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Powered Monitor Electrical Hum

Discussion in 'Sound, Music, and Intercom' started by blademaster, Sep 20, 2007.

  1. blademaster

    blademaster Active Member

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    OK so a few months back (like 2 or 3 months) my church got two Galaxy Audio PA 5X 140 http://72.14.253.104/search?q=cache...140&hl=en&ct=clnk&cd=3&gl=us&client=firefox-a

    Well we just recently started raising volumes and I noticed a 60 cycle hum when we had a input running from the board to the powered monitor. First i though of a bad cable, nope. Next I tried removing the ground from the monitor, nope, then i plugged it into the same breaker as the mixer and amp, nope. Then I raised the ground on it, nope. (all previous test using a TS 1/4 plugin from XLR) then with it properly grounded i did the same test using a balanced XLR to the second input, nope. The only thing i can figure is manufacturer defect. This is happening on both of the powered monitors and only on the signal coming from the mixer. any ideas....
     
  2. SHARYNF

    SHARYNF Well-Known Member

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    Is the output of your mixer a Balanced XLR connection, or are you converting from TS to XLR?

    Have you physically plugged the monitor into the same OUTLET as the mixer, not just the same breaker?

    Have you physically checked the wireing connection on the mic cable XLR connectors?

    Sharyn
     
  3. j_blinker

    j_blinker Member

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    Hey Blademaster,

    Sounds like you might have been trying to lift the ground using an XLR to TS adapter. There's a good chance that adapter is putting pins 1 and 3 on the ground rather than lifting the ground from your output. If you have a ground hum this would only open the cable up to RF interference over the length of the unbalanced run and at best put the hum onto pin 2. To find out if it is a grounding problem inserting a ground lift on the output of you're board, this should simply break the pin 1 connection at the board, not merge it like a TS adapter. If this doesn't make the source of the problem clear there's an easy way to test the cleanliness of your groundings. Grab you're multi meter and check the voltage between hot and neutral, and neutral and ground on all you're outlets and make sure the outlets are the same, or ideally, run an extension cord and test the voltage between your two grounds. If a voltage different exists between the grounds you've found you're problem. Just because they're on the same breaker they aren't immune to leaky voltage, as the paths their grounds follow is likely very different. Good luck.
     
  4. blademaster

    blademaster Active Member

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    the output from the mixer is a true balanced XLR.
    yes i did put it on the same outlet as the mixer
    no i havent checked the wiring on the XLR cable, though it is a manufactured cable not a diy cable.
    I raised the ground on it by using a adapter for the power on the monitor.
    the output on the mixer is a balanced XLR out, however the monitor has two inpouts one 1/4 (dont know if it is balanced or not) then we have a balanced XLR. the only way i know of that my church has the capability to lift grounds is using a DI Box,
     
  5. SerraAva

    SerraAva Active Member

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    Sound of power? Said it is only heard when turning it up. How much more dB does turning it up entail? The other problem might be what is else is plugged into your breaker box, or the breaker box might just be a sub box to another larger breaker box. Trace it to make sure its just your power and no one else's. There might be a shorted ground some where else that you don't know about.

    Try unplugging everything and just starting with the monitor plugged in. If you have the 60 cycle, lift the ground. Still there, its in the monitor. If not, go back and add things one at a time to find what the cause is. Might take a while to do it, but you will find exactly what the source is this way. This is assuming you have never had a 60 cycle through the mains before, or have and knew what the problem was. Good luck, hope you find that 60 cycle gremlin.
     
  6. blademaster

    blademaster Active Member

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    Naw this isnt in our Mains this is for our Stage monitors. we own the buildings so its our power,period. the power for all the tech stuff is run off a sub-breaker box
     
  7. SerraAva

    SerraAva Active Member

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    So that means you can eliminate anything that is in loop with the mains. So its in the monitor, a ground short some where in the monitor loop, or just turning it up too loud that its amping nothing. Have you tried a different send off the board just out of curiosity, maybe the send is bad? Also, try an iso-transformer. Its a ground lift that goes through a transformer instead of just cutting off the ground. It goes in line of the mic, or this case send, cable.
     
