Hum problem, not a ground loop... ?

[ThA tRiXtA]

Hey guys... I've recently hooked up all my new equipment with a new set of cables from BJC and have been battling a bad hum between my two subs.

I know this issue normally reeks of a ground loop, but I have done my research, changed a few things here and there and still cannot seem to isolate it. I am not one to beg for help, but I've pretty much exhausted my bag of tricks on this one.

I have contacted BJC about it, and I am going to copy and paste my opening correspondence with them regarding what I am experiencing.

May a lifetime of prosperity and beautiful ladies grace the gentlemen to solve this.

I wrote to BJC:

Thanks again for your attention and help on this issue.

I have attached a topographical floor plan of how my equipment is setup.

My equipment is as follows:

Onkyo TX-SR875 AVR
Mission speakers
2 x AV123 MFW-15 subwoofers
Pioneer plasma
SA 8300HD PVR
APC H15 power center

That's all the relevant stuff, I have other items in my rack like a PS3 and a Wii, but they are not really relevant to the issue in my opinion.

So, all display sources are connected to the AVR via BJC HDMI cables. The Wii is component video. One HDMI cable goes from the AVR to the Pioneer.

The sub splitter is connected to the back of the receiver in the sub pre out. Then there are the two BJC sub cables connected to the two legs of the Tartan sub splitter. One sub cable is 7 feet long, the other one is 37' long. I had reservations about a sub cable being that long but one of your sales associates said it would be fine before I purchased it. Each cable heads in opposite direction and is connected to the LFE input of each sub.

All cables run in wall through 2 inch wiring conduit along with the BJC 10 white speaker cables, and the wiring conduit does pass by one electrical wire inside the wall, but they cross at a 90 degree angle to each other and with about 3 to 4 inches of seperation between them.

I built this room myself and I carefully routed all wiring as not to have hum / interference ingress problems, so I am fairly confident that the electrical wiring is not the root cause. I have all of the home theater equipment on it's own dedicated 15 amp circuit coming straight from the breaker panel. Nothing else except the TV, two subs and AV rack equipment is plugged into this circuit.

All the equipment at the front of the room is connected into my APC H15 power conditioner, however the sub at the back of the room is obviously not. It is plugged straight into the wall on the dedicated circuit. (This is one of my last remaining ideas as to what is causing the hum, could it be that the sub that is not being surge protected / AC line filtered be the culprit?) I am fairly confident that subs shouldn't hum just because they are not plugged into a power conditioner, as any sub I have owned in the past were never connected to one, but I've seen crazier things happen.

As I mentioned to you, I have properly grounded the incoming TV coax cable to the shared ground on the cold water pipe that comes into my house. I know that often these problems are a ground loop problem, but I assure you I have done my research and this does not seem as such.

I have also attached a basic diagram of how the subs are connected. If only one sub is connected to the splitter, there is no hum. If both subs are connected to the splitter, there is hum. This issue is independent of whether or not the splitter is connected to the sub pre out on the back of the AVR.

I noticed on the 37' sub cable that there are copper strands sticking out of the top of the crimped connection and they make contact with the plug itself. I am not sure if this makes a difference or not, as if you have the shielding braid strands contacting a signal conductor on coax TV cable you get massive noise, but when this sub cable is connected just by itself there is no issue.

That's all I can think of at this time, I just wanted to stress that I only get loud unwanted noise when both cables are both connected to the splitter at the same time.

Any help you guys can offer me would be greatly appreciated, as I am at my wit's end troubleshooting this mess.

Thanks again,

Rob

So, any thoughts?

 

[highfigh]

I don't see any specific steps taken to isolate the source of the hum, so I would:

Unplug the feed to one subwoofer. If it goes away, you have a ground loop. I assume you did this already.

Unplug the cable feed- if it goes away, you need to filter the signal but you'll need the right kind, since they use bi-directional communication for PPV. If you have a scrap of coax, terminate one end and strip the other end so the center conductor is about an inch long, then insert it into the cable box input connector. If the hum goes away, it's still an issue that requires isolation.

With a piece of any kind of wire you have, connect from one subwoofer amp chassis to the other. If the hum goes away, you have a ground loop. I won't assume that you did this.

Plug an extension cord into the AH-15 or the same receptacle as the sub closest to the system, run it over to the second sub and plug the power cord into that. If the hum goes away, you have a ground loop.

Fun, ain't it?

 

[lsiberian]

I would move the TV to a different plug, but I assume you did that already.

Do you have the issue with the TV off?

