Hi every one. I need some help on debug the SSMH amp DIY (12AU7 version). I decide to make SSMH as my very first DIY project. I plan to seperate two chanels to two board. So I did one chanel first.   But when testing:   - When no input, output | input V = 0, output V = 0.75V (not ok?). When connect headphone (not input), I hear eh eh eh eh eh eh eh ru ru ru ... in series. - When connect input, output. I hear the music but have some ru ru ru in...