  8. blademaster

    blademaster Active Member

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    The sound thats coming from it is not the same sound as when you are amping nothing, given you can hear it most clearly when it is not running anything but it is a diff sound. no i havent tried using a different out, we have used this one for other things no problem.. I dont have a iso-transformer but i was looking at one...i'll check with the shop today.
     
  9. SHARYNF

    SHARYNF Well-Known Member

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    You might make up a cable that is xlr to xlr but has the shield disconnected use it at the amp end, and try that also disconnect the xlr cable from the mixer, leave it attached to the monitor and see if the problem is still there

    BUT:
    It is possible that the monitor has a pin one problem where they are using the shield and ground connected to gether, Many times I have found when you have an input that has balanced AND unbalanced, this occurs, since the unbalanced TR Jack connects the two together along with the chassis. In general, I go into the connection plate and disconnect the tr 1/4 jack completely, since in effect what this is probably doing is converting the xlr input to unbalanced. An other option is to get a balancing Isolation transformer, like Whirlwind makes, but If they will let you I would pul the plate and carefully disconnect, make sure that the ground and shield is not connected together, and and tape up the wires individually. I would be willing to bet that is the problem

    Sharyn
     
    Last edited: Sep 20, 2007
  10. blademaster

    blademaster Active Member

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    ok the problem disappears when it is disconnected from the board at either end. we dont have a iso-transformer. would me hooking a di box into it help me at all....
    the likelyhood of me being able to pop it open is almost nil it almost certainly voids the warranty and right now i am disliked by most ppl in my church except the for one of the two ppl that can give me permission to open it up... but i honestly dont think it is unless it is a part of the manufacturer's specs, we have two and both are behaving like this, they are from the same batch those...
    tried a diff send and we still have issues...
     
  11. SHARYNF

    SHARYNF Well-Known Member

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    I'd try to disconnect the ground on the XLR pin one cable connection at the speakers.(open up the connector on the cable )

    Di is not likely to do the trick, typically the output is mic level and the input is unbalanced line level

    What the test you did is telling us in all probablity is it is not the cable acting as an antenna, but it is the ground potential difference between the two systems, in all likely hood it is what I call a pin one problem, so as I said above lift the ground on the INPUT CABLE, not the power cord

    Sharyn
     
  12. avkid

    avkid Not a New User Fight Leukemia

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    If you use one these devices, you don't have to go cutting cables:
    http://www.fullcompass.com/product/231502.html
    Yes, I know it's expensive.
    But it's Whirlwind, so it will outlive us all!
     
  13. blademaster

    blademaster Active Member

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    Thanks avkid, that would help me a lot if i could afford it. it really will outlive us all. My church almost a year ago got two DI Boxes made by Groove Tube, well we are extremely abusive on out equipment, they stopped working after a few gigs. our two IMP2 are still running fine. :D indestructible

    I can try to check with the local shop and see if they have one in their.


    bit of useless knowledge, Whirlwind salespeople are willing to drive over their DI Boxes to prove how indestructible they are.
     
  14. avkid

    avkid Not a New User Fight Leukemia

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  15. mixmaster

    mixmaster Active Member

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    I wouldn't be so quick to discount the DI box. Most DI boxes are basically impeadance matching transformers and work both ways. I suspect this is not a ground problem as much as an impeadance problem.
    I had a similar situation once. A drummer wanted to use the amp with his Electronic kit as a powered monitor so we fed his amp off the board with a XLR to TRS adapter, and dialed in a mix that was heavy on the kit. Anyway, his amp buzzed like crazy. To add to the confusion, this was about 10 min before curtain. Tried lifting the AC ground on his amp (unsafe) without any luck. We replaced the adapter with a DI with the ground lift switch, everything was great. Unfortunately, because of the time factor, we couldn't do any further testing.

