It doesn’t hum when it’s on whether music is playing or not. It’s a very quiet amp. It doesn’t hum when it’s off either once the plug is fully inserted. It only does it while you’re inserting the plug. Thanks for responding I just did some further testing. It does hum with the plug in but not until around two o’clock on the pot. I usually listen at around 10 o’clock. If it’s a grounding issue because of floating ground on amp and non grounded source is there any danger to my headphones or myself? Under normal operation it sounds great.