If you want something cheap (as in $11) this is what use:
http://www.amazon.com/dp/B002KO63X2/
It's not powered but it gets the job done.
https://www.amazon.com/PA-Horn-Speaker-Plug-Wire/dp/B002KO63X2/ref=asc_df_B002KO63X2/?tag=hyprod-20&linkCode=df0&hvadid=311990496852&hvpos=&hvnetw=g&hvrand=5114349924815077413&hvpone=&hvptwo=&hvqmt=&hvdev=m&hvdvcmdl=&hvlocint=&hvlocphy=1027944&hvtargid=pla-570136442879&psc=1