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.
And with that, it was time to build this giant “portable” speaker!