Rotel Diamond Series DT-6000 CD Player-DAC Measurements

Link: reviewed by Roger Kanno on SoundStage! Hi-Fi on April 1, 2023

General Information

All measurements taken using an Audio Precision APx555 B Series analyzer.

The Rotel Diamond Series DT-6000 was conditioned for 30 min at 0dBFS (4.3Vrms out) into 200k ohms before any measurements were taken.

The DT-6000’s primary function is that of a CD player; however, because it offers digital inputs, the DT-6000 was evaluated as a standalone DAC. The DT-6000 offers three digital inputs: one coaxial S/PDIF (RCA), one optical S/PDIF (TosLink), and one USB. There are two sets of line-level outputs (balanced XLR and unbalanced RCA).

Comparisons were made between unbalanced and balanced line level outputs for a 24/96 0dBFS input, and aside from the 6dB extra voltage over balanced and a significant difference in output impedance (see primary table below), there were no appreciable differences in THD+N. In terms of digital input types (i.e., USB, coaxial, optical), THD ratios were essentially the same across all three; however, noise levels were about 10dB higher with the USB input (10Hz to 90kHz bandwidth).

Published specifications vs. our primary measurements

The table below summarizes the measurements published by Rotel for the DT-6000 compared directly against our own. The published specifications are sourced from Rotel’s website, either directly or from the manual available for download or included in the box, or a combination thereof. With the exception of frequency response, where the Audio Precision bandwidth was set at its maximum (DC to 1MHz), assume, unless otherwise stated, the coaxial digital input (24/96 1kHz sinewave at 0dBFS), and the worst- case measured result between the left and right channels.

Our primary measurements revealed the following using the coaxial input and the balanced line-level outputs (unless specified, assume a 1kHz sinewave at 0dBFS, 200k ohms loading, 10Hz to 90kHz bandwidth):

Frequency response (16/44.1, 24/96, 24/192)

The plot above shows the DT-6000’s frequency response as a function of sample rate. The blue/red traces are for a 16-bit/44.1kHz dithered digital input signal from 5Hz to 22kHz, the purple/green traces are for a 24/96 dithered digital input signal from 5Hz to 48kHz, and, finally, orange/pink represents 24/192 from 5Hz to 96kHz.

The behavior at low frequencies is the same for the different digital sample rates—perfectly flat down to 5Hz. The behavior at high frequencies for all three digital sample rates is not quite as expected and deviates from Rotel’s published specs of 20Hz-20kHz (+0dB, -0.15dB) and 10Hz-70kHz (+0dB, -3dB). Here we find slightly more high-frequency attenuation than is typical for a brickwall-type filter. At 20kHz, all three sample rates are down -0.55dB. The -3dB point for each sample rate is roughly 21.1, 45.7, and 54.2kHz, respectively. In the graph above and most of the graphs below, only a single trace may be visible. This is because the left channel (blue, purple or orange trace) is performing identically to the right channel (red, green or pink trace), and so they perfectly overlap, indicating that the two channels are ideally matched.

Phase response (16/44.1, 24/96, 24/192)

Above are the phase response plots from 20Hz to 20kHz for the coaxial input, measured at the balanced output. The blue/red traces are for a dithered 16/44.1 input at -20dBFS, the purple/green for 24/96, and the orange/pink for 24/192. We can see that the DT-6000 inverts polarity (i.e., -180 degrees of phase shift), with a worst-case phase shift (from the baseline -180 degrees) of about 80 degrees at 12kHz for the 16/44.1 data, 40 degrees for the 24/96 data, and less than 20 degrees for the 24/192 input data.

Digital linearity (16/44.1 and 24/96 to -120dB)

The graph above shows the results of a linearity test for the coaxial digital input (the optical input performed identically) for both 16/44.1 (blue/red) and 24/96 (purple/green) input data, measured at the balanced line-level output. The digital input is swept with a dithered 1kHz input signal from -120dBFS to 0dBFS, and the output is analyzed by the APx555. The ideal response is a straight flat line at 0dB (i.e., the amplitude of the digital input perfectly matches the amplitude of the measured analog output). The 24/96 data was perfectly linear down to -120dBFS, while the 16/44.1 data was perfect down to -100dBFS, and only +2dB (left) and +2.5dB (right) at -120dBFS.

Impulse response (24/44.1)

The graph above shows the impulse responses for a looped 24/44.1 test file that moves from digital silence to full 0dBFS (all “1”s) for one sample period then back to digital silence into a 200k ohm-load for the left channel only. The DT-6000 does not use a typical symmetrical sinc function type filter, but rather one that exhibits no pre-ringing. Another thing to take note is how the plot first moves downward at 53.6ms, followed by an upward movement as it nears 53.7ms, which is the opposite of what is usually seen. That is another indicator that the DT-6000 inverts polarity.

