MEASUREMENTS: RME ADI-2 Pro FS R Black Edition as DAC.

 

Well friends, it's time to publish some measurements of my new RME ADI-2 Pro FS R Black Edition ("FS R BE") unit as DAC. For background, refer to the Preview from August and a couple weeks back, I had published results discussing the ADC function.

As you might be aware, since 2018, I've been using the older RME ADI-2 Pro FS ("Pro FS") for ADC and DAC duties in my measurements. It has performed like a champ and in fact, given that the ADC function is almost identical between the two units, there's certainly no reason to retire the "Pro FS" from measurement duties.

What we know is that as a DAC, the "FS R BE" should perform better than the "Pro FS" due to the fact that the internal AKM chip has been upgraded to the AK4493 from the previous AK4490. On paper, this represents a +5dB improvement in SNR and +1dB improvement in THD+N so we're not looking at Earth-shattering differences in specs numbers. Let's see if we're able to measure these differences on the test bench and identify any other changes to the DAC's performance along the way...

To start, let me show you what the equipment test set-up looks like:

For USB digital source, as you can see, I'm using a DIY Raspberry Pi 3B+ "Touch" sitting on top of the "FS R BE". I'm running Volumio on it loaded with the test signals in the USB stick poking out on the left and it's connected to the RME by a 6' length of generic USB cable. Notice that I've also taken out my trusted Logitech Squeezebox Touch. I suspect that a few of you might still be using TosLink and coaxial S/PDIF - the Touch (connected to Logitech Media Server via WiFi) can provide S/PDIF input to the "FS R BE".

For consistency, I will use the "Pro FS" as the ADC measurement device for much of these measurements. However, there will be times where I'll do some loopback of the "FS R BE" itself which I will point out. Unless indicated otherwise, measurements will be of the XLR analogue output from the "FS R BE" sent to the "Pro FS" by 6' lengths of Monoprice Premier cables. Some generic USB then will connect the "Pro FS" to my Intel NUC for data processing.

As with the previous ADC measurements, "FS R BE" firmware is USB 238/DSP 98, and "Pro FS" version 213/98.

Part I: Digital Filters and "Microscopic" Properties

I typically start by looking at some of the "microscopic" properties of the DACs. Usually, I'll hook the DAC up to my oscilloscope and have a peek at square waves and output voltage levels. For the "FS R BE", let's not worry about this because it's the same as the "Pro FS". Furthermore we already know the reference output balanced levels which are +4/+13/+19/+24dBu (corresponding to 1.23/3.5/6.9/12.3Vrms). See here if you want to check out pictures of square waves off the "Pro FS".

One area of interest over the last decade has been selectable digital filters. For the "FS R BE", we have the choice of 6 filters (1 more than the "Pro FS"). Here they are with pictures of the impulse responses (note that I tried to casually center these rather than being reflective of relative impulse latencies) along with the Juergen Reis-inspired "Digital Filter Composite" graphs I've been using for a number of years to examine filter frequency-domain characteristics and to look for overload behaviour:

The first 4 filters are relatively "orthodox" settings. Basically we're looking at minimum vs. linear phase settings with either sharp roll-off or short-length filters with resulting slow roll-off but more imaging/"aliasing" artifacts beyond the Nyquist frequency. In the graphs above, I used 44.1kHz signals so Nyquist would be at 22.05kHz. Remember that filtering is primarily of value for 44.1/48kHz signals; with 88.2/96+kHz samplerate, unfiltered artifacts would be at least an octave beyond human hearing.

The DFC graphs look even better than the older "Pro FS". No hint of overloading at all. This means that a few dBs of overhead have been added. Nice.

Now the last 2 filters are different "by design":

"NOS" setting is our "Non-OverSampling", unfiltered option which allows artifacts beyond Nyquist to pass as you can see in the DFC graph (NOS discussed previously). And for fun, we can have a peek at a 16/44.1 1kHz sine wave through this filter:

Oh those are pretty stair-stepped "digital" waveforms, right? ;-)

The "SD LD" filter stands for "Short Delay Low Dispersion" and we've seen this AKM option in other DACs, most recently the Topping DX3 Pro (V2). It looks like an intermediate phase filter of sorts discussed in the RME manual on page 83. It's steep enough to attenuate most ultrasonic distortions, provides low latency for realtime monitoring work, and maintains relatively linear phase characteristics within the audible spectrum. While there's no intersample overload behaviour between the 0dBFS and -4dBFS white noise signals on the DFC graph, note that the ultrasonic noise floor is a little higher than digital silence; not that this is an audible issue of course since we're looking at below -120dBFS.

