Living at home with my parents during the COVID-19 pandemic, there was one thing that started annoying me after a while. I’d be sitting in my room happily, then my mum comes in, and the first thing she says is “the air in here is awful, open the window”. But… the air feels fine to me, it’s nice and warm, and it’s cold outside… but there’s no point in arguing, I have no way to prove that.
After a while, that annoyance turned into a project idea: What if I build a device to objectively measure the air quality in my room? That way, I would always know when I really need to open a window, and at the same time, have a convincing argument against any potential nagging. 😉
Of course, as with any project, I wanted to get more out of it than just the finished item – I wanted to learn lots of new things while building it. After some research into existing solutions, I came up with the following project goals:
Use an actual CO2 sensor (no “CO2-equivalent” estimations)
Measure temperature too, optionally humidity
Display the measurement values in a clear and visually pleasing way
After the completion of the BlockBox v1, I really thought for a while that I’d be done with this idea. And it only took a few months for me to prove myself wrong, with the BlockBox v2 project.
But this time it felt different – I really was done, at least for now. The BlockBox v2 has its flaws, there are certainly ways to improve it further, but overall… I’m just really happy with it, and very proud of the results of my work – especially if I look at both speakers next to each other.
Let’s look at the features of the final product:
Two-way cross-over speaker system with digital audio path
100W 10″ low-frequency driver
50W hi-fi tweeter
-3dB bass roll-off frequency around 38Hz
Theoretical maximum sound pressure level around 110-113dB (@ 1m)
Bluetooth audio playback (SBC or AAC), effective range ~10-15m
Touchscreen interface with lots of audio settings and playback control
EQ-optimised sound profile (to the best of my abilities)
(Roughly) music-synchronised LEDs
166Wh battery (in practice, lasts for more than 10 hours of continuous playback)
Battery protection and fast charging
2x External USB (5V 1.5A) for phone charging
Optional analog AUX input (not implemented due to lack of practical need)
Looking back at this huge project of building two speakers, I can only say one thing: It was absolutely worth it.
Now, are the speakers by themselves worth it? No, not really. Because I had to buy everything in single-piece quantities, restrict myself quite heavily in terms of what I could work with using the tools I had, and over-engineered many things “just to be safe”, they ended up being wayyyy too expensive to compete with commercially available speakers, at around 1500€ (for both combined) – and that’s if I ignore the cost of the months of work that went into them.
But the main value of this project, for me, was not in the speakers themselves, but the incredible amount of learning and experience that I gained throughout this project, including audio engineering, mechanical design, CAD and manufacturing, electronics design and soldering of course, and microcontroller firmware development.
This is where my speaker-building journey would end for now, but I had a feeling I would be back to it sooner rather than later, in another project.
The digital audio processor included in the BlockBox v2 design (part of the audio PWM modulator) has multiple configurable digital filter stages, specifically, biquadratic filters (“biquads”). These allow the implementation of practically any second-degree infinite-impulse-response (IIR) filter, defined by five filter parameters each.
The baseline for these filters was, of course, the cross-over between the two speaker drivers (low-pass for the bass driver, high-pass for the tweeter, both with the same cut-off frequency). Due to the drivers’ respective effective frequency ranges, I opted for a cross-over frequency of 2000Hz.
Aside from that, the remaining filters were available to fix any frequency response inconsistencies, acting as an equaliser. Therefore, I needed to measure the frequency response of the speaker without any filtering – ideally, this would be done in an acoustic chamber using a calibrated measurement microphone, but I did the best I could: Measuring the response in my room, using RoomEQWizard and the best microphone I had available (a Zoom handheld recorder), opting for measurement methods that try to eliminate most environmental reflection effects (gated and near-field measurements).
Example combined measurement for the low-frequency driver (near-field + gated)
To configure the audio processor filters, I wrote a small piece of helper software that I called SpeakerEQDesigner, which allowed me to visualise the effects of various filter configurations and export the resulting filter coefficients in the fixed-point number format required by the audio processor.
I ended up creating two filter profiles – one that targets maximum flatness of the frequency response (“Hi-fi” profile), and one that sacrifices a little bit of flatness for a higher maximum loudness (“Power” profile).
With these, I should theoretically have achieved very good speaker response characteristics – though I probably didn’t get anything quite as good as the software predicts, because my measurement equipment was quite rudimentary.
Just like before, the build process started with the electronics. So it was time to order new circuit boards and components.
Building the electronics for this second speaker followed the exact same procedure as I described for the BlockBox v1, so I won’t repeat it here. In the end, I had another nice circuit board that looks just slightly different from the first version (shown without the amplifier heat sink here).
