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Buzz from rack-mounted amps

Discussion in 'Amps and Cabs [BG]' started by Planet Boulder, Mar 12, 2003.


  1. Planet Boulder

    Planet Boulder Hey, this is a private residence...man Supporting Member

    Nov 10, 2001
    6,482 feet above sea level
    I once had impure thoughts. Oh, and I pluck my ear hair.
    Okay, here's the deal: I have a rack setup with two Trace Elliot amps, a power conditioner and another rack effect. I am finding that, if i plug a 1/4" into the line out on the front of one of the amps (so i can send signal to the other), I immediately get a horrendous buzz coming through the speakers.

    In fact, if I simply wave the 1/4" end over the metal sides of the rack, I get the buzz. This has happened with two completely different amps in the same rack and this obviously sounds like a grounding issue with respect to the rack because I don't have the same problem when i plug the two units into one another outside of the rack.

    What do I need to do to solve this grounding problem? Is there a specific type of shielded cable that I should use?
     
  2. Planet Boulder

    Planet Boulder Hey, this is a private residence...man Supporting Member

    Nov 10, 2001
    6,482 feet above sea level
    I once had impure thoughts. Oh, and I pluck my ear hair.
    bump
     
  3. jondog

    jondog

    Mar 14, 2002
    NYC metro area
    I dunno. If you really think it's the rack, maybe you should try those Humfrees gear isolation tabs.
     
  4. Planet Boulder

    Planet Boulder Hey, this is a private residence...man Supporting Member

    Nov 10, 2001
    6,482 feet above sea level
    I once had impure thoughts. Oh, and I pluck my ear hair.
    Did not know about these (Humfrees). I'm thinking of ordering a couple of sets from Musicians' Fiend - does anyone here have any experience with them?
     
  5. Petebass

    Petebass

    Dec 22, 2002
    QLD Australia
    I had this same problem at a recent recording studio session. We tracked the problem to my effects pedals, some of which were sharing a power supply via a daisy chain. We hunted down a couple of extra power supplies and the problem went away.

    Is anything in your rig sharing power?
     
  6. Planet Boulder

    Planet Boulder Hey, this is a private residence...man Supporting Member

    Nov 10, 2001
    6,482 feet above sea level
    I once had impure thoughts. Oh, and I pluck my ear hair.
    Well, everything in my rack is plugged into a Furman power conditioner, so I don't think the problem lies there.
     
  7. Sounds like unbalanced interconnects are in use. Are the 1/4" plugs two or three conductor (Stereo)?

    A balanced 1/4" plug is always the stereo type plug.
     
  8. Planet Boulder

    Planet Boulder Hey, this is a private residence...man Supporting Member

    Nov 10, 2001
    6,482 feet above sea level
    I once had impure thoughts. Oh, and I pluck my ear hair.
    I was thinking that as well. The 1/4" are the single "line" type (for lack of better terminology), so they aren't stereo.

    But would that make a difference when the buzzing occurs if i simply wave the end of the 1/4" over the metal edges of the rack without plugging it in to anything? I wouldn't think so.
     
  9. Fuzzbass

    Fuzzbass P5 with overdrive Supporting Member

    I hesitate to mention this because gear might've changed over the last few years, but I remember reading one article that stated if all rack units are chassis grounded via the rack, then all but one should have the ground disconnected from power (two prong plugs instead of three prong). I actually did that myself at one time (using power plug adapters) and it seemed to help. Hopefully someone who actually knows what they're talking about can chime in so don't hurt yourself or your gear!

    Fortunately my rack is almost always whisper quiet. I don't use adapters or Humfrees or anything like that... most of my components are connected with XLR cable, however.
     
  10. Planet Boulder

    Planet Boulder Hey, this is a private residence...man Supporting Member

    Nov 10, 2001
    6,482 feet above sea level
    I once had impure thoughts. Oh, and I pluck my ear hair.
    Okay, sorry to bring this one back up, but the Humfrees didn't work. Still getting horrendous buzz when i do a line out or effects send from my amp to any of the rack unit sor fro a seperate preamp into my AH600 head, all of which are contained in the same rack.

    Can anyone confirm what fuzzbass cited in his post above or does anyone have any ideas?

    Do I need a different rack (I have the standard 8-space SKB racj now) which would cause less of a grounding problem?
     
  11. ganttbos

    ganttbos The Professor Supporting Member

    Dec 18, 2001
    New Orleans area
    You mentioned that the buzz is gone when you hook up the units outside of the rack. Could it be proximity to the Furman power strip? Take it all out of the rack and try it with and without the Furman sitting on top of the Trace Elliots??
     