J-Test (coaxial input)

The plot above shows the results of the J-Test test for the coaxial digital input measured at the balanced line-level output of the DT-6000. J-Test was developed by Julian Dunn the 1990s. It is a test signal—specifically,  a -3dBFS undithered 12kHz squarewave sampled (in this case) at 48kHz (24 bits). Since even the first odd harmonic (i.e., 36kHz) of the 12kHz squarewave is removed by the bandwidth limitation of the sampling rate, we are left with a 12kHz sinewave (the main peak). In addition, an undithered 250Hz squarewave at -144dBFS is mixed with the signal. This test file causes the 22 least significant bits to constantly toggle, which produces strong jitter spectral components at the 250Hz rate and its odd harmonics. The test file shows how susceptible the DAC and delivery interface are to jitter, which would manifest as peaks above the noise floor at 500Hz intervals (e.g., 250Hz, 750Hz, 1250Hz, etc). Note that the alternating peaks are in the test file itself, but at levels of -144dBrA (fundamental at 250Hz) down to -170dBrA for the odd harmonics. The test file can also be used in conjunction with artificially injected sinewave jitter by the Audio Precision, to also show how well the DAC rejects jitter.

The coaxial SPDIF input shows some significant peaks in the audioband, with levels reaching nearly -95dBrA. This is an indication that the DT-6000 may be sensitive to jitter.

J-Test (optical input)

The optical S/PDIF input shows a very different—and much better—response to the J-Test than the coaxial input, with the most significant peaks reaching -110dBrA.

J-Test with 10ns of injected jitter (coaxial input)

The coaxial input was also tested for jitter immunity by injecting artificial sinewave jitter at 2kHz on top of the J-Test test file, which would manifest as sidebands at 10kHz and 14kHz without any jitter rejection. Above is an FFT with jitter injected at the 10ns level, and significant peaks can be seen at -70dBrA. This demonstrates that the DT-6000 DAC is quite susceptible to jitter.

J-Test with 10ns of injected jitter (optical input)

The optical input was also tested for jitter immunity by injecting artificial sinewave jitter at 2kHz on top of the J-Test test file. As was the case for the coaxial input, the FFT above shows significant peaks at -70dBrA at the 10kHz and 12kHz positions.

J-Test with 100ns of injected jitter (coaxial input)

Above is an FFT with jitter injected at the 100ns level, and significant peaks can be seen at -50dBrA. This is further evidence that the DT-6000 DAC is susceptible to jitter.

J-Test with 100ns of injected jitter (optical input)

The optical input was also tested for jitter immunity by injecting artificial sinewave jitter at 2kHz on top of the J-Test test file. As was the case for the coaxial input, the FFT above shows significant peaks at -50dBrA at the 10kHz and 12kHz positions.

Wideband FFT spectrum of white noise and 19.1kHz sine-wave tone

The plot above shows a fast Fourier transform (FFT) of the DT-6000 balanced-line level output with white noise at -4 dBFS (blue/red) and a 19.1kHz sinewave at 0dBFS fed to the coaxial digital input, sampled at 16/44.1 (purple/green). The sharp roll-off above 20kHz in the white-noise spectrum shows the implementation of a reconstruction filter with relatively steep attenuation. There are absolutely no imaged aliasing artifacts in the audioband above the -120dBrA noise floor. The primary aliasing signal at 25kHz is at -100dBrA, with subsequent harmonics of the 25kHz peak at or below this level.

THD ratio (unweighted) vs. frequency vs. load (24/96)

The chart above shows THD ratios at the balanced line-level output into 200k ohms (blue/red) and 600 ohms (purple/green) as a function of frequency for a 24/96 dithered 1kHz signal at the coaxial input. The 200k- and 600-ohms data are nearly identical up to about 2kHz, above which the 200k-ohm THD data outperformed the 600-ohm data by about 3dB from 10 to 20kHz. THD ratios are very low from 20 to 500Hz, between 0.0001 and 0.0002%. Above 500Hz, there is a steady rise in THD, up to a peak of 0.002% at 10kHz into 600 ohms.

THD ratio (unweighted) vs. frequency vs. sample rate (16/44.1 and 24/96)

The chart above shows THD ratios at the balanced line-level output into 200k ohms as a function of frequency for a 16/44.1 (blue/red) and a 24/96 (purple/green) dithered 1kHz signal at the coaxial input. The 24/96 data out-performed the 16/44.1 data by about 5dB from 20Hz to 500Hz, where THD ratios were as low as 0.00015%. Above 1kHz, both THD data sets were identical, reaching a high of only 0.0015% at around 10kHz.

THD ratio (unweighted) vs. output (16/44.1 and 24/96) at 1kHz

The chart above shows THD ratios measured at the balanced output as a function of output voltage for the coaxial input into 200k ohms from -90dBFS to 0dBFS with 16/44.1 (blue/red) and 24/96 (purple/green) dithered input data. Once again, the 24/96 outperformed the 16/44.1 data, with a THD range from 1% to just above 0.0003% at 4.3Vrms, while the 16/44.1 ranged from 5% down to 0.0004% at 4.53rms. It’s important to highlight that large discrepancies in THD ratios at lower input levels are due to the inherently lower 24-bit noise floor of the 24/96 data (i.e., when no signal harmonics are measurable in an FFT, a THD value can only be assigned as low as the noise floor permits). The 24/96 data also shows a slight rise in THD between around 50mVrms and 200mVrms.