Then, it was time for firmware development. This would be a much bigger task than it was for the BlockBox v1, again, because I had to implement a central system controller, as well as touch display communication. I was able to find a community-made library/driver for the FT81X graphics processor in the display module, which certainly made the job easier, though I had to make quite a few modifications to make it work on the PIC32MZ microcontroller. Aside from that, I implemented my own serial communication drivers for controlling the digital audio processor, Bluetooth module, LEDs, and other peripherals, as well as some central system management logic connecting them together, and of course a user interface for the touchscreen. The firmware source code is available in the same GitHub repository as the electronics design files, though I cannot guarantee anything about its correctness, completeness, and especially code quality (I was quite careless sometimes back then).
Clearly, still some bugs to iron out…
When it came to building the enclosure, however, I knew that my methods from the BlockBox v1 would not be viable any more. Making smaller and simpler shapes using my hand saws may have been possible (even if tedious), but the more complex and much bigger pieces needed for this project would just be too much to ask for. Especially considering the idea I had for LED-lit logos on the sides, which require quite significant precision. The translucent parts for the LEDs, as well as some black decoration and cover plates, were laser cut out of acrylic plastic, which I was able to order relatively inexpensively.
As for the main enclosure walls, I decided to use CNC milling – thankfully, though a personal connection to a machine shop at my university, I was able to do so for free, as long as I brought the materials and my own CNC programs. Materials were easy, I just ordered some plain 12mm plywood panels, already cut to the sizes I needed, and got a small piece of aluminium plate stock for the connector panel. The CNC programs were more complicated – Fusion 360 has some great tools for CNC programming based on my CAD design, but as I’d never done anything like that before, it took me a while to research, understand, and learn the basics of machining and CNC programming. In the end, it was time for some machining.
This could be milled much faster with a higher tool RPM, but this machine can’t do more, as it was mostly designed for hard materials. Anyway, I was not in a rush.
With that, I had some rough-looking side panels, which would need quite a bit of post-processing to remove extra tabs and pieces, sand down sharp/rough edges, file away unwanted internal corner radii, and so on.
After post-processing, they looked much better, and also fit the acrylic pieces nicely.
Now it was time to assemble the speaker. I built the main wooden box by screwing the walls together using metal corner pieces, then hand-cut some extra plates with LED strips to go behind the acrylic inserts, installed the drivers, port resonator, display, circuit board, connector plate, battery, and some plastic wool for acoustic damping inside the enclosure.
With everything connected and closed up, the BlockBox v2 was complete.
When I started designing the BlockBox v2, not much time had passed since the completion of the BlockBox v1, so my level of knowledge and experience was similar. I had noticed some mistakes in the original design that I would correct, but decided to leave most of the circuit design untouched, wherever I couldn’t see anything immediately wrong with it.
The first major difference I had to address was the addition of the touchscreen display for user interaction. I looked around at different display options, eventually picking a good and affordable 3.5″ module by Newhaven Display, which offered a sunlight-readable display, resistive touch, and a built-in graphics chip, which would make using the display much easier than if I had to render every part of the user interface in my own firmware.
Still, even with built-in graphics, I would need a much more powerful microcontroller, as it needed to handle the display commands, as well as act as the central system controller – for the BlockBox v1, I offloaded that responsibility to the Bluetooth module, but with the touchscreen, that was no longer a viable option. Also, making my own central system controller would give me much more fine-grained control over everything going on, which was obviously desirable. With these considerations, I decided to use a microcontroller from the Microchip PIC32MZ EF family – it’s likely that I could have picked something lower-cost, but for a prototype project like this, it made the most sense to go with something really powerful, just in case I could use the extra performance.
Another thing that was causing me trouble in the first version was the analog audio signal path. I realised that I was simply not experienced enough with analog circuit design to achieve a high-quality signal in such a way, so I decided to switch to a fully digital audio path for the BlockBox v2. This would also allow me to feed digital audio data to the system controller, for better music-synchronised LED lighting. The first problem with this plan was the Bluetooth module I had used for the first version – it only had an analog audio output. Therefore, I chose its slightly more advanced successor for this design, the Microchip BM83, which also features a digital audio output.
Aside from that, this meant replacing my analog pre-amp and filter stage with a digital signal processor (DSP), and the amplifier with a digital one. Thankfully, Texas Instruments had a great solution for me, which would work as a relatively simple drop-in replacement for my existing circuit: The TAS5548 modulator with integrated DSP, as well as the TAS5634 PWM-input audio amplifier. This amplifier’s pinout and interface was nearly identical to the one I used for the BlockBox v1, allowing me to reuse most of the output stage design – just making sure to use bigger and better inductors, as well as film capacitors for low-distortion filtering this time.
