Class D Design needs your Advice

[alijoorabchi]

Hello everyone
i am majoring in electrical engineering. My senior design project is to build a Class D audio amplifier. It should be all discrete and should provide 50W into 8 ohms. I have been working on it for a long time and have used a lot of information’s from you guys in this forum. I’m planning to graduate next month and the deadline for my project is in about two month. I want to keep it simple and am not looking for perfection.
hot
I have attaché a schematic of my design with simulation results which is done using NI’s Multisim workbench.

Circuit description:
The comparator is inside the black lines in the picture.
The input to Q3 is the triangle wave with frequency of 780 KHz. I’m using a XO type Crystal Oscillator which gives me a rectangular wave with 50% duty cycle to create the triangle wave.
i have build the circuit once. It kind of works but with a lot of noise and the quality is deferent depending on the type of music. As I mentioned before I don’t want it to be perfect, I just want to get it to the point which I can get my degree.
PLEASE give me some suggestions on how to make it better.
attention
Note that I shouldn’t be using any IC’s or OpAmps.

Sincerely
Ali

[alijoorabchi]

This is the simulation results
Yellow is the output
Red is the input at 20KHz

[alijoorabchi]

This is the spectrum simulation

[v-bro]

You casn perhaps mail 'classDfromrussia' on the forum, or perhaps he''ll participate here later. He builds very nice discrete D amps....

[v-bro]

Here:http://www.diyaudio.com/forums/membe...eadid=91773&s=

[Eva]

Excuse me, but I think that your circuit is working in linear mode rather than in class D. Check the voltage waveform at the switching node before the output filter.

[luka]

Hi

Can you send me mtisim file, I'd like to have a look.

luka_kurincic@t-2.net