hum in long cable

[mathyou9]

Hi, this is my first post here (I've lurked for a bit.) To preface, I'm not a die-hard audiophile or home theater buff (so there's a lot I don't know.) I would consider myself a dilletante, though, when it comes to such matters. So I'm not completely ignorant of the physics involved (e.g., one of my favorite college courses was "Physics of audio and video" that I took wholly as an elective course.)

Anyway, I've been using a 25 ft. RCA stereo cable to connect my computer to my TV/receiver. From the getgo, there's always been an audible hum. I just assumed the source was the computer (being a ground loop or something.) Since my computer is not the center of my home theater experience, this was never a big deal to me.

So I recently was experimenting with said cable to find the cause of the hum. I completely unplugged it from the computer but left if plugged into my receiver; still there was hum. Considering it's not plugged into a source signal, I'm not sure where the hum is coming from.

Is the 25-ft. length a factor? Or that it's a [relatively] cheap cable? Or both? Or what?

Anyone care to enlighten me? Thanks.

 

[rnatalli]

Welcome!

Did you try switching the cable to see if the hum persists?

 

[TLS Guy]

Hi, this is my first post here (I've lurked for a bit.) To preface, I'm not a die-hard audiophile or home theater buff (so there's a lot I don't know.) I would consider myself a dilletante, though, when it comes to such matters. So I'm not completely ignorant of the physics involved (e.g., one of my favorite college courses was "Physics of audio and video" that I took wholly as an elective course.)

Anyway, I've been using a 25 ft. RCA stereo cable to connect my computer to my TV/receiver. From the getgo, there's always been an audible hum. I just assumed the source was the computer (being a ground loop or something.) Since my computer is not the center of my home theater experience, this was never a big deal to me.

So I recently was experimenting with said cable to find the cause of the hum. I completely unplugged it from the computer but left if plugged into my receiver; still there was hum. Considering it's not plugged into a source signal, I'm not sure where the hum is coming from.

Is the 25-ft. length a factor? Or that it's a [relatively] cheap cable? Or both? Or what?

Anyone care to enlighten me? Thanks.

An unterminated cable will usually hum. Short the cable and see if you have hum, I bet you don't. I think this is a ground loop. Before hooking it up lift the the ground from the computer or TV/receiver.

 

[mathyou9]

Did you try switching the cable to see if the hum persists?

Yeah, I've tested other [non-terminating] cables (all short) and there was no noticable hum. Although my cable is 25 ft., the minimum length I need is 12 to 15 ft.

Short the cable and see if you have hum.

Not sure if I know how to do that; do I just touch the non-terminating end to an electrical conductor (e.g., metal, electrolyte solution, tongue, etc.)?

I think this is a ground loop. Before hooking it up lift the the ground from the computer or TV/receiver.

And another I'm-not-sure-how-to-do-that-exactly moment. Sorry.

 

[TLS Guy]

Yeah, I've tested other [non-terminating] cables (all short) and there was no noticable hum. Although my cable is 25 ft., the minimum length I need is 12 to 15 ft.


Not sure if I know how to do that; do I just touch the non-terminating end to an electrical conductor (e.g., metal, electrolyte solution, tongue, etc.)?


And another I'm-not-sure-how-to-do-that-exactly moment. Sorry.

Short the pin and the shroud of the RCA plug that is not plugged into your receiver.

To avoid ground loops you need one ground. That means ONE three point plug. Break the others by using those two the three pin adapters. Also if the hum starts disconnect your TV system from your cable system and see if that stops it. You might have to isolate the cable system ground. Let us know the results.

 

[Rickster71]

As TLS guy suggested, you need to connect everything to a common ground.
To do that, you need to power your computer and TV/receiver from the same circuit. To test for a common circuit; get a small lamp to use, as a test light.
Shut off the breaker to your TV, check to see if the plug for your PC is still on.
Your goal is to find outlets that are on the same circuit; and to use those.
The other way to do the same thing, would be to power the PC and TV from the same power strip.

I don't recommend using 'cheater plugs' or two to three pin adapter plugs.
(unless it for a temporary test)
It's an unsafe practice to circumvent a grounding system.

 

[TLS Guy]

As TLS guy suggested, you need to connect everything to a common ground.
To do that, you need to power your computer and TV/receiver from the same circuit. To test for a common circuit; get a small lamp to use, as a test light.
Shut off the breaker to your TV, check to see if the plug for your PC is still on.
Your goal is to find outlets that are on the same circuit; and to use those.
The other way to do the same thing, would be to power the PC and TV from the same power strip.

