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Im very intrested in building my own car audio class D amplifier but runing into problems trying to find info on the subject.Ive got a few books on home audio or AC power supply based amps but they rarely go into any kind of detail of the class D design.I find it very intresting any help on the subject would be helpful.Is there any schools that I might attend for this.Thanks
Posts: 25 | From: Dallas | Registered: Apr 2004
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Class D and car amps are not very well documented, not like home audio. I assume you know the basics of how class D works, and they aer pretty simple devices from an operation standpoint. I doubt there are any schools you could go to, and probbaly no books to read. You're best option would probably be to find the datasheets and app notes for the PWM drivers used in class D amps. I don'ty know if the Tripath app notes are still available on the web, but they are a very good resource, I might still have all of them at home if you can't find them. Read them thoroughly and you'll probably have enough info to design one. You still won't be able to build one cheaper than what you can buy them for these days, but you could build one anyway.
Posts: 2575 | From: GA | Registered: May 1999
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Class D operation is switching, hence the term switching power amplifier. Here the output devices are rapidly switched on and off at least twice for each cycle (Sampling Theorem). Theoretically since the output devices are either completely on or completely off they do not dissipate any power. If a device is on there is a large amount of current flowing through it, but all the voltage is across the load, so the power dissipated by the device is zero (found by multiplying the voltage across the device [zero] times the current flowing through the device [big], so 0 x big = 0); and when the device is off, the voltage is large, but the current is zero so you get the same answer. Consequently class D operation is theoretically 100% efficient, but this requires zero on-impedance switches with infinitely fast switching times -- a product we're still waiting for; meanwhile designs do exist with true efficiencies approaching 90%. [Historical note: the original use of the term "Class D" referred to switching amplifiers that employed a resonant circuit at the output to remove the harmonics of the switching frequency. Todays use is much closer to the original "Class S" designs
Hope this helps a bit Try Class AB, much more musical.
-------------------- E.C. Wuz here Posts: 1057 | From: Vancouver, British Columbia, Canada 'eh | Registered: May 1999
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Thanks for the feedback but im wondering why it would be more costly to build then buy.When I say planing to build its mostly likly planing on coping a design that I find works well and is simple yet powerfull and can be duplicated with ease.For instance the MTX 1501D or crossfire 1000D.Basicly im also wondering what manufactors expect from a desgin eng. or product eng.I have plenty of info on AB class design but none on 12volt audio power supplies unlike home audio.What tools would be needed for this task?
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The reasons that it is more expensive to build than buy is economy of scale. Sure the amp you are buying is marked up significantly from what it costs the manufacturer to build it. But because they build thousands the custom parts like heatsinks, PCBs, transformers, inductors, and connectors aren't nearly as expensive as they would be for someone building one or two. You will find places willing to build you a few custom toroidal transformers, and PCB buildhouses will make you one or two. If you can find a surplus heatsink and some connectors that will work, that definitely saves some serious money, but if you want nice ones like you see one some amps you'll have to have them machined. Plus when you buy hundred of thousands of resistors and capacitors, they become really cheap, though this doesn't impact the cost all that much at least compared to the parts I mentioned previously. Plus if you want something that looks as good as what you'll buy in a store, you'll end up spending lots of money on things like heatsinks, silkscreening, and connectors.
I've build prototype amps, I never build one as cheap as I could have bought a comparable one in a store or online. Some have died, one never worked in the first place, and some live on. Only one is something I would consider a good looking product, and I spent more on the heatsink and connectors than what I could have bought decent amps for. When you add in the time it took to design, build and test, if definitely wasn't cheap.
With those things being said, if this is something you want to do to learn more about the amp topology then by all means go for it. I would start on a smaller scale if that is your goal and work up. You could make something scalable pretty easy and as you add more power you get a larger power supply and use more switching FETs. And if you need any help along the way feel free to ask me any questions you want, I'll answer them as best I can.
Posts: 2575 | From: GA | Registered: May 1999
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I received a quote to fabricate a printed circuit board that measured 6.5" x 10.75", 4 layer design and it would cost about $30 - $50 per PCB, but that price is based upon investing $2k initially.
If it's a DIY 'professional' project where you design your PCB on CAD AND you plan to build a few amplifiers for yourself, then you have to ask yourself if spending around $2k is worth it just for these bare circuit boards.
I haven't even factored in the electronic component costs and heatsinks.
I did have plans to DIY a home audio amplifier but my plans are to build many multichannel amplifiers for personal use. Even so, it's an expensive venture considering that you can get QSC 1800W prosound amplifier for $400 via online retailers. Is my home audio DIY project worth it too if I can get these prosound amplifiers for this low price ? /LOL/
Posts: 47 | From: California | Registered: Feb 2003
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If you can get down to a 2 layer design then you can get PCBs for roughly the same price without the 2k NRE charge. They aren't as good as the ones that you would get the other way, but good enough to use and not have problems. You can't vary the trace thickness, or board thickness so high current traces might need to be supplemented with some solid metal or wire, but most of my designs do that anyway. I think you can get 4 layer as well, and can get soldermask and silkscreening from some vendors. Most are pretty quick turnaround, but in most cases engneering support is pretty lacking.
Posts: 2575 | From: GA | Registered: May 1999
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Here's some helpful information. Zap Pulse Amp These guys make an amp that is CREDIT CARD sized. One PA use is to bolt the amp right to the basket!!! Their Zap Pulse pages will fill you in on what you need. The US distributor is DIY Cable. There's plenty of evals at TRIPATH also. Good luck, db
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I'm in the wrong business, I should be making 200-300 clocks for cd players
Interesting amp modules, but since you still need a power supply with them they aren't that small overall.
Posts: 2575 | From: GA | Registered: May 1999
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I feel class D is actually the most simple. All you are doing is switching the output devices on and off, you don't really even need a class D driver chip to do it. Send the switched signal through a filter and you'll have a sine wave. You can do class D very badly and it will still work and give you a sine wave. Class AB while still being simple, takes more care to get a correct output, and even more to get a good signal without zero cross distortion. Actually all amp topologies are pretty simple, just very hard to get correct.
Posts: 2575 | From: GA | Registered: May 1999
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