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Old 23rd November 2007, 12:41 PM   #1
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Default Phantom power supply arguement

Background...

This 7 year old series of Whirlwind contractor boards has phantom power on the mic input, input number one and allows the phantom power to also be on the 3 outputs that are available on each board.

The outputs of each channel are fed to 3 mixers at once. Even with the phantom supplies turned off on the mixers they are getting an indication of phantom power on the boards.

The fix...

I was to cut zero ohm jumpers on the boards and install 10mfd bipolar caps in their place. There was also 10K resistors going from the plus and minus to ground. This was to issolate the phantom power and keep it from back feeding the board.

My question is this...

Why in the world was it necessary to modify these boards when I'm going to assume the phantom power would not cause any problem?

The 6.8K resistors will only allow 7mA per leg worst case so opposing phantom supplies should cause no problems.

I would appreciate some enlightenment here if possible.
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Old 23rd November 2007, 09:26 PM   #2
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Sounds like someone didn't want to pop for the cost of splitter transformers. What you are describing a 1 in to 3 out "Y".

Why not check out the Jensen Transformer web site.

Given the age of the system, if nothing has died and the system is not generating snaps, crackles and pops as a direct result of the phantom power, then what is the problem?

DC at a microphone input is not a problem in and of its self. The better consoles have input transformers that will block DC. Many good ones and all of the rest have blocking capacitors.

On the other hand, I could see having a problem with the consoles interacting with one other because of their relatively low input impedances.

Chris
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Old 24th November 2007, 02:44 AM   #3
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The arguement seems to be when the phantom is shut off on a mixer board they get an indication of phantom power that is being fed from the Whirlwind's phantom supply.

I have some sound guy crying and thinking this is going to damage his board. I say there is no problem even if both phantom supplies are on at the same time because current limiting is being done by the 6.81K resistors on each leg of the phantom supply.
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Old 24th November 2007, 02:51 AM   #4
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Quote:
DC at a microphone input is not a problem in and of its self. The better consoles have input transformers that will block DC. Many good ones and all of the rest have blocking capacitors.
There is no need to block the phantom even if the mic being used doesn't need phantom to function.

Paralleling mic outputs and sending them in various directions shouldn't cause a particular loss in frequency or signal with 3 paralled.

When you start looking at the cost of transformers for over 50 mics going into the splitter cards and 3 outputs per card the cost runs up in a hurry. Therefore if it isn't absolutely needed its a cost they won't go for.
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