Progression?

[Steve Eddy]

Also take the example of a single transistor input stage that I mentioned above. Old designs utilised it because it was the only option. Then differential input (two transistor) topology came about.

Naah. Differential topologies had been around well before the introduction of the transistor and those same topologies work for transistors just as they do for tubes.

If an input stage used just one transistor it was for one reason; to save money.

se