I don't recommend using 'cheater plugs' or two to three pin adapter plugs.
(unless it for a temporary test)
It's an unsafe practice to circumvent a grounding system.[/QUOTE

I intended it for a test to see what is going on.

 

[Rickster71]

Hi TLS, I know you intended it for testing; just wanted to make sure the OP was aware.

 

[mathyou9]

Ok, so here's what I've discovered:

Shorting the RCA cord removes any humming. And when the RCA cables are hooked up to my computer, I realized the hummer is a bit louder than when it's unterminated (so I'm assuming the more quiet hum is just the "natural" hum that TLS Guy mentioned regarding unterminated cables.)

Anyway, I checked my circuit breaker circuits. The receiver is on a different circuit than the computer (seems like a ground loop.) And the outlets are wired correctly too. However, the receiver's plug is NOT a three-pronged plug; it's just a standard polarized plug. So I unplugged it from the [grounded] powerstrip and plugged it [the receiver] directly into the wall.

IOW, the receiver is not even grounded, but the computer is grounded on another circuit and the [louder] hum is still present when the RCA cables terminate at the computer.

I thought a ground loop requires at least two grounds? Or does the unused ground at the receiver's end still play a part in a ground loop?

 

[TLS Guy]

Ok, so here's what I've discovered:

Shorting the RCA cord removes any humming. And when the RCA cables are hooked up to my computer, I realized the hummer is a bit louder than when it's unterminated (so I'm assuming the more quiet hum is just the "natural" hum that TLS Guy mentioned regarding unterminated cables.)

Anyway, I checked my circuit breaker circuits. The receiver is on a different circuit than the computer (seems like a ground loop.) And the outlets are wired correctly too. However, the receiver's plug is NOT a three-pronged plug; it's just a standard polarized plug. So I unplugged it from the [grounded] powerstrip and plugged it [the receiver] directly into the wall.

IOW, the receiver is not even grounded, but the computer is grounded on another circuit and the [louder] hum is still present when the RCA cables terminate at the computer.

I thought a ground loop requires at least two grounds? Or does the unused ground at the receiver's end still play a part in a ground loop?

Are you sure you don't have another ground? Is your receiver connected to a cable system, or satellite system? What else is connected to the receiver and computer?

If there really are no other grounds, do you hear a hum if the computer is unplugged but connected to the receiver? Let us know and we will get this solved.

 

[Rickster71]

In a main panel, the ground and neutral bars are bonded.
What you are experiencing is the different ground resistances, of two different circuits. Due to the fact, a typical equipment grounding conductor can go through a dozen or more poor splices and connections before it gets back to the building grounding system.
So, you still have to get both pieces of equipment on the same circuit.
Even if you use an extension cord (for test purposes)

 

[mathyou9]

So I turned the computer off and the ground loop hum was still was present. Then when I pulled the 3.5mm jack out partially (i.e., I'm using a 3.5mm-to-RCA adapter) I picked up a radio station. At this point I realized that I completely forgot that my cable line connects to my computer's TV coax input for Windows Media Center.

The hum appears to be a combination of my cable line and the circuit that the computer is on. Here's how my cable run goes:

-From the wall I have a splitter.
-The short leg goes directly to my television.
-The long leg goes to my computer's TV coax input.

-Expectedly, when the splitter is disconnected from the wall, the gound loop disappears.
-When the short leg to the TV is disconnected, the ground loop disappears.
-When the long leg to the computer is disconnected, the ground loop is still present.

It would seem that the cable line to the back of my TV is somehow the culprit. Because whether I plug the RCA cables directly to the TV or into the receiver, the ground loop hum is present. But disappears when the TV is disconnected from the cable.

And of course, when I unplug the computer's powerstrip altogether, the ground loop disappears.

Would a ground isolator for my cable line be sufficient? Or should I attempt to get everything [safely] on the same circuit? Or both?

 

[Rickster71]

The cable installer should have installed a grounding block (see link) and a ground wire should go from the block to the closest part of your homes ground system.
http://electrical.hardwarestore.com/15-54-tv-wire-cable-fittings/single-digital-grounding-block-619058.aspx

Can you trace this line back to it's origin into your house? The block is probably there.
Make sure it has a ground wire, and the connections are tight, and not corroded.

