Hello everyone,
I feel that I have to comment on the review from Bob Katz on this new ‘multi-path’ DAC design, because people in this thread seem to think the measurements of the unnamed 'standard' DAC were from an ADI-2 DAC FS. As you will see this is definitely not the case. Furthermore the review measurements show very unusual, bad results, that need both clarification and correction. Quotations from that review start with ‘BK’, as now:
BK: Next, I lowered the test tone level to -70 dBFS, which produces -56 dBu analog level. I amplified the input level 60 dB with a high-performance microphone preamplifier to overcome the analyzer ADC performance. Then I recalibrated the reading to reflect the actual dBu levels from the DAC. As you can see (Fig. 3), at this low level, the sine wave is corrupted by noise and distortion within the single path DAC. At this level, the single path DAC produces 2.18%/2.24% THD (without noise), Left/Right channel. The noise is significant enough so that THD+N rises to 3.25%/3.47%. These unimpressive low level characteristics are similar to that of all standard (single-path) DACs.
There is so much objectively wrong here, so let’s get into details. I tried to replicate the setup as good as possible: sending a -70 dBFS 1 kHz sine at 96 kHz sample rate from the SPDIF output of the APx555B to the ADI-2 DAC, the analog output goes back to the APx. Without that 60 dB gain amp, because – as shown later – it doesn’t make any difference for these measurements (it would do in specific others). I also set the APx to an (unusual) 1M FFT size, and made sure the Analyzer results are capped to SR/2 bandwidth, like in the review.
Pic 1 shows the result: This ‘single path DAC’ produces -51 dBu output level (quite similar to the review), but THD+N is a mere 0.4% instead of 3.3%. That is 8 times better than the ‘standard DAC’ used in the test. And I do not claim that this is an unusual result – DACs with top AKM and ESS converters (as tested all the time here at ASR) can easily show even better results (noise floor in FFT around 8 dB lower, so even lower THD+N values).
As the FFT also clearly shows there is 0 (zero) distortion. If there is it would be hidden in the noise floor, but even with averaging none became visible. That means THD here is at least -100 dB, or below 0.001%. This is in stark contrast to the continued mentioning of distortion in the review, where there might have been no distortion at all but only noise, or the computed distortion values do not make any sense as SpectraPlus was not able to differ noise from harmonics. Modern ‘standard DACs’ do not distort at lower levels, not at all.
While the review does not include FFT screenshots from this measurement , we can grasp what’s going on with the next measurement at -90 dBFS. Once again my results in pic 2 show a very different picture of today’s ‘single path DACs’. The difference in THD+N is so big (3.8% to 38%) that it becomes obvious something is very wrong in the review. The respective FFTs have nothing in common. The noise floor from the ADI-2 DAC’s APx measurement is about 30 dB (!) lower, and there is (again) zero distortion. THD is unknown, but at least -65 dB, or lower than 0.056% - world’s apart from the stated 6.6% (not to mention that strange 5 Hz needle in the review’s FFT).
BK: As you can see, the single path DAC's waveform is hardly recognizable as a sine wave, as there is so much noise and distortion
There is no distortion visible – only noise. The APx Scope of the ADI-2 DAC signal shows a bit noisy waveform, but still a good sine. I wanted to know what it looks like using an ADI-2/4 Pro as ADC to capture this signal, similar to the setup in the review. The result matches the APx, see pic 3.
So while the D1 shows a perfectly clean sine (it should), any other DAC will show a bit noise on the signal – but nothing like what is shown in the review.
BK: Never Before Seen in Print. No one, I mean no one, has ever seen a clean -90 dBFS sine waveform from any DAC, until today!
I would have to disagree. All DACs reaching 120 dB SNR are capable of that – and that is quite the norm these days. Maybe the D1 looks even more clean and smooth, but the notion that they all look like the mess shown in the review is wrong.
I also did a screenshot of what the APx could analyze, see pic 4. For evaluating the D1 I expect values like THD, THD+N and SNR/DR are limited on the AP side, and need either an external preamp or cannot be measured correctly at all. But the FFT itself seems more than capable to show a difference between a single path and multi-path DAC (BTW, I would have preferred the term dual-path). Here is the FFT with 3x averaging and inputs terminated with 150 Ohms. The noise floor is then around -165 dBu. That is more than enough to see any of the suggested advantages of the D1 (and also if the multiple spuriae from the review measurements are from the D1 or originate somewhere else).
Also note that I tried to use my standard +34 dB preamp and was not able to get any different (improved) results from the -70 dBFS and -90 dBFS tests above. For this kind of stuff the APx seems sufficiently equipped already.
But the designers are pushing the agenda thats this is audible not just proves in engineering which it actually is , incredibly soo . They even claim an uncontrolled eureka moment when it was "obvious" 
