class d amp

Nice circut.I gotta make note of the simulator relay trick

Thanks roops. Yep, useful trick.

the choice of inductor and capacitor depends on corner frequency and resistive load to not ring. Its called critical dampening. 1/2/√C/L=5.67Ω so from dampening its a little missalignet and from corner frrquency its 2kHz which is also not for hifi audio. All in all there is only one result to match both creteria corner freq and dampening for a given load. E.g. for 4Ohm and 40kHz (40kHz is taken to not have to much ohase shift impact at 20kHz) this is 470nF and 30uH is perfect. If you buy class D amplifier on ebay you mostlikely get this tandem LC 470nF and 30uH for 4 Ohm load

if you know the exact PWM modulator frequency (some chips you can programm that with external components) you can improve the filter with an extra notch and filter out the fundamental frequency, here 200kHz as example for PAM8403 check this http://everycircuit.com/circuit/6090651394113536

one more hint, if your triangle is +-1V never go to the maximum in modulation and apply (+-1V audio) go to max +-900mV and leave 10% headroom or you run into the case the modulator does NOT modulate

thanks hurz. Useful suggestions here. I'll look over this stuff in greater detail after work

So I worked out the transfer function and it makes sense given the two capacitors in filter. The troubling part is in the numerator: s^2*C1/(C1+C2) in the numerator will grow along with s^2 + s*x in the denominator. As s-> infinity Vo/Vi = 1 so... Vo = Vi*1... My question is: Wouldn't this filter be better off without the cap in parallel with the inductor? Thanks for your help btw. I am curious what sort of intuition/experience led to you choosing the inductor and capacitor at the output?

as pure low pass it would perform better without this edtra cap for sute, cuz this 21.1nF cap together with 470nF is like a voltage divider and gives independed of frequency and amplitude to its ratio like a resistor divider. But the nice thing is this cap together with the L is a blocking notch for the fundamental frequency 200kHz! The next harmonic is at 600kHz and the next is at 1MHz those are pretty good handeled by the low pass filter cuz the corner frequency is at around 40kHz very low in comparison to these high harmonics. And the harmonics are also 1/3 and 1/5 in its amplitude before filtering, 200kHz is quite close to the corner and with the notch almost all its energy is taken away. Hope this help. With laplace i am very rusty and its tooooo long time ago to calc in s domain, sorry.

Great! That clears it up for me. I appreciate the help. Probably going to order a book on passive filter design. Any suggestions are welcome.

This basic idea has a corner frequency of 2kHz with 900uH and 7uF, by far not HiFi. Dampening is critical (which means perfect) @ 22.6 Ω. With 8Ω its overdamped, maybe better then underdamped, but far away form what is commonly used in todays class D amplifiers. Realy just a basic idea nothing you can take from values to build a working filter in real life. Sorry @crake, im sure it sound nasty for you but actually i dont care. Hope you understand.