the K8004 is a good kit if you want to begin to understand the basics of PWM control -- you can also use PWM for myriad applications BUT...
if you are driving a turntable platter PWM is probably NOT the way to go -- while you can tightly control the pulse width through the error amplifier, you are still pulsing the motor so the speed is always accelerating and decelerating a bit -- the integration is provided by mass and velocity of the turntable platter !
the SG3524 or SG3524, coupled with an H-Bridge chip like the LMD18200 or LMD18201 (or equivalent chips from TI and others) are great devices for robotics. http://www.tech-diy.com/jobo.htm
Somewhere on Teres site you can find a circuit of Manfred Huber's supply. It is a CCS fed shunt reg with a microcontroller providing some non-essential "cosmetic" functions. A pot can easily replace the MC and remove some potential annoyances in the process.
The 317 circuit is just an implementation of the data sheet.