Turn everything even the lights off and leave both subs on. Add 1 thing at a time until you get the hum. (Guessing you did this too.

I wonder if a Filter between the subs and the power source along with a filter between the receiver and subs would help.

 

[Rickster71]

Since we don't know if the room's AC was wired professionally;
I agree with Highfigh's method of going systematically.

Are you sure the water pipe is copper in it's entirety?
Some newer lines are poly from the street, and change at the foundation to copper.
Going to the systems ground rod is the best bet.

I'd try disconnecting the coax, (just to rule it out), and continue with Highfigh's suggestion.

Good Luck

 

[ThA tRiXtA]

I don't see any specific steps taken to isolate the source of the hum, so I would:

Unplug the feed to one subwoofer. If it goes away, you have a ground loop. I assume you did this already.

Unplug the cable feed- if it goes away, you need to filter the signal but you'll need the right kind, since they use bi-directional communication for PPV. If you have a scrap of coax, terminate one end and strip the other end so the center conductor is about an inch long, then insert it into the cable box input connector. If the hum goes away, it's still an issue that requires isolation.

With a piece of any kind of wire you have, connect from one subwoofer amp chassis to the other. If the hum goes away, you have a ground loop. I won't assume that you did this.

Plug an extension cord into the AH-15 or the same receptacle as the sub closest to the system, run it over to the second sub and plug the power cord into that. If the hum goes away, you have a ground loop.

Fun, ain't it?

Hey Highfigh, thanks for the reply.

I'll first fill you in on the stuff you suggested that I already tried so we can eliminate possibilities.

I originally did have a ground loop with the coax cable, which is why I properly took the steps to ground it to the cold water pipe that the breaker panel was already grounded to. This solved that problem, but when i got my whole setup going down in my basement I discovered another hum.

Regarding unplugging one sub cable from one of the subs, the hum will go away, but only if I unplug the sub in the back of the room. If I unplug the signal cable on the sub in the front of the room, the hum is still there.

It seems to me that the sub in the back of the room hums a fair bit and when I join the two subs via the splitter, it feeds back hum to the first. Does what I am saying make sense? The first one doesn't hum at all until I join it to the second one by plugging them both into the splitter.

Regarding plugging the sub in the back into the same receptacle/power conditioner as the one the front sub is on, I wired the room so that the receptacle that powers the back sub runs on to the front of the room to power the equipment receptacle too.

Those are the only two receptacles on that line and no other equipment is on them except what I have listed.

Is it possible to have a ground loop between two devices that are plugged in to the same circuit? If not, I don't need to do the extension cord test, do I?

One last thing, I also tried plugging the back sub into a different circuit other than the dedicated HT equipment circuit... no change in hum.

As for the rest of the stuff in your post, I will try the remaining steps tonight and report back with my findings. I hope this works, it's really frustrating. Thanks friend.

 

[ThA tRiXtA]

I would move the TV to a different plug, but I assume you did that already.

Do you have the issue with the TV off?

Turn everything even the lights off and leave both subs on. Add 1 thing at a time until you get the hum. (Guessing you did this too.

I wonder if a Filter between the subs and the power source along with a filter between the receiver and subs would help.

Hi Lsiberian and thanks for the reply. I am not the most knowledgeable when it comes to this stuff so please elaborate... what does plugging the TV into a different plug do for this issue? the TV is only connected to everything else via a HDMI cable and the TV can be off, unplugged from the wall etc and I still get a hum.

As for the systematic elimination of component to component, I am (begrudgingly) going to tear the whole system apart and build it back together to see if I can isolate the hum.

In regards to your comment about the filters between the subs and receiver, please note that the subs still hum even when not connected to the receiver whatsoever.

 

[MatthewB.]

I have found over the years two things to combat ground loops. One is a good surge protector (this almost always combats the 60Hz hum causing ground loops) the other is a ground loop isolator sold at Radioshack. These work about 75% of the time, the other 25% of the time I find the isolator actually increases the sound level of the hum.

 

[ThA tRiXtA]

Since we don't know if the room's AC was wired professionally;
I agree with Highfigh's method of going systematically.

Are you sure the water pipe is copper in it's entirety?
Some newer lines are poly from the street, and change at the foundation to copper.
Going to the systems ground rod is the best bet.

I'd try disconnecting the coax, (just to rule it out), and continue with Highfigh's suggestion.

Good Luck

Hey Rickster,

I am positive of the city water feed being solid copper. I also am extremely confident of the ground connection to it. Before I eliminated the ground loop due to improper ground, the ground clamp and pipe were very poorly put together and quite oxidized.

