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DI on amp noisy.. Would ground lift help?

Discussion in 'Amps and Cabs [BG]' started by Magneto, May 25, 2005.

  1. Hi..

    Hi.. I'm running the DI out on my bass head to the mains. At a couple of gigs, I've been having some buzzing noise being sent to the mixer. My amp does not have a ground lift switch, and my sound guy said that this might cure the problem. I'm not getting any noise from the speaker cab whatsoever.
    The amp is a Behringer BX3000T.. please don't bash me! It's all I could afford at the time.

    Since the amp does not have a ground lift, could I rig a mic cable to lift the ground, or would this have to be done to the amp? Any info would be helpful. I'd rather not go with a DI box when I have the DI on the amp, but I can't be having these noise issues either..


  2. seanm

    seanm I'd kill for a Nobel Peace Prize! Supporting Member

    Feb 19, 2004
    Ottawa, Canada
    You are probably going to have to get a DI :(

    But, you can try disconnecting I believe it is pin 1 on the mic cable.
  3. I have the same problem in certain places. Put one of those 3 prong to 2 prong adaptors on the powercord & try that before you buy anything else.
  4. RevGroove

    RevGroove Commercial User

    Jul 21, 2002
    Burlington ON Canada
    Manager, Account Services: Long & McQuade Ltd. (Burlington); MTD Kingston Basses International Emerging Artist; Bartolini Electronics Emerging Artist
    Another solution would be to get an isolation box and run that between the DI. I've had a similar problem in some venues, though not all, and I use one everywhere now just to be on the safe side.
  5. Thanks for the helpful ideas..

    seanm: We have some DI boxes. I also have an older Peavey one that runs between amp head and speaker cab with a balanced line out on it. Didn't have it with me the other night, so it didn't get tried.

    barkatozz: That's the first thing we tried. Tried using the adapter on my amp and rack EQ, didn't help at all. I did notice that the Behringer head gets ground hum when connected to improperly grounded AC power. But no luck with this..

    RevGroove: Isolation box? Is that like a DI box, or is it something more? I'd like to hear more about this if you have any info, and perhaps the make/model of the one you're using.

    The Behringer also has an unbalanced 1/4" line out. Wouldn't that be virtually the same as running a balanced XLR with the ground lifted? Man, I really need to know more about this. We have a good sound man, but we didn't have enough troubleshooting time to fix the problem before the gig started..

    Thanks again..

  6. seanm

    seanm I'd kill for a Nobel Peace Prize! Supporting Member

    Feb 19, 2004
    Ottawa, Canada
    No, there is more to a balanced line than just the ground.
  7. I began some much-needed research on this. Thanks for the reply.. If you have any links to helpful information regarding this issue, I'd love to read them...

    Thank you..
  8. tplyons


    Apr 6, 2003
    Madison, NJ
    I'm gonna move this one over to amps for safekeeping.
  9. billfitzmaurice

    billfitzmaurice Commercial User

    Sep 15, 2004
    New Hampshire
    Owner, Bill Fitzmaurice Loudspeaker Design
    The source of the noise is either a ground loop, caused by having two ground paths from the amp to the board, or an incomplete ground, caused by faulty wiring in the venue. One ground path from the amp to the board is via the ground of the AC wiring, the other is the shield of the cord from the DI to the board. Make up a dedicated cord to run from the send to the snake with the shield disconnected at one end. Depending on the AC wiring of the venue use either it or a regular cord, whichever works best.

    Only if you enjoy the possibility of electrocution.
  10. tbitsky


    Apr 15, 2005
    ProCo DB1, which includes a ground lift. $63 at zzsounds.com, and they sound good.
  11. Maverick Blues

    Maverick Blues Being a Thumper is all about ATTITUDE!

    Apr 28, 2005
    Richmond, VA
    I'd like to throw more weight behind Bill's comment. You (generic "you," I'm not picking on anyone) should never, ever run without ground on the AC side if you value your safety. I also urge you to A) carry an outlet tester and check for correct wiring before you plug in (every gig, every location, even if you've checked on a prior date -- you never know when someone's rewired something), B) use an inline GFI or a GFI-equipped outlet strip or extension cord, and C) invest in an AC indicator (either a cheap VOM or a neon light probe thingum or something similar) and check between your bass and nearby mic stands etc. with everything turned on before you go touching stuff.

