As promised, here is the initial design.
Schematic
It's basically the exact same thing that can be found on page 7 of the
LT1033 datasheet, where it is described as a "dual tracking 3A supply."
Components
C1-C4 are 2200uF electrolytics.
R1,R2 are 1.3ohms rated for 5W
C5,C6 are typically 10uF tantalums, but I have 100uF low ESR electrolytics on hand
R3, R4 are 12k ohms
R5, R6 are 120 ohms
R7 is a 10-turn 10k ohm pot.
C7, C8 are 220uf 100mOhm ESR electolytics
Justification
The choices for C5,C6,C7, and C8 are based on the following research:
http://www.tnt-audio.com/clinica/regulators2_impedance1_e.html
C1-C4, R1 and R2 form a CRC filter with a corner frequency around 60Hz to filter the power before it even hits the regulators. This is somewhat overkill and requires large resistors, but I had the space.
I don't bypass the LM317 or LM337 with a small ceramic capacitor as the indication for those is being more than 3 inches from the input filter capacitors.
The other resistor values are simply necessary to have a board which accepts a 10k pot (a common value) and uses its entire range to vary from 1.25v to 37V. I should make a better writeup of selecting these values as it is somewhat tricky and results in a slightly non-linear voltage curve as you turn the pot.
Board Layout
I kind of just trusted gEDA PCB when I imported my schematic to get a set of ratlines. I can't help but feel like like either D5 or D8 is oriented in the wrong direction. Some thought needs to go here. I also haven't really taken advantage of my ability to make really thick traces for the power lines. They are currently 85mil and the signal lines are something like 40mil.
One thing that's difficulty to understand from this (since it will be self-etched) is the grounding scheme. The board has copper on both sides, and what we're looking at is the bottom layer. I intend to use the top as a ground plane, and the 5 vias you on the midline of the board are places where the ground plane attaches to the bottom layer. The other two vias at the extrema of the midline are actually just holes for terminal blocks. The ground plane will be tied to the polygon fill and the chassis in one spot where I will put a standoff. I haven't decided the best place for this.
I'm really looking for some advice about my grounding scheme. I've read loads about the subject here and on diyaudio but I just see a number of conflicting ideas.
Thanks,
-Kris