Digital Processor Reviews

Art Dudley received the v.1.1.4 firmware upgrade for PS Audio's PerfectWave DirectStream ($5999) toward the end of his time with this superb-sounding, DSD-capable D/A processor, so was able to write only a brief report on the change this made to the review sample's sound. "For those who've waited for a computer-friendly DAC that offers, with every type of music file, the best musicality of which DSD is capable, the PerfectWave DirectStream may be in a class by itself," he concluded. In my own auditioning of the PS Audio with the v.1.1.4 firmware, I was also impressed by its sound, feeling it had a smooth quality that made the otherwise excellent-sounding Luxman DA-06 D/A processor ($4990), which AD reviewed in July 2014, sound a touch too immediate in direct comparisons.

However, my measurements had revealed some problems in the PS Audio DAC, even with what was then the latest version of the firmware. In particular, I was bothered by the DirectStream's ultimate lack of resolution with data capable of higher-than-CD resolution, and by the poor linearity at low frequencies. When I discussed these issues with PS Audio's Ted Smith and Paul McGowan last October, at the 2014 Rocky Mountain Audio Fest, they told me that because the DirectStream's data processing is performed by Field-Programmable Gate Arrays (FPGAs) rather than by dedicated chips, these problems could be addressed with a firmware upgrade. That update, v.1.2.1, was already available for download before RMAF (footnote 1) and as Robert Deutsch had just purchased a DirectStream DAC, I asked him to write a Follow-Up comparing the sounds of v.1.1.4 and v.1.2.1. That enthusiastic Follow-Up appeared in our February 2015 issue (p.129); meanwhile, as I still had the original review sample, I upgraded its firmware and got it up on my test bench.

As always, I performed the tests with my Audio Precision SYS2722 system (see the January 2008 "As We See It"), which had just been returned from Audio Precision for service and recalibration. As before, all the measurements were performed from the processor's balanced outputs with the volume control set to its maximum.

Fig.1 shows the spectrum of the DirectStream with v.1.1.4 firmware while it decoded 24-bit data representing a dithered 1kHz tone at –90dBFS, plotted from –60dBFS downward. Fig.2 shows the spectrum with the v.1.2.1 firmware. You can see that both the noise floor and the already low-level power-supply products at the left of fig.1 have dropped by 3–5dB with the new firmware.

Perhaps more important, figs. 3 and 4 compare the PS Audio's reproduction of a dithered 1kHz tone at –120dBFS with the old and new firmware. With v.1.1.4, the fundamental tone was suppressed by 5dB, and it was the second harmonic that peaked at –120dBFS. With v.1.2.1, the 1kHz tone peaks at the correct level, which suggests a significant improvement in low-level linearity.

Figs. 5 and 6 similarly compare the spectra of the DirectStream's output while it drives a full-scale 50Hz tone into 100k ohms with the two firmware versions. With v.1.1.4 (fig.5), the second harmonic reaches a high –43dB (0.7%) and the third harmonic –52dB (0.2%). By contrast, with v.1.2.1 (fig.6), both harmonics have dropped to –60dB (0.1%) or below, and the higher-order harmonics visible in fig.5 are also much lower in level. While the v.1.1.4 version's linearity was better at high frequencies than at low frequencies, v.1.2.1 was even better.

The spectra of the upgraded PS Audio's output while it decoded an equal mix of 19 and 20kHz tones (fig.7) revealed that the difference tone at 1kHz had dropped by more than 10dB, to –90dB (0.003%), and that the noise floor was cleaner than it had been with the earlier firmware.

The measured reduction in low-frequency distortion with the v.1.2.1 firmware is dramatic, the improvement in resolution less so but still worthwhile. Kudos to PS Audio for designing a product so that its performance can so easily be upgraded by its customers. The new firmware is a gift to end users that will keep on giving.—John Atkinson