Inputs and Supported Formats/Resolutions
The RDAC offers three inputs - S/PDIF via COAX (RCA) and TOSlink, and USB via a micro USB socket. These are selectable, sequentially, via the “Input” button on the front of the unit.
The USB input IS galvanically isolated , and the USB input does require USB host power to function as a result. As is typical for isolated USB interfaces, USB power drives the source connection side, and power from the DAC itself drives the DAC side of the USB interface. This also means the unit will show up on your computer even if it has not been powered on.
This isolation seems to be pretty effective as using the COAX or TOSlink inputs did not yield a useful, audible, difference in quality in my setup. Similarly, trying a variety of USB-to-USB “purifiers” and/or DDCs in the chain did not yield a conclusive improvement. So, while you may experience different results, depending on your precise chain, the USB input is the way to go with this unit in my opinion. Though the option for TOSlink is particularly nice if you want to pair this with a TV/AVR/game console, or are having issues with ground loops.
In general, though, to get the most out of the RDAC you’ll want to use the USB input. This will give you the broadest range of bit-rate and format support. Officially, USB will let you play PCM up to 24/384 kHz and double-rate DSD (DSD128). With S/PDIF and a native DSD stream you can get to DSD128 and 24/192 kHz for PCM. S/PDIF using DoP will limit you to DSD64 (single-rate).
Interestingly, and I’ll stress that this is NOT something that is officially supported, nor guaranteed to work , I’ve been able to run the unit at 24/768 kHz and DSD256 (quad-rate) without any issues at all via USB.
As mentioned, the Massdrop x Airist Audio R-2R DAC is natively a multi-bit PCM converter. It can accept native DSD input as well as via DoP, but that DSD content will be converted to PCM before being fed to the actual DAC ladders.
Absent completely discrete PCM and DSD conversion stages, all DACs must do some kind of conversion from one of these formats to the other. For example, the PS Audio DirectStream models convert everything internally to DSD before doing anything else, whereas units like the RDAC or the Soekris units convert DSD to PCM.
What matters most, of course, is what the end result sounds like. Interestingly all of the positive traits I found in the unit are preserved with DSD source material – the same sense of clarity and “pristine” nature of the sound is fully in evidence, and the tone retains the hint of sweetness that I keep going on about.
In practice, this turns out to mean that the relative positioning of this unit vs. the others I compared it to, remains almost identical regardless of whether you’re feeding the RDAC PCM or DSD. I will say that the low-end of the iFi Micro iDSD Black Label cleans up a little when fed with native DSD content, but at best that brings it on par on that one aspect and overall, I still prefer the presentation from the Massdrop unit.
No Snap, Crackle nor Pop!
It is very common for DACs, even expensive ones, to exhibit nasty pops, clicks, and other noises when switching between DSD sample rates and between DSD and PCM content. So, it’s nice to find that the RDAC makes no utterances during such switches. The unit mutes momentarily during these changes. In general, this is transparent unless you’re skipping tracks. There was no intrusion into the replay of a normal playlist of mixed rate DSD and PCM files.
Tweaking DSD/PCM Conversion
You can pre-empt the internal conversion by using something like HQPlayer or Audirvana+ to convert your DSD files to PCM ahead of feeding them to the DAC, using any of a number of more sophisticated routines if you wish. The differences are often audible, if quite subtle, though as an on-going theme with approach I didn’t find any of the them to necessarily be preferable to just letting the DAC do its own thing.
Don’t be daft and convert to DSD from PCM ahead of sending data to the RDAC, however!
Power & Power Supplies
Power to the Massdrop x Airist Audio R-2R DAC is supplied via an external switching PSU rated for 3W @ 5v DC and uses a standard barrel connector. The external nature of the supply means that you can, if you are so inclined, substitute a suitably rated linear power supply instead. Note that all of the listening performed for the main part of this review was performed using the stock PSU.
Out of curiosity, once the main listening was done, I did try a couple of different LPS units here:
The Teddy Pardo “Teddy5/3” yielded a small reduction in the noise floor, a side effect of which meant very low-level detail was more easily discerned vs. the stock PSU. Though it’s worth bearing in mind that this particular PSU costs more than the RDAC itself, and the very minor change in performance probably isn’t worth it, nor is it likely to be routinely audible.
The iFi Power, which is technically underrated for this application (2.5A instead of 3A), did not seem to offer any audible improvement at all. Though this is also an SMPS, just with more filtering and “active noise cancellation”.
And my lab-grade supply - which gave a larger improvement than the Teddy Pardo unit, but at an even more ludicrous value proposition (several times the price of the RDAC).
My recommendation is, worry about your headphone and amplifier first and when you eventually have nothing else to adjust, maybe add an LPS to the RDAC!
The Massdrop x Airist Audio R-2R DAC is a 24-bit discrete R-2R (multi-bit, resistor/string ladder) sign-magnitude, oversampling, converter, using low phase-noise NDK clocks and a custom linear phase filter.
That’s an impressive configuration for a DAC at this level, particularly the implementation of sign-magnitude conversion. This dramatically improves linearity vs. a simple ladder (as much as two orders of magnitude at -60 dB). In listening to other DACs using simple ladders vs. a sign-magnitude implementation it has been my consistent finding that a sign-magnitude approach yields clearly better results - even when compared to much more expensive units.
Sign-magnitude converters use two separate ladders per channel to improve conversion linearity. With a single ladder, any non-linearity in the ladder winds up being relative to the full-scale signal. A simple R-2R ladder specified with 0.001% distortion actually has increasing distortion in inverse proportion to the signal level, reaching 1% by -60 dB. With a sign-magnitude implementation, as used in the RDAC, the specified distortion remains constant … so 0.01% at 0 dB stays 0.01% at -60 dB.
Critical listening was done with the RDAC being driven via it’s USB input (after determining this was the best way to drive it in my system) via Audirvana+ and Roon. Source material was primarily 16/44.1 FLAC format CD rips, with some native high-resolution content and multi-rate DSD albums to test those capabilities.
Primary listening was performed via an iFi Pro iCAN and a Woo Audio WA234 MONO Mk2, with some additional amplifiers included for pairing/system matching commentary. Headphones ranged from the HD650 to the Focal Utopia and Abyss AB-1266 Phi.
Direct comparisons were performed in a hardware assisted blind(ish) fashion. Not strictly a fully blind comparison, but a lot closer than simple sighted listening.