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broken preamp & odd looking waveforms

Discussion in 'Preamps / Channel Strips' started by spinchris, Jun 4, 2006.

  1. spinchris

    spinchris Guest

    so my berhinger mic200 tube preamp has been acting up. it's been dying out during use, producing weird sounds, etc. i tried replacing the tube with no such luck. so i'm out $60, whatever.

    i'm just very curious if any electricians out there know what usually goes wrong with preamps, besides the tubes dying (and being of berhinger descent). i tried recording with it anyways today, and i got some very different looking waveforms than what i'm used to seeing(along with different sounds as well). maybe they are totally normal, maybe i'm just seeing what i think i should be seeing. i guess i'd just like a second opinion.

    i know it's a shot in the dark (and probably even a stupid question), but if anyone has any idea what kind of mechanical failure would produce such waveforms, i'd be very interested to hear it.

    shure KSM27
    berhinger mic200 tubepre
    berhinger autocom pro compressor
    aphex aural exciter
    presonus firebox
  2. Boswell

    Boswell Moderator Distinguished Member

    There might be a clue there in the curly tail in the first screenshot just after the trace goes quiet. You haven't indicated a timescale, but it could be the sign of either a coupling capacitor going open circuit, or (more likely) the d.c. conditions changing. It could be the HT volts going low, cathode resistor going high or open or just a plain dry joint in the power circuitry. Because there is still some output in the quiet bits and the transitions are clean (it's not crackling on and off), I would suspect power problems.

    Could you post an expanded view (with timescale) of the transitions into and out of a quiet section?
  3. spinchris

    spinchris Guest

    thanks so much for the response. the reason i'm so interested in this is because i'm in school majoring in electrical engineering.

    thanks again.
  4. Boswell

    Boswell Moderator Distinguished Member

    Yuk. Horrid waveforms. Your DAW software isn't exactly helping by filling in the space between the curve and the horizontal axis, but we get the general idea.

    If you can get at the insides of the mic200 while it's playing up, I suggest you go round prodding things with a wooden stick (NOT a pencil!) and see if you have a mechanical sensitivity there. Put a low-level sinewave from a generator through the pre-amp and listen for gaps in the sinewave as well as for pops and crackles from manipulating the PCB.
  5. Thomaster

    Thomaster Guest

    i have one of those too.
    replacing the tube seems like a fun experiment to upgrade from crap to a little better.

    is it ok if i just pop in a 12AX7 tube? (i was thinking groove tubes)
    or should i pay attention to something else?
  6. spinchris

    spinchris Guest

    it turns out that after some exhausting troubleshooting, the bad recording was caused by a bug in the new version of my DAW.

    the preamp still does die out though every so often, and i have to unplug it, wait a minute or two, then plug it back in.

    i recommend replacing tubes. mine sounds slightly better, even to my very untrained ears. i got a groove tubes russian 12ax7.

    thanks for the help, and sorry for wasting everyones time. everyone understands how frustrating these kinds of situations can be.

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