Defeating Hum

mtshark · #1 08-29-2010, 02:13 PM

Whenever I use my amp at church I get this nasty (60 cycle?) hum when we try to plug it into the sound system (via the direct XRL output.) I don't really know what to do except somebody suggested removing the ground when I plug in my amp (risking my health) and our pastor for some reason is hesitant on buying a hum eliminator. So yeah... Any ideas or suggestions?

walker rosewood · #2 08-29-2010, 03:05 PM

how is lifting the ground risking your health? If there is a ground loop, then that is what you need to do. Several ways to accomplish this, if your amp doesn't have a ground lift switch then I would suggest getting a DI box that does.

T-Bird · #3 08-29-2010, 11:08 PM

Hi.

Quote:

Originally Posted by walker rosewood

how is lifting the ground risking your health? If there is a ground loop, then that is what you need to do. Several ways to accomplish this, if your amp doesn't have a ground lift switch then I would suggest getting a DI box that does.

Because 9 times out of 10 when non-informed people talk about lifting the ground, they mean the mains ground. On the days of old (

), the ground lift on tube amps did just that. Which in turn led to unfortunate events.

Of course, when You lift the mains ground, the signal ground (loop) is interrupted as well. Whether the people who used such a "feature" back in the day thought about it that deeply is anyones guess.

There has been a few DI threads in TB also where someone has stated that lifting the DI (signal) ground will be a shock hazard. Fortunately more knowledgeable people have corrected the misunderstanding.

Using a separate DI box is usually the best way to ensure relatively hum-free signal transfer.

Regards
Sam

mtshark · #4 08-30-2010, 05:33 PM

Weve got some DI boxes, but I have no idea on how I would use it. Theyre the kind with the 1/4" in/out on one side and the XRL output that goes into the mix on the other.

timbledum · #5 08-30-2010, 05:41 PM

What hes suggesting by removing the ground is snipping off the ground pin on the power plug. I've seen it done and it works, and nothing bad has come of it.... yet. But I wouldn't recommend it.

okcrum · #6 08-30-2010, 06:04 PM

Ground lift on a DI output is not a hazard. Ground lift on the power cable can be.

KeithAlanK · #7 08-30-2010, 09:03 PM

Most likely causes of your hum:
Your amp is getting it's AC from a different circuit than the PA mixer--plug in somewhere else.
The XLR cable has a broken wire on either pin 2 or pin 3.

And when you lift ground, use an actual ground lift adapter--$1 from any hardware store, WalMart, Radio Shack, etc.
DON'T break off the ground prong!

mtshark · #8 08-30-2010, 10:46 PM

Like I said, I have no clue how to use a DI box... But if lifting the ground on the cable is my only option I might try it (I still don't know what the problem is.) Maybe I'll grab a pop filter or something so I don't take all that voltage to the mouth creating a scene worthy of an AC/DC album cover...

projectMalamute · #9 08-31-2010, 08:59 AM

Don't modify the power cord on your amp.

Jesus christ people, if you don't know what you are talking about don't give advice.

If the problem is a ground loop(which is likely) it can be solved by disconnecting the ground in the XLR run from the DI out on your amp to the console. Your amp should have a ground lift switch by the line out, if not you could wire something up. Ground is pin 1.

Or use the DI box. Guitar -> DI box, XLR out of DI to console, 1/4" out of DI to amp. The DI box will have a switch to lift the ground.

Joe Nerve · #10 08-31-2010, 09:06 AM

Quote:

Originally Posted by projectMalamute

Don't modify the power cord on your amp.

Jesus christ people, if you don't know what you are talking about don't give advice.

There's a really good sarcastic response to this, but I haven't thought of it yet. Will get back soon.

mtshark · #11 08-31-2010, 10:47 PM

So after doing a little research, I find out that my amp (a GK MB115) does in fact have a ground lift. Sorry for asking a noon question, but do I only have to take the screw out?

jazzblade · #12 08-31-2010, 11:11 PM

Usually there is just a small button right next to the DI output marked Grnd/Lift or Grnd on/off.

Not sure about any screw.

Munjibunga · #13 09-01-2010, 01:40 AM

Quote:

Originally Posted by timbledum

What hes suggesting by removing the ground is snipping off the ground pin on the power plug. I've seen it done and it works, and nothing bad has come of it.... yet. But I wouldn't recommend it.

This is extremely dangerous.

mtshark · #14 09-01-2010, 08:44 AM

Quote:

Originally Posted by jazzblade

Usually there is just a small button right next to the DI output marked Grnd/Lift or Grnd on/off.

Not sure about any screw.

Take a look...

phsycobass · #15 09-07-2010, 09:54 PM

It does not appear that the GK mb115 has the ground lift feature built into the DI. Clipping the ground pin on a mic cable would work, but is dangerous if the mic cable is used for any other purpose.

Your best bet is to use one of the church's DI boxes. You plug your bass into the 1/4 inch input, the 1/4 inch output goes to your amp, and the sound system gets the XLR feed (same as plugging a mic cable into your amp). 99% of DI boxes have a switch that separates the ground that your amp is plugged into from the ground that the PA is using (effectively stopping the hum). This way the amp and the sound system remain safely grounded and the hum is gone.

PS: NEVER cut the ground pin on a power cable or use a 3-prong to 2-prong adapter, it's just not safe.

if you have any questions on the subject PM me.

Petebass · #16 09-19-2010, 01:03 PM

Quote:

Originally Posted by projectMalamute

Jesus christ people, if you don't know what you are talking about don't give advice.

+1. 15 posts in and no-one has even mentioned that there's more than one type of hum and the ground loop button only fixes one of them.

Try the groung lift and if that doesn't work, ask questions:-

Does everyone's amp hum, or is it just the bass amp? You may have to look at something with an isolation transformer. Some DI's have them, you can even buy them on their own.

Are you by any chance playing a bass that has single coil pickups? Is the grounding ans shielding on the bass itself in good order?

Also check that you haven't got any power cables running parallel to any instrument ot mic cables.

DubnBass · #17 11-12-2010, 02:19 PM

My GK Backline 600 sends a hum through my PA system whether I've engaged ground or lift. I've got a DI box, but I want to have the built-in distortion coming through the PA... But without the hum. How do I go about this? By the way... My bass doesn't have a single-coil pickup & I'm not using a modified cable, just a typical mic cable. Any help is much appreciated!

Johnny Crab · #18 11-12-2010, 04:33 PM

Try this: http://www.sescom.com/Product.asp?item=IL-19
Cheap enough to get for yourself to carry in your bass case for use anytime it's needed IMO.

DubnBass · #19 11-13-2010, 03:57 AM

Will look into that! Thanks!

uhdinator · #20 11-18-2010, 07:11 PM

Ground Loops are caused by AMP grounded to one circuit and Mixer grounded on different outlets/circiuts in the Bldg. Pin 1 in the mic cable from an amp DI is grounded to the mixer.

So the mixer and the amp have two paths to ground and it is like a "big loop"

GO buy a 3' or 5' mic cable (shortest you can find) open a connector on one end and cut the wire to pin 1. Put it back together and spray paint it. (so you know this is the ground lifted cable.

When ever you play somewhere that you have this prob. add this cable on the end of your mic cable from your DI. This forces the Mixer to go to its ground and your amp to go to It's ground. Breaks the loop.