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Old 7th May 2003, 03:53 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, 08:42 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, 08:49 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, 01:59 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, 03:24 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, 04:55 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, 04:11 AM   #7
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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, 04:14 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, 06:52 AM   #9
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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, 09:05 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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Old 8th May 2003, 09:07 AM   #11
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Has anyone seen a simple (ish) PWM amp for the beginer?
I'd be interested to build one that I could later build on to make a monster car amp.

RE: quality... if PWM amps are used for low frequencies won't it mean that you won't exactly notice the added distortion? Bass is mainly distortion anyway.
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Old 8th May 2003, 10:31 AM   #12
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Quote:
Bass is mainly distortion anyway.
Me as bass-player can't share this opinion of course . Although the ear is less susceptible to THD at low frequencies it is always nice to have low distortion bass reproduction

One problem with switching amplifiers is getting enough feedback
specially with the necessarily low unity-gain point involved. It is even desirable to take feedback off from the amp's output to correct for the nonlinear behaviour of output inductors, though it makes things even more complicated.

OTOH feedback-takeoff from the switching stage makes it easy to achieve a phase- marging that is above 65 degrees (one goal that sometimes gives linear-amp designers headaches).

It is quite simple to achive large NFB factors at low frequencies but more difficult at mid/high frequencies, but still not impossible.

The output filter has very weak influence at low frequencies in terms of amplitude response. But FR is influenced by the interaction of filter and load at upper-mid- to high- frequencies.

Since the output devices don't operate linearly, no SOA protection is needed (only max current limiting, if any at all).
This, combined with the low filter/load interaction makes them very good performers for bass-reproduction but sometimes less good above.

At very low frequencies there is some drawback: The output filter inductor makes the amp act as an inverter, leading to supply voltage runaway (also called supply pumping).

Regards

Charles
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Old 9th May 2003, 03:54 AM   #13
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Well, things went a bit better than I had hoped. After I has sorted out a few pcb tracks that I had cut that I shouldn't have, messing up the earthing of the driver and output stages, I powered up the thing at it just went! No flames, no fuss. At first I only used +/- 10v rails on the mosfets just in case something was dreadfully wrong, and it sounded a bit dissapointing - very reminiscent of crossover distortion although it wasn't of course. Later on I got bolder (or more reckless) and in stages finally got the rails to +/- 35 volts, the limit of the supply I was using. By this time it was sounding heaps better. Switching frequency was 50Khz at first but I can crank it to something past 500kHz although it didn't seem to make a whole lot of difference to the sound.

The output inductors I used were 800uH each, 55 turns of 1.25mm wound on a Microlite MP4010PDGC toroid core. This core is made of a long strip of metal wound up like in a normal mains toroid, but is actually a distributed gap or something because it is meant specifically to handle a dc flux. Weird stuff. Output cap is just some 1uF 250v block thing; not really suitable actually. With these, the frequency response is -3dB @ 890 Hz (!) A bit more work to be done there. It sounds pretty much like an AM radio at this point, pleasant to listen to but by no means hi-fi. Would probaby be just fine for voice only PA use though. Winding the volume up, this amplifier is not afraid to get down and get with it; it keep saying "more, more" but it was 11:45 pm by this time so I had to keep it down a bit.

Power output is a bit limited at the moment because the current sense resistors in the fet source leds that go to the drivers are way too big so it cuts in too early. I can see right here and now, to make a top end amp using this aproach would be a huge job, but that was never my intention anyway. I *am* pleased.

P.S. Will try and post some pics tonight, about 8 hours or so from now - lunchtime 12:55 pm.
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Old 9th May 2003, 07:40 AM   #14
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If you have already implemented NFB then there's a simple explanation why it sounds better with the higher supply voltage:

The gain of a switching amplifier output stage is basically proportional to the ratio of supply-voltage/peak-voltage of the triangle used for the PWM modulation.

With increasing gain the feedback factor raises proportionally.

Regards

Charles
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Old 9th May 2003, 08:11 AM   #15
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Quote:
Originally posted by Circlotron
the current sense resistors in the fet source leds that go to the drivers
That should be "in the fet source leads"


Quote:
Originally posted by phase_accurate
If you have already implemented NFB then there's a simple explanation why it sounds better with the higher supply voltage:
Nuh, no feedback yet. Much fiddling remains to be done.
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