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Old 19th August 2010, 03:43 PM   #371
opor is offline opor  Thailand
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Hi,
I am back. With some modification at input and feedback stage. I found that noise is less than before and it may normal level. Now I have plan to use 4 of IRF640 in parallel. Any comments are welcome !! . I do not know is it work or not.
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Old 19th August 2010, 05:11 PM   #372
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I replaced the 220pF caps with 150pF and now i get 3.7µS per period wgich should be around 250-350kHz.

Amp is sounding good, i like it and the carrier resudial on trhe output is about a volt and its squeeky clean, cant even tell its switching.
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Old 19th August 2010, 06:05 PM   #373
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This looks like a nice project, and I'm tempted to try it myself. One thing about the single and double sided PCB's, is somehow possible to make double sided PCB's at home using the toner transfer method by simply sandwiching together 2 single sided PCB's back to back? I guess you'd have to make sure everything was soldered on both sides, but this may allow a more noise free design if it works? Or is there some reason why this would not work?
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Old 19th August 2010, 06:42 PM   #374
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The easiest way to make double sided is using the press n peel film and copperclad withj copper on both sides.

I've upped the switching frequency to ~312kHz now, using 100pF caps in the phase lead network.
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Old 19th August 2010, 09:12 PM   #375
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This is the schematic of whats currently playing music on my bench:
Click the image to open in full size.
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Old 19th August 2010, 09:44 PM   #376
Ryssen is offline Ryssen  Sweden
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What pcb layout did you use,and is it stable with the above schema?
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Old 19th August 2010, 10:08 PM   #377
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It's similar to this layout:
Click the image to open in full size.

And its rock solid, the carrier resudial is so clean that one could think it was the output of a function generator, not even the slightest hint of switching noise.

Here are a few fresh pics of the ugly prototype:
Click the image to open in full size.
Click the image to open in full size.
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Old 19th August 2010, 10:17 PM   #378
Eva is offline Eva  Spain
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Quote:
Originally Posted by Tekko View Post
I replaced the 220pF caps with 150pF and now i get 3.7µS per period wgich should be around 250-350kHz.

Amp is sounding good, i like it and the carrier resudial on trhe output is about a volt and its squeeky clean, cant even tell its switching.
First, you have to look at carrier residual with several amperes flowing from and to the output of the amplifier, most ringing is hidden when the amplifier is idle because it's switching in resonant mode.

Second, a 25Mhz oscilloscope may not be able to display the ringing frequencies with its proper amplitude (serious attenuation and phase shift above 25Mhz). I can get ringing as high as 110Mhz in my IRFB4227 output stage if I remove the snubbers (high ringing frequencies in hard switching circuits are a proof of good layout).

Third, what you tell is not enough criteria to ensure that the self oscillating loop is properly compensated.

Paralleling dissimilar output capacitors may result in peaks and dips in output impedance that will disturb carrier residual only for certain pulse widths (it looks fine with no signal but may misbehave as the amplifier approaches clipping).

Turning your oscilloscope on and off may produce brief EMI spikes, usually picked up by and heard as harmless clicks in audio equipment, faint in good designs, loud in bad designs. One of these clicks can easily clip an amplifier for a small fraction of a second. If the amplifier has some hidden design flaw, it may show up...

For example, yesterday one of my prototipes got into protect mode as my mobile phone started to ring (no damage). It was unusually close. These things happen...
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Old 19th August 2010, 10:29 PM   #379
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Im NOT parallelling dissimilar output capacitors, just provinding the ability to use more than one size, like 2.5mm and 5mm.

I tried using an am radio close to the amp as well as in another room and nothing, with the antenna just millimeters away i was abe to get a barely hearable signal around 1MHz with the radios volume on max. Compared to my previous ones which were clearly hearable all over the am band.

Btw eva, you do this stuff for a living so you always try to look down at others designs in fear of competition and such just to remain the queen of class d on here.

The only way to check the carrier resudial with several A of current is to use DC on the input and a dummy load, otherwise the resudial will just be a blur on the screen.

Turning the scope on/off does nothing, not even a click in the speaker.
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Old 19th August 2010, 10:46 PM   #380
Eva is offline Eva  Spain
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I don't fear any competition. I would like to see you designing good stuff. In fact, why have your quality standards improved so much in the past 1-2 years? Your prototypes are starting to look like my early work (or in other words, good).
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