Hello, yesterday i finished building my first kit (effect pedal) and i have 2 serious unanswered questions.
I have attach the pdf of the schematic.
Question 1) What's the use of the C6(100μ) at the circuit and what is it doing?
Question 2) (I think that the D1 it's a protection diode, is it true?) and if it is, is it gonna work that way? (i think that if it's a protection diode it will need a resistance to work properly).

Thank you!

 

Question 1) What's the use of the C6(100μ) at the circuit and what is it doing?

C6 provides local power to the IC. It might be superfluous but is cheap insurance against local noise on the power pins of the IC.

Question 2) (I think that the D1 it's a protection diode, is it true?) and if it is, is it gonna work that way? (i think that if it's a protection diode it will need a resistance to work properly).

It's a bit like a one-way fuse to protect against reverse polarity hookup. It's rated to 1A though, and that might absorb all the 9V battery or adapter can dish out. For a little while, at least. It would make more sense to me to put it in series with the power. The downside would be the diode voltage drop, but it would be better protection.

 

C6 is a supply bypass capacitor that is used on all electronic circuits.
D1 is called an "idiot diode". It shorts the AC/DC adapter (might cause a fire) if an (you-know-what) connects it backwards. Usually a much safer series diode is used that prevents the circuit and adapter from being damaged if the adapter is connected backwards.

The circuit is wrong, designed by an (you know what). An LM386 is a Power Amplifier. Instead the circuit should use a dual opamp.
R2 and R4 cut all high audio frequencies producing a very muffled sound.

 

IIt would make more sense to me to put it in series with the power. The downside would be the diode voltage drop, but it would be better protection.

You can replace the series diode with a P-MOSFET (source to V+, gate to ground, drain to circuit) for reverse polarity protection with very little forward voltage drop (just the MOSFET on-resistance times the circuit current).

 

A diode voltage drop is nothing when the circuit is powered from 9V. Besides, the circuit is a guitar distortion pedal and will produce even more distortion with the diode voltage drop.