I took the whole thing apart, threw away the original ground clamp, bought a new one and scrubbed the water pipe with sand paper and steel wool.

It was so shiny and clean after I was done. I then clamped the new clamp on and tightened down the screws until I couldn't turn the clamp screws anymore. Then I cut a new piece of 14/2 romex and ran that from my cable demarc to the ground clamp and clamped it in super tight.

It is side by side with the ground cable coming from the breaker panel.

Can you see any steps that I missed or did wrong? It solved the original hum right away and if I remove the coax cable now it doesn't relieve any of the new hum, so I assume it is all good, right?

 

[ThA tRiXtA]

On a side note, I am wondering if the back sub having such a long sub cable could be a cause of the noise. Could it be acting as an antenna? As soon as I connect the second sub to the first with the splitter it seems to cause the first sub to hum too, at the same volume, pitch, everything. Can the second sub send hum back to the first sub, through the splitter?

Unplug the second sub and hum is gone... but plugged in on it's own and it hums.

 

[highfigh]

Since we don't know if the room's AC was wired professionally;
I agree with Highfigh's method of going systematically.

Are you sure the water pipe is copper in it's entirety?
Some newer lines are poly from the street, and change at the foundation to copper.
Going to the systems ground rod is the best bet.

I'd try disconnecting the coax, (just to rule it out), and continue with Highfigh's suggestion.

Good Luck

The electrical service is supposed to be grounded using copper stakes at the point of entry, not the water line. Things can be grounded to that as a common location but the service panel isn't supposed to use that as its ground. The house wiring is grounded to the ground buss and the neutral buss is connected to the ground in the panel specifically so current won't flow between the neutral and ground.

 

[highfigh]

On a side note, I am wondering if the back sub having such a long sub cable could be a cause of the noise. Could it be acting as an antenna? As soon as I connect the second sub to the first with the splitter it seems to cause the first sub to hum too, at the same volume, pitch, everything. Can the second sub send hum back to the first sub, through the splitter?

Unplug the second sub and hum is gone... but plugged in on it's own and it hums.

You have a ground loop. Make sure the second sub's receptacle is on the same circuit as the first and if it's not, reassign it at the panel.

 

[ThA tRiXtA]

You have a ground loop. Make sure the second sub's receptacle is on the same circuit as the first and if it's not, reassign it at the panel.

I absolutely guarantee that the second sub's receptacle and the receptacle behind the av rack are on the same circuit, straight from the breaker panel. It runs from the breaker panel, to the receptacle behind the second sub, then straight to the receptacle behind the av rack. Those are the only 2 drops on the circuit.

UPDATE:

Tried the extension cord test.

If I plug an extension cord into the apc and plug the second sub into that extension cord the hum is reduced by about 75%. So, do I need an APC like device on the second sub, or a line filter?

Thanks for the continued hep guys.

 

[mperfct]

I absolutely guarantee that the second sub's receptacle and the receptacle behind the av rack are on the same circuit, straight from the breaker panel. It runs from the breaker panel, to the receptacle behind the second sub, then straight to the receptacle behind the av rack. Those are the only 2 drops on the circuit.

UPDATE:

Tried the extension cord test.

If I plug an extension cord into the apc and plug the second sub into that extension cord the hum is reduced by about 75%. So, do I need an APC like device on the second sub, or a line filter?

Thanks for the continued hep guys.

Would a Hum-X help in this situation? I had to use one on my MFW-15, and coupled with rewiring the amp, the hum is pretty much gone.

 

[Rickster71]

The electrical service is supposed to be grounded using copper stakes at the point of entry, not the water line. Things can be grounded to that as a common location but the service panel isn't supposed to use that as its ground.

I afraid the water pipe Does make up part of the System Grounding. It falls under Permitted Grounding Electrodes.
NEC's Article 250.52 on Grounding.
The service entrance is grounded with two ground rods and the water pipe (the only part of the pipe used in residential grounding is to be within 5 ft of it's entrance to the building)

I did mention to the OP that - "Going to the systems ground rod is the best bet"

 

[Rickster71]

UPDATE:

Tried the extension cord test.

If I plug an extension cord into the apc and plug the second sub into that extension cord the hum is reduced by about 75%. So, do I need an APC like device on the second sub, or a line filter?

Thanks for the continued hep guys.

No.
Recheck all your ground connections on that circuit (physically tighten all ground connections)
Your two receptacles are at to different ground potentials.
How long is that circuit? And what gauge wire did you use?

