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The New DMX Microphone

Discussion in 'Lighting and Electrics' started by achstechdirector, May 25, 2009.

  1. achstechdirector

    achstechdirector Active Member

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    The other day I was setting up a sound and lighting system when our 100ft 3pin DMX cable broke.

    I started thinking and said I would use a line off of the snake to send the DMX signal to the lighting.

    This just being an assembly in a multipurpose area I used a 8x4 snake and used channel 1 for lighting

    One of my assistants was setting up sound and I plugged the snake in

    (Snake:channel)
    2:1
    3:2
    4:3
    so on

    He didn't get the picture and was plugging a mic into channel 1 of the sound board and when he plugged the mic into channel 1 of the snake (lighting) the microphone started making a sound. He screamed for me and asked me to figure out what the problem was. He plugged it in and it started making the sound and I was dumbfounded. Then, suddenly I realized that it was in the lighting channel. I unplugged the mic and ended up not using that particular mic. I haven't tested it yet but I hope the mic still works It was a Shure SM58. What do you think?
     
  2. STEVETERRY

    STEVETERRY Well-Known Member

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    I think that the SM58 is a popular mic and easy to acquire from a number of dealers, which is a good thing, because you need a new one!

    The current sourced by a DMX512 EIA-485 transceiver at 5 volts is plenty to damage the voice coil of the SM58. If it made noise, that was the voice coil acting as a speaker. Kiss it goodbye.

    ST
     
  3. adude23

    adude23 Active Member

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    {Does this not come under 'Sound'}
     
  4. derekleffew

    derekleffew Resident Curmudgeon Senior Team Premium Member

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    It's a shame that those USITTians in 1986 didn't have the forethought to specify another connector and forbid the use of XLR3 for DMX.:(
     
  5. waynehoskins

    waynehoskins Active Member

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    Data always goes on the last pairs of a snake (when it has to go down a snake), never the first. Same with RTS. That's a perfect example why.

    Though this whole problem can probably be traced back to NSI way back in The Day, who were one of the first, I believe, to use XLR3 for multiplexed lighting control.
     
  6. STEVETERRY

    STEVETERRY Well-Known Member

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    Last edited: May 26, 2009
  7. waynehoskins

    waynehoskins Active Member

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    That's a good article. I wonder what the stats look like now, ten years later?

    The only DMX stuff I have at the church are essentially console-and-dimmers (actually, it's Response Converter from AMX and then Response Demux to analog). If ever I get any DMX toys, things like Studio Colors or the like, probably one of the first things I hope to do is rip out the 3-pin connectors and put the right 5-pin jobs there.
     
  8. achstechdirector

    achstechdirector Active Member

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    IT WORKS

    I tried it today and the mic still worked. I don't know how but it worked. It sounded fine and had great pickup. I don't know how long it will last but it worked for about 45 minutes this morning.
     
  9. avkid

    avkid Not a New User Fight Leukemia

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    What voltage and amperage would one see on a DMX line?
    The only things I can find suggest it is 5 volts.

    Phantom power is a maximum of 10 mA @ 48V DC.

    I know that an SM58 can take phantom power all day long, in fact all dynamic microphones that don't have a ribbon or tube will.
     
  10. Grog12

    Grog12 CBMod CB Mods Premium Member

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    Seconded. Snakes are not good places for DMX.
     
  11. KeeperoftheKeys

    KeeperoftheKeys Member

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    Well you can calculate the current if you know the impedance of the mic, U(V) = I(A) * R(Ohm), the line driver will be capable of a certain current etc. and if the impedance of the mic is significantly lower than what is expected in a DMX system the current could get out of hand and damage DMX equipment on the line (like the line driver).

    Ideally termination of a line is done with a resistor of 100-120 Ohm, so assuming that the current would be roughly 0,05A (50mA).
     
  12. derekleffew

    derekleffew Resident Curmudgeon Senior Team Premium Member

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    The best quote from the article:
    However, the "5-pin 'XLR'-style microphone connectors" could have been worded more clearly.

    Death to the 3pin XLR, except for audio purposes!
     
  13. TimMiller

    TimMiller Well-Known Member

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    ah... nothing like hearing your dmx. When you get into trouble is when the audio guys send phantom power to your lighting console. YAY for monitor engineers not paying attention when patching in the snake. Trust me, on studio spots its not worth the hassle of installing 5 pin. I tried it, yes i got it to work with some help of the dremel tool, but then I had odd ball studio spots and were always getting confused on which were 5 pin and which were 3. I'm sure your mic will be just fine, i'd be shocked if it wasnt. Also always run specialty lines in a snake such as clear com and dmx (of course not recommended but works) down the last channels of the snake to keep it away from the normal audio feeds.
     
  14. IamQuestar

    IamQuestar Member

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    As you have discovered, the mic is fine, and as it's an SM58, it'll last another 20 years. They're built like tanks and can take a lot of abuse. I remember a long time ago, when I was a kid, playing with an old Radio Shack High Impediance mic that had a 1/4" TS Phone connector on it. I pluged it into a headphone jack and was quite tickled that I could hear music comming out of it. I had turned it into a speaker! Later I learned that a dynamic mic is structurally just like a speaker with a diaphram attached to a voice coil suspended in a groove of a magnet.

    While I wouln't recomend making it a common practice... if your mic is working fine, then there is little to worry about. The biggest danger would have been if a large amount of current could have been supplied, then the very thin wires in the voice coil could have heated up and melted/fused then you'd be out of luck. As it it, wipe your forehead, consider it a leason learned, and forget about it.

    --Andrew
     
  15. Trimble

    Trimble Member

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    Well, try not to forget about it. :) All "lessons learned" should be remembered.
    And this one should be remembered just on the merit of being a good story to tell. ;)
     
  16. 00AVD

    00AVD Active Member

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    That's because phantom power is equally on both lines, referenced to pin 1. As the dynamic mic insert has no connection to pin 1, no damage is done.

    DMX on the other hand has a differential voltage between pins 2 & 3, so this ends up being applied across the insert.

    Although the (unloaded) DMX signal can supply 5V, the relatively high impedance of the insert will limit the current to a few milliamps.

    I'm not surprised that the mic still works (well, as good as a SM58 does anyway), but that doesn't mean the insert wasn't damaged in some way by getting slammed around like that.
     
  17. STEVETERRY

    STEVETERRY Well-Known Member

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    Thanks for the clear explanation on Phantom Power. DMX driver voltage is different, as you correctly pointed out.

    As a former sound engineer who has been in really uncomfortable situations in front of thousands of people, I would gladly throw that SM58 in the nearest dumpster. Murphy's law dictates that it will work only until you really need it. Whatever keeping it saves in dollars, it's not worth it!

    ST
     
  18. avkid

    avkid Not a New User Fight Leukemia

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    Sounds like a new talk back mic to me.
     

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