Surface Mount Technology - Portable Bluetooth Speaker (SMT-PBTS)

SMT-PBTS is an open source, Hi-Fi Bluetooth speaker PCB designed for the smallest, best sounding portable Bluetooth speaker applications. Just smaller than a credit card, it features multi-channel configurations for an immersive soundstage, two programmable audio DSPs, Bluetooth v5.0 technology with external antenna support, 24-bit 192kHz Hi-Res audio support, and more.
Source code and technical details: https://github.com/DaGooseYT/SMT-PBTS

Background:
I have purchased numerous portable Bluetooth speakers in the past only to be disappointed in their fidelity. Unfortunately, many manufacturers cut corners for costs that greatly impact the acoustical quality of the speaker and the application as a whole. These cost-saving methods include rolling off the bass at a higher frequency for "longer" battery life, using cheap ceramic capacitors in audio decoupling filters, using highly compressed and terrible Bluetooth audio codecs such as SBC, and using Class D amplifiers to drive tweeters for the highest efficiency. Regardless of the amount of corner-cutting that portable Bluetooth speaker manufacturers appear to make, their products are often ridiculously expensive and are still constructed with cheap materials. With these problems in mind, I took it upon myself to construct a small, high fidelity yet efficient Bluetooth speaker circuit board that corrects the faults of nearly all commercial Bluetooth speaker products.

Objectives:
It was important to make the PCB as small as possible primarily because the speaker enclosure it will be housed in will also be small and portable. For revision four of SMT-PBTS, I selected PCB dimensions of 75mm X 55mm and attempted to place as many components on the board as possible. Both sides of the PCB were utilized for placing components.
I was unable to provide any means for the board to accept wired auxiliary inputs, which is the most optimal method for a high fidelity audio source. However, the HI-RES audio support via Qualcomm aptx-HD in the CSR8675 was implemented to compensate for that by providing a similar to wired quality listening experience. On top of the aptx-HD support, SMT-PBTS also supports SBC, AAC, MP3, Faststram, aptx, and aptx-LL Bluetooth codecs as alternative means for A2DP music streaming.
SMT-PBTS also had to output up to 35W of unclipped audio power per channel to the bass drivers to allow maximum performance. Two Class D amplifier chips were used as they are highly efficient and do not require bulky heat sinks, or in this case no heat sinks at all. However, for the tweeter drivers, small Class AB amplifiers were used as linear amplification of an audio source sounds more natural than their Class D counterparts, where Class D amplifiers implement pulse width modulation for amplification. Although Class AB amplifiers are not near as efficient as Class D, they typically require less external components which is a must for the small PCB dimensions requirement. The combination ("hybrid") of the Class AB and the Class D amplifiers gives the overall application unparalleled audio quality while still maintaining good power efficiency.
The components used for SMT-PBTS are top-notch. All of the capacitors used for the audio decoupling filters require the Panasonic acrylic resin series of film capacitors. While ceramic capacitors are attractive to most manufacturers due to their small size, high capacity, and low cost, they are generally inconsistent with maintaining their rated capacitance at varying voltage levels and are microphonic when used for audio decoupling purposes, resulting in added noise to the audio signal. Film capacitors for use with audio decoupling and filtering are most optimal because they do not suffer from the disadvantageous characteristics as ceramic capacitors with regards to audio. However, they are much bigger in size and are much more costly than the ceramic capacitor counterparts.
Since most stereo mixes typically have an imbalance between the out-of-phase and in-phase musical content, there is an on-board hardware CODEC with software coding that has been implemented for the tweeters to split the phase and then boost the out-of-phase content by around +1dB. The result is a more immersive soundstage, since most instruments are panned to the left and right in a stereo mix.
Although not specific to the SMT-PBTS circuit board, a 14.4V lithium ion battery pack with the highest capacity cells available should be used as a power source to account for the speaker enclosure efficiency limitations. The associated battery management system should be able to output up to 8A of current.

Challenges:
The associated speaker enclosure designed for SMT-PBTS is very small and is port tuned to reproduce bass frequencies as low as 40Hz. In comparison, the average 12-inch subwoofer would have a tuning frequency of around 32-38Hz making the frequency response characteristics for the SMT-PBTS enclosure design to be extremely impressive. It was also designed with medium density fiber wood as it's one of the most resonant enclosure materials for speaker cabinets. However, there is a huge trade-off. According to Hoffman's Iron Law of speaker enclosure design, you can select two of three parameters, but will have to compromise the third. The parameters being small enclosure size, high efficiency, and deep bass reproduction. The enclosure heavily focuses on deep bass reproduction and small enclosure size which leaves the third parameter, high efficiency, out of the equation. This is why the PCB for SMT-PBTS had to be the smallest possible so that the absolute most enclosure volume can be taken advantage of to produce the deepest bass and the most efficiency possible. While the speaker enclosure for SMT-PBTS is severely inefficient, it is compensated through the use of small yet high power speaker drivers with huge excursion capabilities, powerful and efficient Class D amplifiers to drive the bass speakers, and the largest capacity battery cells for their class being used in the battery pack.
SMT-PBTS has very low total harmonic distortion of ~0.03% and SNR of ~110dB.

Other notes:
SMT-PBTS has been in development since the year 2021 and is still to this day. In each revision, DaGoose works to make the PCB smaller, more efficient, and sound better. For revision four alone, it took nearly 80 hours to develop the PCB, coding, schematic files, and perform tests.