The mystery of ground hum-Why it happens and how to stop it?

[wheresgrant3]

I have a few questions involving my keyboard rig and ground hum that occasionally eminates from it. I've been playing keuboards now in the same band for over seven years. Although pieces of gear over time have changed, my setup configuration is usually the same...

 

3-4 synths/keyboards sending stereo signal using dual TS cables into a sub mixer>stereo Submix to stereo DI> Stereo DI splits two stereo signals... one that sends stereo mix to the FOH... one that sends stereo mix to my monitoring amp at my feet. What is always consistent is a degree of noise or buzz at varying volumes. Sometimes it's barely noticable... other times it so loud that when the band pauses there is a noticable hum. It depends on the room of course.

 

Over the years I've depended on two pieces of gear that help to diminish the 'hum' and again depending on which room we play determines which piece of gear I use. One is a Samson active DI... the other is an ART Isolation Transformer that I just call Dark magic. I usually use the DI box... however in some rooms the DI seems useless and that's when I break out the IT box. When both fail I'm foooked... as I was a few weeks back at an auditorium gig. I ripped apart my keys rig, eliminated the submixer, and plugged just two boards directly into my amp and sent the signal silently through the amp's DI.

 

I play in a band that incorporates alot of electronic music. Various synths and samplers to get the sounds I need. Just wondering the best way to manage all that gear with the quietest of operation.

[Coaster]

there is no mystery; multiple paths to ground with can cause hum. this is all on the electric side of things, but can be caused by unbalanced audio connectors by mixing the ground reference with the audio ground.

 

use basic troubleshooting and process of elimination as well as suitable power. i suspect it is mostly power that is your issue, somewhere something is plugged into something electrically that has a different reference to ground and causes your hum.

 

while it may seem like a mystery it is actually pretty simple and common. i use unbalanced lines on my keyboards and have no hum issues. my band uses a distro.

[srp72ee]

If you use DI boxes, you can isolate the ground loops and prevent the hum. They will give you balanced outputs from an unbalanced source.

[Keyrick]

What kind of mixer do you have? Does it use a wall wart power supply or does it have the power supply built in?

[oldschooler]

Are the outputs on your sub mixer not balanced? If they are, there is no need for the DI.

[Coaster]

 

Are the outputs on your sub mixer not balanced? If they are, there is no need for the DI.

 

 

 

there is if it happens to be routed to the xlr ins of a global phantom board with phantom on. i've had it happen to me before..

[TimmyP]

 

Are the outputs on your sub mixer not balanced? If they are, there is no need for the DI.

 

 

Whether a line is balanced has no bearing on there being a ground loop. It's a matter of current flow through the shield of the cable that connects two pieces of gear who's chassis are at different ground potentials.

[oldschooler]

 

Whether a line is balanced has no bearing on there being a ground loop. It's a matter of current flow through the shield of the cable that connects two pieces of gear who's chassis are at different ground potentials.

 

 

My comments just points out that if there is a balanced out on the mixer, there is no need for a DI. One less piece of equipment to hook up.

[Dogoth]

Noise is usualy caused by one or a combination of 3 things.

 

1)induced hum. This is electromagnetic hum from ajacent AC lines. It is only an issue with unbalanced or poorly balanced systems. When I say poorly balanced I mean uneven twists in the cable or a malfunctioning balancing differential amp (the balancing circuit inside your equipment). Keeping unbalanced lines as short as possible helps alleviate this problem.

 

2)Ground loop hum. Audibly indistinguishable from induced hum, this is caused by having a different path (with different resistance) back to ground (I'm refering to the white neutral wire which is a current carrying ground path) which share a common ground. In other words two connected pieces of equipment plugged into two different outlets that have a drasticly different neutral potential (sometimes as much as a couple of volts). The problem also occures if the two AC circuits in question get their neutrals from a different physical place on the earth (say 500 to 1000 feet apart). Strangly enough Ground is not an absolute potential and can vary (again by as much as a couple of volts) depending on where you put the copper rod into the ground. The most likely and best cure for this one is to try to make sure all of your equipment is powered from circuits from the same distro pannel (in some old buildings this is hard to know for sure and you just have to experiment) even then, old corroded connections can add resistance enough to cause the problem. Another cure is to lift the ground path connecting the pieces of equipment (this is where a DI with a ground lift or your iso transformer come in handy).

 

3)Electrostatic Buzz. Sometimes this is called hum but I call it buzz (hum with a lot of high frequency content (more like sawtooth waves)). It is also an induced noise but MUCH harder to get rid of. This can be induced into both balanced and unbalanced lines. It is usualy much more of a problem in high impedence circuits (I.E. single coil guitar pickups or piezzo pickups). The only way to minumize this is to keep cable lengths as short as possible and have everything shielded as well as possible (you will sometimes see shield ratings on cable like 60% or 100%). The problem is a 100% shield cable is a foil shield and not suitably flexable for a guitar (it's usualy more for permanent installs). In cheaper cable the shield can also break down with age and become less effective. The best cure is to eliminate the source (anything that causes high voltage arcing like neon & sometimes flourescent lighting, poorly filtered lighting dimmers, worn motor brushes and spark plugs to name a few).

 

There are a few other much rarer causes like power supply ripple (time to change the filter caps in your equipment) but the first 3 are by far and away the most common. A word of warning. Don't use AC ground lifts to fix noise problems. this can be dangerous. If this fixes the problem it usualy means there is something wrong with the building wiring (or at least the outlet you're plugged into). find another AC source but DON'T USE AC GROUND LIFTS - They are unsafe.

