Designing a 10W Class-D amplifier

Let’s start with the age-old question of why?
I know, ready-made modules are available out there, that can do the same for the same cost or lower – but I didn’t make any of those.

During one of my searches for samples over at the maxim website (Maxim-ic.com) I came across a nice chip – MAX9768, a mono 10W Class-D amplifier chip. I put some in my samples cart – audio amps are alwaysa big fun to play with. This one would work filterless and still meet EMC standards if speaker leads were under 10cm. perfectly doable. I did however, forget to look at the package. a 24-pin TQFN with 1mm pitch. Ouch!

The chip lay dormant in my ‘samples’ box for more than a year. then, I got contacted by some of the amazing people from FabLab Danmark and a project emerged. The task was to make a small boombox for people to make at the Roskilde Festival Makerspace. Challenge accepted.
I found the MAX9768 chip and compared it to some of the other amplifier chips I had. It won by being filterless and very easy to implement. The boombox were going to be a single-speaker solution, so this mono chip would be perfect for the job.

Now, on to the prototyping. I had to mount this chip somehow. This is where my apparent fetish for SMD soldering-by-hand comes to life. I present to you – 24-pin TQFN dead-bug style:

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Sorry about the poor quality but I think you get the idea of how small it is. The break-out is 0.1″ breadboard friendly size. the chip itself is 4*4mm.

Breadboarding the rest of the system was quite easy – in total 9 components are required to make this amplifier work. this includes a potentiometer for analog volume control! The chip also have I2C volume control and a sync feature that allows multiple amplifieres to be cascaded. I don’t know if it will allow to set them up in parallel, but right-left systems should be possible using the first chip as a master, providing the clock signal for the rest of the chips.
It quickly turned out that the breadboard is not meant for high-frequency design like this. The datasheet states that output is a spread-spectrum modulation with a center frequency of 300kHz, changing +- 7kHz to minimize the emitted noice hence allowing filterless operation.

Obvious solution to this was to mount all components directly to the pegs on the breakout board which proved to be somewhat difficult because the heat transfered through the ‘bonding’ wires melted the solder on the chip. I soon found myself resoldering the small wires on the chip with huge through-hole components very close by obscuring my view. but I succeded! It worked! And although the 10W is at 10%THD and the numbers look really ugly on paper, it plays really nice! I mean, comon! It’s meant to be taken to a festival! People will rather have a boombox that plays really loud than one that plays with 0.0001% THD…

Next thing – PCB design. Stay tuned.

10W Class-D amplifier – The layout

So with prototype done and tested It was time to make a proper PCB for the project. well, so I thought. The thing was that while I had designed the amplifier other people had been hard at work too. The idea was for people to be able to buy a kit containing all the needed materials for a boombox where they could plugin a their favourite player and listen to music.

The boombox project was only one of multiple kits that people could get their hands on – a DIY guitar was also available. this meant that I had to figure out a way to implement a guitar pre-amp into my system. also, since I only had one speaker and a mono amplifier, I should do a stereo->mono conversion in my design.

let me highlight the features of the project:

  • 10W amplifier
  • Guitar pre-amp
  • stereo-to-mono summing
  • make 100 kits ready for the festival!
  • oh, here’s a fun one: the participants should be able to solder the board themselves.. goodbye SMD parts – Hello debugging!

I knew from the prototype that the design should be relatively low-noise since I have high frequency running around on the board.

I spend some time looking for a VERY simple and cheap guitar pre-amp and found this. At the time I was runnin out of time so I had to go with the design and hope for the best. I figured that I could put a potentiometer instead of the 1K and 2K resistors to change the amplification of the op-amp but I couldn’t test it. Also, I don’t have a guitar so I was taking a long-shot here.

The stero-to-mono summing is was based on the design found here. It made sense and again – I was out of time.

The only thing left was to figure out a way to change between ‘line’ and ‘guitar’ in a sensible way. I chose a 2-pole switch which would allow me to bypass the op-amp circuit (or put additional gain on my line signal). The stereo to mono converter would be the first thing the unit would see – in retrospect this was maybe the reason the pre-amp was not working properly.

This is what I came up with

schematic

 

As always I’ve used DesignSpark PCB for the schematic and layout. Most of the parts I had to design myself but it works perfectly. The schematic was translated to PCB and looks like this

blaster-layoutI tried to make a small PCB but during the component design I forgot the with of the capacitors. The result was that some of the mouting got interesting. but it worked!

As I noted before the participants should have a hands-on experience with this project so the PCB comes with the chip aldready soldered. oh, where do I get that done? I spent some long nights with a heating gun, solder paste and a toothpick. that’s how.

So, how did it go?
We successfully made all the amplifier and boomboxes we had prepared. Okay, during my late nights of soldering, I screwed up about 10 amps but I had ordered enough extras so I could make new ones at the festival. yup, some more fine-pitch SMD soldering!

Also, You shouldn’t miss out a picture of a pair of the units. the signal is split before entering the amplifier and the summing part is left out as well as the guitar pre-amp. The system is mounted in a small backpack I use for skating trips. The power comes from a 6.8Ah LiPo power pack which sits behind the amplifiers. Sorry for the cable mess!

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Here’s what you want. The design files: Rev.1
I’m working on Rev.2 and 3 which is just the amp with PTH and SMD components respectively. Stay tuned!