Digital filter options are an interesting feature but IMO unless the filter is highly anomalous (like NOS or the highly relaxed PonoPlayer/Ayre filter), there's really not too much to say or hear. I'm happy with the linear phase "Sharp" filter for music enjoyment which I keep as default for both DAC and ADC. (Or I might fool around with the Ms. Goldilocks filter for fun! :-)

Another "microscopic" test that's nice to show as a reflection of the resolution for a DAC is the 1kHz -90.31dBFS undithered 16-bit tone:

We can easily see the defined digital levels of the undithered lowest 16th bit and the DAC is able to better reproduce the intended -90.3dBFS 1kHz sine waveform at 24-bits.

I usually show this overlapping for the two channels so we can examine for any imbalance and DC offset - no problems here. Basically this tells us that the RME DAC is capable of resolutions beyond the basic "standard" 16-bits and has excellent channel balance even at such low levels.

Part II: RightMark Test Suite

Okay, so we know that the filters have been improved and we know that the "FS R BE" is obviously capable of high-resolution reproduction... The question then is "How much?" in comparison to the older "Pro FS" and compared with a reference (let's use my Oppo UDP-205).

While it rarely yields useful information, I do like to start with standard 16/44.1 measurements still simply because most of our music remains "standard resolution". Here's a summary table and the specific graphs for comparison (all measured with XLR output, RME devices set to +13dBu reference, linear phase "sharp" filters used throughout):

Note for the frequency response graphs, I used the "sweep sine" measurement here. The "multitone" result can be skewed when presented with slight clock differences.

Seriously guys and gals, 16/44.1 is reproduced to perfection these days by higher quality DACs. We're looking at distortion levels down at the 0.00*% range or less! Furthermore, as discussed last year with the "Standard Resolution Blind Test", differences in audibility are already very minimal.

The only notable difference in these charts is the IMD+N sweep where we can see that the "FS R BE" and Oppo perform better than the older "Pro FS".

Next, let's go into 24/96 Hi-Res territory:

Nice. Looks like the "FS R BE" is equivalent if not surpassing the Oppo UDP-205's ES9038Pro "SABRE" noise level and dynamic range and within spitting distances of the extremely low distortion with USB input. It's likely that we're also hitting on the limits of the ADC measurement system.

Notice that there are slight differences in the measured values between USB and S/PDIF (TosLink & Coax) digital inputs. These are microscopic differences but speak to the resolution at which the measurement ADC is able to pick out little changes!

For interest I've included the data from the Topping DX3 Pro V2/LDAC in the mix.

As you can see, the main differences here are between the Topping DX3 Pro (V2) and the others. The DX3 Pro has RCA outs which is why the crosstalk is comparatively higher. We can also make out the slightly higher THD+N for the "Pro FS" across much of the audible spectrum.

For completeness, let me show you the data for even higher samplerates on the "FS R BE" DAC:

As you can see, we have 24/192 and 24/384 numbers here for the "FS R BE". As noted above, I used the +13dBu (3.5Vrms) reference level for these RME measurements. Notice that I included a 24/96 result at the highest +24dBu (12.3Vrms) output level and was able to get slightly better measured dynamic range although likely this is hitting against the limits of the measurement ADC again.

Oppo UDP-205 and Topping DX3 Pro V2 results follow to the right in the summary table. I find the Topping DX3 Pro rather impressive for a little ~US$200 DAC!

Here are the graphs done with loopback 24/384 for the RME ADI-2 Pro FS R Black Edition, "Sharp" digital filter:

At 24/384, we have a flat frequency response with less than -0.5dB at 100kHz. Note that with loopback, RME has implemented a compensation filter on the DA side to keep the results essentially flat into 100kHz. This is a thoughtful nod to those who want to use the ADI-2 Pro for measurement purposes (described in the manual on page 79). Noise level is well controlled into 50kHz. Beyond the rising ultrasonic noise of sigma-delta conversion, there is no evidence of issues arising from the stock switching power supply used. Notice how precisely the two channels are balanced.

