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When processing DSD, even a little instability in power causes distortion, hence Ted’s avoidance of an all-in-one commercial DAC chip. On-chip low-pass filtering is equally problematic, hence most manufacturers revert to some digital filter assist. For DSD this results in a give or take of 4 bits with noise shaping instead of the more desirable 1 bit with noise shaping. Moreover 4-bit ‘precision’ means that only 1 of 16 values is correct yet that’s the signal which is fed to a low-pass filter inside a converter chip whose tiny capacitors are struggling to do the job properly. To overcome these limitations manufacturer add capacitors to the boards which get used at random. The end result might be more satisfying but isn’t perfect.

filter board

Ted’s low-pass filter relies on full-size matched capacitors which are doubled up when necessary to obtain the best filtering. In any ultimate DSD DAC, it is simplicity which rules for its designer. All you need is really clean power. The incoming signal is sent either to the high power rail for the ones; or the low power rail for the zeros… before it encounters the filter. Besides the cleanest possible power one also needs a clean clock. That’s not the incoming clock signal which gets completely discarded. Finally one needs the cleanest possible switch to steer the signal at the right power rail for each moment.

In hard figures these requirements come more to life. Ted Smith states that in respect to the power supply, a value just 0.1 too high results in a pulse that is 0.1 too big. Likewise power that is by 1/1’000’000th too high results in a pulse that’s -120dB too big. 1/1'000'000th of 2 volts is 2μV. Working with micro volts calls for an extremely stable power supply. Looking at the clock, 0.1 too late means a pulse 0.1 too big. 1/1’000’000th too late means a pulse -120dB too big. That 1/1'000'000th at twice the DSD rate of 5.6558MHz equals 0.177 pico seconds.

To deliver that clean power, Ted designed an elaborate power supply which "filters, filters, filters and regulates" and that many times over. For his clock Ted did not pursue one that would be stable for aeons but one that would be precise from moment to moment. No more is needed. After evaluating all manner of CMOS-based switches, a simple class A switch worked best. Finally, the low-pass filter became a resistor – capacitor – transformer – resistor – capacitor circuit.

power supply with Bridge

When all these requirements were met, the DirectStream DAC would retrieve literally unheard-of qualities from PCM and DSD alike. And only a discrete DAC can. All chip-based designs suffer from their inherent close proximity of micro parts. A discrete DSD DAC is not only linear—as DSD is linear—it permits the use of real transformers. In the DSD DAC design the only extreme proximity is between master clock and reclocker. Only a few millimeters separate the two to prevent incoming jitter. According to Ted, jitter sucks. In his design there’s no need for a PLL circuit nor any attention paid to the incoming clock at all. To quote Ted, "track a clock and you track its jitter". And, "a PLL is just a low-pass jitter filter".

When the collaboration between PS Audio and Ted Smith began, one of their first challenges was to reduce the design’s footprint. It was Paul McGowan’s wish to have it fit inside the existing PWD enclosure so the PerfectWave DirectStream DAC as the new product would become known could be a field upgrade whilst saving development costs for a new enclosure and enhancing customer trust in PS Audio the company. Here the collaboration between the team’s new member and PS Audio’s head of engineering Bob Stadtherr was crucial and successful. Together they managed to squeeze Ted’s initial layout and parts into the established PerfectWave dimensions.

We received the PerfectWave DirectStream DAC loaner inside the now signature floating contents box. Inside the second of two sturdy cartons sit two strong but elastic transparent foils which float the machine in between. This packing is really clever and offers maximum shock absorbancy in all directions. The DAC itself sits in a soft cotton sleeve whilst the piano black lacquer polished wooden top plate is additionally protected by a piece of sticky plastic. Because the unit under review was to be sold on—demand was high—we left the protective sheet on and also refrained from opening the unit for our usual candid shots to rely instead on stock photos. In a plastic bag at the bottom of the box sat a remote control with batteries and a variety of power cords from our Dutch importer but no owner’s manual which we instead downloaded from the PS Audio site. Whilst there we also downloaded the USB driver for later use with a Windows computer.

The DirectStream DAC’s front is no different from the PWD MK1 or MKII, with a touch screen at the right side next to the remote’s IR eye. At left the company’s logo doubles as stand-by button. At the business end and from left to right there’s the IEC power inlet with the mains switch above. The expansion slot meant for the Network Bridge—not included in our review sample—sits above the SD card slot intended for firmware upgrades. Six digital inputs provide for AES/EBU, coax and Toslink S/PDIF signals, USB type B and two HDMI connectors for PS Audio’s I-squared-S input from their matching PWT transport. Analog outputs are on XLR and RCA. All digital inputs except Toslink accept up to 24-bit/192kHz PCM, the optical input is limited to 24/96. All S/PDIF inputs also handle DSD over PCM aka DoP.