Don’t let anyone ever tell you it’s easy.
In laying out the Kicad component footprints I have started to look at this area again – the phono socket earth ring, how does that connect to the PCB? There are so many ways earthing / grounding can be done and some of those ways are disastrous while many are adequate but not great.
From the circuit point of view, there are the grounds for the de-coupling capacitors, the grounds for inputs, grounds for the feedback circuits, grounds for the output transistors (the black wire from the loudspeakers). There are the left and right channels to take into account – note that the Jack I/P and O/P sockets only have one ground available! Plus there are the battery earth points (one each for left / right channels, one each for pre-amp and power-amp so four in total), then there are the power supply transformer grounds and main earth needs to connect to all of them at some point.
It is common practise to take grounds back to a Star Point. However, this is only part of the story. There needs to be more than one star point and these star points need taking to more common star points and these points go back to the centre of the battery packs which is the earth point between the batteries. But the mains earth must not be connected to this final star point or you get a lot of ‘hum’. Plus there is the additional mains earth that is created when you connect another piece of equipment, such as a signal source, which only too often creates a lot of hum and noise. This should not happen.
Normally this is all relatively straight forward but it fries my head when you drop into the mix the fact that I have two power sources – the batteries have their centre points which act as circuit earth but the power supply unit (which can be in parallel with the batteries) has its own earthing point which is returned to the mains earth / ground.
This area HAS to be got right. Time for another good long think to make sure I have …