  12. Are you sure you installed the Humfrees correctly? Actually, you may need to use custom cables, ground lift on some equipment, and/or humfrees. You've definitely got a grounding issue.

    I'd try Fuzzbass's recommendation by going out and buying a handful of the 3-to-2 prong adapters that will lift the ground prong coming from some of your equipment.

    What you want is to have only (1) path from any ground point to any other ground point. If all your grear uses 3-prong plugs, and it is all mounted in the rack with metal-to-metal contact, then you've got ground loops. Also, probably some of the interconnects are actually connecting the chassis grounds together too. So you've got to get it so that each chassis has only one ground path, either through the power cord, through the rack, or through the interconnect. Hard as hell to trace down, but at least it's fairly obvious what the problem is. If you can hook all your gear up together outside of the rack and you don't have a hum (and it is stacked so that the relative positions are still the same), then you're half way to having it fixed.

    Chris
     
  13. Planet Boulder

    Planet Boulder Hey, this is a private residence...man Supporting Member

    Nov 10, 2001
    6,482 feet above sea level
    I once had impure thoughts. Oh, and I pluck my ear hair.
    throb - sounds good. I'll be snaggin' me a handful of those adapters tonight to work on this with.
     
  14. Planet Boulder

    Planet Boulder Hey, this is a private residence...man Supporting Member

    Nov 10, 2001
    6,482 feet above sea level
    I once had impure thoughts. Oh, and I pluck my ear hair.
    Dredged this one up again, sorry.

    Well, I bought the Humfrees and they didn't do much (nor did the ground adaptors), so I finally (last night) just gave up and removed the friggin' power amp from the rack, leaving only the preamp, Compressor, Sound Processor/Enhancer and Power Conditioner.

    So now my gig setup includes said rack unit with the power amp sitting outside of the unit.
    Downside: more trips to make when hauling stuff around
    Upside: No more buzz and the rack unit is now easier on my back! :D

    Anyway, has anyone else resorted to this configuration?
     
  15. Bob Lee (QSC)

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

    Jul 3, 2001
    Costa Mesa, Calif.
    Technical Communications Developer, QSC Audio
    Someone suggested removing the safety ground on some of your gear. That is never a smart idea.

    If the hum was worst when you waved the plug around your rack, and it has gone away with the amp out of the rack, I think that indicates a shielding problem. Perhaps your amp(s) just leaks a very strong hum field. I don't know what's inside the TE power amps.

    Just to make sure what the symptoms are, it's a buzz (zzzzzzzzzzz) and not a hum (mmmmmm), right?
     
  16. Planet Boulder

    Planet Boulder Hey, this is a private residence...man Supporting Member

    Nov 10, 2001
    6,482 feet above sea level
    I once had impure thoughts. Oh, and I pluck my ear hair.
    Bob - yep, it's sort of a low, loud buzzing sound.
     
  17. Bob Lee (QSC)

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

    Jul 3, 2001
    Costa Mesa, Calif.
    Technical Communications Developer, QSC Audio
    With everything connected and running, and your bass plugged into the rig, does the tone and/or intensity of the buzz change when you vary the position and/or orientation of your instrument?
     
  18. Planet Boulder

    Planet Boulder Hey, this is a private residence...man Supporting Member

    Nov 10, 2001
    6,482 feet above sea level
    I once had impure thoughts. Oh, and I pluck my ear hair.
    No, not in this instance. I have another issue with one of my basses that results in this for which I'm having fixed by a luthier. It's basically a constant loud buzz.
     
  19. Bob Lee (QSC)

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

    Jul 3, 2001
    Costa Mesa, Calif.
    Technical Communications Developer, QSC Audio
    Okay, that at least eliminates the pickups as prime suspects. Pickups are a frequently overlooked source of buzzes.

    Tell me the details of what you've got and how you're hooking them up, like what's plugged into what.
     
  20. Planet Boulder

    Planet Boulder Hey, this is a private residence...man Supporting Member

    Nov 10, 2001
    6,482 feet above sea level
    I once had impure thoughts. Oh, and I pluck my ear hair.
    Right on - thank you for your help with this, Bob!

    My setup is as follows:
    Trace Elliot AH200 (used as a preamp) line out (1/4") -> ART compressor channel 1 -> ART compressor channel 2 (via 1/4") -> Behringer sound processor (via 1/4") -> Carvin FET1000 power amp (via either XLR or 1/4")

    Everything is plugged into a Furman Power conditioner (also in the rack). I've fiddled with the adapters for all, some and none of what is plugged into the power conditioner.

    Note that I have this issue whether or not I also go through my effects pedal board.

    The rack itself is the standard SKB plastic/steel type with 8 spaces.

    Let me know if more info is needed. Thanks again!!