Gramophone Dreams #69: Trance Dancing on Maxwell Street & the Rotel DT-6000 "DAC Transport" Measurements

Measurements, from March 2023 (Vol.46 No.3)

Herb Reichert reviewed the Rotel Diamond Series DT-6000 DAC Transport—fundamentally a CD player with digital inputs—in his February 2023 Gramophone Dreams column, where he described it as a "well-built, great-sounding, reasonably priced CD player." He had so much fun using the Rotel—"rediscovering the Joy of CDs: one-box simple, stress-free, plug'n'play, and something physical to touch, scrutinize, and collect"—that he suggested I examine its measured performance.

I tested Herb's sample of the Rotel DT-6000, serial number 272-2221004, using my Audio Precision SYS2722 system and signals I burned on a CD-R, as well as by feeding serial data to the S/PDIF and USB inputs. For the latter, I used both my MacBook Pro's USB port, with the laptop running on battery power, and the USB port on my Roon Nucleus+ server.

The Roon 2.0 app recognized the DT-6000 as a Roon Ready device. Looking at the Rotel's digital inputs, the optical and coaxial S/PDIF inputs accepted data sampled at all rates up to 192kHz. Apple's AudioMIDI utility revealed that the DT-6000's USB port accepted 16- and 24-bit integer data via USB sampled at all rates from 44.1kHz to 384kHz. Apple's USB Prober app identified the player as "ROTEL USB Audio 2.0" from "ROTEL" and confirmed that the USB port operated in the optimal isochronous asynchronous mode.

I used the Pierre Verany Digital Test CD to check the DT-6000's error correction as a CD player. It successfully played the tracks with gaps in the data spiral up to 1.5mm in length, but there were audible glitches when a single gap was 2mm long and with two closely spaced 1.5mm gaps. As the Compact Disc standard, the so-called Red Book, requires only that a player cope with gaps of up to 0.2mm in length, the Rotel's error correction is superb.

The DT-6000's single-ended output impedance was an extremely low 0.5 ohms at 1kHz and 20kHz, rising to a still very low 2.4 ohms at 20Hz. The balanced output impedance was very much higher, at close to 4k ohms at 20Hz and 1kHz, dropping to 3.65k ohms at the top of the audioband. A 1kHz signal at 0dBFS resulted in an output level of 2.155V from the single-ended outputs, which is 0.65dB higher than the CD Standard's recommended maximum level of 2V. The balanced output level with full-scale data was 4.514V.

Fig.1 Rotel DT-6000, impulse response (one sample at 0dBFS, 44.1kHz sampling, 4ms time window).

Both outputs inverted absolute polarity, which can be seen in the Rotel's impulse response (fig.1). This graph indicates that the player's reconstruction filter is a short minimum-phase type, with all the Nyquist-frequency ringing following the single sample at 0dBFS.

Fig.2 Rotel DT-6000, wideband spectrum of white noise at –4dBFS (left channel red, right magenta) and 19.1kHz tone at –3dBFS (left blue, right cyan), with data sampled at 44.1kHz (20dB/vertical div.).

With white noise at –4dBFS, sampled at 44.1kHz (fig.2, red and magenta traces), the DT-6000's response rolled off relatively slowly above 20kHz, not reaching full stop-band attenuation until the sample rate. The shape of the ultrasonic rolloff is familiar: It is typical of the filter used by MQA-capable devices for non-MQA data (footnote 1). An aliased image at 25kHz of a 19.1kHz tone at –3dBFS (blue and cyan traces) is suppressed by just 18dB, but higher-frequency images are all much lower in level. The distortion harmonics of the 19.1kHz tone all lie at or below –110dB (0.0003%).

Fig.3 Rotel DT-6000, frequency response at –12dBFS into 100k ohms with data sampled at: 44.1kHz (left channel green, right gray), 96kHz (left cyan, right magenta), and 192kHz (left blue, right red) (1dB/vertical div.).

Fig.4 Rotel DT-6000, spectrum of 1kHz sinewave, DC–1kHz, at 0dBFS (left channel blue, right red; linear frequency scale).

The Rotel's frequency response with data sampled at 44.1, 96, and 192kHz is shown in fig.3. The responses all follow the same basic shape: flat to 20kHz with then a gentle rolloff disturbed by a sharp dropoff just below half of each sample rate. Channel separation (not shown) was superb, at >120dB in both directions below 1kHz and still 96dB at the top of the audioband. The low-frequency noisefloor was very clean (fig.4), with no visible power supply–related spuriae.

Fig.5 Rotel DT-6000, left channel, 1kHz output level vs 24-bit data level in dBFS (blue, 20dB/vertical div.); linearity error (red, 1dB/small vertical div.).

Fig.6 Rotel DT-6000, spectrum with noise and spuriae of dithered 1kHz tone at –90dBFS with 16-bit data (left channel cyan, right magenta) and with 24-bit data (left blue, right red) (20dB/vertical div.).

