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Class D Switching Power Amplifiers and Power D/A conversion

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Old 21st January 2005, 01:31 PM   #21
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Charles,


These are the values i am currently using.. I will refer to the schematic for reference as to which parts i am talking about.

The intergrator Cap is a 681 ceramic

R28 ( feedback resistor) 5k6. ( this is probably too small) ??
R21,R27 - 4k7
R34 - 330k
R20 - 6k8
C38 - 121 Ceramic


Anyway any thought / suggestions as to better values would be great.

Regards

Peter.
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Old 21st January 2005, 03:44 PM   #22
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If your designations are still the same as on the schematic of your 2nd post then R28 (parallel to integrator C) is much too low since you have almost no NFB that way. A value around 100k would be a good value to start with.
For the integrator cap I'd use 330pF. I would leave the rest as is if you really want that huge gain of 70.
The above calculations are valid for carrier-based operation using a 2Vpp triangle of 250 kHz and feedback takeoff before the output filter. I'd try it that way first and then change the NFB network for post-filter takeoff.

Regards

Charles



Pierre, how is your one working BTW ?
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Old 21st January 2005, 09:30 PM   #23
Pierre is offline Pierre  France
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Thanks, Charles.
I will do some experiments tomorrow, mainly adding the series resistor with the zero and increasing the integrator resistor. I will post what I find.

Best regards
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Old 21st January 2005, 10:00 PM   #24
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Thanks Charles,

I will make those changes and I will let you know how it goes.

Regards

Peter
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Old 22nd January 2005, 08:48 AM   #25
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Charles,


I have made those changes. It is now running sooooo well... I havn't yet run it though RMAA. But the sound quality is fantastic. Bass, Mid and trebble resonse are REALLY good.

Even when the amplifier is clipping it still sounds really good. I will fix up my dummy load and try and get some measurements with RMAA at near full power into 4 and 2 ohms.

Thank you for helping me, It has made such a difference.

Regards

Peter
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Old 22nd January 2005, 11:22 AM   #26
Pierre is offline Pierre  France
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Congratulations if you are happy with the results. We hope to see some RMAA results. Is the feedback taken now BEFORE filter?
If you have read the thread I started, I was very happy also with this mode, but wanted to have a load-independent freq. response, so I started to think about POST-filter feedback.

I am going to post some more info in this thread today.

Best regards
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Old 22nd January 2005, 12:15 PM   #27
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Thank you. I am still trying to determine why i am getting such A high thd & imd reading in RMAA, with only just a loop back cable from speaker out to line in.. Until i sort that out my readings will all be thrown off...

Yes I am now taking my feedback from before the output filter. I may attempt to move it later by setting up a PID as charles has suggested.

However I did my first test run at 2 ohms (5 minutes) and all is stable. Although my Fets are starting to get a bit toasty. I think i need lower values of gate resistors. ( currently 8 ohms).

I measured the power output while running just at clipping into 2 ohms on that run, I managed to 1650Watts RMS. I am really happy with this, Considering how small the circuit is and how little heatsinking there really is.


I will post RMAA results soon..

Regards

Peter.
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Old 22nd January 2005, 12:42 PM   #28
Pierre is offline Pierre  France
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What mosfets are you using?
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Old 22nd January 2005, 04:21 PM   #29
Pierre is offline Pierre  France
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In order to get valid results with RMAA, you must ensure you get the maximum from your soundcard in loopback mode. Use a cable that you know is good.
RMAA is very tricky with the levels, if you don't get the right one, you will get too high distortion or the program will warn you telling that it has too low signal. Use the "Adjust I/O levels" button, and select "loopback test". In the version I have (v5.3), it then shows a spectrum analyzer and level bars. Open the Windows volume controls and adjust both line in and wave out / master out so you get the correct level.

Have in mind that RMAA allows you to change the test frequency under the "Test Options -> test signals" menu. Start with 1 KHz and sort things right with your soundcard alone.

When you insert the amplifier, be sure to have a voltage divider / potentiometer so you can get the best signal. You will have to play with the levels also to get the optimum noise level.

I will do some measurements next week with my amp, also, so if I find something useful I will post it here or the other thread.

Have luck and keep us posted, please!

Best regards.
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Old 23rd January 2005, 05:53 AM   #30
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Hello Pierre / phase_accurate,

I have just run some more tests with RMAA.

The IMD and THD have improved tremendously.

Have a look at
http://tesla.reidconsulting.com.au/r...Comparison.htm

At 230Watts RMS the THD is about 0.4% and IMD is about 0.3%. I think these reading should actually be less as the THD of my sound card is already 0.191 %

Pierre, The fets i used were IRFP260N's 50A, 200V HexFets.


Regards

Peter
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