Our gym has an input for a mic (3-Prong XLR), and I would like to plug in an MP3 player for our pregame music. I purchased a 3.5mm Stereo to XLR cable to accomplish this, but all I get is screeching similar to feedback... I'm guess a level incapatibility is the problem...I saw a guy use a rotary box in between his MP3 player and the XLR cable and it works fine. Does anyone know what this box is, where I can get one, or how else to get my current set up to work. Thanks for any help...
The mixer and amp are in a remote location behind the bleachers. The jack has been wired into the bleachers themselves by the announcers table. It is a 3-prong female XLR. The cable I have reads as follows: 3.5mm stereo male to XLR male. The box the other fellow had was a male XLR wired to the box which also has a rotary knob, and a 3.5mm male coming out of box for the MP3. I have read there are to types of 3.5mm, a regular like for a MP3, and a TRS, which carries a different signal...
Ok. (I had to google "bleechers" 
)
You got a problem, there's a couple answers, pick your best.
1) "you don't have a problem" , just tell the guy operating the PA system to play that MP3 himself at the sound booth/table.
Wonder why somebody at the "playing field" needs to do that.
2) you can cheat and use that particular line as an un-balanced line level cable.
It will work because your MP3 signal is loud and low impedance, so you don't really need it to be true balanced like with a mic.
If acceptable, I will post a circuit for an adapter for a 3.5mm stereo plug to unbalanced mono male XLR .
Most mixers can work properly even if you unbalance the XLR:
a) standard XLR: Pin 1 screen/ground, Pin 2 Hot , Pin 3 Cold
b) unbalanced XLR : Pin 1 screen/ground , Pin 2 Hot , Pin 3 :wired to pin 1
If that suits you, I'll draw the very simple circuit which you'll have to solder and test, with your MP3 player plugging straight at the Mixer XLR input.
If it works (it should) , plugging that adapter at the solitary XLR in you have available should be the same, just a few yards away.
3) buy a real Direct Box which has a balanced XLR output, check one of these:
Direct Boxes | Sweetwater.com
they usually have an 1/4" jack input, you'll need an adapter 3.5mm stereo plug to 6.35mm mono plug. (what guitar amps use).
)You got a problem, there's a couple answers, pick your best.
1) "you don't have a problem" , just tell the guy operating the PA system to play that MP3 himself at the sound booth/table.
Wonder why somebody at the "playing field" needs to do that.
2) you can cheat and use that particular line as an un-balanced line level cable.
It will work because your MP3 signal is loud and low impedance, so you don't really need it to be true balanced like with a mic.
If acceptable, I will post a circuit for an adapter for a 3.5mm stereo plug to unbalanced mono male XLR .
Most mixers can work properly even if you unbalance the XLR:
a) standard XLR: Pin 1 screen/ground, Pin 2 Hot , Pin 3 Cold
b) unbalanced XLR : Pin 1 screen/ground , Pin 2 Hot , Pin 3 :wired to pin 1
If that suits you, I'll draw the very simple circuit which you'll have to solder and test, with your MP3 player plugging straight at the Mixer XLR input.
If it works (it should) , plugging that adapter at the solitary XLR in you have available should be the same, just a few yards away.
3) buy a real Direct Box which has a balanced XLR output, check one of these:
Direct Boxes | Sweetwater.com
they usually have an 1/4" jack input, you'll need an adapter 3.5mm stereo plug to 6.35mm mono plug. (what guitar amps use).
How much does it bother you you're only getting half the stereo? You could wire a couple of resistors to mix the two sides, and drop the level some while unbalancing it at the same time (I'm rather afraid that your present cable is wired pin to pin and you're just amplifying the difference signal - not good for bass instruments). You'll be mono, obviously, but at least all of the mono. And it's normally possible to conceat the resistors inside the XLR, so you don't need a separate box.
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