I've got this one from Reveelsound. It is designed and developed by BGST ( whish is very famous with BSG QOL Signal completion stage ). It has an headphone amp inside but when you switch to "bypass" mode, it plays with no amp at all. I pair it with my Sony Arco S and Grado SR225 , it significantly enhance the sound quality. Music comes along with some micro details which I have never heard before ( even with head-amp i've owned such as Lisa 3 or Pico Slim ), without boosting to volume ups. Very very great !