In 2012, a group of Direct Stream Digital (DSD) enthusiasts within the audio industry (led by dCS) released the DSD-over-PCM (DoP) standard, providing a way to “trick” traditional PCM audio circuits into playing native DSD files on suitably-equipped servers and DACs without first converting them to PCM. Today, few new DACs dare venture into the marketplace without DSD capability. Designing a DSD DAC has become almost routine; just order up some DAC chips capable of handling DoP inputs and use them in standard digital and analog circuits.
There is, however, always someone who wants to toss conventional practices over the side and start from scratch. In the case of PS Audio’s new DirectStream DAC, the original thinker is a computer scientist/audiophile named Ted Smith. A strong admirer of DSD recordings, Ted felt that the ideal DAC should work entirely in the DSD domain, so he set out to build a DAC which was entirely DSD-based. Of course, it had to accept PCM files; there are too many of those (e. g., CDs) to ignore, but they would be internally converted to DSD upon input. On his own, Ted took seven years to design a working example of his DSD DAC. Almost by chance, his design came to the attention of PS Audio’s CEO Paul McGowan, who liked what he heard (a lot), ultimately resulting in the subject of this review.
What’s so great about DSD anyhow? Here are some of the advantages PS Audio cites for DSD playback:
- DSD is simple to convert to analog: just run it through a low-pass filter. It doesn’t need a brick-wall filter like PCM, which can affect the sound. The DirectStream DAC uses a 24dB-per-octave low-pass filter, the same as you’ll find in many loudspeaker crossover networks. [In theory, a DSD bitstream can be converted to analog with a single capacitor.—RH]
- DSD is inherently linear; it’s hard to build a PCM DAC that always takes the same-sized step in the output for any possible unit increment of the representative PCM voltage value.
- DSD soft clips when overdriven, more like magnetic tape.
- All bits in a DSD stream have the same weight; a single-bit error anywhere is barely measurable, let alone audible.
PS Audio describes the DirectStream DAC’s circuit as follows:
All incoming data, PCM and DSD, are upsampled to 30 bits running at ten times the standard DSD rate and then back down again to double DSD for noise-shaping. The ten-times DSD sampling rate was the lowest common rate attainable through integer upsampling of 176.4 and 192kHz PCM files. An internal volume control maintains complete precision. Except for the sigma-delta modulation process itself there is no rounding; a full 50 bits are used. Consequently, there is no degradation from using the digital volume control. After the volume control, the signal is converted to DSD and downsampled to double-speed DSD (often referred to as DSD128). The double-speed DSD rate allows the low-pass filter to begin rolling off at 80kHz. A higher output rate would would have increased jitter.
There is no conventional analog output stage. The output of the DSD engine is fed directly into high-speed, high-voltage, high-current symmetrical video amplifiers and from there into the passive output filter. Rather than use an active output section, a balanced wideband transformer (which is part of the low-pass filter) drives either balanced or unbalanced interconnect cables. The output impedance is 100 ohms (unbalanced)/200 ohms (balanced), which should drive any reasonable load.
Off-the-shelf DAC chips can’t perform the functions described above, so Ted used a Xilinx Spartan 6 field-programmable gate array (FPGA) chip instead. An FPGA is a computer chip that’s a blank slate; you can program it to do whatever you want, and that’s what Ted did. A single master clock is used, but it’s unrelated to the input sampling rate.
I don’t usually spend this much space describing the design and functions of gear I review, but since the DirectStream DAC is such an innovative design I thought it would be worthwhile; if you’re interested in learning more about it, I highly recommend a visit to PS Audio’s Web site. Suffice it to say that Ted Smith has completely rethought how a DAC should operate and has designed a unique and innovative DAC.
The $5995 DirectStream DAC replaces PS Audio’s PerfectWave DAC. The two DACs are virtually identical, and that’s not an accident; PS Audio has a program for updating existing PerfectWave DACs by converting them to DirectStream DACs. One of the options for doing that involves gutting the PerfectWave DAC and replacing it with the DirectStream DAC’s interior parts. Apparently that’s easy enough that an owner can do it, but since some owners won’t feel inclined to take on that project, PS Audio has other options for updating the PerfectWave DAC. See the PS Audio Web site for details and pricing.
Like the PerfectWave DAC, a DirectStream DAC is a black or silver-gray box that measures 17" x 4" x 14" and weighs 19 pounds. Its fine-grained metallic chassis has rounded corners, a color touchscreen towards the right end of the front panel, and a high-density fiberboard top panel finished in piano-gloss black. Its elegant and refined styling would look right at home alongside the fanciest components. In a nutshell, I’d describe its looks as classy. A plastic remote control is included. Some manufacturers provide remote controls hewn from ingots of solid metal, but the first time you drop one of those on your coffee table (or your foot), you’ll really appreciate a light plastic remote—don’t ask me how I know this. The PerfectWave Media Bridge, an optional expansion card that plugs into the PerfectWave DAC and enables you to connect it to a network, also works with the DirectStream DAC. The PerfectWave Transport, an advanced optical drive in an enclosure stylistically and dimensionally identical to the DirectStream DAC, is still available and makes a natural match with the DirectStream DAC. In other words, PS Audio has bent over backwards to protect the investment its customers have made in other PS Audio equipment.
Like the PerfectWave DAC, the new DirectStream DAC provides a wide variety of digital inputs: SPDIF on coaxial RCA and TosLink inputs, USB, AES/EBU on an XLR connector, and two I2S inputs on HDMI connectors. Although HDMI connectors are used for I2S inputs, these inputs don’t carry HDMI video signals. Interestingly, while all the inputs will accept DoP-encoded signals, the I2S inputs will accept raw DSD signals direct, without DoP encoding. One raw DSD source is PS Audio’s NuWave Phono Converter, which combines a phono preamp with a high-resolution PCM and DSD analog-to-digital recorder.