The rest of the circuit remained nearly unchanged. The battery management circuit seemed to work great in the first speaker, so I reused it exactly as it was, along with the choice of battery cells themselves and their 4s4p configuration. As for the voltage converters, I only made a few small modifications – adding a trimmer potentiometer to the power amplifier supply, to be able to lower its voltage for driver safety and efficiency, as well as making the 3.3V supply based on a switching converter as well (instead of just a linear regulator), since the new controller and display needed more power. Unfortunately, I still hadn’t understood the importance of power supply noise reduction in audio circuits, meaning the new power supply design would also be quite noisy – but I didn’t know that at the time. Also, just as with the BlockBox v1, I was still limited in my component choices by the abilities of a simple soldering iron.
I created the modified PCB design based on these changes, deciding to keep the board dimensions and layout the same as in v1, just in case I ever wanted to replace the v1 electronics with the new version as well.
As with the first version, the full schematics and PCB design files (for Autodesk EAGLE) are available on GitHub here. While this design is certainly better than the BlockBox v1, it is still not good overall, and not recommended for direct reuse. After all, it was still designed with some beginner mistakes and flaws.
Since the main source of my “disappointment” with the first BlockBox was the lacking low bass frequency range, that exact point became the springboard for the design of the second version.
Knowing that I wanted a lot more bass, I started looking into better speaker drivers for it. I found quite a few options that would offer a complete bass extension down to ~20-30Hz, but nearly all of them meant sacrificing quite a bit of efficiency. That would mean either increasing the maximum power, which is difficult on battery-powered systems and makes the circuit design harder, or making the speaker quieter. But come on, there’s no way I would allow the second version to be quieter than the first version – that would just make it feel inferior from the start.
So high efficiency remained a very important consideration in my driver selection, and eventually I found a new low-frequency driver that looked great for my needs: The Beyma 10BR60V2. Since it had a lower effective frequency range than the BlockBox v1 driver, I also needed a new tweeter with the ability to pick up higher midrange frequencies. I ended up choosing the Monacor DT-28N, which was also just a higher-quality driver compared to the ones I used for the first version.
There was, however, a very significant difference from the BlockBox v1, with this driver choice: Size. While the first speaker used a 6.5″ driver that could achieve good performance with an 8-9 litre enclosure, this new low-range driver was quite a lot larger at 10″, and according to SpeakerBoxLite, its optimal enclosure volume was somewhere around 61 litres. Yikes.
This prompted me to take a step back and think about it for a moment. Would such a huge speaker even make sense for what I wanted? It’d certainly be much more difficult to carry around. However, I realised that with the right form factor, it may be possible to attach some straps to the speaker and carry it around like a giant backpack itself. The idea was certainly cool enough to make me want to try, so I continued with this choice of drivers.
The enclosure size I ended up deciding on was 55 x 40 x 35 cm, with a 10 cm vent port (Monacor BR-100HP), and walls made of 12mm plywood:
According to the SpeakerBoxLite calculations, with this design, I should be able to achieve a -3dB bass roll-off frequency of 38Hz – certainly a lot better than the first version. It wouldn’t be quite as “precise”, with a slight over-emphasis around 57Hz (+3dB), but I was willing to accept that, especially since audio pre-processing could fix that if it annoyed me later.
Based on this, I once again created a CAD design in Fusion 360, allowing myself to be a bit more ambitious with the features this time. The BlockBox v2 would have some protective rubber corners, hooks for backpack straps, much better LED lighting (including lit logos on the sides), a nice aluminium connector panel on the side, and even a touchscreen display for user interaction (instead of the basic buttons of the first version).
Of course, the electronics would need some changes to go along with these new features, as well as some upgrades to address some of the shortcomings I noticed with the first version. So, naturally, that was the next step of the design.
With the end of the BlockBox v1 build, I had finally accomplished my long-time ambition of building my own Bluetooth speaker – it really felt like I had reached a new level in my electronics projects, with the ability to build more complex systems.
Let’s summarise the features of the completed speaker:
Two-way cross-over speaker system
100W 6.5″ bass/midrange driver
Two 15W tweeters
-3dB bass roll-off frequency around 72Hz
Theoretical maximum sound pressure level around 110-113dB (@ 1m)
Bluetooth audio playback (SBC or AAC), effective range ~10-15m
(Roughly) music-synchronised LEDs
166Wh battery (in practice, lasts for more than 10 hours of continuous playback)
Battery protection and fast charging
External USB (5V 1.5A) for phone charging
Optional analog AUX input (not implemented due to lack of practical need)
Diagnostics LCD for showing battery and Bluetooth status
Sure, there are many imperfections and mistakes in the design, as is expected for my level of experience at the time, and I tend to focus on these imperfections too much myself when looking back at it.
