Quote Originally Posted by Wegamus View Post
Why would a power amp have an input sensitivity of 0.5V? Is it because most of the circuits were basically designed before 2.0V digital sources appeared so no one wants to change them? I’ve noticed old Quad amps are 0.5V too, and seen modded ones where it’s been changed to 1.5V presumably for use in a modern setting. My old Nagra power amp had a switch to change it from 1V to 2V which was very handy. I don’t understand the wide variation and the purpose of it when we’re all using the same sources - 2V for digital and a few mV for vinyl. It’s like volume controls too, they’re all 9 o’ clock at best for normal listening when there’s a whole pot to use, just why?
The (arbitrary) 0.5V was arrived at through the years of the early 'evolution' of amplification, and became a default standard, as so many other things in society have done, and I presume that this level was regarded as sufficiently high to be relatively immune to external interference. It is very easy to with hindsight, criticise earlier things when a greater understanding has been achieved.

In broadcasting a voltage standard of 0.775V was a standard derived from a power level of 1mW in a 600 ohm resistor, this derived form maximum power transfer theory in telecommunications, and became the standard. But power transfer efficiency is not a real consideration with signals in which low currents are involved, but maximum S/N ratio is very much so. Arguably since there are metres of cable in broadcast studios carrying signals at this level, it may be advantageous to use a higher level to improve S/N ratio and crosstalk.