Amplifier Capacitance

Aewhistory, Time constant is amount of time it takes to bring voltage to 63.2% of desired value. For instance applying 10 volts to 1000uF capacitor thru 10ohm resistor will result in 6.32V on capacitor after time equal 10ohm x 1000uF = 10ms. Same would apply to discharging from 10V to 3.68V (10V-6.32V). This time is called time constant RC.

Instead of saying "Time necessary to charge capacitor" we say "time constant". It is shorter and more precise. In our case it just means amount of time to have significantly lower supply voltage (discharge supply capacitors) when playing very low frequencies very loud. We want this voltage steady since any variations might compromise amp's operation (output affected by supply voltage changes).
We are looking at small changes in supply voltage (not 63.2%) but using terms like "time constant" or "-3dB frequency" just to have some reference point. From that we can, if necessary, recalculate exact percentage changes at particular frequency.

Kijanki, thanks for that explanation, it was excellent. Do you teach electrical engineering by any chance? You, bombaywalla, and Atmasphere have done a magnificent job explaining very esoteric (to me) subjects. This is difficult to do, especially when speaking to a layman. I've taught college history now for 12 years and it can be daunting getting people to relate to human history, so I've always wondered how people in the sciences make their subjects more approachable and relatable. Discussing matters here certainly gives me an idea how this can be done as I've never encountered a group with the combination of technical savvy and possessing the ability/willingness to explain as here at Audiogon.

Teaching? No, Almarg would be much better at this. He explains things with much more clarity. History was always difficult for me.

but once we draw less than max current, power supply voltage modulation at low frequencies becomes compensated by amplifier (since it is regulated).

Kijanki, I don't understand this statement. Power amp power supplies are not regulated (I don't see a feedback loop around the power supply). The power amp itself has minimal feedback around itself for sonic purposes (as you already know) & this feedback is for the music signal & not for any vairations of the power supply. The amp is counting on the power supply to be essentially DC. So, any power supply modulation will modulate the output voltage signal (AM-AM modulation/distortion).
I'm afraid that I'm not seeing how the power supply modulation at lower/bass freq is compensated by the power amp. Thanks.

Bombaywalla - Amplifiers are line regulated. It means that amplifier supplied from 40V and set to produce 5V output voltage will still produce 5V output with supply lowered to 35V or increased to 45V. There will be small error because regulation is not perfect but it is in order of one percent or less. There is an easy way to test it - just set your amp at moderate listening level and then reduce line voltage from 110V to 90V. Your amp will play at the same level. You could measure it with test tone and voltmeter.