This was taken from Zed Audio's manual.
"Stiffening capacitors 
These are of NO use with our amplifiers due to the fact that the amplifiers have fully regulated power supplies. The power supplies will compensate for small volt drops which exist on the +12v power cable. The amount of current drawn by a particular amplifier would drain a fully charged 1 Farad capacitor almost instantly. Consider the theory. Energy (Joules) = Power(Watts) x Time(Seconds). The energy in a 1 Farad capacitor = 0.5CV.V= 0.5x1x12x12= 72 Joules. Let us assume a medium size amplifier such as Draconia. Let us assume that we are playing it such that all four channels (into 4 ohms each) are just clipping on the loudest musical peaks. This means that we are delivering 300 watts on peaks. The amplifier's average efficiency is about 45. The peak to average power ratio is about 20 so average power is 20 of 300 = 60 watts. The input power is therefore 133 watts. If the 1 Farad capacitor was charged to 12 volt and we remove the main source of power  the battery, the amp would remain playing for 0.54 seconds! (Put the numbers in the formula (E=PxT above and solve for time T). Now compare this to the battery. The amplifier will play for some hours (depends on actual battery of course) as compared to 0.54 seconds! So what good is a 1 Farad capacitor? Assign the above example to a Megalith playing into a 1 ohm load. Going through the mathematics the 1 Farad capacitor would keep the amplifier playing for 0.14 seconds. So if the voltage at the amplifier's power terminal dropped by say 1 volt the amplifier's power supply will correct for this on a continuous basis. The stiffening capacitor however has nowhere close to the energy reserve to compensate for a 1 volt drop. The main internal power supply capacitors hold 100 Joules of energy and these are more effective than any stiffening capacitor since they do not have to contend with the inefficiencies of the amplifier's power supply."
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