    Yeah, I'm paranoid, but you only get to be electrocuted once. :)

    One safe thing you can try is to run a long and heavy extension cord and plug your rig into the same outlet or circuit that the PA's on. If the ground loop is related to the electrical wiring that may cure it. It also potentially brings other problems (sags from too much current on too long an extension, too much load on that one circuit, etc.) but if nothing else it may help you isolate the source of the problem, and you can then work out a proper solution.

    Good luck with this, I know it can be frustrating! Just go about it safely, the world needs as many talented bassists as it can get.

  12. RevGroove

    RevGroove Commercial User

    Jul 21, 2002
    Burlington ON Canada
    Manager, Account Services: Long & McQuade Ltd. (Burlington); MTD Kingston Basses International Emerging Artist; Bartolini Electronics Emerging Artist
    I'm using an Apex AIB1 Dual Isolation Box from Apex Electronics. Takes RCA, 1/4" TRS and XLR in/outs, 2 channels for each. I go from the DI out from my amp XLR into the AIB1, and then XLR from there to the snake or board. It's not a DI box per se, it's more of a junction that isolates and removes noise between two systems.

    I won't say more due to the CUP, but if you want to know more, then PM me and I'll be happy to fill you in.

    Oh yeh...definitely do NOT remove the ground pin, it's not safe...
  13. Bob Lee (QSC)

    Bob Lee (QSC) In case you missed it, I work for QSC Audio! Commercial User

    Jul 3, 2001
    Costa Mesa, Calif.
    Technical Communications Developer, QSC Audio
    It sounds like at least one item has a "pin 1 problem." Lifting the shield might alleviate some or much or the symptoms. If I had to, I wouldn't lift the shield in a long mic cable because that would be a waste; I'd do that in a short one, maybe a foot long, so I could insert it as needed. In general it's more effective to lift the shield at the receiving end of a balanced cable than at the sending end--despite the fact that DI boxes usually have ground lift switches and mixer inputs usually don't. ;) If none of your gear had a pin 1 problem (or if their pin 1 problems were fixed), you wouldn't need to lift any shields. Google "pin 1 problem" and you'll find a lot of information.

    I agree with Bill Fitzmaurice and others that you should never lift a safety ground with a 3-prong to 2-prong adapter. There is a reason it's there, and it is to safely shunt AC current to ground and trip a circuit breaker if an internal short circuit developed; without it, that AC current could shunt through you. If you sing, it could shunt through your lips when they touch the mic screen. That is very unpleasant.


    Here are some resources on hum and buzz:


    You might notice that much of this stuff was either written by or refers to Bill Whitlock. He is a guru on this topic, and very open and willing to share knowledge.

    Also, the June 1995 issue of the Journal of the Audio Engineering Society was dedicated totally to issues of grounds, earths, and shields. It is very useful for understanding how to eliminate hums and buzzes, and unlike most AES journals, it is devoid of esoteric math, arcane Greek letters, etc. It is available as a reprint for $15 ($10 if you are an AES member) here: http://www.aes.org/publications/journal_issues.cfm (the issue on auditory illusions is really interesting, too, if you'd like to learn more about when you can and can't trust your ears).
  14. Jerrold Tiers

    Jerrold Tiers

    Nov 14, 2003
    St Louis
    One cannot just go around willy-nilly disconnecting the shield on lines between equipment. While that IS usually the source, and yes it's probably a "pin 1 problem", you may create a bigger problem lifting the shield.

    If you do NOT have a transformer isolated DI out, then lifting the shield may either cause a garbling of the signal, OR damage to the source or receiver circuits.

    This is because the "green wire" in buildings (equipment grounding conductor), which your 3rd (grounding) prong on the plug is connected to, MAY NOT be at the same voltage at all outlets. Yes, it is supposed to be. No there is not supposed to be any current in the green wire, and hence no voltage drop.

    In practice, induction, leakages, defective equipment, etc, etc ,etc. may cause a current flow. I have measured as much as 15 A flowing in the main ground wire of a residence, the one going to the water pipe and ground rod. Some of that may flow in the ground wires, although it isn't supposed to.

    So, there may at least be current flow, and possibly a significant voltage, between the ground wires of different outlets. Unprotected, unisolated "DI" outputs can be damaged. Usually it is only a few volts, but it could be more.

    When you plug between pieces of equipment, that voltage may exist between them. So current may flow, if the shield is not lifted.

