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Constant Ampeg SVT-4 Pro Hum. Any Ideas?

Discussion in 'Amps and Cabs [BG]' started by BassIan, Mar 21, 2005.


  1. BassIan

    BassIan Supporting Member

    Apr 27, 2003
    Cupertino, California
    I've had my Ampeg SVT-4 Pro head for a bit over six months. I purchased it new, and have been absolutely adoring it through my PR-410HLF cabinet. The only thing I've noticed that actually really bugs me is a hum through the speaker cabinet. I have checked every method I can think of to trace the hum, but nothing seems to make a difference. Here are some facts about it:

    1) It is independent of master volume and gain. Even with one/both of these at 0, the hum is still there, and its volume does not change. I am not confusing this with standard amp hiss, which is almost nonexistent on this amplifier until the master volume is near full. This I am not concerned with, it's the hum that bothers.

    2) It is independent of pedals/rack effects. This is sort of proven by number 1 too, but I had to check it anyway.

    3) It is not because of the extra power outlet on the back. I have my Korg rack tuner plugged into that, and removing that plug did not make a difference whatsoever.

    4) It does not depend on the electrical circuit. I've tried it on a variety of circuits and always the same result. The hum is the same volume and never goes away. It is not due to bad line power.

    So I'm left wondering why this hum continues. I cannot for the life of me determine where it comes from since it must be happening downstream of even the master volume control on the amplifier. Perhaps Jerrold Tiers could help here too.

    If anybody has any comments on this, or suggestions, please let me know.
     
  2. If its not a pre tube, which youve proven it cant be, maybe its a transistor?
    Isnt there a fan on it too ? could be that
    Could also be the speaker cab (faulty tweeter? i think they have them), unless it is definatly from the amp and im picking you up wrong
     
  3. BassIan

    BassIan Supporting Member

    Apr 27, 2003
    Cupertino, California
    There is a fan in the amplifier mounted on the power block heat sink. I'd be very surprised if this was it, although interference from the fan or the transformer (it is not a toroidal transformer, rather the heavier iron-core laminate type) is a possibility. In this case, I'm sure other people will have had similar experiences.

    I've ruled out faulty tweeter because the noise is present from all drivers. Thank you for your suggestions.
     
  4. So the sound is actually in the signal then, i wasnt sure if you meant it was just a humming from the amp itself or not

    Simple, but could be, cable maybe?

    I dont know, just an idea
     
  5. Jerrold Tiers

    Jerrold Tiers

    Nov 14, 2003
    St Louis
    I can think of a couple dozen things that COULD be it, but I have nothing to go on here for a real diagnosis....

    In any case, 'tain't right, so since it is under warranty, your first fix is to take it in and get it fixed....let the tech and us worry about it.

    Otherwise...

    I assume this is hum thru the speakers, right (not direct from amp acoustically)?

    From how far away can you hear it? (only with ear near speaker, audible at 5 feet, "loud" at 5 feet, etc)

    Is it still there if a plug is put in the "power amp in" jack? (there is some circuitry after the master and before the power amp).

    Are you running mono? If not in mono, are both channels giving same hum?

    If no outside issues are involved, i.e. it hums loudly in the speakers if they are the only thing plugged into the amp, warranty action sounds very appropriate.

    It's the sort of problem that local repair techs sometimes don't understand (the better ones do)....don't let them chase you away.
     
  6. BassIan

    BassIan Supporting Member

    Apr 27, 2003
    Cupertino, California
    Jerrold, thanks for replying here. I appreciate this. I'll have to take the amplifier in, but I'd like to troubleshoot as far as I can first.

    The hum is through the speakers, not from the amplifier itself. The sound is audible at 5 feet, but not "loud". It does not distort my playing, but when the band is talking, not playing, it's audible.

    I will have to try taking a plug into the Power Amp In jack. I currently run the amplifier bridged mono into the PR-410 HLF cabinet, and do have the Speakon connector wired correctly.

    If any more information could help you narrow it down, that's awesome, but I understand that I'll have to send it in. Thanks in advance for any further help.
     
  7. Jerrold Tiers

    Jerrold Tiers

    Nov 14, 2003
    St Louis
    Send it in? As in "to us"? Hopefully not.......

    Nobody near you is a servicing dealer or warranty repair outfit? Local service would be your best approach, normally.
     
  8. BassIan

    BassIan Supporting Member

    Apr 27, 2003
    Cupertino, California
    Pardon my ignorance since I'm pretty unfamiliar with the underpinnings of the musical instrument industry - I've been deeply involved in the cycling industry for years and the standard practice of manufacturers there is decidedly "different." Of course my first stop would be my music store, they know me well and should be able to help out.

    Thanks again for your help.