Discrete Class D Amp For Fun

Thread Starter

iimagine

Joined Dec 20, 2010
511
Assuming that my 7 transistors triangle wave generator works as expected (As I am still waiting for 2N2369's and 2N5771's to arrive), I need your help on completing this amp for fun and educational purpose because I am not quite sure if this is right

Thanks in advance
 

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tindel

Joined Sep 16, 2012
936
Full disclosure: Audio is not my forte...

A few points...

Most amplifiers require feedback to remain stable... it looks like your gain at 20kHz is only about 2.7... so maybe it doesn't matter much with this small of gain.

It appears that your input will have 1.4V DC offset... how do you plan on achieving this?

You have a a DC offset on your speaker... making the average output power be 1W @ 20kHz... I'm not sure that most speakers can handle that.
 

Thread Starter

iimagine

Joined Dec 20, 2010
511
Full disclosure: Audio is not my forte...

A few points...
It appears that your input will have 1.4V DC offset... how do you plan on achieving this?

I have completely forgot about this :D, thanks for pointing it out. This can be solve by adding a cap and two resistors. I have updated my circuit to include this.

You have a a DC offset on your speaker... making the average output power be 1W @ 20kHz... I'm not sure that most speakers can handle that.

I need help with this, I'm not understanding at all, can you please elaborate? Is there any reading material on this matter you can point me to?

Thank you for helping
 

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#12

Joined Nov 30, 2010
18,224
It's simpler than it sounds. Speakers don't like DC because it displaces the cone and heats the windings. A 60 watt speaker might not care, but a 3 watt speaker would suffer.
 

RichardO

Joined May 4, 2013
2,270
I ran the Spice simulation and noticed that the drive voltage to the FET's does not go to ground. I have not analyzed why this is happening.

Also, the circuit would be more efficient if the FET's were run common source instead of as source followers. This does, however, need different FET's than the ones you have in the simulation. For this to work, the drive has to go close to both the power rail and ground and the FET gate thresholds must be around 1 to 2 volts.

One final comment. You may want to get rid of the large coupling cap to the speaker. You can do this by driving the ends of the speaker 180 degrees out of phase using 2 output stages. This is commonly called a bridge amp drive. An added benefit of the bridge amp is you can supply 4 times the power to the speaker with the same power supply voltage.
 
Or using two supplies with a midpoint centertap, though you loose the automatic dc protection you get from the output cap.

Also the reason your circuit doesent pull all way to ground is that it is more or less just your average class b amplifier circuit, wheres what you want is a comparator.

Look at the philips UcD class d circuit for inspiration.
 

Thread Starter

iimagine

Joined Dec 20, 2010
511
Also the reason your circuit doesent pull all way to ground is that it is more or less just your average class b amplifier circuit, wheres what you want is a comparator.

Look at the philips UcD class d circuit for inspiration.
I have been looking at the Phillips UcD circuit but could not get anything out of it. Its a little too complicated for me :confused:

Could you provide me with a comparator circuit for this please? Thanks
 

Thread Starter

iimagine

Joined Dec 20, 2010
511
I managed to drive the gate at ~0.4-5v. The output is much better now, although I dont understand how it can be slightly above 5V. Please advice

Thanks :)

I'm working on driving the ends of the speaker 180 degrees out of phase using 2 output stages as richardO suggested
 

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I built a little bridged class d amp once using two UC3710 igbt gate driver ics and a LM311 comparator ic.

These gate driver ic's in TO220-5 package makes nice output stages for a low power class d.

As for a more practical class d amp, i built this: https://www.youtube.com/watch?v=asC3zPaB9mY

Uses a MAX913 comparator, a IR2010 halfbridge gate driver and IRF540 mosfets for the output stage.
 

Thread Starter

iimagine

Joined Dec 20, 2010
511
I built a little bridged class d amp once using two UC3710 igbt gate driver ics and a LM311 comparator ic.

These gate driver ic's in TO220-5 package makes nice output stages for a low power class d.

As for a more practical class d amp, i built this: https://www.youtube.com/watch?v=asC3zPaB9mY

Uses a MAX913 comparator, a IR2010 halfbridge gate driver and IRF540 mosfets for the output stage.
Very nice! Can you share those schematics? I would like to build one to study
 

Thread Starter

iimagine

Joined Dec 20, 2010
511
Solved that spiky ripple problem! :D

I'm only doing this to study, learn and to see if there is any other approach, alternative way of doing things :). Any one have any ideas or suggestions please share. Although I do not think many here cares about this little project of mine too much too bother, but its alright I can always turn to Experimentonomen, RichardO and #12 for help :D.... I think
 

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