Even with the DIY version, do not build it unless you know with absolute certainty that your amp's negative terminals are a shared ground.
These converter boxes are basically just L-pads that merge the negative terminals together. Be aware that this will also potentially pooch your output impedance. Depending on your headphone, this can affect the frequency response.
edit: for all the hooplah, you can also just buy a proper headphone amp for cheaper than a converter box