One mic, two preamp channels - no phantom overload!

[Dags]

Hi everyone and welcome to 2013.

Got a [edit: couple of] question for the electronics/electrical/tech gurus out there.
I sat in on a recording session for creating SFX and the technician had one phantom-powered shotgun mic connected to his portable recorder by a Y-cable to allow the same signal to pass to two preamps and recording channels set at different levels so that if the louder channel clips, the softer one maintains the peak of the waveform which can be edited in later.
What did my head in was that the unit supplied phantom power across all channels, so presumably the mic was seeing phantom power from both preamp channels and it didn't blow up the mic or damage the portable recorder.
He's been using that technique for years.

My first question is, how does the mic handle multiple phantom power sources in this way? Is it because it is connected to a single unit sending phantom power on all channels simultaneously that it somehow addresses any power feedback issues?
My second question is, is this power handling something that is unique to shotgun mics or could studio condenser mics also be used with a Y-cable connected to two preamps, and if so, would you only need to supply phantom on one channel or both to get a signal to each preamp?

If anyone is able to shed some light on this technical mystery and let me know if it is *safe* to attempt this at home that would be awesome.

......anyone game enough to try this with one of their condenser mics & a couple of preamps and let me know the outcome?

Dags

 

[Dags]

Update:
Bit more research shows that the cable solution can be hit or miss, and may require modification of the Y-cable to isolate the second output from phantom power.
As my technical ability to do this with any confidence is zip, I have opted to give the Art ProSplit box a go.
https://www.swamp.net.au/microphone-splitter.html

Always good to know that some company out there has already developed a solution!



Dags

 

[ashcat_lt]

When the phantom power comes from the same unit, it's the exact same thing on both lines, there's no voltage difference, and cant be any damage. It's just a redundant path.

When it comes from separate units things are more complicated. There absolutely will be a voltage difference between the two, and current will flow between the two phantom power circuits and one or both could be damaged. I don't think that the mic will be hurt either way. Parallel voltages don't really add. They can sometimes provide more current if the load demands, but they won't "push" any more than just the one might. The problem being that each supply will have a very low source impedance, and will look like a serious load to the other. They will try really hard to pull each other to their own voltage, and a fight to the death can ensue.

A transformer isolated splitter like that ART unit are good for other reasons as well. Phantom power will be blocked, and usually the grounds are (or can be) isolated as well.

 

[Chili]

When the phantom power comes from the same unit, it's the exact same thing on both lines, there's no voltage difference, and cant be any damage. It's just a redundant path.

When it comes from separate units things are more complicated. There absolutely will be a voltage difference between the two, and current will flow between the two phantom power circuits and one or both could be damaged. I don't think that the mic will be hurt either way. Parallel voltages don't really add. They can sometimes provide more current if the load demands, but they won't "push" any more than just the one might. The problem being that each supply will have a very low source impedance, and will look like a serious load to the other. They will try really hard to pull each other to their own voltage, and a fight to the death can ensue.

A transformer isolated splitter like that ART unit are good for other reasons as well. Phantom power will be blocked, and usually the grounds are (or can be) isolated as well.

That's a great answer. It's great because it is exactly what I was going to say....

 

[Dags]

Thanks for the reply ashcat.
That makes perfect sense as to me as to why he is able to use a simple Y-cable splitter if the phantom is supplied from the same unit and why I'm glad I didn't try this at home
I'm guessing that using mixing desks that have one button that supplies phantom power across all XLR inputs at once would also be suitable for this purpose.
But yes, I also think the Art unit will be the best way to go to go with experimenting with this method of tracking.
Much safer!

Dags