 

[cbraver]

My bet is that you need to lift the ground between you're computer and audio equipment.

For a quick fix, go to Radio Shack and get a RCA ground-loop isolator.

More perminently, I use a Whirlwind direct box out of my computer which has a ground loop lift. You might want to browse www.whirlwindusa.com ... or search through bhphotovideo.com for direct boxes with ground lifts. Going directly from you're sound card to you're receiver is generally a headache. Most direct boxes will be RCA in and XLR out... so you'd need two XLR to RCA adaptors.


I find it interesting that although we have threads like this often, when someone asks about the benefits of balanced connectors I'm the only one that says that they are handy.

 

[TLS Guy]

Jensen cable ground loop isolator

Well now you know the problem. I had a feeling there was ground you had not told us about! Cable systems are notorious for these types of problems.

Jensen make he best gear for this sort of thing. I would put one of these between your cable outlet and splitter.

http://www.cs1.net/cables/products/jensen_transformers/VRD-1FF.htm

 

[cbraver]

Well now you know the problem. I had a feeling there was ground you had not told us about! Cable systems are notorious for these types of problems.

Jensen make he best gear for this sort of thing. I would put one of these between your cable outlet and splitter.

http://www.cs1.net/cables/products/jensen_transformers/VRD-1FF.htm

If he has closed-loop digital cable (anyone with On Demand and such), that will most likely prevent return transmissions, making things like On Demand, some DVR functions and Pay Per View unusable. If he just uses it going to the computer, that should be good. But, definetly not to the cable box. Basically, use it on the split line that goes to the computer and not the cable box.

 

[TLS Guy]

If he has closed-loop digital cable (anyone with On Demand and such), that will most likely prevent return transmissions, making things like On Demand, some DVR functions and Pay Per View unusable. If he just uses it going to the computer, that should be good. But, definetly not to the cable box. Basically, use it on the split line that goes to the computer and not the cable box.

If the functions he is using now pass the splitter, I think they should pass the Jensen isolator. Unless there is something phantom powered which I don't think he has, my plan should work. That way both his computer and TV will be isolated from the cable system ground.

 

[cbraver]

If the functions he is using now pass the splitter, I think they should pass the Jensen isolator. Unless there is something phantom powered which I don't think he has, my plan should work. That way both his computer and TV will be isolated from the cable system ground.

Most RF filters will remove too much, the nice ones will work. This Jenson isolator you recommended looks really nice (they even present a frequency response) and might not filter out too much of the return signal. Cheap ones most definetly will. Like a Radio Shack isolator, I know from experience, won't work with Comcast's On Demand.

If he gets it, and connects it into the wall and his On Demand no longer works, then he can just move it to the computer side of the split and everything should be fine.

The reason I say to also get a direct box with a ground lift, is that if he doesn't the ground loop will occur through the audio connection from the computer to the receiver. He needs isolation from both the cable and the audio so that his audio gear doesn't share the ground with his computer.

 

[TLS Guy]

Most RF filters will remove too much, the nice ones will work. This Jenson isolator you recommended looks really nice (they even present a frequency response) and might not filter out too much of the return signal. Cheap ones most definetly will. Like a Radio Shack isolator, I know from experience, won't work with Comcast's On Demand.

If he gets it, and connects it into the wall and his On Demand no longer works, then he can just move it to the computer side of the split and everything should be fine.

The reason I say to also get a direct box with a ground lift, is that if he doesn't the ground loop will occur through the audio connection from the computer to the receiver. He needs isolation from both the cable and the audio so that his audio gear doesn't share the ground with his computer.

I don't think he will have a ground loop with my plan. His receiver is two prong. His computer is grounded. If he isolates from the cable ground with what I suggested, he will have one ground and no hum.

 

[cbraver]

I don't think he will have a ground loop with my plan. His receiver is two prong. His computer is grounded. If he isolates from the cable ground with what I suggested, he will have one ground and no hum.

If he doesn't have ANYTHING grounded in the audio setup, yeah. I know you asked that earlier, but I don't see a response answering you from him about that. If his TV or subwoofer or something is grounded, the loop will persist because it will still be connected to the computer. Or if he adds something that is in the future.

I agree with you that if nothing else is grounded over there, then the cable isolator should be enough, but...I just can't imagine nothing being grounded over there. Just about all my gear has a ground.

I still believe in the direct boxes for doing computer to receiver connections though, for impedance matching alone.