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Old 7th May 2003, 02:53 AM   #1
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Default My very first Class D pwm (switching) amplifier.

This is something I have been tellling myself for years that I should do - build a Class D pwm type audio power amplifier. Nothing too ridiculous (at least not yet buahahaha) just something to try the water and get the feel of things and gain a little confidence. As usual I'll start at the output and work back. I'm shooting for an output of about 150 watts RMS into 8 ohms. The output fets will be driven by a pair of IR2121 & IR2125 isolated fet gate drivers, and the pwm controller a UC3637. Switching frequency to be 100kHz to begin with. Also thinking of adding output "slices" to make the thing multi-phase to lower output switching ripple by raising the effective switching frequency, at the same time raising the output current capacity. This is the output stage topology in the pic. It is already PATENTED by Crown so beware, you can't make and sell any, only make one for your self.
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Old 7th May 2003, 07:42 AM   #2
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This is the BCA topology which was already discussed once:

how they do that?

Regards

Charles
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Old 7th May 2003, 07:49 AM   #3
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got any pics/links/schematics of what you did?
Class D's somefin I always wanted to learn about but it looked like the only option where those Class T driver and the National class D amp.
Well done
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Old 7th May 2003, 12:59 PM   #4
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Default Fess-up time.

OK. I may as well admit right up front that I am not starting totally from scratch. The test bed for my project is a 75VAC 25 Hz 30 watt telephone ring current generator that is actually part of a larger dc power supply. It has all the physical setup for the fets, opto's, DC rails, and some of the pwm generating stuff. At this stage I have to make some external supply to feed the rails though. Have changed the rails down from +/- 160vdc, put in low voltage high current fets instead of the opposite, and changed the output topology which used to be a conventional half bridge.

Charles, thanks for that link. There seems to be three kinds of people - those that have never seen or heard a Class D but say they sound rotten, Those that have listened and say the same, and thirdly those like yourself that have actually designed and built one and declare that the sound is a difficult to aquire taste. I hope to become one of the last group. I don't expect this amp of mine will set the world on fire or anything, only that after it is all said and done I might in fact learn something and have half an idea of what I am talking about. Besides, I have never made one before, so that's enough reason I say. BTW, I'd love to see the details of the lab amp you did.

fr0st, no pics or stuff yet, but for sure I'll post a blow by blow story just as I did with my similarly titled thread on my Class A amp. All schematics and component values will be included.

Just thinking a little; I have a couple of 500v 32 amp mosfets. Now if I ran just two of them on +/- 250vdc rails they ought to pump out ~3900 watts RMS into 8 ohms. Hmm....
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Old 7th May 2003, 02:24 PM   #5
jcarr is offline jcarr  United States
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Graham:

>There seems to be three kinds of people - those that have never seen or heard a Class D but say they sound rotten, Those that have listened and say the same, and thirdly those like yourself that have actually designed and built one and declare that the sound is a difficult to aquire taste.<

AudioPhysic has a poweramp called the "Strada" that incorporates class-D technology that was designed by Mircea Naiu, a Rumanium mathematician who worked principally for Siemens. He has the following power amplifier patent, and the Strada also uses the "anaco" (analog arithmetic computer) technology:

http://patft.uspto.gov/netacgi/nph-P...S=PN/5,825,247

IIRC, the version currently produced will put 250W into an 80ohm load, and 2500W into a 1-ohm load. Sounds pretty good, too. In fact, I would suggest that it sounds subjectively better than all but a handful of top-notch analog power amps. OTOH, the technology is still under development, and the internal componentry has turned out to affect the sound to a surprising degree, so seemingly trivial parts substitutions in production can lead to a very different-sounding batches of amplifiers.

regards, jonathan carr
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Old 7th May 2003, 03:55 PM   #6
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Another thing to take into account with any switching amplifier topology is PCB layout.
You can make a clever circuit sound very bad by mediocre board layout. So it is very difficult to get good performance from a veroboard lab version.
A switching amp is basically an RF circuit that is used to process audio signals.

Though I don't have the mathematical proof, I have the suspicion that delta sigma amplifiers are less susceptible to EMC issues than PWM types. If anybody is interested in an explanation I would try to do so.

I think I remember vaguely that Graham works with a company that build smps or the like, so he might have some experience with circuits of this kind.

