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After the I²S is reclocked and stripped of jitter it hits designers Guido Tent's and Bart Van der Laan’s choice of PCM1792 DAC for its musicality over other comers once a low-jitter input is assured. This Burr Brown part enjoys a very high S/N ratio of 132dB combined with large dynamic range of 129dB. These specifications make the chip a quite ultimate choice in conjunction with its THD+N of -108dB. Let’s also mention that this is a dedicated 2-channel chip.

The PCM1792 outputs differential currents which must be converted to voltage, then amplified and buffered. For I/V conversion a pair of LM4562 opamps do the job in a 2nd-order low-pass function. A third LM4562 handles the single-ended output. For DC filtering Tentlabs of course picked their favorite coupling capacitors.

The SRC not only creates the I²S stream, it also generates the S/PDIF output which the b-DAC reclocks for a second time with its own XO clock before it is buffered and calibrated for a precise 75Ω output impedance. This turns the b-DAC not only into a digital-to-analog converter but also USB-to-S/PDIF D/D converter or coax in/out reclocker.

For his power supply, Guido turned to fellow Dutch firm Amplimo to develop a special toroidal transformer to deliver the needed voltages to the rectifier diodes. For ripple suppression there’s a pair of capacitance multipliers which enabled the use of smaller capacitors. Another Tentlabs signature solution are the shunt regulators. They stabilize the analogue and clock power supplies and isolate them from cross talk. On the digital side the power supplies use a dedicated stabilizer and to reduce cross-talk here each integrated circuit chip is equipped with a supply-decoupling network

As the photos show the whole circuit is laid out neat and clean as the result of many years of experience in ultrasonic technologies. Jitter, cross talk between digital and analogue domains and electro smog should be avoided at all times wherein a good layout becomes crucial. This arrives at a DAC that accepts 16-bit/48kHz over USB  and 24/192 via coax/optical. The analog sockets output 2 volts at 30Ω, the coaxial output is exactly 75Ω.

How does one operate this box with just one control and no remote? Tentlabs opted for a multi-function wheel which can be pushed and turned to access various menu layers, albeit via occasionally cryptic abbreviations. As of May 2011 the b-DAC has been equipped with a 3-digit display. Our loaner still ran off two digits where power on/off was displayed as logically ‘On’ and ‘OF (mind the upper and lower cases). Unlike more contemporary displays the b-DAC runs a nostalgic 7-segment LCD which takes some getting used to but otherwise works fine. User manual at hand the device is fully operational within a few minutes, giving you a volume setting and choice of input.

As a mini preamp the b-DAC attenuates in the digital domain or outputs an industry-standard 2V when the display reads '-'. Digital attenuation eventually decimates resolution. We found 60 increments down from unity gain entirely benign below which the sound got grainy. Guido opines that any negative effects from the volume control would be noticeable immediately, i.e. even at the mildest attenuation.

Holding the control knob down for a few seconds accesses ‘ES’ for extra settings whereby one can control display brightness, sample rates for the DAC and S/PDIF input and digital filter slopes. The DAC’s oscillator runs at a default 24.576MHz, i.e. at multiples of 48kHz. This means that its sample rate settings offer 48, 64, 96 and 192kHz option, the latter still displayed as ‘92’ due to our two-digit constraints. With a jumper setting and different oscillator available by special order the default oscillator can be set to multiples of 44.1kHz to make the sample rate options 44.1, 58.8, 88.2 and 176.4kHz. The reclocked coaxial output can be set to 176.4 and 192kHz depending on what an outboard converter will sync to.