    If the shield is lifted, the voltage may be applied to the internal DI circuits. It may just garble the sound, or it could damage the typical op-amp-based line out.

    An isolation transformer, common to nearly any DI box (and several brands of amplifiers), will withstand far more voltage than even most types of fault could put on the ground wire. It is the best isolation, and allows lifting the shield with no worries.

    BTW, if you didn't look up the references, a "pin 1 problem" is what you have if current on the shield can cause a signal to be induced in the piece of equipment. It is typical of equipment where "pin 1" of an XLR goes through circuit board traces to get to ground, as opposed to being direct to chassis.
  15. 44me


    Jun 17, 2002
    Bedford, NH USA
    I’ve never seen a good reason to ground both ends of the shield. Grounding at both ends will almost always result in current through the shield. If you need to ground both ends to get an acceptable signal, you’re trading one problem for another, and you should be using an isolated DI instead.

    - John
  16. A big thank you for everyone taking the time to share their knowledge with me. I appreciate the explanations and the links to information as well. Some of this is quite technical, and I'll have to read it a few times to let it soak in.

    So far I've played through the mains a total of 5 times. Twice the noise problem was bad (at different gigs), once it was mild, and twice it was non-existent.

    Here at home my AC wiring is not grounded properly, and I am using a dedicated ground wire run to a ground rod outside. Before doing this, I had a bit of ground noise from this amp. My bass's electronics are shielded quite well, and I've never noticed any noise being picked up by it. I also am not getting the slightest bit of buzz or hum from my amp cabinet at these PROBLEM gigs. It's only coming from the DI-mixer connection.

    I have a gig at a new place on Sunday, so I'll be testing various options. I have a DI box that goes between amp head and cabinet, taking a balanced line to the snake, or I can try one of the band's DI boxes from my amp's 1/4" line out, or I can try rigging a mic cord with the ground lifted.

    I am going to buy a AC outlet tester tomorrow to keep and test outlets where I play. Maybe this will help shed some light and hopefully keep things safe.


  17. Bob Lee (QSC)

    Bob Lee (QSC) In case you missed it, I work for QSC Audio! Commercial User

    Jul 3, 2001
    Costa Mesa, Calif.
    Technical Communications Developer, QSC Audio
    With a microphone or any phantom-powered gear, it's necessary. ;)

    But really, if the various pieces of gear don't have a pin 1 problem, there should be no problem with having the shields connected at both ends, and it saves having to dedicate balanced cables to either mic or interconnect usage.
  18. RevGroove

    RevGroove Commercial User

    Jul 21, 2002
    Burlington ON Canada
    Manager, Account Services: Long & McQuade Ltd. (Burlington); MTD Kingston Basses International Emerging Artist; Bartolini Electronics Emerging Artist
    What kind of DI is it? If it has a ground lift, then that should solve the problem.

    I had the same issue...problems in some places, other places, no noise (direct from the amp). The iso. box changed all that.
  19. 44me


    Jun 17, 2002
    Bedford, NH USA
    Right – I should have been more specific. Anything that is floating benefits from continuing the shield connection to the device (mic is a prime example). I should have said I’ve never seen a good reason to ground both ends of the shield when equipment at both ends of the cable are already grounded.

    - John
  20. I was looking at the APEX unit you suggested. Probably just what I need. However, couldn't find it advertised at any of the main music houses.
    The DI I have personally is an older Peavey EDI, with no ground lift. I used it in earlier bands. It took the signal from between head and cabinet.. has a timbre(treble) adjust control to help with the sound. ONE THING THAT CONCERNS ME is that it is rated at an input sensitivity of 39 volts RMS. I'm not sure how that translates to the 300 watt head into 4 ohm cabinet setup I'm using. I am not running my amp wide open, and I'm probably not coming too close to the 300 watts either, but I like to know I have room to breathe, and blowing this thing during a set would not be my idea of fun..
    I did see another spec sheet on a DI box that has a sensitivity of 69V RMS that translated to about 600 watts at 8 ohm load.. Can't be sure how much wattage this EDI could handle.
    I do not know what type of DI boxes the band has. I think they only have the 1/4" input/outputs and single XLR out. I don't know if they have a ground lift switch..

    Since we have plenty of spare mic cables, I've decided to rig mine with the ground lifted. I'll test between the 2 cables and see if this helps. This should work until I can check into isolation boxes more thoroughly..


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