Amplifier Capacitance

Okay, naive question of the day: except for cost and perhaps size limitations inside the casing, why don't manufacturers go bonzo large on capacitance? For instance, I'm thinking of replacing the caps in this damaged Hafler DH220 I've got and, while I probably won't find any that fit, I started to wonder why limited myself to two 18-19k cans? Why not 50k, 80k, even 100k if I could fit it? (I doubt any of those would, but you get the drift.).

much can be written about this but I'll try to provide a compact answer ;-0
large amounts of power supply does not come for free. you'll need a high(er) current transformer to supply the current to charge up those huge caps in rapid time so that the power supply voltage does not sag when a big slug of charge is demanded by the power amp electronics. Large power supply caps are huge reservoirs of charge, true, but they take time to recharge. Often this recharge time is too slow to keep pace w/ the music hence you'll often find smaller caps (like 10,000uF) in parallel w/ the huge power supply caps. these smaller caps are there to provide the charge for sharp transients thereby taking the load off the large power supply caps. The smaller caps are more nimble in terms of recharging hence they work in complimentary w/ the larger power supply caps.
So, note the transformer current delivery capability before you go "bonzo" on increasing the caps. if you did, you'll saturate the transformer core & bring in distortion into your power supply damning your Hafler sonics. Over time you'll also damage the power transformer.
Also, the larger the power supply cap, the larger the power-on current surge. My amp has a lot of power supply capacitance & it takes 37A in-rush current upon power on!! :-o. The amp designer has to consider this when designing the power supply wiring & choosing the AC fuse(s).

Capacitance seems like once of those things that should really make a big diffence in amp performance, no? In fact, shouldn't it particularly help offset a somewhat weaker power supply as well?

yes, power supply capacitance can make a big difference in the power amp's power supply ONLY if the power supply is designed to take advantage of the larger capacitance amount.
Huge amounts of capacitance cannot hide the flaws in a weaker power supply. In fact, large capacitance will quickly bring a weaker power supply to its knees as it'll expose its weakness quickly.
hope that this sheds some light on the matter....

Thank you for a marvelous reply! This explains quite a lot to me, especially using a combo of larger and smaller caps for nimbleness as well as keeping a large reserve. I forgot that I had seen this before and wondered why.

I still have some questions though. How does the amp draw power from the PS and caps? In other words, are the caps charged then remain as a reserve while the PS powers the amplification? Then the caps are simply drawn upon during the difficult moments that exceed the PS capabilities? OR is it something more like a cycle of the PS charging the caps and the caps supplying the power for amplification. As such, as power is pulled from the caps it is replaced by the PS and during the intense moments the amp can use as much power as is available in the caps while the PS recharges the caps? Or am I completely off?

I am also wondering why my idea for "bonzo" caps would be harmful for the PS, but I have an idea or two why this might be the case. I had assumed that extra large caps would allow a modest PS to deal nicely with heavy transients. OTOH, having expended that charge, this modest PS would have to recharge my "bonzo" caps AND continue to supply amplification power. This is why it would cause damage isn't it? It would essentially push the PS to it's limits far too often?

I'm an engineering newb (actually, newb would be generous) but I've been trying to learn more lately, so I apologize if some of these questions seem remarkably stupid. Of course, it is nice being 'remarkable'. ;)

Thanks, Aaron

Aewhistory, Look at this chart: http://www.powerint.com/en/community/papers-circuit-ideas-puzzlers/circuit-ideas/careful-rectifier-diode-choice-simplifies-and-

Top graph shows amp's supply voltage. Capacitor is charged from transformer thru rectifier only in short moments of time when voltage goes up (bottom graph is charging current). Picture is greatly exaggerated - in reality line representing voltage is almost straight (very small ripple) and capacitor charging happens in very short high current "spikes". Amplifier current demand from capacitors might be constant (class A) or vary a lot with the music (class AB). Very large electrolytic capacitors are characterized by capacitance (opposition to change in voltage), inductance (opposition to change in current) and ESR (effective series resistance) that represents pure resistance. Obviously we want a lot of capacitance to store energy but we don't want inductance since it is opposing rapid current changes. Best solution to lower inductance would be to use less inductive capacitors (expensive) or to use more of small capacitors in parallel (capacitance increases, inductance decreases, ESR decreases).

Bombaywalla mentioned two problems with a lot of capacitance - rush current and over-stressing power supply. Initial current will be higher and last longer to charge larger capacitance resulting in blown fuse or damaged rectifier. Rush current could be limited by soft start circuit - basically a temporary current limiter but amplifier has to be designed for that.
Second part is a little more difficult to explain. Imagine perfect capacitor with a lot of capacitance, no inductance and no ESR. What will be the shape of the voltage on the upper graph? - almost straight line with very, very small ripple. Charging time of capacitors will now be very short (only when voltage goes up) while charging current spikes will have higher amplitude (to deliver same average power) limited only by transformer and power line. This large current spikes might damage rectifier or overheat transformer. Again, it is a little more complicated with transformer since average amp's power is technically the same. The problem is that core of transformer will be heated with high frequency component (iron losses) of narrow spikes, while copper windings will be heated (copper losses) more since, in spite of the same average value, RMS value of current (representing heat) will be much higher.
There is also possibility that ripple current (charging current) peaks might now be too high for caps you selected. Anything can be done (carefully), but linear power supply is not that simple to design properly.