THD+N ratio (unweighted) vs. output (16/44.1 and 24/96) at 1kHz

The chart above shows THD+N ratios measured at the balanced output as a function of output voltage for the coaxial input into 200k ohms from -90dBFS to 0dBFS with 16/44.1 (blue/red) and 24/96 (purple/green) dithered input data. The 24/96 outperformed the 16/44.1 data, with a THD+N range from 10% down to 0.001%, while the 16/44.1 ranged from 50% down to 0.002% at the maximum output voltage of 4.3Vrms.

Intermodulation distortion vs generator level (SMPTE, 60Hz:4kHz, 4:1, 16/44.1, 24/96)

The chart above shows intermodulation distortion (IMD) ratios measured at the balanced output as a function of generator input level for the coaxial input into 200k ohms from -60dBFS to 0dBFS with 16/44.1 (blue/red) and 24/96 (purple/green) dithered input data. Here, the SMPTE IMD method was used, where the primary frequency (F1 = 60Hz) and the secondary frequency (F2 = 7kHz) are mixed at a ratio of 4:1. The SMPTE IMD analysis results consider the second (F2 ± F1) through the fifth (F2 ± 4xF1) modulation products. The 24/96 outperformed the 16/44.1 data, with an IMD range from 0.5% down to 0.001%, while the 16/44.1 ranged from 2% down to 0.002% at the maximum output voltage of 4.3Vrms at 0dBFS. The 24/96 data exhibited a slight rise in IMD between -20dBFS and -15dBFS.

FFT spectrum – 1kHz (digital input, 16/44.1 data at 0dBFS)

Shown above is the fast Fourier transform (FFT) for a 1kHz input sinewave stimulus, measured at the balanced output into 200k ohm for the coaxial digital input, sampled at 16/44.1. We see the second and third signal harmonics (2/3kHz) at -120dBrA, or 0.0001%. There are also higher-order signal harmonics at and below this level. There are no power-supply noise peaks to speak of to the left of the main signal peak.

FFT spectrum – 1kHz (digital input, 24/96 data at 0dBFS)

Shown above is the fast Fourier transform (FFT) for a 1kHz input sinewave stimulus, measured at the balanced output into 200k ohm for the coaxial digital input, sampled at 24/96. Due to the increased bit depth, the noise floor is much lower compared to the 16/44.1 FFT. We see signal harmonics ranging from -120dBrA (left/right), or 0.0001%, at 2/3/9kHz, and other signal harmonics down to -140dBrA, or 0.00001%. Even with the lower noise floor (-145dBrA), there are still essentially no visible low-level power-supply noise-related peaks on the left side of the main signal peak.

FFT spectrum – 1kHz (digital input, 16/44.1 data at -90dBFS)

Shown above is the fast Fourier transform (FFT) for a 1kHz input sinewave stimulus, measured at the balanced output into 200k ohm for the coaxial digital input, sampled at 16/44.1 at -90dBFS. We see the main peak at the correct amplitude, and no signal or noise harmonics above the noise floor within the audioband.

FFT spectrum – 1kHz (digital input, 24/96 data at -90dBFS)

Shown above is the fast Fourier transform (FFT) for a 1kHz input sinewave stimulus, measured at the balanced output into 200k ohms for the coaxial digital input, sampled at 24/96 at -90dBFS. We see the main peak at the correct amplitude, and no signal or noise harmonics above the noise floor within the audioband.

Intermodulation distortion FFT (18kHz + 19kHz summed stimulus, 16/44.1)

Shown above is an FFT of the intermodulation distortion (IMD) products for an 18kHz + 19kHz summed sinewave stimulus tone measured at the balanced output into 200k ohms for the coaxial input at 16/44.1. The input dBFS values are set at -6.02dBFS so that, if summed for a mean frequency of 18.5kHz, would yield 4.3Vrms (0dBrA) at the output. We find that the second-order modulation product (i.e., the difference signal of 1kHz) is at -115dBRA, or 0.0002%, while the third-order modulation products, at 17kHz and 20kHz, are just above -120dBrA, or 0.0001%.

Intermodulation distortion FFT (18kHz + 19kHz summed stimulus, 24/96)

Shown above is an FFT of the intermodulation distortion (IMD) products for an 18kHz + 19kHz summed sinewave stimulus tone measured at the balanced output into 200k ohms for the coaxial input at 24/96. The input dBFS values are set at -6.02dBFS so that, if summed for a mean frequency of 18.5kHz, would yield 4.3Vrms (0dBrA) at the output. We find that the second-order modulation product (i.e., the difference signal of 1kHz) is at -115dBRA, or 0.0002%, while the third-order modulation products, at 17kHz and 20kHz, are just above -120dBrA, or 0.0001%.

Diego Estan
Electronics Measurement Specialist