AC ground loop ?

[dude]

Not sure if this is the right category to post but I'm having issues with hum in my amps, pedals etc when playing in a band situation.


I have hum from my amp which says the same, constant hum but when played alone with another guitar players amp plugged in, no hum. Sometimes hitting a pedal I get major hum, turn it off hum is less from amp.


I believe it's a ground loop in the ac from my receptacles. It's an old house and I only have two receptacles in the jam room, lots of extension boxes.


Both receptacles "might be the same circuit or line side". Taking the amp to another room plays fine. Can someone explain why I have the hum?


If I put in another separate circuit to the panel on the other side of the panel's 120v line and then have the amps plugged in different circuits, would this help?


I'd like to know a little theory on ac ground loops from the same circuit. Or direct me where to find info.


thanks,
al


 




 

[sluckey]

Check the outlet with your voltmeter. There should be 120VAC between the narrow blade and the wide blade. There should be 120VAC between the narrow blade and the round pin. There should be no voltage between the wide blade and the round pin.

[PRR]

> I only have two receptacles in the jam room,

Don't use both. Use GOOD multi-way boxes, so everything is on the one outlet.

That's for a "house" jam, where you don't have several 500 Watt bass amps. Obviously if you are filling large arenas with only stage-amps, you need more juice than one outlet can supply.

[PRR]

I really like Belkin F9D1001-15
Home Depot has discontinued, but Amazon has it:
http://www.amazon.com/Belkin-10-Outlet-Metal-Protector-15-Foot/dp/B001E8LZ5Q

Amazon notes "There is a newer model" at a lower price:
http://www.amazon.com/Belkin-10-Outlet-Metal-Master-15-Foot/dp/B000BVC0WO
I have not examined the new one and wonder what cheap-cheat they did to get price down by 1/3.

HomeDepot will sell you a Tripp Lite Model # UL800CB-15 which looks like a sturdy thing.
http://www.homedepot.com/p/Tripp-Lite-10-Outlet-120V-15-ft-Multiple-Outlet-Strip-UL800CB-15/203348221

Note that many other "metal" outlet strips I have got recently turn out to be plastic on the back. This is not necessarily bad; but when they boast about metal on the side you see in the box, and use plastic on the side that is hidden in the box, I wonder what else they are hiding.

[drgonzonm]

PRR is correct,
If both receptacles are on the same line, it doesn't matter to the wiring and breaker/fuse whether you are on the same receptacle,  Most house wiring will have one room on the same breaker.  Most receptacles are rated at 15 amps,  The wiring installed will be rated at 15 or 20 amps.    The breaker will throw if you over draw the breaker's amperage rating for any decent length of time.   

WARNING: do not replace a 15 amp breaker with a 20 amp breaker because you are throwing the breaker.  You need to know that the wiring is rated for the higher power draw. 

PRR is several posts has taken a position in several posts that house wiring can not be trusted, and, I agree.  Get you one of those three prong wiring checking devices with three LEDs.  Those devices will tell you if the receptacles are wired correctly. 

If not, and you have the electrical experience, then you can correct the wiring.   

[dude]

Slucky, no miss-wiring. 120v at line side on all outlets, grounds and neutrals all tied together and at the panel, 120v across all neutral and line receptacles and grounds.


I miss-wrote about the hum, one amp alone no hum, when we get together to jam and several amps are on that's when the hum comes. someone stomps on a pedal and the recording gets screwed up. Even me and the keyboard player alone with one guitar amp on, hum. He's through the PA and a monitor amp. Yeah, I forgot the PA, 600 watts per side, in the rack.


I've have two pic's of the jam room. See all the extension cords, all the stuff near the fireplace and stairs in plugged into one 15 amp outlet. I have keys, Roland amp, several small 20 watt amps, 300 watt bass amp. I probably pulling at least 15 amps when four people are here jamming, 15A circuit (never flipped). I have several pedal wall warts, I need another receptacle I know and circuit. The panel is just under the jam room so I can easily put in a new beaker, 20 amps. I wired this whole house, had knob and tube, built in 1873. I know wiring but I don't know about ground loops.


I need to get some education on ac ground loops so I can rewire this room right.


Plus, I have a recording mixer set up to my PC with Sonar, 16 channels, pic's don't show. The PC is hooked up to a separate circuit though. Also several racks for effects. I'm sure the three receptacles in the room are on more than one 15A breaker as I  never flipped a breaker even when seven or eight people were playing amps, so I'm pretty sure there's at least two circuits to the three receptacles.


This is the only room I didn't rewire, the knob and tube wiring had been replaced in this room back in the forties with BX cable and old cloth wire. I did check it all out and it was grounded properly years ago but it wasn't a jam then. I replaced the BX wire with new RomanX as the cable was the ground, untrustworthy.


