What is D6 and R17 for? What are C11 and C12 for? I assume these parts are for protection and or noise suppression. Just want to understand power. Thanks, Steve
D6 and R17 are there diode for polarity prtection and r17 for filtering
A more thorough explanation since you wanted to understand power sections:
Diodes conduct in only one direction. The 1N5817 allows DC to pass if positive DC appears on the anode side of the diode, which is connected to the outer ring of the power supply plug via the DC jack. If negative DC appears on the anode (if you plug a center positive DC adapter in), the diode will not conduct, and the 9v connection will be for all intents and purposes severed.
R17 places a small amount of resistance in series with the power supply to form a low-pass filter with C11 and C12. Switching power supplies create a steady high-pitched noise that can, in some circumstances, become audible, plus some lower pitched hum. Your power supply usually has its own output impedance, which could make R17 redundant, but R17 is more or less there to ensure consistency, and it won't be much additional resistance if the power supply has much of its own. C12 is a film cap because film caps are better at filtering high frequencies.