- What C2 condenser stands for? What is doing in the circuit? Is that value correct for my purpose?
It filters the DC out of the signal. DC is zero Hertz, and this capacitor (along with the load resistance/impedance) actually forms a high-pass filter. 10uF is large enough that it should pass all audio frequencies with any reasonable load resistance.
- I was also looking to how change R value with a pot but I found out that with lesser R value in RC circuit, Freq.Cut value is highier. Which circuit invert the behaviour of resistances?
You can't reverse the basic physics.
All material you can suggest me on web is welcome, also about designing more complex filters (2/4 poles - with adjustable resonance / analog filters driven by pwm arduino signals etc. etc.)
You'll need an active filter (with an op-amp). In the old days, everybody used the Active Filter Cookbook
, but I assume that information is available online now. Or, you can use a switched capacitor filter which requires a special filter chip.
Realistically, if you are going to add a chip you might consider getting a proper (PCM) DAC, instead of using PWM. You Should be able to fine a DAC with a built-n filter, but it will probably surface mount which makes it more difficult for hobbyist use.