A preamp with a theoretical minimum THD+N value of 0.00064%. In reality due to thermal noise (as it is most prominent at these levels) and nonlinearities of the semiconductor devices the value would be something like 0.005% or so, maybe even a little higher, depending on the components used, the layout of the topology, the stability of the supply rails etc. Basically this result is best case scenario, without all the deterioration factors. Use ultra low noise transistors like BC550 or BC547 and their complementary counterparts. The ripple rejection ratio is also very good at -120dB. I found out that keeping the 6.8kOhm resistor greatly improves the ripple rejection and the overall noise. It gives the input a ground reference, which is usually prone to picking up noise from all sorts of places