The red trace in fig.5 plots the error in the Rotel's output level as a 24-bit, 1kHz digital tone fed to the TosLink input steps down from 0dBFS to –140dBFS. The amplitude error is less than 0.5dB until the signal lies below –120dBFS, which implies high resolution. An increase in bit depth from 16 to 24 with dithered data representing a 1kHz tone at –90dBFS (fig.6) dropped the DT-6000's noisefloor by 18dB, which implies a resolution of 19 bits from the player's ESS9028PRO DAC chips. This is more than sufficient to accurately decode CD data.

Fig.7 Rotel DT-6000, waveform of undithered 16-bit, 1kHz sinewave at –90.31dBFS (left channel blue, right red).

Fig.8 Rotel DT-6000, waveform of undithered 24-bit, 1kHz sinewave at –90.31dBFS (left channel blue, right red).

When I played undithered CD data representing a tone at exactly –90.31dBFS, the waveform was symmetrical, with no DC offset, and the three DC voltage levels described by the data were cleanly resolved (fig.7). Repeating the measurement with undithered 24-bit S/PDIF or USB data gave a well-formed, noise-free sinewave (fig.8).

Fig.9 Rotel DT-6000, balanced outputs, spectrum of 50Hz sinewave at 0dBFS, DC–1kHz, into 100k ohms (left channel blue, right red; linear frequency scale).

As seen in fig.2, the DT-6000 featured very low levels of harmonic distortion. The third harmonic was the highest in level from the balanced output into 100k ohms (fig.9), but this lay at just –110dB (0.0003%) and didn't rise in level when I repeated the spectral analysis with the more challenging 600 ohm load. The third harmonic from the unbalanced output was similarly low, now joined by the second harmonic, though this lay at just –120dB (0.0001%).

Fig.10 Rotel DT-6000, balanced outputs, HF intermodulation spectrum (DC–30kHz), 19+20kHz at 0dBFS into 100k ohms (left channel blue, right red; linear frequency scale).

Intermodulation distortion with a full-scale mix of equal levels of 19 and 20kHz tones was extremely low in level (fig.10). The levels of the aliased images at 24.1kHz and 25.1kHz were much lower than I was expecting from fig.2.

Fig.11 Rotel DT-6000, high-resolution jitter spectrum of analog output signal, 11.025kHz at –6dBFS, sampled at 44.1kHz with LSB toggled at 229.6875Hz: 16-bit CD data (left channel blue, right red). Center frequency of trace, 11.025kHz; frequency range, ±3.5kHz.

Fig.12 Rotel DT-6000, high-resolution jitter spectrum of analog output signal, 11.025kHz at –6dBFS, sampled at 44.1kHz with LSB toggled at 229.6875Hz: 16-bit TosLink data (left channel blue, right red). Center frequency of trace, 11.025kHz; frequency range, ±3.5kHz.

I tested the Rotel's rejection of word-clock jitter with the undithered Miller-Dunn J-Test signal (a high-level tone at one-quarter the sample rate over which is overlaid the least-significant bit toggled on and off at a frequency equivalent to the sample rate divided by 192). With this signal played back from CD, the Rotel reproduced the odd-order harmonics of the LSB-level, low-frequency squarewave very close to the correct levels (fig.11, sloping green line), and no other sidebands were present. However, when I repeated the spectral analysis with 16-bit S/PDIF data via TosLink (fig.12), the sidebands closest to the high-level, high-frequency tone were much higher in level. This graph was taken with optical TosLink data. The behavior with coaxial S/PDIF data was identical.

Fig.13 Rotel DT-6000, high-resolution jitter spectrum of analog output signal, 11.025kHz at –6dBFS, sampled at 44.1kHz with LSB toggled at 229.6875Hz: 16-bit USB data (left channel blue, right red). Center frequency of trace, 11.025kHz; frequency range, ±3.5kHz.

The J-Test signal is not diagnostic with data where the bit clock and/or word clock are not embedded in the datastream. However, I use it to check the performance of a processor's USB and Ethernet ports. To my surprise, the DT-6000's USB input did poorly with this signal, whether the J-Test data were sourced from my MacBook Pro (fig.13) or from the Roon server. The presence of low-frequency audio noise broadens the base of the spectral spike representing the 11.025kHz tone, and a large number of power supply–related spuriae are present. Perhaps this is why, as HR noted in his review, the most noticeable differences between the Rotel's CD playback and streaming files via Roon "were in energy delivery and viscosity: Music from CDs sounded denser and more fortified than music from Qobuz and Tidal. ... With the DT-6000, streaming sounded less physical than phono or CD, but it was quite nice for late nights."

With its high resolution, low noise, superbly high channel separation, and vanishingly low levels of both harmonic and intermodulation distortion, the Rotel DT-6000 offers some of the best measured performance I have encountered from a CD player. However, because of its high levels of jitter, as a streaming DAC, its performance is not in the same class.—John Atkinson

Footnote 1: See, for example, fig.2 here.

cognoscente's picture

It is good to read a review of an audio component that is affordable for most of us.