Have fun

Charles
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Old 8th May 2003, 03:11 AM   #7
Diode is offline Diode  United States
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I work on Crown Macro-tech 3600's and the like. Those are some really cool amps!!!!!!!! They have all kinds of controls that are fine tuned. From my understanding, they drop the ground and add another power supply when the power demand gets to a certin level. Bridged into 2 ohms, it can produce 3600WRMS. It requires a 30A breaker at 120V input. They look at 2 ohm loads and seem to say, "Gee, can you give me a challange?" and nod back off to sleep. I'm totally impressed with the amps. Crown makes a superior product, in my opinion. Class D. Well, it is nothing more than a modulated switchmode power supply. I worked for Lucent, now Tyco, power division for a few years, before the big lay-off but, we made big 2400W 24V. That's 100A!!! At any rate, we had to test it's closed loop and open loop performance and this meant that we had to inject a signal into the voltage feedback path and modulate it. My engineer told me that at this point, it is just a fancy audio power amp. We used a frequency sweep generator. He told me that the bandwidth would be extremely limited but the low frequencies would do pretty well. Of course the power transformer wasn't designed for audio purposes but just the same, one could be. We switched at 100KHz so what does that tell you? You would need to switch at much higher frequencies to get the whole audio spectrum. EMI is a huge concern. If you switch the huge currents needed to get high power, you will need to not only pay close attention to layout and lots of filtering. Not only that, it will need to be enclosed in a metal box that is filtered and grounded properly. Our power supplies were in 2 pieces. 1: the AC-DC board and 2: The DC-DC power board. Each half bolted to it's own half of the case and when we had them running with the case open, they would wipe out FM radios for at least 50Ft. EMI is a huge concearn........ Without taking your amp to a place that measures and qualifies RF gear, I wouldn't even think of doing my own as the FCC gets really upset at unwanted noise. You don't care?..... Try listening to your radio and have the thing running at the same time. You may screw up the TV too, and maybe your neighbors, if in an apartment..... NOT cool. Just because you put it into a metal box doesn't mean thet your filtering is good enough to keep RF off of your input and output wires.... YES, it matters.... If you start putting ferrite chokes on your speaker wires, what will that do to your "beloved" sound quality. You really need to know what you are doing, to be successful, that's why those engineers get the big money to design good clean power supplies.

Good luck,

Chris
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Old 8th May 2003, 03:14 AM   #8
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Quote:
Originally posted by phase_accurate
Another thing to take into account with any switching amplifier topology is PCB layout. I think I remember vaguely that Graham works with a company that build smps or the like.
Yes and yes, otherwise I would be really scared of making one instead of only a bit scared, as well as having not the right parts in the junkbox.

Layout is indeed a big deal as you say, Charles. I should have something going by Saturday if all goes well. Will post pics sometime then.
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Old 8th May 2003, 05:52 AM   #9
djk is offline djk
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"I work on Crown Macro-tech 3600's and the like. Those are some really cool amps!!!!!!!! They have all kinds of controls that are fine tuned. From my understanding, they drop the ground and add another power supply when the power demand gets to a certin level. Bridged into 2 ohms, it can produce 3600WRMS. It requires a 30A breaker at 120V input. They look at 2 ohm loads and seem to say, "Gee, can you give me a challange?" and nod back off to sleep. "

The Crown amps are OK. The VZ3600 is rated at 1800W/2R X 2, and considering each channel is already a bridge amp, it would be folly to try and bridge it at 2R.

When Crown says 2R they don't mean four 8R speakers in parallel, the ODEP lights come on and the amp locks down in the low power (-6dB) mode.

You can run it on 120V if not banging it hard, if you are going to run it full tilt wire it for 208/240. On rock music at clipping it draws about 7KW into two 2R loads.

The big brother VZ5000 runs on two 82V supplies per channel. At low volume these are paralleled, at high volume they are switched in series.
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Old 8th May 2003, 08:05 AM   #10
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Quote:
Originally posted by Diode
we made big 2400W 24V. That's 100A!!!
EMI is a huge concern.
Good luck, Chris
I was directly involved in the design (i.e. I did all the hack work) of a 54.6 volt 65 amp smps with pfc for telecoms stuff so I appreciate what you mean. Luckily also I have access to an EMI room and mega-fancy HP spectrum analyser. It is only about 20 paces away. Anyway, I doubt if I'll be using the amp that much, just play with it a bit to see how well it goes and then go back to good ol' Class A.
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