Thank you so much,
@tomb! The veil is beginning to lift from my eyes! I think I have confused ripple current rating of capacitors with ripple voltage - it's ripple voltage that one wants to reduce, and that is probably more influenced by the overall design of the amp than by my capacitor selection, right? Selecting a capacitor with lower impedance may help to reduce ripple voltage by a tiny amount though, right?
You are right that I am limited to a 18 mm max diameter for all the capacitors, and further limited to a 7.5 mm hole spacing. I have no height limit, as these caps will hang "upside down" from their PCB in the subwoofer enclosure with the only limit being the bottom of the enclosure, which is several inches away. I went ahead and ordered 5 x Nichicon PW series 4700 uF 35 V capacitors (25 V, for the slightly smaller singe cap in the photo, was out of stock at Digi-Key). I calculated the ESR for the FW series caps and the original caps from a formula on the Digi-Key website, and it looks like ESR is lower for the FW than stock, and ripple current at 120Hz is a touch lower in the FW series as well.
I also sought advice over at the DIY Audio forums. Here is a link to that thread in case anyone here, or anyone across the interwebs, is facing the same situation as me and searches this out:
https://www.diyaudio.com/forums/pow...pcitors-class-amp-power-supply-recapping.html
I think you're still confusing things, but I don't want to get known as a nit-picker. Ripple current rating is the amount of ripple, period, that the capacitor can reduce. Ultimately, the power supply supplies power, encompassing both voltage and ripple. Nothing is going to reduce "voltage" - whether ripple or not - with a capacitor, unless you consider that blowing the cap by exceeding its voltage rating will open the circuit and stop the power supply. In fact, large capacitors after the rectifier portion of a power supply will slightly
increase the power supply voltage, not decrease it.
It's infected current flow that you want to reduce with the ripple. The ripple, if it exists, is going to be imbedded in the power supply voltage as a secondary harmonic of the primary voltage. As such, it will
always be less voltage than the primary voltage. Think of it in terms of distortion of the primary voltage, except that the ripple is remaining AC that is corrupting the DC the power supply is supposed to produce. It's very specific - usually considered as 120Hz, twice the normal 60Hz of the AC line source. You can have ripple at other harmonics as well - 180Hz, 240Hz, etc., except those are quite small in a reasonable quality power supply. Unfortunately, the higher the frequency of the ripple, the more noticeable it becomes in an audio circuit.
Rectifiers themselves convert the 60Hz to DC. About the only way 60Hz gets into a DC circuit after rectification is through leakage: somehow the original line source is contaminating other parts of the circuit. However, ripple results because while the rectifiers chop off the tops of the 60Hz AC sine wave, there's still little spaces left inbetween the waves, as in, the DC voltage is distorted. The charge capacity in a capacitor can remove those spaces, leaving a straight line voltage that doesn't vary with time (DC as opposed to AC).
In the real world, however, nothing is perfect - neither is a capacitor. Some capacitors are better at this smoothing than others. Technically speaking, the ripple current rating of a capacitor is a measure of its ability to filter ripple without over-heating or decreased life. By definition - if you don't push the capacitor to complete failure - that's still a measure of its overall ability to filter ripple.
I'm dismayed by some of the responses you got on DIYAudio. Like a lot of forums - including this one - you are sometimes playing roulette in terms of who will answer and how. That includes me, too, of course. However, I tend not to stand on absolutes when it comes to analog circuits, performance, and the human ear. The idea that better-rated ripple capacitors will not make a linear-regulated power supply perform better is farcical. Yes, one can make that statement from a perspective of theoretical principle, but analog circuitry rarely behaves perfectly according to theory. Power supply performance, like a lot of things, can depend to a large degree on circuit layout, grounding, and the like, none of which are based exclusively in theory.
Quality of components absolutely plays a part.
With headphones, we are able to hear differences in a few microvolts of ripple from a power supply. A good linear power supply will measure down below 50uV of AC (predominately 120Hz ripple). A
great linear power supply will measure down into the single uV of AC. Neither one can be measured that low with even the best Fluke DMM. A low-noise measurement pre-amplifier is required. In any event, good practice is to use electrolytic capacitors with a high ripple current rating and low ESR in a linear-regulated power supply, the same as in any other power supply. Once you have a good circuit design, it's the little things that will make even more of a difference, especially in audiophile applications.
