(Pictures below...)

Purpose: to confirm that a given XLR jack (such as that on a snake in a club) is "cold" before plugging equipment into it that may be damaged by the presence of phantom power (or any other strange power). Devices that send balanced signals into XLR jacks (such as a keyboard or a computer sound interface, using a 1/4" - XLR converter) may be harmed unless the jack is known to be safe (e.g. the sound person didn't turn off phantom power on the correct channel, or strange wiring is afoot, etc.)

Also, in cases where phantom power is desired, it was hoped that the tester could indicate the presence of correct phantom power or miswired phantom power or other wiring anomalies before risking the device in question.

Note: the primary purpose of this is not to check phantom power wiring, but rather to detect the presence of power of any kind; it just happens to also do the latter to some degree.

For the first issue, a DI box would protect the device in question in most cases, and for the second, testers are available. But something smaller and cheaper was desired. Total cost for parts: about US $5, excluding shipping. About 3 hours to build and test.

If you just want a phantom power wiring checker, you might build this instead.

My first sort-of-audio DIY project! Feedback/corrections welcome (comment form below).

This thing uses a standard male XLR jack, four resistors, and four bi-color LEDs. It indicates regular phantom power as well as most strange miswired situations. It is designed to "light up" in almost all cases of energy being present on the wires, rather than to be informative as to what the miswiring actually is. In other words, it is not primarily a cable-tester or a diagnostic tool, but rather a protective device. It does, however, serve a diagnostic purpose to some degree.

There is at least one similar commercial product, The "Rat Pack" (US $50) which seems to fill a very similar role, with the added benefit of an optional battery-powered phantom source, and I'm assuming some more sophisticated circuitry. Not sure if it detects everything this detects (e.g. voltage to mains ground, and I'm not sure how to differentiate pin 1 lifted vs. no phantom present, etc, since their tester is designed to confirm proper phantom power and cabling rather than to confirm the absence of phantom power), but what it does detect I'd guess it detects more professionally. :-)


Big thanks to Scodiddly and The Scum on the Tape-Op message boards for their mentoring. The transcript of the creation process contains more information about the rationale behind things.


Schematic:

 

Materials: (see the design notes for next time before buying anything!)

LEDs used: 4 standard 2-legged "T-1 3/4" 5mm reg/green bi-color

Resistors: 4 standard carbon composite 1k8 (1,800) Ohm, 1/2-Watt rated. (1k8 is maybe a little on the small side, intentionally; see the transcript for more info...)

XLR jack: 1 standard male, with an electrically-exposed metal chassis


Testing procedure: (note: before inserting in any jack, make sure that jack isn't "live" - i.e. connected to something that will make any noise, as big clicks and pops are to be expected.) (Another note: it's been mentioned that even though LEDs are low-power, it may not be a good idea to plug this device into boxes that are using firewire-supplied power since it may draw too much from it or other devices using the same power source, etc. Just be aware.)

Check it periodically in a normal phantom-powered mic jack to make sure it's working, at least on some level. Insert it into the test jack. If anything lights (even the two green lights that indicate "normal" phantom power), the jack is probably not safe for devices that need "cold" jacks. If the two "normal" green lights illuminate, then normal phantom power is present (though the quality of it is not known, i.e. what voltage, etc.) If nothing lights, we're lookin' good, but not safe yet: leave it plugged in, take the nearest good 1/4" cable or wire, touch one end's tip to the chassis of the tester, and touch the other tip to a grounded part of the system being tested (e.g., if testing an XLR plug in a snake, an exposed grounded part of the snake). Try a few spots. If nothing lights, then the jack is "probably" safe. There are cases that may not be shown, but more paranoid users should isolate their device properly anyway.

It's recommended to not leaving it plugged-in longer than necessary, especially if the lights are glowing brightly, as miswired situations may push too much current through the LEDs. Use of the device is basically risk-free to the system being tested, even if things short out or mistakes are made in the construction or use (as long as the note about not testing jacks that are live-to-speakers is followed...)

A few various combinations of miswirings were tested (e.g. pin 2 not connected, pin 3 not connected, pins 2 and 1 switched, pins 3 and 1 switched, pin 2 not connected and pin 1 carrying 48V, etc.) and they all worked. They also generated different light combinations, so a user could chart those out to determine what light combinations indicated what. I didn't bother.

It was not tested on lower-power phantom systems, like 12V or 24V systems. The LEDs light well under 48V, and my impression is that they would at least glow dimly under those other systems, but I don't know.


The innards. Tough to make sure things don't bump into each other when it's put inside the chassis, etc.:
In the casing... So cute:
Lighting up under standard phantom power, the LEDs connecting 1 - 3 and 1 - 2 light up green:
Bad wiring example 1 - pin 3 disconnected:
Bad wiring example 2 - pin 1 disconnected, pin 3 connected to ground:

Building notes:

  • Took off the rubber boot from the XLR jack
  • Cut off most of the internal connection that would normally be used to connect the cable shielding to the XLR jack chassis (to save space). Left a bit of there it to connect to.
  • Soldered it all up according to the schematic. Space is limited, plan ahead.
  • Used a plastic sleeve wrapped around the innards to prevent short circuiting against the chassis
  • Care is taken so that the orientation of the LEDs causes them to light green under normal phantom power. The LED between pins 2 and 3 is of arbitrary orientation. The LED between pin 1 and the chassis connector I oriented to be red when current flows towards the chassis.
  • Maybe 3 hours for a beginner/intermediate person to build and test.

Design notes for next time:

  • use the smallest LEDs you can find. These 5mm LEDs had to be sawed down to size to fit through the XLR case. It was a pain. I think 3mm LEDs would fit, but it might be tight (for this particular jack, anyway.) Or use a light bar or some other product that has multiple elements in a small package.
  • it is suggested to use LEDs with the smallest current requirements you can find. Since you want the lights to light even if small amounts of current are present, look for "high efficiency" LEDs that need like 4-6 mA to light, rather than 15-20 for standard LEDs... This may be necessary for 12V and 24V phantom power systems.
  • Make sure length of the LED leads is such that the LEDs don't poke out past the XLR chassis, so they won't get bumped, etc, and to make it easier to glue a protective cover over the end.