Excellent question / observation regarding the dac's input choice.
db Audio Labs purposely chose not to include the interfaces suggested for very specific technical and MOST importantly, sonic reasons! Basically, for all the interfaces you suggest, some form of circuitry, albeit a chip or discrete componentry, must be included which connects to each of those interfaces and provides an I2S output signal which in turn provided to the dac chip.
So, in essence, each and every interface that you suggest (as well as the USB interface) must output an I2S signal to the dac. Typically, some sort of switching is used to select the respective I2S signal the interface desired. In order to optimize the I2S connectivity to the dac chip, the I2S data signal lines MUST be treated as RF transmission pathways and laid out accordingly. This is seldom if ever done in audiophile consumer products. But, we do it this way with the Tranquility design. In other words, audiophiles desiring numerous interface inputs as is what is typically seen from other dac products, more often than not, prevents optimal RF layout as the I2S signals must traverse the switching necessary thereby adversely affecting the overall sonic capability of those dacs period. So, less inputs makes RF layout manageable leading to better sound.
We also recognize a multiplexer could be used rather than switching. However, multiplexers are not without their own attended problems. As they too impede layout in their own right.
Another thing to consider when adding interfaces, since they are often in the form of some dedicated interface chip and are likely to require some clocking, i.e. an oscillator, for their operation. Those oscillators give rise to the potential of interfering and intermodulating with the oscillator that is in use for the interface selected. Obviously, this like switching, gives RISE to the potential for jitter and usually does, without exception!
dB Audio Labs selected to use USB because there is no correlation between the clock and the audio data. “Jitter”, as properly defined, requires that there is some sort of clock associated with the data. Absent that, there is NO jitter.
SPDIF includes data correlated with a clock thereby allowing for jitter to occur (not as good).
Firewire does not include data correlated with a clock and thereby does not give potential for the rise in jitter nor does USB (much better).
It should appreciated that both Firewire and USB have more than enough bandwidth necessary for the transfer of digital audio data.
However, there is one major distinction between Firewire and USB that is typically overlooked by audiophiles. That is to say, a dac manufacturer who decides to include a Firewire interface on their dac must provide, or be in the business of providing, developing and/or maintaining a Firewire device driver for EACH operating system, (Windows and MAC) for their digital product. And while some boutique manufacturers many be capable of doing such, there is obviously an intended cost to the consumer. Moreover, there exists the potential for a customer to have, for at least some brief period of time between operating system upgrades, for their dac to become inoperable as their current driver could be incompatible as a new version of their operating system is rolled out.
In contrast, USB dacs typically make use of those drivers found in both the Windows and Mac operating systems. USB specifications and standards require this! So, as long as the dac meets the “USB 1.1 / 2.0 standards”, you are golden. And yes, the Tranquility dac meets these standards!
So, rather than spending our time developing and maintaining Firewire drivers, we’ve selected USB connectivity for inclusion in the Tranquility design and put our efforts into developing the balance of necessary circuitry, e.g optimization of transmission paths, power supplies, dac optimizations, I/V stage, analog output stage, parts selection, etc..
We hope that you consider each and every one of these salient points we’ve outlined when selecting your own personal digital to analog converter. We obviously have, as from the foregoing explanations outlined above.
And that is why you only see a single solitary USB input on the Tranquility DAC ;-)