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Gear / Technical Help => Microphones & Setup => Topic started by: JNT on June 20, 2007, 02:55:09 PM
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I need to use some unbalanced, battery powered condensor mics with a mixer that supplies phantom power.
Any problem with this arrangement?
Am I correct that the mics will not even see phantom power? I don't want to cause any damage.
Thanks,
Joe
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Can you turn the phantom supply off?
Roger
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Yes but I need the phantom power for the onther mics I'm using.
Joe
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Most often, unbalanced microphones are connected to balanced inputs by grounding one of the two wires that would (with a balanced mike) both be carrying audio. Those are the same two wires that carry the phantom powering.
So this is a little tricky, because if you're not careful, you'll short-circuit half the phantom powering on the input where you're connecting the unbalanced microphone. This will draw about 7 mA, which some mixers are fine with and others are not. ("Not fine" = the supply may fail catastrophically, or its voltage may drop below spec or fall out of regulation, thus compromising the performance of your balanced, phantom-powered microphones.)
Are you sure this is really necessary? Could you possibly boost the unbalanced microphone's level with some piece of outboard gear and come in at line level instead?
Otherwise, an input (1:1 isolation) transformer would be my recommendation. Jensen sells a nice little box (model MS-2XX-P) with two such transformers, which allows you to connect any combination of two balanced or unbalanced microphones to any combination of two balanced or unbalanced inputs. As I recall Lundahl sells an in-line 1:1 microphone input transformer in an XLR barrel, which might be nice for this application.
Jensen and Lundahl are top-of-the line. A lower-cost version seems to be available from Sescom, the IL-20. Beyerdynamic has miniature 1:1 transformers that can be used for this purpose--they're quite good considering their tiny size--but unfortunately they don't sell them built into XLR barrels as they used to.
--best regards
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Simple solution, use a passive splitter box (with the ground lifted) on the channels you dont want to pass phantom power to. a passive splitter will not pass phantom unless the ground is connected at both ends, so if you lift with a ground lift switch on the splitter it wont pass phantom. Passive splitters arent very expensive and its good to have a couple around anyway. Or buy some transformer isolated splitter boxes (like Whirlwind makes) and use those. Transformer Iso splitters will not pass phantom at all.
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Simple solution, use a passive splitter box (with the ground lifted) on the channels you dont want to pass phantom power to. a passive splitter will not pass phantom unless the ground is connected at both ends, so if you lift with a ground lift switch on the splitter it wont pass phantom. Passive splitters arent very expensive and its good to have a couple around anyway. Or buy some transformer isolated splitter boxes (like Whirlwind makes) and use those. Transformer Iso splitters will not pass phantom at all.
No ground with an unbalanced signal = No audio.
Chris
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You could also try runnig the unbalanced XLR from Mic (assuming it takes XLR) > 1/4" to a DI box first. After thinking about it though it seems like it would be simpler to just run the unbalanced mics into the snake, then before plugging into the mixer use a xlr > 1/4 adaptor an plug it into the 1/4 channel input on the board instead of the XLR input. clean input and no phantom.