In addition to bias points (operating points) filter caps need to charge up and they charge on an exponential curve. So that last nth degree takes a while. Filter caps are usually electrolytic and electrolytics are funny things, sort of a cross between a real capacitor and a battery (more on the cap side of things though). They are also used as coupling caps in solid state gear and for that reason need some time.
Transistors have a non-linear capacitive element that is inherent in their junctions (which affects the kind of distortion they make). This aspect is affected by heat and voltage (most radios these days are tuned by varactor diodes that take advantage of this effect). That quality of the devices has to stabilize and if the designer did his homework, that happens after a period of time rather than just after the amp is turned on.
IME solid state gear really needs to be on all the time to sound its best, though I have run into to some pieces that actually sounded worse after a day or so.
One might ask why such tiny effects might be so audible and the answer is that it has to do with distortion, and tiny amounts of it at that. But the distortion involved happens to be of a kind to which the ear is actually more sensitive than most test equipment, which is to say that we can measure that the distortion is there in trace amounts but to the ear its pretty obvious. This is because the ear/brain system uses the harmonics that are affected by this distortion (5th harmonic and above) in order to figure out how loud a sound is. So the ear brain system has to be pretty sensitive to do that as our range of hearing is pretty wide from soft to loud.
This is why the tube/transistor thing has been going on for so long BTW. Tubes inherently make less of these higher orders than transistors, which is why they sound smoother.
Transistors have a non-linear capacitive element that is inherent in their junctions (which affects the kind of distortion they make). This aspect is affected by heat and voltage (most radios these days are tuned by varactor diodes that take advantage of this effect). That quality of the devices has to stabilize and if the designer did his homework, that happens after a period of time rather than just after the amp is turned on.
IME solid state gear really needs to be on all the time to sound its best, though I have run into to some pieces that actually sounded worse after a day or so.
One might ask why such tiny effects might be so audible and the answer is that it has to do with distortion, and tiny amounts of it at that. But the distortion involved happens to be of a kind to which the ear is actually more sensitive than most test equipment, which is to say that we can measure that the distortion is there in trace amounts but to the ear its pretty obvious. This is because the ear/brain system uses the harmonics that are affected by this distortion (5th harmonic and above) in order to figure out how loud a sound is. So the ear brain system has to be pretty sensitive to do that as our range of hearing is pretty wide from soft to loud.
This is why the tube/transistor thing has been going on for so long BTW. Tubes inherently make less of these higher orders than transistors, which is why they sound smoother.