But taking a step back, looking at the bigger picture of what I built in the BlockBox v1 – I’m very proud of it overall. It’s a powerful, functional, portable speaker, and while its audio quality isn’t “hi-fi” by any means, I would also never call it bad, it’s certainly good enough for most use cases.
However, just being proud of it was apparently not quite enough for me. Yes, it was a good achievement for me, but I wasn’t quite satisfied. The bass roll-off was too high for my taste, the button control scheme was too basic, and the low-volume audio output was too noisy.
The first part of the BlockBox build process was the circuit board. Once my custom boards arrived, along with the components for them, I decided it would be best to build and test the power supply circuits first. Because battery management is not trivial in itself, I would start with external adapter power – which I needed to get from somewhere. So, it was time to modify my laptop power supply, adding a compatible connector.
With that done, I could solder the necessary components for basic power path. This was followed by constructing the main switching power supplies (52V, 12V, and 5V), allowing me to test that they work and produce the desired voltages.
Next, I added the Bluetooth module and auxiliary controller, as well as the corresponding connectors and support components. I also soldered the power amplifier chip, since its small pin pitch would make it annoying to solder with more components in the way later.
To do any real testing, I would also need the user buttons mentioned in the previous post. They were soldered on a separate little board, to be inserted into the enclosure later. I chose some nice tactile buttons with LED backlights. They also support custom labels to be inserted into them, for which I took some printable transparent foil, resulting in presentable, self-explanatory buttons.
After adding the rest of the analog audio path and amplifier output stage, the circuit was ready for testing and firmware development. And yes, some of the power amplifier output components (especially the inductors) were severely undersized in hindsight, leading to more distortion at high power. As you can see, the amplifier also got a heat sink, along with a temperature sensor and a temperature-controlled fan, which worked quite well.
During firmware development, I also realised that my initial idea for LED music synchronisation didn’t work – so I had to work around it, and the results were quite mediocre, but still did work somewhat.
Finally, all that was left was the battery protection and charging circuit, as well as some transistor heat sinks, and we have a complete and working circuit board!
Speaking of the battery – that needed some assembly as well. I ordered 16 Li-ion cells (in the standard 18650 size) with pre-installed spot-welded nickel tabs. To keep them arranged nicely and safely, I also got some plastic cell spacers.
Safety note: DO NOT mess with Lithium batteries unless you really know what you’re doing. They can be dangerous and start fires if mishandled.
These cells were then arranged into four groups of four, creating the desired 4s4p (four series, four parallel) configuration. I soldered the four parallel groups together, before using some solder wick to connect them in series. Soldering isn’t ideal, as it could overheat the cells – spot welding is the way to go, but needs tools I don’t have, so I just had to carefully solder the nickel strips – it ended up working out quite well.
Now I could get to the last piece of the puzzle: The enclosure. I didn’t have access to the ideal tools for such a build (CNC mill, or at least woodworking power tools) – so I made everything by hand out of plywood stock, using a drill, various hand saws, files, and sandpaper. The resulting cuts were far from perfect, but good enough for this project.
For the RGB lighting, I glued RGB LED strips to some acrylic rods, before gluing everything (except for the back wall) together to create the enclosure (just missing the side compartment in this image).
The only thing left to do now was to install all of the components inside it, and I had a complete BlockBox!
This would be the time to say something along the lines of, “That concludes the BlockBox project, thank you for reading”. But… let’s not rush it.
One reason why I initially dismissed a Bluetooth speaker project as “too complicated” was, put simply, the Bluetooth part of it. At the time, I barely understood most conventional electronics, so the idea of getting involved with something as complicated as RF communication seemed completely out of my reach. Looking back, I was absolutely right about that feeling – trying to design my own Bluetooth receiver would have been a bad idea. Even years later, at the time of writing, I’m not sure I could succeed at that task, unless I spent many more hours researching the topic – and even then, it would be quite a lot of work for a single person, due to the hardware and software complexity involved.
In light of this, there was only one thing that made me feel confident in pursuing this project anyway: Discovering the existence of Bluetooth audio modules – in particular, for this project, it was the Microchip BM62.
I found that such a module struck the perfect balance: It satisfied my desire to have the system be “designed by me” to an acceptable degree, while abstracting away the complicated task of maintaining Bluetooth links and sending/receiving wireless audio data, instead providing a relatively simple interface.
