Let's build a Class D amplifier!
A couple of weeks ago, my good friend Andrew (from down under) told me he had acquired a cheap class D amp off ebay. It was the cheapest of the cheapest. I think he paid for 3 dollars AUS, which is something like 12 cents American. The amplifier came from eBay, the seller is arjenhelder_electronic : http://stores.shop.ebay.com/arjenhelder-electronic__W0QQ_armrsZ1
He runs a pretty good store there, lots of PCBs, parts, kits and whatnot. Anyway, I made fun of Andrew and said that he had bought a piece of shite, neener neener, etc. At the time, I went ahead and used an 8Audio board to build the Chairman Mao, which turned out rather nicely.
But then it started grating on me. How could I make such a proclamation unless I tried it? Nothing like casting aspersions without prior experience. So I decided to procure one of these 3 dollar Class D amplifiers off ebay.It arrived and I marveled at its cheapness. Some kind of shrink wrapped input capacitors, suspect looking parts, and fit and finish that was, well let's just say....lacking. Time to put my money where my mouth is. First off, let's see what's behind those shrink-wrapped input caps.
WTF? A super-cheap 1uf poly cap in parallel with a a super-cheap 100uf electro. I recall reading somewhere that this technique has some type of advantage, but I can't recall the reasoning behind it. Also paralleled caps aren't typically seen at the input of power amplifiers.
So, like nasty boils, these had to be removed. Here's where to desolder:
Next up, the power supply caps. Now I'm sure that 220uf is fine for a 12 volt/1A supply, but we are talking about hifi here. Time to dump those suckers. Here's where they are on the board:
And here's where I desoldered them from the bottom:
Now the all caps are gone:
Now run wires to where the power supply caps where. The stock caps where paralleled, so I only had to run two wires.My plan is to replace the two small power caps with a pair of much bigger ones. But they won't fit in the small space left on the PCB. So I'll do those as off-board components. To prepare for this, I ran a red wire to the + pad and a black wire to the - pad.
Time for a Listen!
My plan was to put the whole thing in a nice enclosure with knobs, switches and speaker lugs. But before I went to all that trouble, I decided to hook it up to the test rig to see what it sounds like after the mods.
My source is the Line Out from my iPod Classic (using the super excellent SendStation PocketDock.) From there I set up a pair of old Radio Shack Minimus 7's. To my surprise, the sound was amazing, especially given the cost. It drove the speakers well and even handled the bass and mid-bass parts of the sound with competence.
Let's put it in a box!
For an enclosure, I decided to start with something a bit non-traditional when it comes to DIY hifi stuff. I chose a Hammond 1590BB which is an aluminum alloy enclosure with a screw on bottom. Dimensions are 119 x 94 x 34mm and it looks like this:
I gathered up all the parts for the project, which are listed here:
Time To Measure and Drill
To get my drill holes right, I first used a pair of digital calipers to find the center line on the enclosure and drew a line using a fine-tip sharpie marker. If you don't own a pair of digital calipers, go buy one now! It is a life-saver and makes measuring soooooo easy! I use this model.
Then I laid the components along the center line to find the best fit. With those in place I used a sharpie to mark the drill points.
Now the same with the components for the back side of the enclosure
Time to Drill!
I took the newly marked enclosure over to the drill press and used a small bit to make pilot holes at each mark
After all the pilot holes are drilled, I use a stepped drill bit, also known by its trade name: Unibit. Like the digital calipres I mentioned earlier, this one tool makes drilling enclosures so much easier because you can drill several different hole sizes without changing bits. Harbor Freight carries a nice set of three stepped drill bits for 15 bucks. Can't beat that!
Here's what the drilled enclosure looks like:
Finishing the Enclosure
There are hundreds of different ways to finish enclosures. You can paint them using automotive spray paint cans, hand paint them with enables, etch them, polish them, etc. One of my favoroite easy+reliable techniques is to simply use a wire wheel attached to a power drill to get a really nice shiny-but-tough finish. Here you can see the enclosure clamped into a vise and my venerable 20 year old Craftsmen hand drill with a wire wheel in place.
When you use the wire wheel, you don't need to use a lot of pressure. In fact, the weight of the drill is often enough pressure. Just guide the wire wheel back and forth and check frequently for missed areas.
After brushing the case, I use some acetone and a clean rag to get all the dust and any leftover sharpie marker ink off. I clean both the inside and outside to make sure I have no left over drill shavings or dust.
On to the Wood Sides
Now its time to make the wood sides. As mentioned in the parts list, I used strips of Paduak. Paduak is a fairly soft wood so it is easy to work with. It has a visually stunning reddish color to it that really looks nice after sanding and a few coats of poly or tung oil.
I measured the length and height I wanted and made a few cuts on the power miter saw.
To drill the holes in the sides, I used a piece of scrap lumber underneath. That's a neat trick to make the exit hole a lot cleaner.
Since I'm using pan head screws, you can see that they stick out and look unprofessional. The answer? A countersink bit. That makes a nice oval for the screw head to fit into.
Next up, sand the sides with 150 grit paper, the use 220 grit to make a small bevel on the edges.
Finally, a test fit: do all the pieces actually fit? This time it was good luck, things seem to line up pretty nicely.
Now it's time to wire all this stuff up. The first thing I was worried about was the speaker lug mounting. The holes for the solder terminals were as big as I could make them, but there was still a chance that they may short out against the chassis. This is a bad thing, especially on Class D amplifiers. Class D designs don't ground the negative side of the speaker output. So the SHOULD NOT be connected together, nor should they go to ground.
To ensure this wouldn't happened, I used two passes of heat-shrink tubing on the speaker lugs.
Once installed, I used my multimeter in continuity mode to be sure none of the speaker terminals where short to the enclosure or to each other.
Now on to the rest of the wiring. I did the power first. From the DC jack in the back to a ground lug, then ground and v+ running up to the front for the switch and LED power.
It took about 30 minutes to get everything wired up. The up front part with the switch, input, LED and volume control was a bit tight. I think it probably would have made more sense to design this with the long ends of the enclosure being the front and back, maybe that's that form factor I'll switch to in my next 1590BB-enclosure project.
The Power Caps and FInishing Up!
Finally, I had to mount the new big honkin' power caps. To do this, I snipped 4 terminals off a euro-style terminal strip, trimmed the cap leads, and inserted them into the terminal strip holes. I then connected in the two wires (red and black) from way back earlier. Those where the wires that where soldered to the PCB where the stock caps once lived.
So here are various gut shots of the finished unit.
Time to Attach the Sides
For the sides, I decided that I would use short aluminum spacers so the wood panels would be slightly outset, making the whole thing look a little bit like a tie-fighter :) Overall, the look is pleasing and hopefully somewhat different enough to be interesting.
Conclusion and Listening Tests
Since I had already listened to the capacitor-modified amp back at the way beginning of this project, nothing changed on that front, except I now have a convenient all on one unit. The amplifier sounds great with the three different set of bookshelf speakers I tried. With high-sensitivity speakers it will get quite loud, yet still remain well behaved and pleasant sounding. If you push it too hard at the input, or max out the volume control, you will start to hear distortion when the amplifier hits its limits. But since this design is intended as the driver for a small bookshelf system, it is unlikely you would push that hard.
All in all a fun project. It is interesting to note that this article really has been about construction techniques to take an existing PCB-based component and put it in a box. You could repeat the same general ideas and approaches shown above to just about any type of component.
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