No, I mean fan for power amp sections.
Got it.
With class AB amps, unless you have a high quiescent current or an output power capability of over 50W, power dissipation in the output transistors and drivers is seldom a problem, and certainly would not require a fan. Class A amps are a different matter. In fact, by good heat sink design you can dissipate surprisingly large amounts of power without a fan, but the downside is that the heat sinks tends to be large. In general many thin fins are better than a few thick fins. If you need an illustration take a look at the cylinder head and barrel of a typical air cooled aero engine:
With the heat sink running along the entire width of the case and vertical fins, as in my suggested case design on the Transistor Equivalent thread, heat sinking will be good, especially with the NP aluminum casting technique.
Sounds good, but I really would need to see an outline design to comment. One thing that I did not mention before is the question of OV points: you need two OV points: one for signal and one for screening. The way that they should interconnect is difficult to describe, but the screening OV is much less critical than the signal OV.
I have built amps and preamps with both steel and aluminum cases and the most difficult, from an induced hum point of view, has been steel. This is because steel is a magnetic material and it can induce unwanted currents. I would advise that the preamp be built exactly the same as the power amp, except much smaller and, of course the back panel can be plain sheet, rather than finned. Even if you have a steel back panel you can still bolt the preamp stabilised PSU dissipating elements to it to get adequate heat sinking.
You mention soldering, and I assume you mean silver soldering rather than soft soldering. Well, that is a good construction technique so long as you look out for distortion due to heat. Another good approach is to use fibre glass or epoxy resin to form joints, making sure that you have positive bonding between panels of course.
I once made a multi-meter case from fibre glass sheet bonded together with fibre glass resin and mat. Once it had been sanded down and given a coat of epoxy paint, it really looked the part- shame I didn't finish the electronics.