Dave reverse engineers the XuanZu X2-U808 Hi-Fi Headphone Amplifier to see if the valves actually do anything.
What is the circuit topology?
E180F (6J9) pentode valve datasheet
Doug Ford’s Microphone design series Playlist
Reverse Engineering How-To video
from EEVblog – The Electronics Engineering Video Blog http://www.eevblog.com/2016/01/07/eevblog-837-reverse-engineering-a-valve-headphone-amplifier/