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noise at lower frequencies will be quite high due to flicker noise
Yes, but the 1/f flicker corner is (for clean silicon) usually below 1KHz. And total noise is proportional to bandwidth. And what is the difference between DC-20KHz and 1KHz-20KHz? 19KHz versus 20KHz, inaudible.
Plus of course the ear's mid-high boost, and profound bass-loss at low levels (so our own footsteps resonating our head/neck don't deafen us).
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even when the extreme emphasis on low frequencies occurs, such as an RIAA EQ'ed phono preamp, it is rarely the ~1/f noise region that one hears coming out of the loudspeaker
I ran into a situation where flicker noise overwhelmed everything.
ART Research phono preamp into a commodity (Sony?) HiFi amp. Worked fine at moderate level. But this was a piano studio, the speakers were capable of keeping up with a grand piano, yet when I tried it the amp kept shutting down. Even without the needle in the groove. Only phono, not CD. And the woofer flapped frightfully. On the bench, a 1.5V
DC VTVM on the output showed several-tenths volt random jerks. The volt-time product of some of these apparently would exceed the power amp's DC protection threshold as total gain was brought near realistic levels.
The phono preamp implemented the 20Hz corner with a small cap in front of the 47K input resistor. Yes, mega subsonic noise impedance. Looking at the chip's
published noise curves, it all made sense.
I swapped some caps around to keep the input well damped to very low frequency, and took a 25Hz cutoff at the output, with a shunt resistor to swamp load variations. Now it was OK all the way to maximum available gain, which on most phonodisks would be well past power-amp clipping.
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Would lowering the operating current (from the calculated optimum) increase or decrease THD, or would it affect the distortion at all?
If everything is scaled, no change (until you get to extremes, as bcarso says). In fact as you go to too-high currents, 1/Gm becomes "small" compared to parasitic resistances, so you actually get more linear. OTOH thermals can bend bass bad.
For the same power output: lower current is sure to hurt, or at least force higher impedances. THD is almost meaningless unless you specify what power you want.
Some things won't scale. Parasitic resistances, and also stray capacitance. In audio and in video, useless stray capacitance often requires more current than we need to drive the real load on a "voltage amp" stage. You can move from metal TO-5 to plastic TO-92 cans, from terminal strip to PCB, to SMT, to IC, and shed some capacitance (in return for tiny eyes and tools), but you can't get rid of it.