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Let's Build the Chairman Mao Class D Amplifier |
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Amplifier topologies haven't changed drastically in a long time. That is, until Class D amplifiers came along. Class D has a lot to offer, but probably the most interesting points to me are:
All these attributes add up to a very cool project: The Honestly Good Power Amp. Of course, you could just buy a complete Class D amplifier cheap, but through DIY you can build it to your taste. |
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What are we going to build?We are going to build a power amplifier (in other words, the kind you plug speakers into) that is compact, easy to assemble and sounds incredible as a bookshelf or desktop system. It will be simple:
Wait, Can't I Just Buy A Completed Class-D Amp?Sure you can. There are tons of ready-made units out there. Google Class-D or Class-T amps and you can find reasonable units like the Sonic Impact Gen 2 T Amp of search on ebay for prebuilt units. But almost all of them have cheap connectors or crappy power supplies/filtering, and aren't, well, DIY. You don't get to create your own amplifier. What is it good for?This amplifier is great when you want a compact system that will drive a pair of bookshelf-size speakers and don't have to worry about multiple inputs, like a CD player, your xbox, turntable, etc. That's what you want a receiver for with those impressive looking back panels full of jacks. No, what we are building is a single input amp--great to hook up your ipod or portable CD player with a minimum of fuss and a maximum of kickass sound. Why is it So Easy? Isn't Building an Amplifier Difficult?Building a great sounding amplifier can be very difficult if you want it to be. If you have great DIY, soldering, and electronics skills, then there are literally thousands of designs out there, and great forums like www.diyaudio.com where you can learn about making some amazing gear. But this project is about easy of building: can you do a little soldering? Can you drill holes in an enclosure? Can you use google to find various parts you'll need? That's about the extent of the skills for this project. Also, our amplifier is going to use a wallwart AC/DC adaptor so there aren't any dangerous voltages to kill us. The Amplifier ModuleThe reason this project is easy is because we are going to base it
on a pre-built Class-D amplifier PCB. One that is pretty close to done.
All the tricky soldering, SMD mounting and all the exotic robot-assembly-line
hard work has been done for us. The good news is that there are a buttload
of great Class-D modules available. For our project, we are looking for boards based on the Tripath TA2020 or TA2040 Class-D Amplifier chip. (Note that Tripath went tits-up as a business a while back, but loads of their chips are still available. Tripath called their implementation of the Class-D architecture "Class-T" which muddied the waters a bit, but in essence, they made incredibly good Class-D chips that covered a wide range of power requirements.) Both chips are quite similar but the TA2020 puts out a bit more power. Let's take an inventory of some of the types available.
Our ModsFrom what I've seen in the low-cost Class-D PCB category, components other than the Tripath chip itself are usually the first things to be skimped on. Depending on where you source your board, you have to look out for:
So that's what our project will be: take our chosen pre-built PCB, solve its problems, and turn it into a hi-fi grade piece of coolness that will rival units costing much much more. The TA2024 PCBFirst let's take a look at the 8 Audio TA2024 PCB.
I've highlighted the general areas of the board, the parts in red are what we are going to change.
