Designing & building a 70W watt amp is not that easy... You are probably going to fry a few components and maybe even fry a speaker. About a million years ago, I built a preamp. Before the design was finalized & debugged, it oscillated at an RF frequency (which of course I couldn't hear), and it fried my 120W power amp!
Building an amp with an Arduino is probably a great way to learn about class D amplifiers, but I'd start with lower wattage... And If you want to build a good-sounding, reliable 70 W (or more) amp, you are better off using an amplifier chip and the chip-manufacture's recommended design.
It's virtually impossible to build a 70W class A amp, although some crazy audiophile may have done it. 
The most common design is class A/B, where there are separate transistors (or MOSFETs) for the positive & negative half-cycles (the class B part), and a small bias current (the class A part) and usually negative feedback, so that you don't get a "glitch" when the AC signal transistions between positive & negative.