This is the latest version of my tube Stax headphone driver. I have been working on this for almost 2 years. It satisfies all the requirements. First of course it sounds absolutely amazing which is clearly the goal here. There are no capacitors in the signal path. Its fully DC coupled. The quiesent output voltage is 0 volts. No expensive apex parts. Buildable by just about anyone.

Here is how it works. The first stage is a differential amplifier with feedback directly from the output stage. This input stage works equally well with both balanced and unbalanced inputs. For unbalanced inputs ground the minus input. Works well with the step attenuators from goldpoint. The second stage is as far as i know unique in the tube world. It translates the output of the first stage downward by 450 volts with no loss in gain. The 3 0a2's can be replaced with 6 ne-2's to save in the final size of the amplifier. The 0a2's can also be replaced with a zener string but then the amplifier is not fully tubes is it. This middle stage actually runs at a higher bias current than the output stage. The output stage is another differential amplifier. Feedback around the whole thing fixes the gain and keeps the output voltage at 0 volts. The output load resistors are vishay rs-10 noninductive wirewound resistors.

The power supply is a simple one that puts out a regulated +350 and -350 volts, also -15 volts, and 6.3 vac for the filaments. Schematic will be provided in the future when i have more time.