The Interspace Industries PCDI box eliminates hum problems when connecting audio from your presentation PC. Completely passive and bi-directional.
 
Being completely passive (no batteries or external power required) and bi-directional, PC Balance Box can match unbalanced and balanced audio sources, line to mic levels, stereo and mono signals, and troubleshoot hum problems - all in one versatile and rugged unit. The PC Balance Box is the simplest way to interface unbalanced computer or consumer audio signals to balanced AV sound systems.