1) Big. It depends on how hot you are willing to let the output devices get.
2) My Aleph 2s are Aleph 60s on their mother's side.
3) Nelson has tended to favor the 2 in the old series and both the 30 & 60 in the new. Although perhaps he will give a breakdown as to which has better highs, lows, image, whatever. I'll be experimenting with a number of variations in the near future...
There are two factors to consider about heatsink size. The first is how hot do you allow the transistors to get and the second is how hot do you allow the heatsinks to become.
The first is most important, the transistors temperature should be kept as low as possible for best operating life. The main bottle-neck of getting the heat out from the transisor is the insulating washer between package and heatsink. Using power devices in parallel rather than one (for the same total power) is a good solution as then the temperature rise from heatsink to each transistor is halved. Also note that the thermal compound used to coat the insulators is not a good thermal conductor 0 but it is better than air. It's only job is to fill imperfections between transistor - washer - heatsink to keep air out. So apply very sparingly not liberally.
To stop heatsinks getting too hot to the touch then there is no recourse but size (or don't touch them). However, if you don't get the first point right you could have nice cool heatsinks but extremely hot transistors. Getting the heat into the heatsink will make them feel hotter but is actually much better for the transistors (and then make the sink as big as you like).
Also keep in mind that extruded heatsinks used for natural convection (no fan) become more inefficient as they get taller (fins vertical). This is because the air flowing up to the top of the sink is already heated by the bottom of the sink and the top cannot transfer as much heat to the air. For example, to halve the thermal resistance of a 3" extrusion you should make it 12" long whereas you could take two 3" extrusions side by side for the same effect. Having many shorter heatsinks fits well with having many output devices.
Have a look at www.wakefield.com for all the details. click on Services -> Thermal Tutorial.
how hot is hot?
how hot runs comercial aleph 60. i think 25 c more like ambient.
is my calcuation right?
25 c hoter heatsink like ambient
irf 640 (150w, junction to case 1.0 c/w, case to sinc 0.50 c/w)
two heatsinks per chanel
25c/155w=0.16 0,32 per heatsink
each output device disapate 13w
janction to case + case to sink = 1.5 c/w
1,5 c/w x 13w =19,5 c
this is about 70 c junction temperature
is this good?