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Class D Switching Power Amplifiers and Power D/A conversion

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Old 20th January 2005, 11:02 AM   #11
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Thanks for that Pierre,

I will have a good read of the other thread you just mentioned.

I just ran RMAA with my circuit just to test things. The results don't look too bad considering i havn't got the feedback sorted properly yet..

Freq = 200kHz
Output Rails +/-20v

Here is the link if anyone is intersted.
http://tesla.reidconsulting.com.au/r...Comparison.htm

Regards

Peter
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Old 20th January 2005, 02:29 PM   #12
tiki is offline tiki  Germany
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RMAA is not bad for that purpose, is it?
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Old 21st January 2005, 07:02 AM   #13
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Charles,

I took your advise with regards to the Clipping diodes after the feedback, and hence removed them. I am running a little bit of feedback. ( It is still not set correctly). I reduced the resistance of the gate resistors to about 8 ohms. And used lower voltage, higher current HexFets. 200V, 50A. The clipping is now very clean it you can hardly hear it.

The switching output is very clean even while running from +/-80V. I have increased the Carrier frequency to 250kHz. I was able to obtain a continous power output into a 4 ohm load of just over 1400Watts peak ~ 950Watts RMS. It still sounded really good at this power level.

I even tested it running a 4 ohms || 8 ohms ie 2.8ohms Total. And obtained a power output 1230Watts RMS.. I am very pleased with these results.

I just need to improve the feedback now!

btw thankyou for your help up till now.

Regards

Peter
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Old 21st January 2005, 09:18 AM   #14
Pierre is offline Pierre  France
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I am very pleased someone gets good results with IR2xxx chips!
What changes have you done in the feedback network? Did you read the "Help with feedback" thread and used the ideas exposed there?

Have you performed any RMAA test now? Your THD was way too high (10%!) before in the "loopback test" is it better now? The IMD was also too high. Could you clarify what the conditions were in that measurements?

Good job!

Best regards.
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Old 21st January 2005, 09:22 AM   #15
Pierre is offline Pierre  France
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Sorry, but I have a curiosity, perhaps out of context in this thread:
What is the exact function of the resistors between gate and source in the mosfets?
I have seen that in a lot of designs...

I understand that a zener is good to avoid gate destruction due to excessive Vgs, but what is the resistor for? To increase noise and false trigger inmunity?

Thanks! and let's return to this thread's subject. ;-)
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Old 21st January 2005, 09:27 AM   #16
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Hi Pierre,

I have posted updated results at the same web address.

It appears that my sound card has a pretty high THD when i have got the levels really high ( i need to do this for the loop back test) however my most recent tests show the THD is down to about 1.1% at 2.8 ohms and the IMD is down to 3.8% at 8 ohms. I still need to make lots of improvements. btw these results were with the amplifier running from +/- 80V rails.


At the moment the feedback is really hardly doing anything, I have read though some of the "Help with feedback" thread but I am yet to finish reading it.

By the way those tests are all loop back.. The one i labeled loop back was the output of the soundcard connected back to the input of the soundcard (line in). The others have the amplifier inbetween. The output of the amplifier is divided down using a resistive network....


Regards

Peter
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Old 21st January 2005, 11:01 AM   #17
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Quote:
It appears that my sound card has a pretty high THD when i have got the levels really high ( i need to do this for the loop back test) however my most recent tests show the THD is down to about 1.1% at 2.8 ohms and the IMD is down to 3.8% at 8 ohms. I still need to make lots of improvements. btw these results were with the amplifier running from +/- 80V rails.
One important thing to watch out for: Some soundcards will have problems with the switching residual off class-da amps and will therefore show more THD than you actually have.
Have you considered a passive lowpass between your amp and the soundcard's input ?

Some info on measuring class-d amps:

http://www.aes.org/sections/la/Meeti...Amplifiers.ppt

Regards

Charles
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Old 21st January 2005, 11:47 AM   #18
Pierre is offline Pierre  France
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Yes, I also read that tip.
It is curious, however, because there is a guy in "Ucd180 q&a" thread that has measured a UcD-180 without lowpass filter (and a crappy power resistor) and gets good results (even when UcD leaves lots of carrier residue!) But who knows what the inputs of his soundcard are like, anyway...


Charles, do you know what the resistors between gate and source are for? Thanks!

Best regards
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Old 21st January 2005, 12:56 PM   #19
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Charles, do you know what the resistors between gate and source are for? Thanks!
They are there to ensure that no gate charge can stay or be built up if control by the IR2110 is lost. This could happen if the supply voltage for the output stage is still there but the supply of the IR2110 failed. But you don't see them often. In the given dimensioning the pull almost as much current as the driver's static current consumption is.

Regards

Charles
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Old 21st January 2005, 01:03 PM   #20
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I have increased the Carrier frequency to 250kHz.
How about decreasing the value of the integartor's cap, or have you done that already?

Regards

Charles
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