For a while I had been discussing a solution for bringing decent fidelity to a family members home. This actually went so far as to me designing and building a USB DAC/headphone amplifier, sadly this didn't actually work with her laptop, major bummer, so that idea was shelved and my brother got the headphone amp. A little while later she ended up with a tablet computer and microchip released their RN52 bluetooth audio module.
Curious I built a tiny pair of bluetooth speakers based around the BF45 from Visaton, the RN52 module and a TPA3118 class D amplifier from TI. These used a simple passive line level crossover between the RN52 and amplifier to correct for bafflestep losses and to protect the tiny driver from overload via a high pass filter. They sounded fairly decent but the battery life wasnt spectacular. They ran from two 18650, 2600mAh, lithium ion batteries but as no micro controlled was involved, plus only having very basic power management, ran at a fairly high quiescent current. They were however very simple and proved the point that the basic idea was a good one. I used these with my tablet when watching shows on Netflix and Youtube but for a proper room filling sound needed a little more.
First and foremost they needed a more capable driver. Within its own limitations the BF45 sounds very nice but can't really fill a room with music, perhaps a small corner, but definitely not a room. They are excellent for an inexpensive pair of small nearfield speakers, but if you want a little more you need to look elsewhere.
The second issue was with the RN52/amplifier interface and the DAC quality of the RN52. For any real improvement to be had a new DAC would be required and one with an integrated DSP. This would require a micro controller and would bring about the potential for further improvements in power management.
At the time there were no real offerings from semi conductor manufacturers that were compelling for the second issue and none that really impressed me for the first. TI had their TAS5706B chip, which while attractive, wasn't really suitable. It required more voltage than two series Li-ion batteries provided and took up a bit too much space to make it feasible. Tang Band speakers had a few small drive units that would fit into the proposed cabinet but had lacklustre specs for unsupported operation, 0.5mm of xmax really isn't my thing.
Whizz forwards a few months and all of this changed. SB Acoustics added a new full range driver to their portfolio and TI added a preview for a new integrated amplifier. The two items in question were the SB65WBAC25-4 and the TAS5756M.
The latters output stage appeared to have had been built around the TPA3118 allowing it to run from two series Li-ion batteries and also included an onboard DAC/DSP solution that looked very similar to the PCM5142. Indeed the devices programmable registers seemed pretty much identical to those of the 5142 and even with the very poor datasheet I took a chance and assumed my code for the 5142 would work with the 5756M. I am glad I did as this paid off.
SB Acoustics have only been on the scene for a short amount of time, but what they lack in longevity they make up for in engineering ability, having been founded by veterans from the now defunct Danish Sound Technology. Almost all of their drivers measure like champs and are very highly regarded, both in terms of performance and for their relatively high performance to price ratio. The SB65WBAC fit my requirements perfectly. Given SBs datasheet a nice extended response with a stiff aluminium cone that promised pistonic operation across all of the audible band. It also came with what every other driver lacked, a healthy amount of xmax and venting in all of the right places.
From preview to purchaseable took quite a while for both of these products, I waited months for both to become available but neither disappointed.
The attention to detail in these tiny drivers is rather impressive, especially considering their price. They measure far better than Scan-speaks similar offering that costs a lot more.
The enclosure for the project was chosen to be an aluminium box from Hammond. Not only would this reduce the build work for the design but it would also work in the designs favour vs trying to make something out of MDF. CJD, over at the HTguide forums, first gave me this idea with a tiny speaker he built and I thought it was too good to ignore. While the aluminium is prone to ringing it is also very stiff and takes minimal volume away from the overall cabinet. This is very important in tiny loudspeakers as the thickness of the construction material, and hence the volume it takes up, always needs to be taken into consideration when coming up with a box design. The aluminium is far thinner than any MDF that I would have otherwised used so it's a perfect choice. The other nice thing about aluminium is its thermal conductivity is very high and while the electronics weren't expected to give off much heat it's certainly a better idea to put them inside an aluminium box than a wooden one.
As can be seen construction is fairly simple. The original aluminium lid for the box was removed and used as a template for the creation of a replacement made from a piece of yew wood. A piece of self adhesive sealing strip was appied around the edige of the wooden front to provide an air tight seal between the wooden front and the aluminium box. The screws to attach the original lid were reused with the wooden one. Once constructed the driver needed to be measured.
Both nearfield and farfield measurements were taken to fully determine the drivers performance. The nearfield response is fairly predictable, due to the drivers highish Qts and the boxes small volume the end result is a fairly high box f3 and Qtc. This was to be expected and is one of the reasons why the DSP was required for the electronics as it would allow a mild amount of EQ, via a Linkwitz Transform circuit, to give the speakers a more pleasing tonal balance.
The farfield response looks far worse at first glance. The SB65 has an intrinsic rise towards high frequencies and the additional loss towards low frequencies is simply because of baffle step. The SB65 does have a mild depression in the response centred on 1kHz, but this is made worse in my design due to the mild effects of some cabinet diffraction. Thankfully the DSP can help address all of these.
After applying some simple EQ a decently flat frequency response is arrived at.