I would argue that when level-matched, if a supposedly "high fidelity" DAC sounds very different from a high performance machine like this RME or the Oppo, it would be a result of intentional distortion (like this ridiculous "euphonic" audiophile mod), noise/hum, software settings (eg. EQ, DSP applied), user error, or suboptimal system integration (eg. impedance mismatches). The idea that a hi-fi $10,000+ DAC somehow could inherently "sound better" than even a reasonable ~$200 DAC (like the Topping DX3 Pro) makes no sense these days apart from the above factors. IMO, price-performance correlations are meaningless now absent objective evidence.

Part III: Jitter

Here are some high-resolution Dunn J-Test FFTs with the signals played from the old Logitech Squeezebox Touch's S/PDIF TosLink and Coax outputs:

And this is the USB input from a Raspberry Pi 3B+ "Touch" using Volumio, generic 3A switching power supply. I've maximized the levels to peer as deep into the noise as possible:

As I have said for years, if you really think jitter is audible, then stick with the asynchronous USB interface (asynchronous ethernet should work just as well). In general, TosLink is a more "jittery" interface, and depending on the DAC, commonly coaxial input is better than TosLink. [Try out the Jitter Simulation Demo.]

In the tests above on the "FS R BE", jitter is very low regardless of S/PDIF or USB input used. The obsessive audiophile will see that USB input is performing better with a narrower "base" of the fundamental frequency and also lower spurious noise (like ~17kHz with 24-bit S/PDIF).

If you're wondering, I did not use any fancy TosLink or coaxial cables of course. The coax is a homemade 3' length of Radio Shack cable with gold plated connectors I put together 20 years ago. The TosLink is some >20 year old 6' Acoustic Research branded cables with presumably plastic fiberoptic material. 

Dare I say that "Bits Are Bits" with high quality modern devices these days? Of course I dare! ;-) Only low quality digital gear makes bits sound like anything else.

Part IV: A detailed look at THD+N

I thought it might be interesting to capture in high resolution the THD(+N) graphs comparing the "FS R BE" and the "Pro FS" as a loopback (the DAC output --> ADC input on same machine).

 

Using a -1dBFS sine wave at 24/96, 256k-point FFT, averaged over 4 reading, we see that the "FS R BE" with its upgraded AK4493 achieves a THD of -117 to -118dB and measured THD+N of around -113.5dB for both channels (the right channel was a little better even). This is within spitting distance of RME's specs reporting THD+N of -115dB at 0dBFS with my measurement perhaps also limited by the ADC. The "FS R BE" THD+N is about 4dB better than my older "Pro FS".

A THD+N of -113.5dB is in line with and even exceeds the "official" AK4493 spec sheet value of -113dB. If I push the "FS R BE" to +24dBu output level, I've seen the average THD+N better than -114dB.

A result of -100dB/0.001% THD+N is already phenomenally distortion-free for human consumption. Let's also not forget that the dB scale is logarithmic.

Part V: Level and Frequency Step Measurements

Using REW, we can now start looking at the detailed harmonic structure and linearity errors as we vary the generator level (level step).

As you can see, this is a very linear DAC from -120 to 0dBFS. I basically had to zoom in to show that tiny deviation in linearity above. And when we look at the harmonics across the levels even at the highest +24dBu output, it looks very clean with little "saturation" of THD+N approaching 0dBFS:

Likewise, we can look at the frequency step measurements and how the harmonics fluctuate across the audible spectrum:

THD+N is nice and flat across the spectrum. Mainly low order 2nd and 3rd harmonics present.

For "fun", I was thinking it might be good to have a way to simply visualize and estimate the DAC / ADC linearity especially for USB interface devices like these where a loopback can be easily performed. So I created a test signal starting from -144dBFS at 100Hz that increases by 3dB every 100Hz until we hit -12dBFS at 4.5kHz in 32-bits and 96kHz for precision, then the signal was dithered down to 24/96 for playback. The important assumption is that the DAC has a "ruler-flat" frequency response from 100-4500Hz.