From the pictures you can see I need to rewire this jam room as over the years more and more amps, recording equipment, etc were added. I want to add a receptacle too, tripping over extension cords and I need at least two 15 amp circuits with four receptacles, probably three.


Don't want to add like three separate circuits, put receptacles around the room and still have a hum problem. Every amp outside of this room on there own, no hum. It's not the amps.


Just need an education on ac ground loops in household wiring for my application,so I can wire the room right.


al


     

[dude]

second pic

[sluckey]

grounds and neutrals all tied together

There is only one place that neutrals connect to grounds. That's in the main entrance service panel. Neutrals and grounds are kept separate in all sub panels, outlet boxes, junction boxes, etc. Is your house wiring different from this?

[PRR]

> back in the forties with BX cable

I understand the inclination to let it be.

But BX/AC cable is terrible "ground". The ground path is long and twisted and in Iron, which has 7 times the resistance of copper. Later AC had a ground strip, but it was widely ignored. Also all the clamps have rusted.

You are perhaps lucky you have not had fires. The cloth insulation goes bad. My parents fought this for decades, reluctant to rip-and-replace.

The fact that things change as minor stuff is changed suggests some really bad contacts in the "grounding" path.

In a more traumatic wing (bare splices inside inaccessible walls), I ran metal WireMold and low-profile boxes on the baseboards.

Short term, I'd do the good multi-boxes on a fat cord from a room and outlet with proper grounded outlet.

As said, ground does not touch neutral except at the fusebox. (I'm fighting that with upgrades which make my original main panel a sub-panel, but putting in 4-conductor feeder is much tricky work.)

[dude]

grounds and neutrals all tied together

There is only one place that neutrals connect to grounds. That's in the main entrance service panel. Neutrals and grounds are kept separate in all sub panels, outlet boxes, junction boxes, etc. Is your house wiring different from this?

No, only tie to neutral is at panel box like you mentioned; line to breaker, neutral (white wire on 120v) to buss and I usually tie the bare ground to the bottom of the buss bar. I kept several holes open on the bottom of the buss, for the bare grounds from each circuit. Sometimes I put three 14 gauge grounds in these holes. 200 Amp panel.


I have three prong ground plugs on all my equipment, except the wall warts. That might be the problem...? I'll have to check all these amps out, the pic's are years back. I now have several more amp builds, I play these amp hard, every Saturday night for hours at at a time and I haven't checked all the amps out yet for tube wear, some I've been playing for years and no check-ups...? Maybe a have some worn pre-amp tubes as most all are old USA pulls...?


Some of the wall warts don't have the neutral wider on the plug...?

[dude]

> back in the forties with BX cable

I understand the inclination to let it be.

But BX/AC cable is terrible "ground". The ground path is long and twisted and in Iron, which has 7 times the resistance of copper. Later AC had a ground strip, but it was widely ignored. Also all the clamps have rusted.

You are perhaps lucky you have not had fires. The cloth insulation goes bad. My parents fought this for decades, reluctant to rip-and-replace.

The fact that things change as minor stuff is changed suggests some really bad contacts in the "grounding" path.

In a more traumatic wing (bare splices inside inaccessible walls), I ran metal WireMold and low-profile boxes on the baseboards.

Short term, I'd do the good multi-boxes on a fat cord from a room and outlet with proper grounded outlet.

As said, ground does not touch neutral except at the fusebox. (I'm fighting that with upgrades which make my original main panel a sub-panel, but putting in 4-conductor feeder is much tricky work.)

I think I have one circuit in that room with old BX, I'm sure years ago when I checked it out I tighten the ground screws on the cable on both ends.


The old cloth wire has been replaced. On the third flr, I found the knob and tube ceramic tube had broken and the wire was stretched by someone before me adding a receptacle. The hot wire, no insulation was pulled against a 100 year old oak beam, the thin covering had worn off and the "bare hot wire was pulled tight against the beam, wow. Now that was a fire just waiting to happen. It was then that I ripped all the knob and tubing out. I also learn a lesson working with knob and tube, even if the breaker is off on the circuit you're working on, you can have a live neutral wire in that circuit as back them they tired the neutrals wherever they could find them, several circuits on the same neutrals. I get my butt shocked, the circuit read no current but another circuit was using that neutral too and that one was live.


So I take it, if I wire this place to modern codes, I shouldn't have any problem with ac hum in my music room? At this point I thought it was wired ok, the old part. 


Four 20 Amp receptacles around the room, splitting them on two circuits, wired to code then I shouldn't have any ac ground loops no matter where I plug my equipment in?


I'm thinking those wall warts with a non-polarized plug might be causing the problem..?


al
 

[jjasilli]

I have hum from my amp which says the same, constant hum but when played alone with another guitar players amp plugged in, no hum. Sometimes hitting a pedal I get major hum, turn it off hum is less from amp.

