Built sub, finally got the amp, now hum!! Please Help!

Discussion in 'Archived Threads 2001-2004' started by Chris_Campbell, Feb 2, 2001.

  1. Chris_Campbell

    Chris_Campbell Stunt Coordinator

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    I just built a sonosub (one tempest, wired to 4 ohms), and I finally got the amp for it today. The amp is a mackie 1400i pro amp, so it has the standard fare .25" balanced/unbalanced, and balanced XLR inputs. I consulted a previous post, and I'm hooking it to my receiver with an acoustic research RCA cable to the sub-out, with the other end going into an RCA to .25" phono adapter (into the bridged ch). All settings are correct (set to bridged, low cut filter off, etc.), and the amp seems fine (powers up, shows sig present when there is one). However, when I give it a signal, and slowly turn the amp's gain up from 0, it only makes a humming noise! I also called a Mackie tech, and he mentioned that if I pull out the RCA from the back, and the humming stops, then it's probably not a problem with the amp (which is the case). And I know my receiver is working fine because I had it hooked up to a velodyne va1215 just before trying this. For informational purposes, i use an Integra DTR-7 (basically an onkyo 777), and the adapter i'm using is an RCA (brand) bought at home depot that has only one black band on the phono tip (this is mono right?). Please help guys, this is killing me!
     
  2. Patrick Sun

    Patrick Sun Moderator
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    Go back to Home Depot and buy a cheater plug for the Mackie's power cord. It turns a 3-prong plug into a 2-prong plug. This should get rid of the hum. The cheat plug cos about $1.
    Or you could spend $150 on a Mondial unit to get rid of the ground hum.
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    PatCave ; HT Pix ; Gear ; Sunosub I + III ; DVDs ; LDs
     
  3. Chris_Campbell

    Chris_Campbell Stunt Coordinator

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    Thanks for the help Patrick, I went out to home depot and bought the "cheater" plug. Unfortunately, when I plug the amp in with this attached, the signal present light doesn't come on. Maybe Mackie has some sort of protection built in?? I'm stuck. [​IMG] Appreciate the help
     
  4. Dave Campbell

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    Most common cause is a ground loop because your cable is grounded elsewhere from the outlet. I tried most everything but only reduced the hum. I finally got rid of it by running a heavy extension chord from the amp to the electrical point closest to where the cable enters the house.
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    Hideaway Shivas
     
  5. Dustin Haug

    Dustin Haug Agent

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    Try disconnecting your TV cable. If it goes away then go to Radio Shack and pick up a 75 to 300 and a 300 to 75 ohm transformers. Piggyback them and you shouldn't have any noise any more. Worked for me. Although I had to use the method Patman suggested also. With only one or the other I get a small hum.
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    My DIY sub page
     
  6. Kyle G

    Kyle G Auditioning

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    >>go to Radio Shack and pick up a 75 to 300 and a 300 to 75 ohm transformers. Piggyback them and you shouldn't have any noise any more. Worked for me. Although I had to use the method Patman suggested also. With only one or the other I get a small hum.
     
  7. marc seals

    marc seals Agent

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    I had the same problem with my Mackie 1400i! I played around with it for a while but never did get it to work in bridged mode. However I didn't buy it with the intention of running it bridged so I just blew it off. I would be interested if anyone comes up with a simple solution on what is wrong (ie. incorrect settings etc)and the correct way to hook it up though.
    I also had a bad ground loop hum that a cheater plug took care of quite nicely.
    Good Luck
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  8. Chris_Campbell

    Chris_Campbell Stunt Coordinator

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    Dustin: I tried disconnecting the TV cable but it didn't do anything.
    Marc: You have a mackie 1400i hooked up to a home theater receiver? Which receiver do you have? The cheater plug prevents the signal light from coming on for me, how about you? Did the problem not occur when you were not using it bridged? How did you finally fix it?
    I have tried three things to eliminate the hum: used a cheater plug, grounded the chassis of the amp and the receiver with a wire, and tried a ground loop isolator cable from Ratshack. Unfortunately, all of these solutions eliminated the hum, but shut the signal present light off too!
    Thanks everyone!
     
  9. marc seals

    marc seals Agent

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    I never got mine to work in bridged mode at all. It did work with the cheater plug though. I had it hooked up to a Marantz AV550 preamp using 1 channel only.
    Have you tried to play something through it even though the signal light is not on?
    If memory serves me correctly you should also try increasing the subwoofer output and see if you can get a signal. That is what I had to do to get the signal light to work, I think [​IMG]
    Hope this helps!!??
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  10. Chris_Campbell

    Chris_Campbell Stunt Coordinator

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    Well, I'll have to try that. It's interesting, but I think my receiver just isn't putting out enought voltage through the sub-out. I know that the pro audio gear is used to a hotter signal. The guy I bought the amp from mentioned that he had a similar problem when he hooked up consumer grade audio components to it, but said it would be fixed with this product: http://www.musiciansfriend.com/ex/sh...136?pid=189100
    This seems to make a lot of sense. Anyone have any experience with this product? I really didn't want to have to plunk down another 70 big ones... Thanks for all your help.
     
  11. Patrick Sun

    Patrick Sun Moderator
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    This is another reason to get a BFD, it provides a switch to toggle between the +4dbV and -10dBu (I may have gotten those 2 output voltage standards mixed up). I'll re-edit my post when I figure out which is which.
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    PatCave ; HT Pix ; Gear ; Sunosub I + III ; DVDs ; LDs
     

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