Amplifier Capacitance

Bombaywalla, Yes, power supply should be clean but I was talking about modulation of power supply voltage by varying load that amplifier presents. Low frequency will definitely do it (biggest current) and high frequency will do it as well (inductance of the caps). It will be reduced by amplifier's PSRR (power supply rejection ratio) but will still affect the sound. Atmasphere was talking about very low frequency signals causing big sags of supply voltage that bounces back (motorboating). Limiting low frequency response of the amplifier, as he suggested, will help but I can imagine scenarios where it will still happen. Let's play "Kodo Drums" (Shefield) - enormous amplitude of low frequencies repated once a second. That will do it as well. It becomes obvious why good amps have so many caps in the power supply.

As for power supply being clean - the biggest offender there is 120Hz ripple proportional to load. At low sound level we cannot hear it because ripple is very low (light load) but at high sound levels when ripple is strong we cannot hear it either because sound is too loud. It is almost like jitter that is undetectable unless you play louder. There is also high frequency component related to charging current spikes and also limited "softenss" of rectifier diodes (late switch off, fast snap back).

That's why many designers started using switching power supplies instead. Modern SMPS switch at zero voltage/zero current, produce high frequency noise that is easier to clean than 120Hz, have line and load regulation plus protection against overcurrent or overtemperature. Jeff Rowland uses 1MHz SMPS in his newest creation model 625 (class AB) amplifier. There are some other benefits size being perhaps the least important. One of them I can appreciate in my Rowland 102 amp. It works from 85-265VAC or DC voltage to almost 400V making it less susceptible to overvoltage and completely immune to DC on the power line.

Also many Rowland amps have active power factor correction that makes amplifier "look" like resistive load loading power line evenly during sinewave instead current spikes near the peak: http://jeffrowlandgroup.com/kb/questions.php?questionid=144

why not increase the voltage rating, to improve the longevity, ESR, and inductance characteristics instead?

The price of higher voltage, low ESR, low inductance caps is perhaps too high. I checked once site that sells Hypex class D kits. Power supply module was by far the most expensive because of BHC slit foil low inductance electrolytic caps. People try to remedy inductance of large caps by placing small non-inductive caps in parallel. It lowers caps reactance at high frequencies but also creates parallel resonance circuit that will ring under rapid current draw. There is a reason design engineer avoided it.

Bombaywalla, sorry for my tardy response; obviously you got it right.

Kijanki, I thought you might find this interesting. Years ago I worked on a popular tube preamp. I found that it was a copy of the Marantz 7 circuit, sans tone controls.

This preamp tended to have a problem- which was if you played a bassy track too loud, it could thump and do some weird stuff- especially if you were able to watch the woofer cones.

Anyway, it turned out that the timing constants of the preamp section went lower than that of its power supply. I spoke to the designer (a Brit) who was not at all happy to hear of my diagnosis, despite admitting to me seconds before that the bass instability was a problem.

The take-away is this is not an issue limited to amplifiers...

Ralph, that's very interesting. I just looked at Jeff Rowland Capri preamp - the upper limit is 350kHz but bottom is only 10Hz. It could be direct coupled with servo (integrator) in the feedback set to 10Hz. In any case, he did it for a reason. My small Rowland 102, as well as big 625, have both 5Hz bottom -3dB but all his preamps have bottom at 10Hz. He also uses SMPS in preamps. I know he does in Capri, not sure about others. He might feel that limiting rumble or power supply effects should be done as early as possible. Additional 5Hz bandwidth in the amp improves slope, adding second pole, without affecting much overall 10Hz bottom limit.