Here is the Apple support DOC for your question: iMac (Flat Panel), Power Mac G4 (Digital Audio) and (QuickSilver)+ MDD: Sound Output Ports: http://docs.info.apple.com/article.html?artnum=58764 Briefly, the 2.5 mm jack is the audio out specifically configured for Apple pro speakers. I have a G4 Dual 866 and the Pro speakers, they are nice but for "BIG" sound I use the headphone output and an external amplifier/speaker/subwoofer combo. On Jun 11, 2008, at 1:58 PM, Bill Disney wrote: > I'm new to the group and recently bought a Apple G4 Dual 1GHz > (Quicksilver 2002) (M8667LL/A). At the moment I have CA (cheap) > audio speakers plugged in the rear Headphone jack. Is there a > special adapter that can be used on the 1/8'' speaker jack to plug > into the Audio in jack on back of G4? Would the sound quality and/or > volume be enhanced? Should I not be using the rear Headphone jack to > run any speakers but only the Audio in jack? Any suggestions > appreciated - thanks.~Bill >