High pitched whine/noise with mBox h/p o/p into active monitors

[audioboffin]

I work with a number of different Pro Tools rigs, HD systems, old Mix systems, and a number of combinations of LE and M-Powered systems. I have noticed a few times that one of my systems is sometimes noisy when I monitor out of the headphone socket of an original mBox. I'm patching a Y cord from the headphone socket of the mBox into a pair of powered speakers. I have several pairs of monitors and a couple of the first edition mBoxes, and the problem isn't always there.

My computer hardware varies between several generic home built PCs, an Apple Mac Book, several iMacs of various vintage, and an Apple Intel Pro quad core tower. Sometines small movements of the USB cable, and sometimes changing the USB cable would give some improvements, and sometimes lifting power grounds of various bits n pieces would help, but nothing was consistent.

I thought I'd spend some time and dig into what is happening, and consequently wasted a long time on different USB cables, lifting grounds on monitor power cables etc before I hit on the solution.

I have a few of the afore-mentioned "Y-cords" with a 6.5 or 3.5 mm stereo jack connector at one end, and 2 x 3 pin male XLR connectors the other ends. Usually when connecting an unbalanced source (like for example the headphone output of an mBox) to a balanced input like the inputs on most powered monitors, I will tie pins 1 and 3 together to the shield of the cable, and the hot conductor of the cable I connect to pin 2. Under these conditions I was hearing a whine every time either Pro Tools launched, or every time the Digi CoreAudio Manager application launched when accessing the mBox with third party software.

Now here is the solution, and it seems to be repeatable: Lift the connection to pin 3 of both XLRs. Just tie the shield to pin 1 and the hot signal to pin 2 when monitoring with active speakers from the headphone socket of the mBox. I have yet to test this with a passive attenuator patched out of the line output, usually when I have this scenario the attenuator is coupled via a pair of transformers.

Hopefully someone else can benefit from this experience.

Cheers!!