the cable comes in to the room you insert before the DVR/receiver.  That is because the shield has a ground connection outside the building on a ground rod.  This is the inexpensive version of the jensen good transformer for video. 

[Jerome Malsack]

Here is the Jensen document I found this on the home theater side. 

http://www.jensen-transformers.com/an/generic%20seminar.pdf

Around page 17, to 20.   

I hope you find there is a cable feed somewhere that is causing. 

Also the computers can create the same ground loop problem with projection. 
is there a DI on the computer audio feeds? 

[Mike Sokol]

With home theater the powered subs and cable company feeds tend to be the problem. 

Some times adding the video isolation transformer is the answer. 

http://www.mcmelectronics.com/product/DISTRIBUTED-BY-MCM-4170-/33-8700

I have one available to test with. 

as the cable comes in to the room you insert before the DVR/receiver.  That is because the shield has a ground connection outside the building on a ground rod.  This is the inexpensive version of the jenson good transformer for video.

I've felt a small shock (maybe 20 volts AC on dry hands) when connecting the cable tv coax into my digital modem and tuner box. I never really measured the open circuit voltage or closed circuit amperage, but it must be there. When I get a chance I'll put some meters on it and see what it all means. I know the head engineer at Antietam Cable (my cable provider) so we can begin a discussion to see if any of their customers are getting hum from the coax cable feed. Getting interesting. 

[Jerome Malsack]

With Satellite systems they have a phantom power type system to have the amplifier at the antenna. 

The cable company might be doing something similar also to provide power to devices on the line. 
Problem is most are, or have moved to a fiber backbone.  So when they convert from fiber to coax where is the device getting power to do the conversion and such?   

[Jerome Malsack]

Jensen pdf has the hum bar also on page 31. 

[Mike Sokol]

With home theater the powered subs and cable company feeds tend to be the problem. 

Interestingly, I have a fairly modern (6 year old) M&K powered sub in my home theater system, and it uses a non-grounded (2-pin) euro connector. It's a pretty substantial sub (600+ watts) but they must have some sort of double-insulated power supply to pass UL listing without a power cable ground. But it's completely quiet even when the gain is turned up, so they must have known what they were doing. I think I've got the designer's email address, so maybe I'll ask him to weigh in on the sub-woofer video ground-loop question. 

[John Roberts {JR}]

Interestingly, I have a fairly modern (6 year old) M&K powered sub in my home theater system, and it uses a non-grounded (2-pin) euro connector. It's a pretty substantial sub (600+ watts) but they must have some sort of double-insulated power supply to pass UL listing without a power cable ground. But it's completely quiet even when the gain is turned up, so they must have known what they were doing. I think I've got the designer's email address, so maybe I'll ask him to weigh in on the sub-woofer video ground-loop question.

If the powered sub uses a two wire line cord. there should be no competing ground potential issues. It will probably grab it's audio ground from the unbalanced (?) audio input.  May have some issues with a long unbalanced audio feed but for short home theater feed with proper shielded cable should be OK.

JR

[Taylor Hall]

This stuff ain't easy...

Which I've found out very quickly. It's still wildly fascinating, however, and feel it would be foolhardy not to know as much as I can about it. Whether it's being able to identify a wiring problem before I plug in my gear or to make the best of an existing problem that I have no control over, it makes no difference to me. Your posts and others' in this forum have been very helpful in getting me on my way (and adding a few choice tools to my gig bag ).

[Mike Sokol]

Which I've found out very quickly. It's still wildly fascinating, however, and feel it would be foolhardy not to know as much as I can about it. Whether it's being able to identify a wiring problem before I plug in my gear or to make the best of an existing problem that I have no control over, it makes no difference to me. Your posts and others' in this forum have been very helpful in getting me on my way (and adding a few choice tools to my gig bag ).

I've tried to get some manufacturer support of a No~Shock~Zone clinic at an AES convention or something similar, but no traction so far. I have tabletop demonstrations of most of this stuff I've been writing about and that's how you really see how it all works.

Here's a funny story about Whirlwind from two years ago which I'm sure that Al Keltz (from WW and a member of this forum) won't mind me telling. I was having WW's engineering guys do some peer review on my hum current and hot-chassis experimental data, and they all agreed that my hypothesis and empirical data look correct on paper. So when I was in their area I brought in my NSZ Hum/Volt demonstration gear and set it up in their conference room where they all clustered around the table. I then started a cool demonstration where I first bias the chassis ground of a little PA system to 40-volts AC. Of course, there's no hum or sparks or anything visible since EVERYTHING is at 40 volts AC above earth potential. Then I told them I was upping the bias to 80 volts AC, and everyone looks a little nervous but stood their ground. Again, no hums, no sparks, no blue glow, nothing bad visually, except that putting a NCVT anywhere near the mic or console makes it beep like crazy. But then I told them I was upping the bias to 120 volts, and everyone took a step back from the table. It ALWAYS happens like that. Most of them were engineers and had read and verified my shock demonstration. But the first time you see it on a table in front of you it becomes REAL. Am I'm sure they still all remember that a Non Contact Voltage Tester will beep at a foot away from a little mixing board when its chassis is energized to 120-volts AC. 

It's both fun and terrifying at the same time.     

[ProSoundWeb Community]