With a high resolution FFT (256k-points), we can analyze the test tone using averaged multiple readings to reduce noise and capture a graph that looks like this with the "FS R BE":

I superimposed the black line over the peaks of each tone. Visually, we see that the threshold is around -135dBFS, beyond which the linearity clearly breaks down. By -144dBFS the noise level is rather significant for any single reading, the signal level is averaged to be off by about +4dB. Not the most precise test to use for absolute linearity, but results can be had within seconds as a quick "screen" for anomalies. Excellent test result nonetheless from the "FS R BE" showing great performance as both DAC + ADC at the same time.

Part VI: DSD

Not sure how many of you care about DSD resolution these days. This DAC is capable of up to DSD256 playback.

Here are some measurements using the 24/96 RightMark test signal converted to DSD64 (SACD resolution) and DSD128. Note that PCM --> DSD conversion is not lossless. By definition, the DSD performance will not measure as well. We'll at least get a sense of whether DSD playback is of high-resolution and whether there are any gross anomalies.

Clearly the device is capable of DSD reproduction in high resolution. As usual, we see that DSD64 playback contains ultrasonic noise increasing above 20kHz. DSD128 is able to encode the full 48kHz bandwidth of a 96kHz signal with minimal change in noise level.

Notice that I've included results with "DSD Direct" mode turned on. Basically this bypasses any processing of the DSD signal (like volume control) in the AKM DAC chip and since I was playing back the signal without attenuation anyways, we don't see any significant difference in the results.

There is a setting in the DAC for either 50kHz or 150kHz filter to manage the ultrasonic noise. If you play any DSD64, it's advised to leave it at the default 50kHz due to the increasing high-frequency noise.

Curious about DSD J-Test and whether that might show any anomaly?

No problem. It looks beautiful for DSD128 as shown above, likewise the DSD64 playback looks perfect as well (not shown).

Part VII: Headphone Output

Note simple 1/4" to 3.5mm stereo adaptor then 3.5mm phono to RCA cable for headphone out measurements.

The RME ADI-2 Pro FS R Black Edition's "Extreme Power" headphone output circuitry has not changed from the "Pro FS". We're looking at two headphone outs capable of +22dBu (10Vrms) and enough current to drive a 32Ω load at 1.5W. Output impedance is rated at <0.1Ω; excellent for low-impedance headphones.

Since headphone output PH1/2 shares the same AK4493 DAC as the main analog output (ie. the XLR measured above), let's compared this with PH3/4 which has its own DAC chip. Let's measure at both "normal" +7dBu Lo-Power output level and cranked up to Hi-Power +22dBu.

RightMark summary and graphs:

Notice that I used a -1dBFS signal for the Lo-Power measurements for technical reasons; this is due to the fact that I was using RME's +4dBu reference setting on the ADC side and needed the signal to be a little lower so as not to clip the ADC. No problem with Hi-Power output at 0dBFS which produced a +22dBu signal into the ADC's +24dBu reference setting.

An interesting finding is that the PH1/2 headphone output at Lo-Power setting had ~2dB higher average noise level and had a touch more distortion on my unit. I wonder if this is linked to the fact that the DAC output serving PH1/2 is also connected to the rear "Main" analogue outs (XLR/TRS). Obviously there's no need to be concerned; after all, there is no mechanical transducer in this world performing anywhere close to this level of fidelity! Nonetheless, The Ultra-Neurotic Audiophile [TUNA] :-) might want to preferentially use PH3/4 for low-power headphone duties.

Overall the graphs look great.

Basically equivalency between the two headphone jacks. Remember that the headphone outputs are single ended and we can see some low-level 60Hz hum down around -120dB with Lo-Power output and -135dB with Hi-Power. The RME is also capable of balanced headphone mode utilizing both output jacks; alas, I don't have the appropriate cables to test this out.

Part VIII: Subjective Impressions

Seriously ladies and gentlemen, in 2020 with some of the best hi-res DACs, there's really no need to spend too much time on the subjective descriptions. Using the "FS R BE", soundstage was wide and precise with good recordings (like Preservation Hall Jazz Band's Live!**), tonality of instruments sounded natural (like Rachel Podger & Brecon Baroque's Vivaldi: L'Estro Armonico), it had no trouble with highly dynamic recordings with tight transients (like the Jim Keltner's drum improv on The Sheffield Lab Drum & Track Disc), and "microdynamic" details and cues were rendered with finesse (audiophile nugget like Nils Lofgren's "Keith Don't Go").