 

This was probably more info than you wanted but it's something to chew on for a while anyway (and it helped me kill about 20 minutes that needed killing :>)

 

Hope it helps

[The Chinese]

 

Whether a line is balanced has no bearing on there being a ground loop. It's a matter of current flow through the shield of the cable that connects two pieces of gear who's chassis are at different ground potentials.

 

 

Yes, But I noticed it as well. The concept isn't sound; Using a DI off a Mixer Output reduces the SN ratio quite a bit... If the Mixer isn't balanced, then a Line Balancing Transformer 1:1 is the appropriate tool to use. I would advise using the DI's at the output of each Keyboard, then Taking the SubMix out at Line level wherever it needs to go. Agedhorse makes a Line Isolation transformer, which I would recommend after the Split off the Mixer out (to your amp...)

 

Todd A.

[W. M. Hellinger]

 

It's a matter of current flow through the shield of the cable that connects two pieces of gear who's chassis are at different ground potentials.

 

Especially evident if the AC outlets are wired incorrectly.

 

For years our keyboardist had a problem with humm from one of his keyboards. After I instituted a rigorous program of testing all venue outlets I figured out the problem. At one venue we'd played many times, there was one AC outlet our keyboardist had been plugging into that eliminated the humm. Come to find out, that "quiet" outlet had the load (hot terminal) and neutral swapped (and no safety ground... but it was a 3 prong outlet)... but any other AC outlet that was wired correctly would cause his keyboard to humm. So, I dug into his keyboard and discovered the power cord of the keyboard was wired wrong (from the factory). After I corrected the power cord problem with his keyboard, the humm problem went away. No doubt this sort of thing is a rare occurrence, but it existed.

 

I have found that in public venues where the 117v AC outlets get a lot of use (and abuse) that a very high percentage (possibly 50% or more in many cases) are wired incorrectly (typically the load and neutral are swapped).

[gearhead1972]

Definitely carry one of these

Clicky You should always start at the source and work back. Also as mentioned check to make sure all your gear is wired in the right polarity.

[agedhorse]

Definitely carry one of these Clicky You should always start at the source and work back. Also as mentioned check to make sure all your gear is wired in the right polarity.

 

Note that this device (and those like them) will NOT indicate multiple bonds between neutral and ground or a reversal of neutral and ground... both possible causes of hum.

[Scodiddly]

 

Stereo DI splits two stereo signals... one that sends stereo mix to the FOH... one that sends stereo mix to my monitoring amp at my feet.

 

 

So are you saying that you Y-cable the XLR output of the stereo DI to both house and your own amp? Or are you maybe using the 1/4" pass-through jacks on the DI for your amp?

[agedhorse]

Ground loop noise, and hum in general can ve very difficult to troubleshoot because ther are many possible causes, which increase as the system becomes more and more complicated.

 

The first thing to do in this case is to set up the submixer into the main mixer and see ifthere is hum. If so, there is a ground loop between the two. In general, an active DI will not be as effective at eliminating the hum because of the mechanism used to obtain phantom power is at odds with high ground isolation. A (passive) DI may indeed be the best solution here because many mixers don't handle the potential of +4 and higher level that a 1:1 isolation transformer will (or can) deliver. If you have a real pro (main) mixer, then that's not a problem. Also, you REALLY need to isolate between the submixer and main mixer in all situations to guard against phantom power from the main mixer potentially interefering with the output stage of the submixer, as well as the ability to easily and effectively open the audio ground connection between the two mixers.

 

Once the link between the main mixer and the submixer is resolved, the next thing to do is attach ONE keyboard and do the same listening test to see ifthere is hum. If so, then there may be additional local ground loops and additional transformer isolation (via passive DI's) will be necessary.

 

Pro installations will often transformer isolate as a precaution, especially in the broadcast field, where all kinds of uncontrolled situations will be encountered. I carry about $3k worth of isolating solutions that I can use as needed. Usually I end up using them to solve other poeple's problems and ALWAYS between aduio and any connecion to the video world. ALWAYS.

 

This is an area where I probably have more experiece than most folks, I also have to deal with ground loops and ground currents in each piece of equipment that I design. The DI outs on all of our products are considered stellar, it's because they have considered all of this stuff from the practical real world perspective and the designs are as tolerant of real world issues as I can possibly make them (plus a few tricks from up the old horse's sleeve).

[agedhorse]

 

So are you saying that you Y-cable the XLR output of the stereo DI to both house and your own amp? Or are you maybe using the 1/4" pass-through jacks on the DI for your amp?

 

 

Eeek, another possible cause... any slpit should have the second output transformer isolated from the main output. That way the ground to the second piece can be easily and reliably lifted. There are devices made specifically for this purpose.

[Bugzie]

Andy, do devices such as the Ebtech hum eliminator work in a case like this?

[agedhorse]

 

Andy, do devices such as the Ebtech hum eliminator work in a case like this?

 

 

They can, and must be used on each and every connection that has a ground loop. All it takes is one ground loop to cause problems, multiple loops are common.

[The Chinese]

 

Andy, do devices such as the Ebtech hum eliminator work in a case like this?

 

 

I've used these a ton, the biggest achilles this unit has is that it uses TRS jacks. I'd Rather see XLR. But it is effective.

[RoadRanger]

I've used these a ton, the biggest achilles this unit has is that it uses TRS jacks. I'd Rather see XLR. But it is effective.

http://www.zzounds.com/item--ARTDTI

[The Chinese]

From the Manual, Bold is by me:

 

 

The compact black anodized all aluminum case and it

[Bugzie]

It's available in both 1/4"TRS and XLR.