The complete circuit is shown below. The idle current for the first stage is only 3.5 mA. While I usually would use more current, the aim was to limit current consumption to extend battery life. The output stage idle current is set at 10 mA, which implies a peak output current of 20 mA. This equals 60 mW into a 300 ohm load and only 6.4 mW into a 32 ohms load.

Complete Portable Tube Headphone Amplifier

In other words, this push-pull amplifier is better suited to higher impedance headphones because of the current limit. How could the peak output current be increased? Another triode might give a higher idle current, but unless its rp is substantially lower that the 6922's, it to will run in into the Ipk = B+ / 2(Rl + rp) current limit. The B+ can be increased by using a higher-voltage DC-to-DC converter, but at the cost of  diminished battery life. Using the existing circuit with capacitor-coupled output transformer might be a good idea, as even a winding ratio of 2:1 would double the peak current into 32 ohms.

Conclusion

I wish there were a conclusion. But until more information on available output transformers comes our way and until further tests are preformed, no conclusion can be reached. One test that must be done is a shootout with a reality-check, Op-Amp-based headphone amplifier, such as the one shown below. This simple OpAmp-based headphone amplifier boasts some great features, such as DC coupling at the output and a single-ended output stage. In addition, the 1k pull-down resistor at the OpAmp's output forces the OpAmp's own internal output stage to run in single-ended mode. (By the way, almost any good OpAmp could be used; even an OPA637 could be used, if the 30 feedback resistor were replaced by a 75k resistor. And just about any NPN transistor in a TO-220 package could be used, such as the TIP29.) The punchline is: if the tube headphone amplifier does not handily beat this $25 contender, we are wasting our time.

Reality-Check Headphone Amplifier

By the way, always have a reality check handy. When comparing $400 interconnect cables, throw in a pair of $5 Radio Shack specials. When comparing $4,000 8 watt amplifiers, throw in a Dynaco ST-35. Is this a promise for yet another article on tube headphone amplifiers? Eventually, yes.

//JRB

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