My very first Class D pwm (switching) amplifier. - Page 4 - diyAudio
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Class D Switching Power Amplifiers and Power D/A conversion

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Old 13th May 2003, 03:00 AM   #31
djk is offline djk
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"It is vice-versa, i.e. for no input signal the switching frequency is high and it is getting lower with increasing signal level "

I expressed myself poorly, you made the point I thought I was trying to make.
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Old 15th May 2003, 12:51 PM   #32
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Default I hit it with a bigger hammer.

Tried a 33k and 1nF LPF at the input to each comparator on the modulator just in case any noise was getting in. That was it! Sounds much much better now. None of that fuzzy crossover-like distortion. Also earthed the centre of the +/- 40v rails - that fixed the problem when I touched one of the schottky's and the amp would cut out. Then I gave it my 6 ohm load and fed the loudspeaker at the same time via a 100 ohm resistor so I could listen to it while it was pumping some real power to "Sultans of Swing".

Anyway, I noticed something interesting. The amp doesn't have any negative feedback which probably helped the situation, but when driven into clipping on music signals you can hardly notice it at all! Often the wave would flatten for as much as 1.5 mS at a time. Peak output at clipping was +/- 39 volts, but for the last 1.5 volts the wave would slow down exponentially and then hit the clip point, or rather it would approach the clip point and then gradually slow down almost asymptotically and gently stop right on the line. When it was time to move away again it would move away at a right angle. No fuss, no bother, no rude noises or oscillations. As a result, on voice signals in particular, the apparent loudness could be wound up quite a bit further than what maximum power would otherwise dictate. Would make a great PA amp I reckon.

When running like this for about 15 minutes the schottky's got to just above skin temperature and the mosfet heatsink to about 35 deg C at about a 15 deg ambient. Judging from the load resistor temp, the average power output would have been about 35-40 watts so I was shoving it reasonably hard. Ahhhhhh....... niceness is radiating out of this amplifier indeed.
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Old 27th May 2003, 07:51 AM   #33
ssanmor is offline ssanmor  Spain
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Default Veeery interesting

Hello, Ciclotron.
It seems that you have finally put your amplifier to work! Good news!
I am also starting to feel like building a Class D amplifier basing on Crown's BCA technology, but have some doubts:

How do you calculate the ouput filter?
What are the required ratings of the schottky diodes?

By the way, you have used a crap SMPS from a car amplifier or so, aren't you? Or is it a Class D amplifier? (just for curiosity)

Best regards from Spain.
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Old 27th May 2003, 08:47 AM   #34
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Always fun to read about your projects! Nice work...
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Old 27th May 2003, 01:37 PM   #35
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Default Re: Veeery interesting

Quote:
Originally posted by ssanmor
How do you calculate the ouput filter?
What are the required ratings of the schottky diodes?

By the way, you have used a crap SMPS from a car amplifier or so, aren't you? Or is it a Class D amplifier? (just for curiosity)
I didn't really calculate the output filter, I just used something from my junk box that looked about right.

Schottky diodes should have a voltage rating of whatever the total dc voltage rails are. I will eventually have + / - 50vdc so I have used 100v diodes. Schottky diodes are MUCH better than normal fast recovery diodes here - you don't have to worry about reverse recovery current and all the noise that goes with it.

Not sure what you mean 'have I used a crap car amplifier, or Class D amplifier? If you mean what was the pcb originally, it was for generating 75VAC 30VA 25Hz sinewave for ringing telephone bells.
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Old 27th May 2003, 01:39 PM   #36
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Quote:
Originally posted by anelar
Always fun to read about your projects! Nice work...
Wow! Thanks anelar.
I am going to post the schematic one day soon.
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Old 27th May 2003, 01:44 PM   #37
ssanmor is offline ssanmor  Spain
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Thanks, Ciclotron.

And another question...
What kind of MOSFETs do you use? P and N or only N-channel?
If they are N-channel, I would be very interested in knowing how you drive them. It seems that you don't use transformers nor optocouplers. Could you provide more details on that?

Thanks for your soon response!
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Old 27th May 2003, 01:47 PM   #38
jcarr is offline jcarr  United States
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Circlotron:

I very much enjoy reading about your projects, too. The always-present touches of clever imagination and fun are refreshing. If anyone deserves the title of "diyAudio resident mad scientist", it would be you.

best, jonathan carr
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Old 27th May 2003, 01:49 PM   #39
ssanmor is offline ssanmor  Spain
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Soooorrry!
Another question!
You have remarked that adding a RC filter in the inputs of the comparator cured the problem. But, if I am not in a mistake, that limits your audio passband to about 4.8 KHz, doesn't it?

Sergio
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Old 27th May 2003, 02:32 PM   #40
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Quote:
that limits your audio passband to about 4.8 KHz, doesn't it?
I have a distinctive suspicion that he will use it to substitute his subwoofer amp that went up in smoke

Regards

Charles
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