[CDBacklash]

Any analogue signal (assuming the amp is not clipping).
Eg. plugging a headphone amp into the headphone jack on my PSP
or into my guitar amplifiers output.
Just curious because people keep telling me that its not, even though by logic electrical energy flows only one way in a circuit

 

[iriverdude]

You mean output from the headphone amp into PSP? Why would you do that? And yes it could damage your PSP.

 

[CDBacklash]

lol no psp -> amp -> headphones

 

[krmathis]

Should be perfectly safe. Just make sure you keep the PSP volume down when you hook it all up.
Not 100% perfect sound wise though, as you will double-amplify.