Reduce hissing in Ruby's headphone output

[simop]

I built the Ruby amp and added a headphone output to it with the instructions from runoffgroove.com FAQ. How could I reduce the hissing from the headphone output? Now the amount of hissing is very annoying.

[simop]

I put a 100µF capacitor across the hot and ground terminals of the headphone jack. It reduced the hissing enough.