Just when you think the design is as good as it’s going to get and can’t progress much, some small part (relatively) changes and the ripple effect is felt right through the design:
The PSU has moved to a separate block and the batteries are now within the amp box. This does mean that being able to purchase the amp with mains power only is no longer viable so there will be no chance to do that and then upgrade to batteries; it will be batteries with mains right from the start.
The Amplifier Board may now be mounted vertically to reduce signal path lengths and ease construction. This does mean the switchwork needs changing or at least re-thinking since the pins are now at right-angles to where they should be.
Removing the PSU affects the shape of the casework and a more symmetrical design is now possible. The circuitry is not all squashed up against the far end to get away from all those horrible toroidal transformer magnetic fields.
Do I go for a flat, wide box with batteries side by side or I could double them up, one set above the other giving a half-the-width box but double the height? I also have to think about how the control extension shafts get through the batteries.
With the new chip being used (LM4702) there is the provision for a Mute switch. Since I want to have a switch to reduce the gain of the pre-amp stage to cope with driving headphones I can now use this switch in conjunction with a switched attenuator for the volume control instead of the Alps Blue potentiometer I was going to use. Switched attenuators will always be better sounding than a variable resistor.
Using the LM4702 has had the effect of upping the power amp rails from +/- 24V to +/-36V. So I now need to up the rating of the heatsinks. Also they can now go on the back face of the amp which means I do not have to cut holes in the casework to allow hot air to escape.
So changing the design to use the LM4702 means a redesign of the casework, the heatsinks, the switchwork/controls, PSU, batteries, umbilical cord connections …
You might expect all this change of design to be upsetting to me, in fact I am in nerds heaven. It is just the sort of problem I love working on!