Well of course there has to be some active components in an amp in order to get the gain to be called an "Amplifier".
I was always impressed how simple single ended tube
amplifiers could be. A single valve, a resistor or two and an output transformer. So I fashioned a FET amp around the same simplicity of design. One audio grade MOSFET, a couple of resistors and capacitors and a heavy duty well filtered power supply.
I used Sprague capacitors for the input coupling and a large electrolytic on the output with a 10µF polyester bypass capacitor. The four 10W non-inductive wire wound resistors act as the load. The four resistors are 15ohm and 10W each which I wired two in series for 30 ohms and then the two sets of 30 ohms are wired in parallel to give a total resistance of 15 ohms.
Circuit MOSFET Amplifier 2SK1058
These get extremely hot and burn about 30W at idle. Yes, Class-A amplification is very inefficient. I burn over 20 Watts to get only about 4.8 audio Watts per channel! The heatsink I used is rated at 0.784 °C/W
I am using this Class-A
Amplifier with a 12AU7 based valve preamp. It produces the most purest sound. I have no idea of distortion levels etc. but it has a very fine grain and delicately textured quality. With only a few watts of output (the power is less than 5W RMS but I have seen clean 15W pulses on the CRO), sensitive speakers must be used. Bass is better than the design predicts and the amp drives my 12" 63L based 3-way speakers with ease.