 

[Rickster71]

On a side note, I am wondering if the back sub having such a long sub cable could be a cause of the noise.

That is a distinct possibility. How long is that run?

I agree with Highfigh as far as you having a ground loop.

I'm starting to lean toward the long sub cable.
Does the sub have a three prong plug?

EDIT:
Just a thought:
Can you put the splitter at Sub #1 and run from there to Sub#2? I think you should get rid of the long cable.
Sorry, I missed that little tid-bit in your first post.

 

[Rickster71]

Hey Rickster,

I am positive of the city water feed being solid copper. I also am extremely confident of the ground connection to it. Before I eliminated the ground loop due to improper ground, the ground clamp and pipe were very poorly put together and quite oxidized.

I took the whole thing apart, threw away the original ground clamp, bought a new one and scrubbed the water pipe with sand paper and steel wool.

It was so shiny and clean after I was done. I then clamped the new clamp on and tightened down the screws until I couldn't turn the clamp screws anymore. Then I cut a new piece of 14/2 romex and ran that from my cable demarc to the ground clamp and clamped it in super tight.

It is side by side with the ground cable coming from the breaker panel.

Can you see any steps that I missed or did wrong? It solved the original hum right away and if I remove the coax cable now it doesn't relieve any of the new hum, so I assume it is all good, right?

It's required that a product like this be used: http://www.idealindustries.com/products/wire_installation/accessories/noalox.jsp

It's used where any dissimilar metals are used in electrical connections, and to protect outdoor connections from oxidation.
Keep in mind Grounding, is part of a system, and at any point a weak link in that system can cause problems.
I'm not implying that this specific thing will solve your hum issue. Though, this is part of how a professional connection is made.

 

[ThA tRiXtA]

No.
Recheck all your ground connections on that circuit (physically tighten all ground connections)
Your two receptacles are at to different ground potentials.
How long is that circuit? And what gauge wire did you use?

I rewired both receptacles and no change. I forgot to mention both receptacles that are used for the HT equipment are leviton hospital grade receptacles. I don't know if that changes anything.

The circuit from breaker panel to the end receptacle I would approximate being 60 feet +- and the romex is standard 14 gauge.

The back sub's cable is 37 feet long. Whatever the problem is, is it possible that the second sub can feed back to the first? The first sub doesn't hum until I connect it to the second.

Yes, the subs have a 3 prong plug. I'm about to rig up a cheater plug and see if that kills it for me.

If I were to run a sub cable from the AVR to the first sub, then split it there and double back to the second sub, the second sub's cable would be even longer than 37'.

 

[emorphien]

Boy that sounds annoying. I had a ground loop problem and it turned out to be nothing I could fix but instead I had to get the cable guy to come out and put an isolator on the line before it entered the cable box.

 

[highfigh]

I absolutely guarantee that the second sub's receptacle and the receptacle behind the av rack are on the same circuit, straight from the breaker panel. It runs from the breaker panel, to the receptacle behind the second sub, then straight to the receptacle behind the av rack. Those are the only 2 drops on the circuit.

UPDATE:

Tried the extension cord test.

If I plug an extension cord into the apc and plug the second sub into that extension cord the hum is reduced by about 75%. So, do I need an APC like device on the second sub, or a line filter?

Thanks for the continued hep guys.

Usually, the resistance in the ground conductor and/or neutral causes the loop, which is basically just one device trying to do what it does with a little less and since the audio cable is connected to a grounded device, that's where it makes up for the resistance (which causes a slight voltage drop). Ideally, a room that has two receptacles on opposite sides and will be used for a system like this should have one feed to an area that can then be wired to both boxes on equal length wires. That minimizes any resistance differences and unless the Romex is passing by a strong source of EMI, it should be clean. The extension cord test is a good way to verify a loop like this.

It's usually better to use an isolation transformer but sometimes, grounding the audio shield to the receptacle works. Remove the wall plate and use a wire to connect the RCA cable's shield to one of the screws that holds the receptacle in the junction box to find out if this will get rid of the hum.

I'm doing some work on a system in a bar/restaurant and I added an amp in a soffit above the bar. They had a receiver and CD player before and I really didn't listen for hum but once I added the amp and connected it to the network jukebox, it started. I'll be checking with my trusty extension cord but the ground lift adapter didn't help, so I think it may be a radiated noise issue, so I'll have to clip the drain wire in the audio cable once I can get into the jukebox. It has normal and inverted jacks, so I ran it balanced and since I don't have a key, I have to wait. I suspect that it may not go away when I use the extension cord.