What exactly does it do? Could I use this as a compact substitute for a DI box in combination with a padded/attenuated cable?

there are often instances when I want to be able to record one channel of audio from an XLR mic input and another channel from a line level (2V) 3.5mm source, like say phone interviews using a cell phone.

i realize this could easily be done with a mixer, but i often don't have or want to carry a mixer with me. My Marantz PMD661 only allows one set of inputs to be used at once (both XLR or both from 3.5mm line-in).

The VXLR is designed to adapt a 1/8 mono microphone such as the Rode VideoMic to plug it into a single XLR input.

That much I already knew, I was wondering more how the wiring works from unbalanced to balanced.

I found this VXLR schematic, but I don't really know what's going on.

When an XLR is used for balanced wiring, the normal case, pin 2 is signal hot, pin 3 is signal cold, and pin 1 is ground. As the schematic shows, pin 3 is shunted to ground, effectively unbalancing the XLR side of things. Meanwhile, at the 1/8 side, ground goes to XLR ground, left and right signal (should it be a stereo mic plugged into it) are connected together in parallel, combining them into a single mono signal, and sent to the XLR signal hot. So the VXLR is both a physical connection adapter and an unbalancer for the XLR input.

Beware that adapter for phantom powering of your lav. Not all lavs are happy to receive +48V.

Some adapters insert a zener diode or two to limit the voltage to your lav. That might protect it, but won't necessarily power your lav correctly. I bought a Sanken COS-11D and a cheap adapter. Didn't work. I ended up getting the real, $150+ active adapter, which works, sounds great, and makes a truly balanced XLR connection.

Hats off to Rode for making their lav work with a cheap XLR adapter, but note that the XLR cable signal won't be balanced, so avoid long runs.

hmm...my iphone might not like phantom power either.

i didn't know you could send a balanced input a signal on only 1 pin. i just assumed that it would require a signal on both pins.

Well you do have to have a complete circuit. But by grounding pin 3, you complete the circuit. Note that unbalancing an OUTPUT in this manner, such as feeding the output of a balanced mic or other device into an unbalanced input, results in only half the waveform getting used, thus there is a 3dB drop in level. Balanced output to unblanced input results in a signal drop. This adapter goes the other way, unbalanced output to balanced input (albeit functioning as if it was an unblanced input).

So could this adapter work with the Rode Videomic Pro and a recorder - say the Tascam DR-100 with XLR's?

Well, that is the original purpose of the adapter. Rode has another XLR adapter (MICON-5) that provides somewhere between 2-5V of plug-in power for the Rode Lavalier when supplied with phantom, but has a MiCon connection instead of a 3.5mm TRS.