It is an op-amp type of circuit which is built with discrete components (BC559 and BC560 transistors, with a BD139/BD140 output pair). It runs in class A and has no output coupling capacitor.
My own (slightly modified) schematic of the headphone amp is shown in Fig. 1. The first stage uses one-half of a E88CC (6922/6DJ8/ECC88) in a common cathode configuration. This is directly coupled to a cathode follower which employs one-half of a 6AS7G. I have added a volume potentiometer and a grid stopper resistor to the original schematic. The size of the output coupling capacitor was also increased (from 100 to 220 uF), simply because I had this value in stock and also because I intended to use 32 and 60 Ohm headphones. With 60 Ohm headphones, the calculated -3 dB cutoff point is now at 12 Hz, whereas with 32 Ohm phones, it is at 22 Hz.
A prototype which I made on a piece of plywood worked immediately and I really liked it. With good recordings there is a life-like quality to the sound. Voices and instruments are pinpointed on the stage, with lots of musical detail and “air”. Cathode followers have the reputation of “muffled” and “boring” sound, but Rudy’s circuit renders dynamic contrasts very well and it grips your attention. Minor details in recordings become audible. One can hear, for example, the difference between different violoncellos, and the fact that different tracks of a CD are recorded in a slightly different recording venue.The solid-state amp sounds “hard”, somewhat “glassy” and “mechanical” in comparison, with less detail and less precise imaging. This surprised me greatly since the tube amp has an output coupling capacitor which the solid-state amp lacks. Apparently, the absence of global feedback and the simplicity of the tube circuit works wonders for the sound. The single-ended topology of the tube circuit may also result in a spectrum of harmonics which is different from that of the solid-state amp which has a push-pull topology.
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