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unbalanced output hum problem

Discussion in 'Pro Audio Equipment' started by jjg, Feb 11, 2004.

  1. jjg

    jjg Guest

    Am using a Mackie 1604 for live board and came across a noise problem last weekend. The monitor output is an unbalanced TR jack and was connected to a 100 snake via a 1/4 TRS jack. This fed a TRS input into my amp on stage for the monitors.

    Running the unbalanced TR output via a TRS feed 100' produced a noticable hum and white noise (this was not present on my main speakers fed via TRS balanced output from mackie 1604 main outputs). Unfortunately, there is no other TRS balanced output available on the Mackie with independent monitor volume control via a dedicated aux bus.

    Any ideas?
     
  2. AudioGaff

    AudioGaff Distinguished Member

    Joined:
    Feb 23, 2001
    Location:
    Silicon Valley
    Check the Mackie manual. It has descriptions and diagrams for specific cable types. Sure sounds like a ground related problem. This is where a portable DI box comes in handy.
     
  3. realdynamix

    realdynamix Well-Known Member

    Joined:
    Feb 23, 2001
    :) I would check how the tip ring to TRS cable adapter is wired. If one end, say the low end is not normalized, then you get a big gain reduction in the run.

    The cable adapter should present ground on both the sleeve and ring end of the adapter. Otherwise the balanced input of the amp only see's one side of the balanced circuit.

    The other half is open. Hum could be potential differences too, needing iso like Gaff says with DI. But if it's the wire, then it's most likely the added gain needed.

    Just a suggestion,
    --Rick
     
  4. jjg

    jjg Guest

    Gaff - Can you explain what the DI box will do? I use a DI box for getting a high impedence instrument into a low impdence mic input, but never used one for outputs. Thanks
     
  5. AudioGaff

    AudioGaff Distinguished Member

    Joined:
    Feb 23, 2001
    Location:
    Silicon Valley
    The DI box will take the unbal output of the mixer and make it a bal output to go through the snake to the bal input of the amp. The transformer in the DI should also help to eliminate the ground problem.
     
  6. Thomas W. Bethel

    Thomas W. Bethel Well-Known Member

    Joined:
    Dec 12, 2001
    Location:
    Oberlin, OH
    Home Page:
    And depending on the DI box lose 50 dB of signal level which is NOT GOOD.

    I think what you want is an unbalanced to balanced transformer. 600 ohms in 600 ohms out not I DI box that also drops the level.

    Running 100 feet from the unbalanced output of a mixer is basically asking for trouble. You could be picking up light dimmer noise and if you are running it in parallel with AC power cable you could be getting the hum in from the AC power.

    I really sounds like a grounding problem.

    The transformer you want is made by EBTECH

    Here is their website

    http://www.ebtechaudio.com/he-2des.html

    Just an idea.....
     
  7. bgavin

    bgavin Guest

    The IEEE defines a balanced circuit as a differential type. Pins 2 and 3 carry the differential signal where it is summed at the destination and the noise is cancelled.

    The link below shows how it works. Transformer conversion of unbalanced to balanced is the way to eliminate hum. This preserves the voltage at LINE level, and does not drop it back to MIC level, as do most DI boxes.

    Transformer Isolation Graphic
     
  8. AudioGaff

    AudioGaff Distinguished Member

    Joined:
    Feb 23, 2001
    Location:
    Silicon Valley
    I suppose I should have been more specific in my comments. The DI box suggestion was meant as a short term quick fix. It appeared to me that this was not a common problem and something you don't usually encounter. The DI does work and I have done it several times. True, depending on the DI box and if it is passive or active, you will lose gain. It is the trade off to solving the bigger and more annoying problem. The isolation transformer box is the correct way to do it and are available from Ebtech and Furman or you can make your own as I have. Every FOH guy should have 1/2 dozen of them in his tool kit.

    And despite the potential problem of 100ft unbalanced lines, in over 20-years of FOH work in a vast variety of the most crappy and hostile conditions, with a wide variety of mixed PA gear, using a 100ft snake with unbalanced sends for monitors and poweramps always worked out ok with the amount of problems being very few if you take a little care and pre thought in how you run the snake. It the buzz/hum problem is dimmers in a room, balanced lines don't usually fix that.

    I don't remember on the Mackie, but if you had balanced outs on the one of the four subs, you could have used that and assigned channels to the bus for monitors, then living with the FOH mix levels feeding the bus. There is just about always some kind of work around.
     
  9. Kurt Foster

    Kurt Foster Distinguished Member

    Joined:
    Jul 2, 2002
    Location:
    77 Sunset Lane.
    I checked the Mackie website and downloaded their PDF owners manual for the 1604.
    http://www.mackie.com/products/1604vlzpro/index.html

    It does not have balanced outputs for the aux sends as I mistakenly thought but they are low impendence, (120 ohms) and should be capable of driving a balanced load. By using a RTS connection, the amp will balance the run at its end. I have run my SR24 and 1604 both this way through a 100 foot snake and never had this problem, but then I usually run my power line to the mixing position from where the stage amps are plugged in.

    I still don’t think this is the issue. I think you are experiencing a grounding issue.
     
  10. jjg

    jjg Guest

    This hall was an older building with suspect wiring. Several of the electrical outlouts near the stage were ungrounded. I had the two stage amps (mains and monitor)plugged into one supposedly grounded circuit and the mixer and effects into another at the back end of the hall. There WAS a dimmer for a portion of the lighting.

    Short of trading in my trusty Mackie 1604 for a newer mixer with balanced outputs, I am going to try Kurt's suggestion of powering everything from a common circuit at the stage (still using the snake) to see if perhaps it helps eliminate any grounding problems

    Secondly, I think I will get a couple of the Ebtech isolation transformers suggested to add to my FOH bag of tricks.

    Appreciate everyone's input.
     

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