Now you can readily see why push-pull, Class AB, B is so popular. Well then, the answer seems easy enough: use either an inductor or current source to load the output stage. Now a few problems arise. First, finding high quality, high inductance, small chokes is not easy. Second, small, high inductance chokes have a correspondingly high DCR, which will displace some of the precious B+ voltage. Third, active current sources do not function all way down to zero volts; instead, their impedance falls when the voltage differential falls to just a few volts in the case of solid-state current sources, and tens of volts in the case of tube current sources.

    In other words, while these devices are in general good ideas to implement, we may not gain all the efficiency enhancement promised by these devices. What about output transformers? In theory, this would be the best design path to pursue, as a transformer would provide both a better impedance matching between tube and headphone and a means of protecting the headphone from DC offsets. But in practice, it is extremely difficult to find high quality low wattage output transformers that offer bandwidth down to 20 Hz. Additionally, the simplicity of a single resistor and coupling capacitor are truly compelling.