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Class D Switching Power Amplifiers and Power D/A conversion

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Old 2nd February 2013, 12:48 AM   #31
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Yeah, I'm trying my best :/ I'm not too good with analog circuits. I'm more comfortable in the digital level since practically all my classes have been digital for my comp eng. major (hence why the professor gave me an analog individual senior design project).

I've taken all your input into consideration and here's my updated schematic (attached.) R47 since R5 is 18.2k should be roughly 180k but for some reason it just doesn't sound as clean as 100k. Changing it to 47k also had more distortion as well X_x.

It's really weird though because at 5V it plays fine despite the lack of some bass and weak volume. BUT once I wrap my fingers on the top of C24 the audio gets louder AND I get a much cleaner bass. I felt like I had magic hands hehe. Though I'm not sure how I can keep that bass and louder audio without touching it. That's my next goal.

Here's a video of what's happening HERE
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Old 2nd February 2013, 10:32 PM   #32
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The low volume and poor bass is fitting to your input gain stage values.
In simplified view the gain of the input stage is defined by the impedances of R77 and C24. C24=380pF has a high impedance at audio frequencies and furtheron shows a 1/f impedance, which increases the gain of the input stage towards higher frequencies . When touching a 380pF the way you did, your fingers form a parallel impedance to C24 and change the transfer function of the gain stage.
Try to substitute C24 by a series connection of a cap and resistor.
Reasonable starting points would be 1uF (towards the signal generator) and 10k (towards the base of Q77).

R47 is already pretty high with 100k. Going to 180k results in weak driving signals for your gate drive circuit.
The ratio of 100k/18k should be OK.
You could try to step down both in small steps and search fro an optimum.

Last edited by ChocoHolic; 2nd February 2013 at 10:36 PM. Reason: typos and messed eplanation....
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Old 8th February 2013, 07:08 PM   #33
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This is fantastic!

Thanks so much Chocoholic! You've been incredible help and I don't think I would be where I am now if not for your help!

It's sounding as good as I can probably make it right now and I'm off to step two and soldering it on a board. Here's a video to show where it is now (here).

Thanks so much!
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Old 8th February 2013, 10:15 PM   #34
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Great!
....glad to see that you made the race.
Wishing you good luck with your prof.
Asking a digital native who is studying computer electronics to design and build a discrete classD amp, that's a tough project.
And it was positively brave from you to chose such an alien project.
It is always good to see and learn about topics, which are beyond the main stream of your particular discipline.
Enjoy
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Old 9th February 2013, 09:16 PM   #35
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BTW:
Your output filter doesn't filter much.
The amount of HF at your speaker is so huge that it might already be stress for the tweeter.
You have a low switching frequency and a fast filter - not really fortunate.
When changing the filter, take care not to place the filter resonance close to the switching frequency....
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Old 14th February 2013, 07:38 AM   #36
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Ohh that's what's going on? I figured something weird was going on when if I bypass the filter and connect it to the speaker it sounded the same.

D: I actually presented my soldered project to my professor already and he actually was shocked that it sounded pretty clear. He said he's never had a student that turned in something that didn't crackle or popped. It felt great!! Especially since it's not even that great X_x BUT I do still plan on working on it a bit more . I want to make it mobile and perhaps run on 2 9-Volt batteries ( 1 battery for + and - power).

Here's some pictures here of the soldered work. The box just adds to its grundgyness hehe.

I'll see what I can do with the filter as I'm not really sure how to go about that X_x. Slow it down? I recall trying out different caps for the filter and they all sounded relatively the same so I'm guessing I have to figure out a better inductor+cap combo ): . I was trying too hard not to buy a new inductor for convenience :/
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Old 14th February 2013, 05:25 PM   #37
Baldin is offline Baldin  Denmark
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Default Breadboard

Hi darkamikaze
Good to see you are up and going. Just a small trick, when starting up a new design, I always start on a bread board (see picture). It can actually work ok for first test and to make sure the design is ok (don't use high voltages and high currents on the bread board though). This makes it extremely easy to change component values and get the first shot right.
Then I go directly to prototyp on two layer pcb .... it's just so cheap to get manufacturednow, that nothing else makes much sense .... though there is some waiting time though
I have now sifted to ITEADStudio for pcb manufactory, and are waiting on my first batches. (before i used Olimex).
Keep the good work up.
Baldin
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Old 15th February 2013, 02:18 PM   #38
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Finally you made a fashionate retro style casing.

Quote:
Originally Posted by darkamikaze View Post
I'll see what I can do with the filter as I'm not really sure how to go about that X_x. Slow it down?
The filter is not needed for the sound, but to keep the amount of HF at the output low.
To achieve this your LC resonance should be at least 3 times less than the switching frequency. For reasonable audio full range reproduction the LC resonance should be at least 25kHz or higher.
Most likely your switching frequency is to low to allow a properly filtered full range design.
1. Have a look to your switching frequency, what do you find?
2. Calculate the resonance of your filter..., what do you think?
3. Calculate or simulate the transfer function of your filter ...what do think?
Stepping through the points 1..2 and 3 you can easily learn from the search function in this forum and/or google.
It's OK to be a little bit lazzy and ask a lot, but not digging for this on your own is to lazzy

Nevertheless my congratulation that you impressed your prof.
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Old 15th February 2013, 04:12 PM   #39
Baldin is offline Baldin  Denmark
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You don't even have to turn on the pocket calculator
L-C Filter Equations

or

http://www-users.cs.york.ac.uk/~fisher/lcfilter/

.... but you of course need to calculate for different values of RL if you want to use the amp for different speakers .... and donīt want to much peaking in the transfer function
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Last edited by Baldin; 15th February 2013 at 04:16 PM.
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Old 15th February 2013, 07:29 PM   #40
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Hello. Is the demonstrator board which is attendant to UM10155 [ UcD by Philips] still available for sale? Please advise. Thank you.
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