Although I am very fond of the ease of use of nowadays digital music (I have never understood the revival of the record player, or the cassette deck / walkman, reel deck, the discsman and yes, it took a while, but then of course the CD (player) nowadays - sales of CDs are on the rise again) I am also against streaming. For the quantitative reason, I don't need to have all, almost all music at my disposal, also the qualitative reason. Streaming is simply not the best way in terms of quality, as described in the review. I am in favor of a simple and, above all, closed system, without all the noise that you also get from the internet in an open system, where you then need all kinds of unnecessary, and therefore too expensive, peripheral equipment to get it all right again (I'm ignoring the power supply for convenience). Data traffic is also a major cost item for the providers, so they have every interest in not sending you the very best, read heavy full, files. But the main reason is that I don't believe in the platform economy, where only the platform really earns from it. I'm not a hi-res audio denier. I still buy music, not a physical CD or SACD, but on Qobuz. AIFF files, so without compression. The full quality. Usually in the highest Hi-Res quality available, if I think it has added value. That is certainly not always. I put the purchased music on my iPhone (which is the storage and remote control) in the Onkyo HF app (which makes it possible to play Hi-Res files, CD quality is a software limitation on an iPhone, not a hardware limitation ) and play the music via a USB cable connected to my dac (I'm also not in favor of wireless audio, BTW Airplay is again limited to CD quality). This is an intermediate form of listening via a physical (CD / SACD) player and streaming. The ease of streaming and the quality of a physical player.

Anyway: what I mean to say, if you still feel the need for a physical playback device, get the Panasonic DP-UB9000. The whole player, including the transport, is built like a tank, or bunker if you prefer. And a good music dac is probably already in your current set anyway. Although the audio dac in this Panasonic is also very good. You also immediately have an excellent Blu-Ray player, for an everlasting build quality, better than the Rotel, and cheaper player than the Rotel.

cognoscente's picture

oh forgot to write, streaming a song 1x takes the same amount of energy as downloading it 1x. And you keep streaming over and over. Every time and again and again and again. So also in terms of climate change, if you believe in that and want to do something about it, streaming is not the best way.

jtshaw's picture

My experience with a Bryston BCD3 CD Player somewhat mirrors Herb's. It's a redbook-only player that truly sounds fully-optimized for its mission. I'm running it with a balanced interconnect into a Luxman integrated amplifer, and I'm getting the best CD sound I have heard in almost 35 years of CD collecting and listening. We still have a lot of CDs at our house, some still unavailable on streaming services. A few are quite rare now (for example, Gilad Hekselman's Split Life, his first recording, live at Small's in New York) that I am so grateful to have. I'll be listening to CDs for a long time even as I enjoy other media as well.

cafe67's picture

And limiting they didn’t include a digital out of some sort even if just a coaxial out.

Jason Victor Serinus's picture

Marvelous writing, marvelous storytelling.

Herb Reichert's picture

it was a fun story to tell


georgehifi's picture

Like a multi digital input all in one preamp/dac. Great work Rotel!!

But!! and there's always one for me, the only thing missing even though it's on the remote, is a volume control so you can then go direct into your power amp/s.
Hopefully there will be a firmware upgrade to active this, as the ESS9028PRO converter has that ability to easily be turned on. Like Benchmark did on their Dac3 with the ESS9028Pro dac

Cheers George

Ed Oz's picture

This Rotel sounds like a wonderful player; however...for this price point...not being able to play SACD discs makes it a nonstarter. Although I don't have a lot of SACD's, I want one player that will accommodate all the physical disc formats.

ctalcroft's picture

If I may, I'd like to clear up a few details about "And This Is Free," the film, and "And This Is Maxwell Street," the CDs. First, the subtitle ("The Life and Times of Chicago's Legendary Maxwell Street") on the film package you show and that you quote was added later by whoever is marketing the film these days (my brother owns the rights to the sound recordings, not the film). It is not part of the original title of the film. The title of Mike Shea's film was and is simply "And This Is Free."

The CDs were released first in 1999, in Japan, on the P-Vine label, and then in the US (Rooster) and the UK (Katfish), in 2000. The music and street sounds they contain were transferred to the digital domain from Mike Shea's original tapes by Ian Talcroft of Studio IT using the same Nagra equipment Gordon Quinn used to record them on the street. Ian also mastered the discs, created the compilation, and oversaw the project. I designed the CD packages and wrote the liner notes (for which I won a Living Blues Award). Allan Murphy did background research, so it would be more accurate to say that the discs were produced by Ian Talcroft, not the three of us.

In the second photograph, the man described as "another musician" is John Lee Granderson.

Finally, would you kindly edit this review to credit the two photos you've used, to indicate that they are from the "And This Is Maxwell Street" CDs?

Thank you

Colin Talcroft
Santa Rosa, California