    I suggest trying a DI box temporarily. Cable from the output of the mixer to the XLR jack on the DI box. Cable from the a 1/4" jack on the DI box to the input of your powered monitor. Start with the ground in. If the DI box alone solves the problem, you have an impedance mismatch. An line matching transformer could be a permenant fix. If you have to lift the ground then you have a ground loop somewhere.
    Happy Hunting
     
    Last edited: Sep 26, 2007
  16. SHARYNF

    SHARYNF Well-Known Member

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    Difference of experience here, while some of the di's can be run either way, typically they are also padding the signal down from line in to mic out, so you will loose too much signal level, need to boost it back up and that could cause problems

    the Cheapest first test is to simply open up the xlr connector going to the amp, unsolder the pin 1 wire and test it
    I agree, A whirlwind LBS would be the best for this, and well worth the price
    Sharyn
     
  17. j_blinker

    j_blinker Member

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    It's been suggested a few times that a D/I might solve this problem. I'm not saying it won't, but other than most D/I's having a switch that lifts ground (simply disconnects pin 1) I haven't read any explaination as to why it might do so. The only impedance issue I can think of is your board is presenting too high a load for the amp. This would cause voltage loss and would create a pretty nice antenna. Mixmaster, you're making me nervous with you're AC ground lifting. Be careful or we'll lose you to the lampis as you'll be able to light a light bulb just by touching it.
     
  18. SHARYNF

    SHARYNF Well-Known Member

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    The only time in an emergency that I would look at removing the ac ground on a device would be with a notebook ac power supply. If you look at the design, you have three wire in, two wire dc out, and on occasion these can drive you crazy. Amps connected to Mixers is a definite never, if you had a problem you could be sending ac into the ground, back out to the mic, with serious safety issues.

    In my experience an impedance mismatch typically either produces a signal level problem OR a serious restriction in the frequency response.


    Sharyn
     
    Last edited: Sep 25, 2007
  19. museav

    museav CBMod CB Mods Departed Member

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    What is the source at the console and what input are you running into on the Hot Spot? Where are the gain settings on the console and the monitor? The Hot Spot inputs will accept a wide range of signals, from mic to line to instruments, and the PA5X140 supposedly senses whether it is a mic or line signal and automatically adjusts for that. However, it may still simply be a gain structure issue. Have you tried varying the level at the console and does that affect the noise at the speakers or let you turn down the speaker's input level? Do you have another powered speaker that you coudl use to see if it has exhibits the same problems?

    According to the product data, the 1/4" input on the PA5X140 is balanced. There is also 24VDC phantom power on the XLR.
     
  20. mixmaster

    mixmaster Active Member

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    Hey Blinker
    first of all, if you read my post carefully you see that I noted this as an unsafe practice. I would never under any circumstances run a show with any of my equipment grounds lifted. I was shocked once as a result of someone elses' stupidity and one trip to the ER is plenty. I am not advocating this as a permenat solution for anything.
    That said, the problem was one of those last-minute things that make this job fun and I didn't have a lot of time for testing. I knew from previous experience that the ground in this venue was a bit dirty. I did a quick check with the ground lifted to see if removing the possibly dirty ground solved the issue of the buzz and then I restored the ground. This was the quickest test to check the most logical failure point. Had the issue been a dirty ground, we would have abandoned the whole contraption as I wouldn't have let him do the show with a bad ground any more than a missing one. As it was, a DI box fixed the problem and the show went fine. I have used a "backwards" DI box several times since in situations where I need a quick temporary fix for a buzz that doesn't go away by breaking the shield.
    Anyway, under normal circumstances you're positively correct. One should never lift an AC ground. Safety is something I take seriously. I'll leave the light bulb stuff to the lampies:grin:
     

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