Testing for phantom power, blocking it...

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chconnor
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Ok!

Post by chconnor » Sun Feb 04, 2007 4:12 pm

Ok, I'm off! I'll post pictures and so on when it's done. :-)

Thanks both of you for your help! -Casey

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Post by The Scum » Sun Feb 04, 2007 6:25 pm

Before you get moving too fast, a quick anecdote:

I just grabbed the LED drawer from by junk box and went up to a pre (API 512c, just for reference).

By sticking the legs of an LED into the holes on the front panel (I was bridging pins 1 and 3, with the longer leg (anode) into pin 3), I found I could get some of my LEDs to light...some reasonably bright, some a faint glow, and some not at all. No external resistor at all, just relying on the internal current limiting ones.

So you might look for "high efficency" LEDs, that will light with ~5 mA.

Also, transformer isolation is probably the best solution, and isn't necessairly expensive. Edcor make decent transformers that aren't too expensive. Look at their WSM10k/600 and WSM10K/150...for about twelve bucks apiece. www.edcorusa.com

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chconnor
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Hmmm.

Post by chconnor » Sun Feb 04, 2007 7:28 pm

So you might look for "high efficency" LEDs, that will light with ~5 mA.
Oops, I've already ordered some regular LEDs... "minimum operating current" specs not available... I'll see how those work at least in terms of detecting standard phantom and simple miswirings... It's surprising to me (just on an intuitive level) that raw phantom power wouldn't be able to light up any little LED...
Also, transformer isolation is probably the best solution
And the idea there is to use them while operating whatever is going to be plugged in? These protect the box? Is this basically what a passive DI is?

Maybe I'll just start with the LEDs unless the transformer thing is a really simple proposition. :-)

-c

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Post by The Scum » Sun Feb 04, 2007 10:38 pm

And the idea there is to use them while operating whatever is going to be plugged in? These protect the box? Is this basically what a passive DI is?

Maybe I'll just start with the LEDs unless the transformer thing is a really simple proposition.
The thing that occurred to me is that with any sort of detector, it only gets you halfway there. You can tell that there are wiring problems, but you don't have any way to fix them. For phantom power, it is probably simple enough to turn it off (unless it's globally switched, and something else requires it)...but for AC power and ground problems, they aren't always easy to fix with the tools at hand.

From the sounds of this, you'll be using it in the heat of battle, while setting up to play, when the last thing you really need to be thinking about is how the power and sound are wired.

Isolating your rig from the sound system using transformers means you don't have to worry if there's phantom or not. They make it so that there's no electrical circuit between your gear and the sound gear...just a magentic one, which only passes AC, not DC. They'll take phantom power with impunity, and keep the phantom out of the outputs of your interface.

And, yes, a passive direct box fills the bill. A jack leading to the primary of the transformer, and a male XLR hanging off the secondary...with a switch to break the ground connection between the input to the output.

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Post by chconnor » Mon Feb 05, 2007 12:44 am

From the sounds of this, you'll be using it in the heat of battle, while setting up to play, when the last thing you really need to be thinking about is how the power and sound are wired.
True. The way I'm going to build it it's not even going to do a particularly good job of telling me what's wrong, just that "something is wrong". And although it only detects, at least can stop me before I fry me or my precious box. :-)
Isolating your rig from the sound system using transformers means you don't have to worry if there's phantom or not.
So I guess the real answer is for me to suck it up and get a nice stereo DI. Meantime I'll build the detector for fun. Total expected cost for the detector parts w/shipping: $7. Thanks to the helpful folks on tapeop.com, of course. :-)

-c

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Prototype done...

Post by chconnor » Fri Feb 09, 2007 2:45 am

Ok! Got it done, I think. :?

I'm going to throw up a couple pictures soon, but I wanted to ask a couple more questions, if you still have the patience!

I connected a fourth LED from Pin 1 to the chassis, as suggested. In the "field", if nothing illuminates, I'll be subsequently connecting the chassis to various points on the body of the device being tested while the device is plugged in. The question: I can wire in a lead to my little tester for this purpose, but if I can get away without doing so I'd like to avoid having a wire hanging off of it all the time. Ideally, I'd like to use the closest 1/4" cable and just touch the tips to the relevant parts, but I'm not sure if the average lengths of a 1/4" cable would make them inappropriate for this use for some reason. Thoughts?

And: Off the top of your heads, is there any way that mis-wiring or short-circuiting on MY part could fry the various things I'll be testing with it? (That would be a tad ironic, eh?) That only occurred to me as I was soldering it together and realizing how crammed everything was inside the XLR jack. :shock:

I've been plugging it in to stuff over and over again just to watch it light up. :idea:

Thanks!
-c

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Post by The Scum » Fri Feb 09, 2007 11:21 am

And: Off the top of your heads, is there any way that mis-wiring or short-circuiting on MY part could fry the various things I'll be testing with it? (That would be a tad ironic, eh?) That only occurred to me as I was soldering it together and realizing how crammed everything was inside the XLR jack.
That's one of the brilliant things about phantom power: the current limiting resistors also make it very tolerant of various mispluggings. You can short any combination of the lines to each other, and at worst you're drawing 15 mA from the phantom supply.

If things shorted internally, I think the worst thing that might happen is that an LED doesn't light when it should.

If you're worried about the internals, you can insulate things like the LED legs using heat shrink tubing, or "spaghetti tubing," which looks like wire insulation without the wire in it. The teflon kind is cool because it won't melt when soldered.
I'm not sure if the average lengths of a 1/4" cable would make them inappropriate for this use for some reason. Thoughts?
Nothing wrong with that...it's only wire after all...as long at the 1/4" cable isn't broken.

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Done...

Post by chconnor » Sat Feb 10, 2007 3:25 am

Ok, it's all done! Thanks again for the serious guidance:

http://caseyconnor.org/phantastic.html

-c

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Post by Scodiddly » Sat Feb 10, 2007 7:41 am

Pretty cool page!

Now I'm tempted to build one for myself - it would be much easier to carry around for line testing at shows. Though I'd be wanting to make sure that phantom was available, instead of wanting to avoid it.

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Post by The Scum » Sat Feb 10, 2007 1:55 pm

Looks good.

Come to think of it, you could take a handful of how-current white LEDs, and XLR, maybe a switch and piece of copper pipe, and wire them up so they you had a phantom-powered flashlight!

I've done live sound gigs where that would have been pretty handy!

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Post by Scodiddly » Sat Feb 10, 2007 2:55 pm

The Scum wrote:Looks good.

Come to think of it, you could take a handful of how-current white LEDs, and XLR, maybe a switch and piece of copper pipe, and wire them up so they you had a phantom-powered flashlight!

I've done live sound gigs where that would have been pretty handy!
Yeah, but you'd need a flashlight to find a loose mic cable onstage. Chicken and egg, my friend, chicken and egg. :wink:

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Flashlight

Post by chconnor » Sat Feb 10, 2007 4:03 pm

That flashlight is totally my next project. Thanks for the idea. I'll be asking questions later (no time soon.) :-) -c

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