For simplicity, the module has an integrated digital-to-analog converter (DAC), directly outputting the raw audio signal. In my design, this signal would then pass through an analog pre-amplifier and filter circuit, conditioning the signal to the levels I wanted and implementing the driver cross-over (at around 5000Hz). The last amplifier stage included trimmer potentiometers, allowing me to adjust the maximum output levels, in order to not overdrive and damage the speaker drivers.
Looking back at it, this was not a great design by any means – the op-amps I chose are quite noisy, I was using Class-2 ceramic capacitors (which cause distortion), the voltage supplies were not designed with noise reduction in mind at all (likely the biggest single mistake I made here), and so on. But, despite that, it did still work – which is all that mattered to me for this first design.
The pre-amp/filter stage was followed by a power amplifier – namely, the Texas Instruments TPA3255. It was configured in 2.1 channel mode, with two single-ended channels for the tweeters and a differential high-power channel for the bass/midrange driver.
Aside from the audio path, I needed a power supply. This was based on a large lithium battery (4s4p configuration), with corresponding battery protection, balancing, and charging ICs.
With the base battery/adapter voltage available, I now needed multiple voltage converters to generate the various system voltages (3.3V, 5V, 12V), as well as a high-power voltage rail for the power amplifier (52V, which turned out to be overkill for these drivers). For the design of these converters, the Texas Instruments WEBENCH Power Designer proved invaluable, especially for a beginner without any switching power supply design knowledge.
For controlling the speaker, I again decided to go with the easiest option, letting the Bluetooth module’s integrated controller operate on its own, controlled using a set of buttons. Aside from that, I just added a low-performance microcontroller (Microchip PIC18 K42 series), to handle auxiliary tasks like battery charge estimation, diagnostic display (using a text LCD), and controlling some RGB LEDs that I wanted to sync to the music.
Since I only had a soldering iron available (no hot air or reflow tools), I ended up being quite limited in terms of component selection. Thankfully, I was able to find everything I needed in “soldering-iron-friendly” packages, but it did mean choosing suboptimal components in many cases.
Some design work later, I placed and routed everything onto a single 12 x 12 cm PCB.
The full schematics and PCB design files (for Autodesk EAGLE) are available on GitHub here. However, be aware that this design is NOT GOOD overall, containing many beginner mistakes, and the firmware code quality is likely bad for the same reason. Therefore, I do not recommend that anyone actually use these designs, but they may be useful for anyone who’s curious.
As I started my research into designing the BlockBox, I realised one thing: Speakers are really complicated. So many speaker drivers to choose from, priced anywhere between affordable and astronomically expensive, with different sound profiles, efficiencies, features, and drawbacks. And what the hell are “Thiele/Small Parameters”?!
Hours of research later, I figured out what most of these things mean, and found out that the final size (volume) of the speaker would play a major role in the low-frequency sound design. Since most commercial Bluetooth speakers are very small, I always imagined making one of a manageable size too – so that is what I decided on. It’d still be larger than the typical “handheld” mini-speakers, but small enough to fit in a backpack.
Finally, I decided on a set of speaker drivers to use for this project. I wanted my speaker to be loud, so driver efficiency was very important, and my budget was quite limited, of course. Therefore, I went for a two-way crossover speaker, using the 6.5″ Monacor SP-6/108PRO as a bass/midrange driver, and the Monacor DT-74/8 tweeter (or, in fact, two of them – mostly for the sake of symmetry, as you will see later).
In order to get the most out of these drivers, the design of the enclosure was quite important. Using SpeakerBoxLite, an incredibly useful free tool for speaker design, I determined that a vented enclosure with a volume of 8.7 litres would be perfect given these drivers and my size requirements. Combined with a port resonator of the correct size (Monacor BR-50HP), this results in a 33 x 26 x 13 cm speaker enclosure with 10mm plywood walls.
According to the SpeakerBoxLite calculations, this should result in a flat frequency response down to 90Hz and a -3dB roll-off point of 72Hz. Not great, especially for fans of music with lots of bass (including myself), but not terrible either. I’m sure it’s possible to do better within the constraints of this project, but all better options I found back then were too expensive, too inefficient, or just too complicated for me, as a beginner.
Based on this rough sketch, I created a more detailed enclosure design in Autodesk Fusion 360 – featuring all drivers, some translucent bars for LED lighting, a cutout for buttons to control the speaker, a side compartment to house a diagnostic display, the main power switch, and connectors for charging etc., and of course, enough space in the back for the battery and electronics.
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