You may be doing something different when other players are around.  Light fixtures, fans, dimmer switches.  Whatever. 

[dude]

Just thought of something;


This old house's main panel has become a sub-panel, and the main for that has become another sub-panel, etc. I have two sub-panels that each at one time were main panels before I install a big 200 amp main panel.


The wire connecting these now sub-panels, is 100Amp three wire, two 4 gauge wires, wrapped in an aluminum shied that is twisted to form the "neutral and ground". It's not four wires but three...?  Each sub-panel has it's own 3 wire (4 gauge) going to the main serviced panel. does this need to be 4 wire?
 
Do I need two 120v lines and a neutral plus the shield from each sub-panel to the main 200 service panel? This is important, those sub-panels are running almost the entire house. The main 200 amp service panels ground is going outside from the buss to a rod 6 feet in the ground and that is the main ground.


EDIT:  One of the sub-panels does has 4 wire, 4 gauge 100 amp wire to the main 200 amp service panel, the other sub panel has three to the main panel service box but has a separate bare copper ground wire tied to a cold water pipe from it's buss. 


al
 

[PRR]

The feeder from main panel to sub-panel "should" be 4-wire.

Otherwise neutral and ground currents share a wire. Worst-case, significant AC voltage on Neutral.

Ripping FAT but healthy 3-conductor cable for 4-wire stuff is a major expense and headache. Re-rigging panels for truly isolated neutral and separate ground is a real headache. (I did that in my sub-panel as pre-planning, but then jumped the two (actually 4) busses so they both come off the one groundED/groundING wire in my cable.)

You are presumably grandfathered for common ground/neutral, until AHJ decides your changes are Major Upgrade and need to be fully NEC compliant. And in small residential work this bit of Code is not all that critical.

This is not related to your hum.

The 2-pin wall-warts are touch-safe without building ground. They get audio ground via their audio cables. I would suspect dirty plugs/jacks before any major fault in the wall-wires. Second guess is poor grip on power 3rd pins.

I do think ANY audio system should be powered all on ONE source (power strip) whenever possible. If load is truly too large, then divide the system where there are the fewest audio cables (probably between mixer and power amp, if possible) and use audio transformers (at least true differential interface) there.

[dude]

The feeder from main panel to sub-panel "should" be 4-wire.

Otherwise neutral and ground currents share a wire. Worst-case, significant AC voltage on Neutral.

Ripping FAT but healthy 3-conductor cable for 4-wire stuff is a major expense and headache. Re-rigging panels for truly isolated neutral and separate ground is a real headache. (I did that in my sub-panel as pre-planning, but then jumped the two (actually 4) busses so they both come off the one groundED/groundING wire in my cable.)

You are presumably grandfathered for common ground/neutral, until AHJ decides your changes are Major Upgrade and need to be fully NEC compliant. And in small residential work this bit of Code is not all that critical.

So I take that my sub-panels are safe as per grandfathered? One sub-panel is right next to the main service panel and that does have a 4 wire connection. The other sub-panel is across the basement and is 3 wire connected to the main service panel, I did attach a thick bare (added a 4 ground wire) to a cold water pipe, in reality that added ground should go to the main service panel for modern code.


So, am I ok with that added copper ground tied to the water pipe instead of the main service panel or should I take that out..?  I feel the water pipe is the same as going to the ground in the main service panel as the water pipe and the rod outside are earth, just one is a water pipe.


I'm probably a little anal here but I've had no problems with excessive current on any neutral wire, been wired this way for years, all testing of the wiring has no voltage on the neutrals unless I use the line side with the neutral.


Just clarify that I',m safe in your opinion.


As far as the hum, I take it I have another problem and not ac hum from my house wiring? I need to clean up the many cables laying over top of each other and clean the shielded guitar cords ends, check out the amps, etc.  I will add another circuit to the room so I can cut down on all these extension cords all over each other.


Thanks for all the info,
al

[sluckey]

GFCI may not work properly if the neutral is connected to the ground on the load side of a power panel.

[PRR]

I'm not worried about your safety. Ground-on-Neutral feeders are very common.

As Sluckey says, many newer GFI will instant-trip if the neutral and ground are jumped "close" to the GFI. Such as a jumper AT the GFI. I saw this when I was wiring my garage. I was super careful about the black, but a screwdriver ground to white would trip the GFI. They apparently use a short-range radio signal to detect "nearby" G-N shorts. FWIW, I have a GFI 5 feet from my main box and G-N jumper, and it doesn't instant-trip. Actually the first one I put in did, I discarded it as defective, and the second one has held.

I'm not keen on old BX/AC installed by unknown persons, but in a wood-floor room and with awareness of old/rotten insulation, I would not rush to rip it out.

I do think you should one-source all your AC, and go over your audio connections, before you think about major power work.

[dude]

Thanks for all you're input.


al