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Author Topic: pantom power on dynamic mic's  (Read 558 times)

Billy Snody

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pantom power on dynamic mic's
« on: March 05, 2017, 03:38:35 pm »

We have several mic inputs (dynamic mics SM 58's in most cases) on our gl2400 mixer that I am having difficulty getting signal on. With the gains wound all the way up, I sometimes don't light the first lamp (sig) on the channel input meter. All inputs are xlr, none padded, and no polarity reversed. I'm told it should not make a difference if I switch on phantom power with a dynamic mic but it does, and it lets me get signal without maxing out the gain, and of course here comes the feedback issue when I run the faders up. Our snake runs from the mixer through a tube in the floor out the back of the stage to a terminal strip, there it connects to the cables to the floor boxes. I'm guessing seventy feet of cabling each and maybe one hundred or so for each of the three ( not SM58's) choir mics. Knowing what phantom power is doing in this scenario, should I be looking at changing to condenser mics?
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Mac Kerr

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Re: pantom power on dynamic mic's
« Reply #1 on: March 05, 2017, 03:57:00 pm »

We have several mic inputs (dynamic mics SM 58's in most cases) on our gl2400 mixer that I am having difficulty getting signal on. With the gains wound all the way up, I sometimes don't light the first lamp (sig) on the channel input meter. All inputs are xlr, none padded, and no polarity reversed. I'm told it should not make a difference if I switch on phantom power with a dynamic mic but it does, and it lets me get signal without maxing out the gain, and of course here comes the feedback issue when I run the faders up. Our snake runs from the mixer through a tube in the floor out the back of the stage to a terminal strip, there it connects to the cables to the floor boxes. I'm guessing seventy feet of cabling each and maybe one hundred or so for each of the three ( not SM58's) choir mics. Knowing what phantom power is doing in this scenario, should I be looking at changing to condenser mics?

What happens when you take that same mic and plug it into the same input directly into the back of the console through a short mic cable rather than the installed house wiring?

Mac
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Ivan Beaver

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Re: pantom power on dynamic mic's
« Reply #2 on: March 05, 2017, 05:06:43 pm »

There is nothing that would make this happen.

You may have some wiring issues-but I cannot imagine any case in which turning phantom on a dynamic mic would cause it to have greater level.

Something is "missing" in your question or else more than one thing is changing.
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John Rutirasiri

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Re: pantom power on dynamic mic's
« Reply #3 on: March 05, 2017, 09:39:15 pm »

Our snake runs from the mixer through a tube in the floor out the back of the stage to a terminal strip, there it connects to the cables to the floor boxes.

Did this problem recently started happening, or has it always been that way?  Can you post a picture of the terminal strip?  I find that configuration kind of odd.  Usually it comes into a patch panel or directly to a fan end of the snake.

Is it a SM58 with or without switch?
Is it a genuine SM58 and GL2400 and not the Sanway version?

As Mac said, to test this you should bypass the snake and directly connect the SM58 to the back of the mixer with a known good mic cable.
- If problem persists, try same mic/cable combo on another channel
- If problem persists, try another SM58 with same cable on same channel
- Try same SM58 on another mixer

John R.

 
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John Roberts {JR}

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Re: pantom power on dynamic mic's
« Reply #4 on: March 06, 2017, 09:55:49 am »

check the XLR wiring... 1=ground, 2=signal +, 3=signal -.

JR
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Jerome Malsack

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Re: pantom power on dynamic mic's
« Reply #5 on: March 06, 2017, 10:48:14 am »

a good tool to test your phantom power is the Behringer CT-100.  25.00 roughly,

you can test pin 1, 2, and 3,  but you can turn on phantom power and plug the cable on the other end into the box and it will show two led's for a positve phantom power test and that will also be a good cable run. 

This test device can also send test tone down the wire, along with the testing of the shorter runs.   
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Jonathan Johnson

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Re: pantom power on dynamic mic's
« Reply #6 on: March 07, 2017, 12:09:32 am »

With regard to the phantom power issue, there is either a problem with the wiring, the microphone, or the mixer.

As others have suggested, eliminate variables one at a time.

Does the effect happen with different mics on the same channel? If so, it's probably not a problem with the microphone. If different mics behave differently, it's probably a problem with the mic.

Does the effect happen when you bypass the wiring (and/or using different cables)? If it's always the same regardless of the wires/cables in use, it's probably not the wiring. If changing the wiring changes the behavior, you've got a problem with the wiring. (If your wiring starts with the shield/ground on pin 1 at the mixer and connects it to pin 2 or 3 at the jack, that can cause problems, though right now I'm not sure how the problems would be manifest. Swapping the wiring at pins 1 and 2 is an easy mistake to make.)

If you prove the microphone and wiring don't make a difference, it's probably the mixer.

As for the feedback, gain structure has nothing to do with feedback. It doesn't matter where you get your gain. It's the total gain that matters. The way to eliminate feedback (to maximize gain before feedback) is with proper microphone selection, relative positioning of microphone and speakers, proper microphone technique by the talent, proper equalization, and room acoustics. Room acoustics is usually the most difficult to change, but it is by far the most important factor. But gain structure has nothing to do with feedback.

(The only way that gain structure will affect gain before feedback is if some gain stage in your mixer is nonlinear, where it boosts some frequencies by a different dB than other frequencies. That is an indication of failure or poor product design.)

What gain structure DOES have to do with is the noise floor of the system and clipping. Proper gain structure minimizes the audible noise in the system while still preventing clipping. This is known as dynamic range. Proper gain structure also ensures that all inputs result in the same ouput level when the channel fader is at a given point in its travel (i.e., 0 dB) so that your mixing becomes visually intuitive -- so you can tell the relative levels of each channel just by looking. If your guitar is at -10 dB and it's drowning out the vocals that are at +5 dB, you may not have proper gain structure, because at a glance one should be able to tell that the vocals will be louder than the guitar (not considering program dynamics here).
« Last Edit: March 07, 2017, 12:19:35 am by Jonathan Johnson »
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