Phantom Power Question

[tenor_singer]

Good morning Controlbooth!

I had something interesting happen this past weekend at our school's show choir invitational that has me a tad perplexed.

I have a set of AT (something-something) boundary microphones that require phantom power. My console is a Midas and has phantom power capabilities... when the tiny switch is engaged on the back of the board.

I set up the AT's, plugged them into channel 3 and 4 (which is a hardwired port SL). Tested their sound making sure that everything was ok at the board (including the phantom power being on) and got nothing. I switched the AT's to various hardwired ports around our stage and still got nothing.

I then put AA batteries into the AT's pack and voila... they worked. This has never happened before. They have always been able to run off of phantom power without the batteries.

My questions:

1. Do they normally require batteries and all of my previous 12 years of experience has been a fluke?

2. Since the wire run from our booth to the stage is very long...

(I am estimating by the time the wire tray... our school has all wires; electric, internet, microphone, DMX, etc, running in these "cable trays" in the ceiling that look like rib cages... reaches the ports, there is over a 175' run)

... do you think that there is a voltage drop?

3. Is it possible that the people who installed the system miswired some of the ports (because my hanging microphones which require phantom power DO work when patched into the board through our patch bay).

4. I know that they sell phantom power units. Would that be a worthwhile investment or should I stick with batteries.

Thanks!

Tenor.

[stantonsound]

I would try to take my voltage tester (multimeter) and check the voltage at the SL hardwired port. Try it with the phantom power of and off. Phantom power should be +48 volts DC. If nothing at the hardwired port, you might want to check the mixer as well. It could be a problem with the phantom power at the mixer. If so, it could be an easy fix, just move them to a different channel (if there are multiple power supplies, which is usually a feature of midas boards from my experience with them).

I have some mic's that will allow for batteries but rarely, if ever, use the batteries. There is something about the thought of an old battery leaking and destroying the mic. That, and in theatre, the condensers are usually in a hard to get to place (hanging above the stage, for instance) so personally I would get a power supply for them and skip the battery.

[fredthe]

Just as a quick check... there isn't a splitter on the SL line, is there?

[Foxinabox10]

Does the auditorium have any sort of "auto-system", where someone can just come in and plug a mic into the front of the stage and it will work? They just upgraded the system at the high school I went to and put a system in like this. This could cause some confusion with the wiring and thus the phantom power.

As for the battery, that is essentially a built in phantom power supply. I would just use that and make sure that you change the battery before big shows, just as you do with the wireless mics.

To test things, I would first plug the boundary mic (with no battery) directly into inputs 3 and 4 on the board. Make sure both work. Then, since you know that the hanging mics work, plug them into the hanging mic ports over the stage. If they work, you know that the wiring is faulty. If they don't work, they must need more phantom power than the hanging mics and the distance is the culprit.

[silvrwolf]

On Audio Technica mics with seperated transformer units (The box in between the actual mic and mic jack to your console) there is a little switch on the side that determines where the power comes from, at least on mine ther is. I would check to make sure the switch is not set to the battery mode. This may aplly to you or may not, just trying to help.

[Eboy87]

I'd also put a meter between pins one and two, and one and three. Perhaps the run is unbalanced somewhere, whether intentional or not. If the ground/shield is disconnected somewhere, the mic can't use the phantom supplied to it. Highly unlikely, but worth a shot if everything else fails.

[tenor_singer]

Thanks for the advice everybody. I will go purchase a new multi-meter and check things out (my old meter isn't working any longer).

I didn't realize that the AT's had seperate switch settings for phantom power versus battery power. I will also check that out as well.

Thanks!

Tenor.

[CURLS]

I will also add that i know some phantom powered inputs that have cracks in the battery shell over time. In essence when you put a battery in this allows for the incoming +48V to come across the traces along with the battery.
This is fairly hard to explain but its a common occurance in D/I boxes that need phantom.

[Dillon]

I've worked in a space before where there were issues with phantom being run from the board, due to the patch-bay in place in the theater. I'd try just plugging the mic into the board directly (with a known "good" cable). You may have something funny with the wiring in the space. We used separte phantom supply boxes (made by Stewart... but any other brand should do the trick as well) whenever we used condenser mics from one of the stage panels.

[Eboy87]

Oh, that reminds me of something. Do you have patchbays? If you do, are they 1/4"? I've heard of instances where the phantom was on while people unplugged a patch cable from the bay, which shorts out the phantom on the board. I've heard of this damaging the power supply. Unlikely, but something to think about.