I honestly don't know how subjective-only digital audio reviewers spend so much time dreaming about ways to describe the "sound" of high quality gear. Come on... As if they don't already know that much of those adjectives used and anecdotal impressions are just their own projections about how they "feel" about the machines and their makers rather than truly reflective of audible differences that would stand up to sober disciplined scrutiny! I do believe that acknowledgement of biases especially with loaned equipment, friendships/favours from Industry, and financial interests need to be accounted for when it's all subjective impression-making.

From what I'm hearing over the last month, this DAC simply "gets out of the way". It's "transparent" in the sense of what I described as the goal of high-fidelity years ago. If the music has encoded within the signal excellent soundstaging and subtle cues about the space around the instruments, the RME will reproduce those characteristics precisely. If the recording has a wide dynamic range, likewise this will be reflected without any hint of strain in the analogue outputs. The objective measurements already tell us that there's no need to be concerned about hardware limitations.

Conclusions

I can say that no matter what album I was listening to or what tests I've thrown at the ADI-2 Pro FS R BE, the device performed superbly. The "basics" of a good stereo DAC have been achieved like precise channel matching, very low crosstalk, minuscule distortion, excellent dynamic range and concomitantly low noise level. The headphone section remains formidable with high power capability, low output impedance, and superb resolution. We've seen RME evolve the ADI-2 Pro family of devices over time with improved jitter/time-domain performance in the "Pro FS" released in early 2018 and now better overload protection with the digital filter settings in the "FS R BE" to the point where they're "perfect" on my DFC graphs along with resolution gains of the AKM AK4493 over the previous AK4490.

As an audiophile who wants high-fidelity performance, this RME DAC/ADC can do substantially more than I need and IMO performs "perfectly" as far as human perception and cognitive abilities are concerned. Remember that while I'm simply measuring and offering my opinion of pure DAC and ADC abilities, the device has functionality I haven't even talked about which many users will find beneficial (I like the variable headphone crossfeed processing for example).

I believe the RME ADI-2 Pro FS R Black Edition will stay for the longterm in my sound room as a reference device when enjoying music, ripping vinyl, or even to perform the occasional measurements. This is the highest recommendation I can make here at the Musings as a reviewer.

For those with no need for the ADC function, remember the ADI-2 DAC FS (~US$1000). That machine has a more consumer-centric complement of RCA + XLR outputs and 3.5mm front "IEM" headphone jack. Since November 2019, RME has upgraded that model to the AK4493 DAC as well. I suspect objective performance will be similar to what I've published here.

If we're comparing this DAC with others, and if one chases objective numbers, yes, there are even lower THD+N / higher SINAD DACs out there. A friend E-mailed me recently that the Topping D90 with its AK4499 (THD+N 124dB, S/N 137dB!!!) scores phenomenally for example. But seriously folks, how much actual hi-res material do we listen to? How much resolution do we need even if we implemented digital volume normalization and room DSP? For me, objectivism is not about chasing numbers, but having the ability to reliably measure these performance characteristics, appreciating their value, and then correlating the findings to the real-life utility of the device. Be smart. Reductionistic mindless chasing after numbers among objectivist audiophiles encourages unwarranted ridicule just as much as extreme subjectivists proffering delusional beliefs damage the position of otherwise reasonable listeners.

By the way, over the years I've been asked about using the RME ADI-2 Pro for things like room and speaker measurements. Remember that unlike say my previous Focusrite Forte USB audio interface, the RME is not a microphone pre-amp. Typical condenser measurement microphones like the popular and inexpensive Behringer ECM8000 or Dayton EMM-6 will require +48V phantom power. If I need to connect such microphones to the ADI-2 Pro, I'd do it through my Yamaha MG06X mixer and its +4dBu line level output.

Whew... While there are a few other things I can still measure, I think I've tortured this machine enough with test signals.

Time for real music, friends! :-)

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** Remember awhile back we talked about the importance (or lack of importance depending on your perspective) of "Absolute Polarity". The Preservation Hall Jazz Band recordings which feature quite a bit of trumpets and trombones would be prime albums to listen for this with actual music.

Notice the asymmetry in the waveform display of the Live! recording when the brass instruments kick in: