8kHz Hum on DI??

Discussion in 'Microphones & Recording' started by rmccam, Feb 2, 2006.

  1. rmccam

    rmccam Guest

    I am having a bit of an issue with hum at 8kHz, but only on DI'd instruments... everything else is fine.

    I've tried trouble shooting the usual hum causes but nothing seems to work. Does anyone have any suggestions as to what would cause hum at about 8kHz?

  2. moonbaby

    moonbaby Mmmmmm Well-Known Member

    Feb 23, 2005
    8K?!?!?! Are you sure about that frequency? That's more like squeal!
    That can be caused by RFI or some wierd feedback loop problem...
    When you trouble-shot, what did you do? Change cables, DI boxes, try different pre's, different phantom power sourcing, what? And, I swear to the almighty on this one, is there a MiniMoog Voyager in the room? The ones with the optional blue backlighting are giving out a god-awful HF whine from the power supply they use.... Fill us in, dude!
  3. Kev

    Kev Well-Known Member

    Nov 13, 2001
    how are you DI'ing these instruments ?

    is there one DI box involved ?
  4. RemyRAD

    RemyRAD Member

    Sep 26, 2005
    It sounds like you have an inductively induced feedback loop. Are you using an analog multitrack recorder? If you are trying to record on adjacent tracks in sel-sync, this is a common problem. You can make it work by reducing the record level.

    This only happens with DI inputs.....? You have a ground loop on the DI, try lifting ground? Try another DI that is not active but transformer isolated.

    Ms. Remy Ann David
  5. rmccam

    rmccam Guest

    Ya, I realized how messed up it is... hence the posting on this forum!

    okay, where to start... I know it's 8k because I can almost notch it out at 8k.

    Signal path example... bass guitar (active or passive pick ups) > U Audio LA-610 > Apogee Rosetta 800 > Pro Tools


    Any guitar > POD > Radial Passive DI with Jensens > Presonus MP20 > Apogee Rosetta 800 > Pro Tools

    The issue arises with any DI'd instrument (with single coils), with any preamp, with any cable, with or without a DI, with or with out the ground lifted. I might make a subjective judgement in saying that it's a bit worse with instruments with active pickups, and non-existent with humbuckers.

    In as far as troubleshooting goes... I've tried swapping out everything in the chain, lifting the ground on the DI's, moving power supplies/transformers/etc., shutting off the computer monitor, isolating the entire chain, and completely isolating audio cables from power cables. Oh, and everything is on the same breaker.

    I'm pretty stumped. I hope that helps though.

    Thanks everyone
  6. moonbaby

    moonbaby Mmmmmm Well-Known Member

    Feb 23, 2005
    I think that you've answered your own question on this! It is non-existent with humbucking pick-ups! I believe that your problem is the single-coils picking up RF! But from what, I know not. You will have to do a lot of detective work, to be sure. What kind of lighting is in the room? Does the noise change as the instrument is moved around? Did it change when you played with the computer/monitor position? Active pick-ups can be a pain because the little preamp in them is susceptible to the same RF, plus it is there to BOOST whatever the pick-up is picking up. I'm sure that there are plenty of others on this forum who will advise you as to what they've done to minimize RF in the room. Vaya con Dios!
  7. rmccam

    rmccam Guest

    Thanks moonbaby!

    I figured that it was some sort of RF but as you said, from where? I was hoping to find some suggestions on usual/unusual culprits.

    I use only lamps for lighting... no flourescents, no dimmers. There are noise fluctuations when I move around but none seem to be in relation to the computer monitor.

    Suggestions on different RF cures would be fantastic!

  8. moonbaby

    moonbaby Mmmmmm Well-Known Member

    Feb 23, 2005
    OK. First off, all I can do is to tell you to start with whatever you have that is PASSIVE. How proficient are you with opening up the axe and tearing into the rats' nest, err, wiring harness? Examine the pick-up cavity and the back of the pickguard. How well is the instrument shielded? Is it with copper foil tape or metallic paint? How extensive does it go around the cavity? And you DID say that turning off the computer and its monitor did not help, correct? Any digital signal generators (samplers, synths, drum machines) or digital signal processors in the room? are they turned off? More later.
  9. rudedogg

    rudedogg Guest

    what kind of computer are you using? if it is a laptop, try putting it on battery power instead of AC. I had a similar problem with my laptop when doing remote recording, so I switched to using battery when tracking with it and fixed the problem.

    good luck.

  10. Kev

    Kev Well-Known Member

    Nov 13, 2001
    a switch mode power supply perhaps
    go around the room and turn things off and on one by one
    you may find a plug pack causing this ... or a monitor etc
  11. MadMax

    MadMax Well-Known Member

    Mar 18, 2001
    Sunny & warm NC
    Home Page:
    I LOVE these kinds of problems...

    They're either soooooo stupid simple, or such a complex problem that you end up scratchin' yer' haid plum bald figuring it out.

    OK, 8K - Lets look at harmonics and het modulation for a moment...
    8k, 16k, 32k, 64k, 128k, 256k 512k, 1MHz. Anything in the area or vicinity of your room that operates at those frequencies?

    Does anyone have a house for sale nearby? Sometimes realtors have those gawd awful lil' 2 watt AM transmitters that are shielded for squat. They're usually at 800K-900K. (harmonic)

    Do the math on wavelengths of common RF emitting appliances when you heterodyne any two frequencies... like a microwave and your converters, cordless phone and converters, etc.

    Is there a cell tower nearby that just went up? A local AM station? Some dufus with a new Ham rig? (A lot of nimrods use an inverted V which sends all kinds of crap into the ground plane.)

    I've seen Nintendo's, microwaves, clock radios, wall wart power supply's and cable TV boxes do this stuff, so unfortunately, it can get a bit tedious to track down. Any managed switches like a 3Com? What about power strips with telco isolation?

    This is kinda' radical, but if you can, get a decent sized sheet of copper (12"x12") from your local "craft" supply... ideally, a 24" square of copper screen would do it! (You aren't likely to find THAT at yer' local Radio Shack...) You could tie it to ground and wave it around the instrument slowly and see if it's coming from a particular physical direction. (It's either in a direct line of sight path, or 90 degrees.)

    Just some things to look into.

    (I'm gonna keep an eye on this thread... this kind of troubleshooting is interesting to see pan out!)

    Good luck!

  12. CoyoteTrax

    CoyoteTrax Well-Known Member

    May 25, 2005
    Home Page:
    Try taking the POD out of the configuration. Sometimes a multiFX processor can develop what sounds like RF when it's actually a component issue [like a bad IC or capacitor].

    Unplug the power cords for all your powered gear in the signal chain for 1 full minute then plug it back in. A capacitor holding a static charge can cause "squeal" in the freq range you're talking about. The only way to allow that static charge to dissipate is to disconnect the power cable from the wall.

    Make sure you've swapped the guitar cable out too and swap out all your other cables to elimate those as well.

    Also, if you have one of those ionizer air cleaners on the same circuit, they can send noise down the wire.

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