class D amplifier - Page 1
Forum section: electronics / projects

author: Adgj533
author_rank: Contributor
timestamp: January 28, 2019, 10:05:01 pm
content: Hello guys,I want to build a class D amplifier at home. I am a hobbyist. I want the power output around 50W -100 W.I got a schematic from AAC which is alittle complicated and honestly I dont understand it a lot.I was wondering if you guys can provide a simpler schematic, anyone that have worked on it before. It must be working as an end product. I am planning to use it in my car.Thanks

author: nick_d
author_rank: Regular Contributor
timestamp: January 29, 2019, 06:27:00 am
content: I can help you with this, but let me first say that this is not a trivial project (I know this from experience having done exactly what you propose).You have some decisions to make right off the bat.1. Is the time of the output transition infinitely variable? If so, then what you're basically doing is comparing the audio with a triangle waveform and sending this comparator output to the high current output stage. If not, then you're updating the output at some fixed frequency (say 250 kHz) by sending it a bitstream (at say 250 kbits/sec), the bitstream being calculated by digital logic such as a sigma-delta modulator in an FPGA.2. Is the output bridged? If so, then you can output either 00, 01, 10 or 11. These cause the speaker to see a voltage of 0, -1, +1 or 0 respectively. To play a slower waveform such as a 50 Hz drumbeat, what you'll be doing is mixing the 0 and +1 in the correct proportion for the positive part of the cycle (10 ms) and mixing the 0 and -1 in the correct proportion for the negative part of the cycle (10 ms). If the output is not bridged, then you can output either 0 or 1. These cause the speaker to see a voltage of -1 or +1 respectively. For silence you'll be playing -1 and +1 in equal proportions, and for music you'll be varying those proportions. This will require a huge and bulky output filter and will waste power.3. Are you going to construct the waveforms yourself or are you going to use a commercial IC? Frankly, I would recommend going the latter way unless you are prepared to do a fair bit of development. There are loads of class D ICs available. This is because they are used in laptops. Some of them probably have the ability to add your own output stage. In that case you'll have to worry about issues like dead time and so forth.4. Will the output(s) be driven symmetrically? That is to say, will you have a transistor that connects the speaker to +V and another transistor that connects the speaker to -V? That's probably the sensible way to do it (alternatively, you could build it like a class A output which basically has a transistor to pull the output down and a resistor or inductor to pull it up). How will you generate gate drive signals and the dead time?5. Will you use bipolar or FET transistors? Will you use complementary types... so PNP to pull output up and NPN to pull output down in the bipolar case, or P-type FET to pull output up and N-type FET to pull output down in the FET case... or will you use only N-type FETs with a floating driver? How will you drive the high-side transistor or FET? Will you bootstrap it with a capacitor, use an opto-isolator, a transformer?Let us have some decisions on the above matters and I am sure that we can help you further.How are you on op-amp topologies? If you know how to build an inverting amplifier with a virtual earth... then you know how to build an integrator as well. (An integrator has only a capacitor in the feedback path and no resistor). So what you do is create a square wave and then integrate it to get your triangle wave. Put the triangle wave into a Schmitt trigger and feed that back as the square wave to be integrated... and then you get a triangle wave that increases linearly to the Schmitt high threshold, then decreases linearly to the low threshold, and repeats.cheers, Nick

author: mariush
author_rank: Super Contributor
timestamp: January 29, 2019, 07:00:33 am
content: There are a lot of class D audio amplifier ICs... you could simply check the datasheets and use the reference design and pay attention to the explanations.Take for example TDA7498L :https://www.st.com/content/ccc/resource/technical/document/datasheet/03/dd/12/33/f2/bc/42/06/CD00252942.pdf/files/CD00252942.pdf/jcr:content/translations/en.CD00252942.pdfDatasheet also contains circuit board layout examples, low pass filters for 6 ohm and 8 ohm and other things.All you need is to make a boost (step-up) regulator to boost the 12v from your car battery to somewhere around 24-36v (the chip's absolute maximum rating is 45v) ... or you could buy a dc-dc converter.Datasheet says it can do around 60w with 30v input.There's also an evaluation board for it (with TDA7498E) :https://www.digikey.com/products/en/development-boards-kits-programmers/evaluation-boards-audio-amplifiers/789?k=&pkeyword=&sv=0&pv984=10172&sf=0&FV=ffe00315%2Cfffc01f1&quantity=&ColumnSort=0&page=1&pageSize=25It's expensive, so I'm not saying to buy it, but you can read the PDF (LINK) to see the schematic for the evaluation board circuit, what inductors they chose and their specs, capacitors used etc etc ...

author: SiliconWizard
author_rank: Super Contributor
timestamp: January 29, 2019, 08:27:01 pm
content: Just making sure what your goal is.Do you just want a class D amplifier? The cheapest way will be to get ahold of one on eBay or something. A ton of them for little money, and many are actually not bad.Otoh, if you mainly want to learn about class D amplification - then I suggest starting with something lower power, like a couple W, which won't need specific MOSFET drivers for instance.Regarding your posted schematic, it's a very basic design. The triangle signal generated by the LM555 here will be a lousy triangle waveform IMO (looks like a capacitor charging and discharging and NOT at constant current), and not exceptionally stable, and the result will probably be a rather significant level of distortion. Will it work? Seems so. Will it be disappointing? Probably a bit.There are actually better ways of using a 555 to implement a PWM modulator either directly, or to make it generate a better triangle signal.If you need a class D amplifier for immediate use, look up "class D 100W" in eBay - you'll get a decent amplifier for like 20 bucks.As for learning, I suggest starting with those notes:https://www.maximintegrated.com/en/app-notes/index.mvp/id/3977https://www.analog.com/en/analog-dialogue/articles/class-d-audio-amplifiers.html