As you can see, my 12AU7 vacuum tube headphone amplifier/preamp based on a single ended class A design is finally finished! Since the last time I posted about this project, there have been a few changes and additions. I added a nice bright blue LED into the tube socket as a power indicator. I also removed the separate heater/main power switches and went with a single on and off switch. I did this mainly because there wasn’t enough room inside to accommodate the insanely large Cherry switches I bought. This was mainly because I had to redesign the power supply. I originally wanted to use a simple LM317 for the voltage regulation, but the device oscillated and the noise floor was way too high. I had to rip it out and used a zener diode with a darlington emitter follower. It wasn’t pretty because I didn’t have the right parts, but in the end it works and is silent. The only problem with it now is that during the warm-up phase there is intermittent crackling on the output. It goes away once nominal temperature is reached. I am pretty sure that it is because the socket has oxidized or is dirty. I don’t have DeoxIT, but I retensioned the socket and it seems to be better now, but not entirely fixed. It still retains the ability to use both an 18V DC input from a transformer or SMPS as well as a direct unregulated input for 12V SLA batteries. As for performance, I don’t really have a good enough scope to measure the accuracy as well as I need to calculate something like  THD or SNR. All I can say is that it beats my IC-based headphone amp that I have been using until now in both frequency response (especially low end) and output power. I can’t calculate the output power directly, but I assume it is somewhere in the range of 200-400mW. The amp can drive basically any impedance headphones from 8-600 ohms. My Shure SRH-440s with an impedance of 44ohms sound great!