If you have a stereo headphone output on quarter-inch jack and you want to connect it to a single active monitor with XLR input, we describe here how to make the cable you need.
David Greaves wrote: Although many headphone outputs can tolerate their left and right drives being simply joined to sum to mono, many others will not (it depends whether they already have a small series resistor in their output connection after the point where the negative feedback is taken off). To be on the safe side, and to reduce loading on the phone amp, adding your own two resistors is a very good idea. The two resistors need to have the same value as each other, but any value from 100 ohms to 10K will work well. We used 470 ohm resistors.
There was an error in the original diagram XLR pin labelling drawn by David Greaves.
But Mike Senior has redrawn the figure correctly as below:
Here are the resistors placed inside a jack plug. The other end of the balanced cable feeds an XLR. The hot pin on the
XLR, pin 2, should be wired to the hot source at the junction of the two resistors.
Explaining the error in the old figure, Hugh Robjohns wrote: The middle pin on both male and female XLRs is pin 3 which is the 'cold' pin -- and that should be wired to core 2 which connects with the (grounded) jack plug sleeve. Pin 2, the 'hot' pin, is the top one on your book's diagram and that should be wired to core 1 which goes to the junction of the two resistors, the other ends of which go to the tip and ring of the stereo jack plug. Pin 1 is the XLR ground and that is wired to the cable screen which also goes to the jack plug sleeve.
When you're looking into the back of an XLR to wire it up Pins 1 and 2 swap sides between male and female XLRs, but the cold pin (3) is always in the middle!
The following link presents an XLR version of the summer cable: https://recording.de/threads/einen-mono-lautsprecher-an-stereo-amp-anschliessen.210267