Input CapacitorsAs mentioned earlier, input capacitors are where the amplifier board first hears your audio signal. Good quality boards will use good quality capacitors. For example, the 8 Audio board ships with a pair of Wima 1uf Film parts which are certainly good quality. But many of the other designs use cheap electrolytic caps with little regard for how good they sound. In the freakish world that is Audiophile Capacitor Madness, you could end up spending more in input capacitors than the entire amp. But we want to be realistic, we want to use parts that are sound great and don't break the bank. The stock 8 Audio Board ships with 1uf parts, but the specs state that we can go from 1uf to 3uf. Here are some types for our consideration:
The next step is to choose the input capacitors you want. If price is a key consideration, the Wima or Janztens would be a fine choice. If you want to invest more, keep moving up the food chain. But the real question is (an you'll see this same phrase all over the beavis hifi website)
The easiest way to find out is to audition different types. In other words, instead of soldering a pair in, use a temporary connection to make it easy to swap different types in and out. I'll show some easy ways for you to do this a little later on. Power CapacitorThe 8 Audio board ships with a pair of ???uf shamefully tiny electrolytic capacitors for power supply filtering. You can call me a deviant goat herder, but there's one thing I know: big caps in the power supply are a must. So we want to replace that diminutive duo that comes stock on the board with a single big whopper power cap. The good news on power caps is you don't really have to be choosy: they aren't in the signal path. You essentially want to choose a part that has the right capacitance rating and is rated for twice the voltage of unit, as a safety margin. For me, a value of 470uf is about right, and since the amp is going to expect a 12 volt power supply, I'll chose a cap somewhere in the 50v range just to be safe. That's all you have to do, you don't have to "audition" power supply caps. When in doubt: 1. Choose from a mainstream manufacturer 2. Get your farad and voltage values right 3. Find one that has a pretty color :) Swapping out the PCB-Mounted ConnectorsNow we want to move those PCB-mounted do-hickeys off the board. Why? Well, strap yourself in for a beavis hifi history lesson/rant
The parts we are going to de-mount are the volume control, the power jack, the input jack and the speaker terminals. Where also going to pull off the LED so we can choose our own LED color and figure out where in the enclosure we want all the pieces to go. The Power SupplyThese TA2020/TA2024 amps require 12 volts at 2 amps. So a basic wall wart from Radio Shack isn't going to help you. Your best bet is a switching power supply. These look like the devices that power your laptop: wall cord goes into a little black box, then another cord goes to your laptop. Here are a couple of good deals on power supplies for this project: Be sure to get one that matches your house mains voltage (i.e. 120,220, etc.) You will also want to chose one that has a plug that correctly mates with whichever power supply jack you install in your amp. Book 'Em Dano! Let's build a prototype!I really felt groovy about the look of the pcb and thought it would be a capital idea to use my patent-pending "screw it to a piece of wood" approach to designing the prototype. I had a nice piece of hardwood fretboard material, that would be my hunk of wood. Attaching the PCB to the wood with metal standoffs would be easy. But what about all that i/o stuff I wanted go offboard? I would have to fashion a set of front and rear panels that could also be Screwed To the Piece of Wood(tm). I got some aluminum plate at the local hardware bodega for material. Then I stumbled about in Visio creating the drilling template. Then a hacksaw to cut the plate, drill press for different hole sizes, and then a drill-mounted wire brush thingy to shine it up a bit.
Measure Once, Drill TwiceAnd that's what I did when fastening the new backplate to wood base. I drilled pilot holes that were too small. As a result I stripped off a few screw heads and totally farkled one end of the wood. I can always tell when it is time to call it a night when I start working like a sloth on ludes. But I had to keep going or the voices would start again. Back to your regularly scheduled viewing of pictures.
Power Supply Filter CapacitorsEarlier I talked about the power supply filter capacitors I wanted to change. These aren't in the signal path, so bog-standard electrolytic values are fine here. An old engineer neighbor well into retirement told me me once told me an unused secret in power supply filtering--instead of using a single big value, use a larger number of smaller values. As long as the part you use is rated for at least twice the voltage <<twice? yeah I'm conservative on this one:) >>. And even though they don't have to be hifi-rated parts at least we can make them look nicer. I stripped the nasty PVC cover of a set of 220uf value and fashioned up a little stripboard-based holder for the thing.
Final Assembly
Use and Listening TestsThe main question for this project was: would changing a few parts make the stock Tripath TA2024 board sound better? The answer is a definitive YES!. The Jantzen input caps sounded the best of all I tried and were a big improvement over the stock Wimas. I'm would pretend to know all the correct audiophile phrases to use, but it did indeed sound warmer and clearer at the same time. The additional power caps seemed to really help during large transients, like a big change in dynamic range from soft to loud. The stock board seemed to sag a little in that territory. The additional bank of caps appears to have fixed that, although it may have just compensated for my low-quality power supply. Regarding the construction part, it took a lot of time. Making a custom enclosure is not trivial: machining the end plates and getting them drilled correctly took a few tries. The wood mounting base did turn out as a very convenient and easy to machine "chassis" so I may play around with that concept again. In conclusion, there are a lot of excellent Tripath-based audio amplifiers out there in the world. With a little but of work you can turn a great sounding amp into an even greater sounding amp!
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