 

New board revisions

As promised in the previous post here is some newer revisions of the amplifier board.

I have removed the guitar preamp and obviously the switch to control it as well. This makes the board a whole lot smaller so the overall footprint is now:

42.4mm * 22.5mm

PTH

I also have done a mono version, without the summing block. This removes 3 resistors and saves a bit more space. Not much, but it could be the difference between a fit or not! Mono version is now:

33.6mm * 22.5mm

PTH-Mono

Lastly I have done an all SMD version. This is more a test in ‘how small can it get’ but it should be functional. I havent tested it out yet. To make it even smaller I have deleted all component numbers. It might be possible to make it even smaller if the headers were replaced by SMD pads where wires could be soldered directly. the size for the SMD version is only

18.1mm * 20.8mm !

SMD

the SMD version also has stereo-to-mono summing so comparing the SMD and the PTH versions, the SMD version is only 40% of the original size. about 3.8 square cm!

The files:

WARNING! Since this is where the files are, chances are that you will read this warning as well! the linear regulator should be 3.3V and NOT 5V! The systems I’ve made is running on 5V but it is NOT recommended by the datasheet!

Rev.2 – PTH

Rev.2 – PTH – Mono

Rev.3 – SMD

Amplifier project aftermath: 20W stereo version

While finishing my work on the Roskilde boombox project with the MAX9768 chip I discovered that there is a sister-chip called MAX9744. While the 9768 is a 10W mono amplifier the 9744 is a 20W stereo chip! so 4 times the power in a single chip, while the same controls are maintained! This was too good to pass up, so I present to you, the Rev.4 of my amplifier project. This is basically just the recommended system design with SMD parts.

schematic_20w

The schematic layout more or less matches the 9768 design file. The only change is the dual input and output along with a 100uF cap at the output stage supply.layout_20W

 

The layout is quite small as well - 25.4mm * 22.4mm. I have put a lot of thermal vias on this one just to be sure the heat can get away from the chip.

 

So there you have it – go make some noise!

Rev.4 – 20W – Stereo (unverified. Read as I haven’t made a prototype yet.)

Bluetooth speaker

What re you going to do when a good friend shows up with an old speaker system and tells you to make something cool out of it?

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One of the speakers in all its tiny glory. It really isn’t as big as it looks on the picture!

Okay – first you put it away for some time so you can move. When that’s done you order a bluetooth module from ebay and whip up one of those small amplifier circuits that you designed last year. Lastly, you shove it inside one of those speakers and dance because you now have something that will sound way better than the cellphone you would be listening on before!

It really is that simple. Whoever said that building a small stereo should be hard and cumbersome is wrong!

2014-01-14 00.16.12The modules themselves are really small. The right one is of course the amplifier. The left one is an OVC3860 breakout module bought on ebay for about 35DKR (~6$).

The pinout can be a be a bit hard to find, which also was the main reason I haven’t finished this project earlier. however, I managed to find a bit of information and as with so many of the other bluetooth audio modules out there it is really simple.

The only parts that I needed to add besides cables was:
1. 100uF capacitor on supply terminals for the bluetooth module
2. 10K potentiometer for volume control on the amplifier side. the bluetooth module has a high-pitched noise when no music is playing and the amp. volume is on 11.
3. power jack
(4. on-off switch – but only because I had one that was suited. Otherwise I would have done without)

The only downside, which is also reported by other users of the module is that when doing a hard-off. the module doesn’t have time to report to the phone that it’s turning off. this means that the bonding between the module an the phone is kept by the phone. to reattach the module, I need to toggle the bluetooth on my phone.

Well, I promised you som schematics of the module – here they are:

bluetooth pinout

Pin description

footprintThe top one also describes a small speaker/headphone amp, which I don’t think is present on the module. To do soft-reset, connect pin 14 to ground for a second or two. the same to power on again. this approach makes sure that the bonding is done properly.

The two small modules was glued to a piece of Veroboard and put inside the speaker. The heatsink is on the linear regulator providing 3.3V for both the amplifier and the bluetooth. the bluetooth is pretty power hungry!

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There really isn’t anything more to it! power in, music power out!2014-01-14 20.17.40These days nothing is complete without something 3D-printed. So I figured that I could make a cool-looking back panel to hold volume control and power connection. Somehow I managed to screw up 43mm with 45, so the mounting holes are off. I will redo the panel soon. (as if – if it ain’t broken, don’t fix it, right?)

Class-D amplifier conclusions

I’ve completed the last part of the amplifier project. The 20W stereo amplifier board i tested – and works like a charm!

I discovered an error in the layout of the input stage having connected the feedback resistor to the input instead of the feedback pin. whoops, no sound through the left channel. luckily it was a very quick fix with only one track cut and a rotated cap. job done – now the amp is playing along very nicely!

I should do some real power testing to confirm the efficiency claims but I haven’t got the time yet. Although I have some serious 4 and 8 ohm resistors laying arround (>300W power resistors). I might do that one day.

2014-03-30 20.43.36Just a nice quick picture for comparison. the weight? I should get a precision scale. The kitchen scale wont even register the small (10W). both combined is about 3g.. talk about a Power-to-Weight ratio that rocks!!