Since I am creating Kicad footprints for components I need to know how I am going to mount the output transistors. That means making a decision about which side of the PCB they are soldered to (which side all components go on for that matter) and how they are connected to the PCB.
The grey and maroon rectangle at top left is going to be the heatsink – I have decided on the larger heatsink so both maroon and grey is the area it will take up. The lighter grey squares to the right are the switches and the green thin rectangle above shows the PCB. This is all a birds-eye-view.
I have decided to attach the output transistors so they lay flat on the PCB in order not to use plated through holes which is the normal way to mount components and will be the way for all other components. This will improve sound quality a tiny amount as PTHs give a small contact area and use metals other than good quality conductors. The use of a long strip of silver plated track will make a much better electrical connection and should not present any problem if the transistors need changing for any reason.
To do this I will need to bring the PCB forward in the box (downward in the drawing) so that the most forward edge of the PCB (green area) aligns with the most forward edge of the heatsinks. But you can see that the switches will then foul the batteries. But the batteries are 66mm wide and the heatsinks 200mm wide. 3 x 66 = 198mm. If I move the batteries together and leave 1mm between then they take up 200mm. 1mm is not enough to mount them the way I was going to using tie-wraps so I have to think of another way of mounting them and keeping them in place if the whole thing is dropped in transit (most likely with any postal system). The mounting needs to take into account the rather high weight of the batteries (so they will have a lot of inertia in a drop) and the fact I don’t want to use metal any more than absolutely necessary (to reduce magnetic field coupling). My design for the mounting now rests with the design of the casework since I will need the case to hold the batteries down in some way.
In short; in order to work out the mounting footprint of the output transistors, I need to first work out how the case will work and be manufactured. One more minor move in this giant game of chess.