Phantom Power Buffer?

PhatBasstard · #1 02-13-2005, 03:41 AM

OK electronics wizards and gear heads:

I play gigs with a guy that regularly uses condenser mikes which require him to run the Phantom Power on his board. It is not individual to the channels. They're either all on or off. I give him signal off my rig through the XLR out on the back of the head.

Problem is, my amp hates the Phantom Power and starts to distort (more in the signal to the board than my cab). This is the second amp I've owned that does this. I cannot use a regular direct box as I need the "effected" signal (from my effects loop) to get to the board.

The question: Is there a box or adapter that I can put in between my head and the (Phantom Powered) board that will keep the voltage away from my amp?

l0calh05t · #2 02-13-2005, 03:52 AM

Considering that phantom power is dc you could just wire in a 1:1 transformer, although this might change your tone a little bit. iirc there also are special opamps that can do this but i'm not 100% sure about that

EDIT: A simple di box should work as well (transformer uncoupled di box ~= 1:1 transformer)

nonsqtr · #3 02-13-2005, 04:05 AM

Quote:

Originally Posted by PhatBasstard

The question: Is there a box or adapter that I can put in between my head and the (Phantom Powered) board that will keep the voltage away from my amp?

Yes. Basically, all you need are two large value capacitors in series with the two "hot" XLR leads. Many people make boxes like this. Or, you can save yourself 50 bucks and just do it yourself, it's two bucks at Radio Shack and five minutes with a soldering iron. I'd try 10 uF or 22 uF, that should work pretty well most of the time. The transformer idea will work too. Anything that "uncouples" the DC from the signal lines will work fine.

Passinwind · #4 02-13-2005, 10:12 AM

You could consider having the board modified so that a few channels never have phantom power on the inputs.

If you use electrolytic caps as blockers, bipolar ones are preferable, and you might have to go to someone like Mouser or Digikey rather than Radio Shack. You can wire small caps right into an XLR shell, no need for a separate box if you can keep track of your cables.

PhatBasstard · #5 02-13-2005, 12:25 PM

Quote:

Originally Posted by Passinwind

You could consider having the board modified so that a few channels never have phantom power on the inputs.

He's going to have to take his board in to get a channel fixed anyway. I'll ask him about this since it might come in handy for him in the future anyway, although I still want something for myself incase I run into this problem in the future again.

PhatBasstard · #6 02-13-2005, 12:27 PM

Quote:

Originally Posted by Passinwind

If you use electrolytic caps as blockers, bipolar ones are preferable.....

Never heard of these. What are they exactly?

l0calh05t · #7 02-13-2005, 12:32 PM

Quote:

Originally Posted by PhatBasstard

Never heard of these. What are they exactly?

electrolytic caps usually have a specific polarity and will be damaged if reverse voltage is applied to them. this is not so for bipolar electrolytic caps

nonsqtr · #8 02-14-2005, 01:40 AM

Bipolars aren't necessary for uncoupling phantom power (and besides, they're usually very expensive, at least ones that big are). In a phantom power setup, you have +48 volts coming in on "both" of the hot XLR lines, and the ground return is through the shield. So, you hook the "+" side of the electrolytics to the "source" side (in this case, the board), and the "-" to the "output" side (in this case, the amp). That should work fine. Bipolars will work too though, if you're willing to go the extra mile.

PhatBasstard · #9 02-14-2005, 02:21 AM

Quote:

Originally Posted by nonsqtr

Bipolars aren't necessary for uncoupling phantom power (and besides, they're usually very expensive, at least ones that big are). In a phantom power setup, you have +48 volts coming in on "both" of the hot XLR lines, and the ground return is through the shield. So, you hook the "+" side of the electrolytics to the "source" side (in this case, the board), and the "-" to the "output" side (in this case, the amp). That should work fine. Bipolars will work too though, if you're willing to go the extra mile.

Thanks guys,
It's beginning to look like one of these Phantom Powerable Direct Boxes that match impedences (etc., etc.) that also have a female XLR input option will probably do the trick. We'll see.

Passinwind · #10 02-14-2005, 10:20 AM

Quote:

Originally Posted by nonsqtr

Bipolars aren't necessary for uncoupling phantom power (and besides, they're usually very expensive, at least ones that big are).

I made that recommendation not for when phantom power is there, but for when it isn't. Polarized electrolytic caps work better when they are polarized by an external voltage, so if you happened to plug your box/cable into a board with the phantom turned off, the bipolar ones may exhibit less distortion. Frankly, I can't always hear the difference myself, but I'm going off the Jensen Transformers recommendation. As far as cost, an eletrolytic bipolar 22uF, 63 volt cap is 43 cents in single unit quantities in the current Mouser catalog; hardly a bank-breaker, eh?

They used to be a lot more expensive, so I rarely bothered with them in the past. I'm guessing you are thinking of mylar caps though, which are still quite expensive in those values?