Rotel Diamond Series RA-6000 Integrated Amplifier-DAC Measurements

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

General Information

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

The Rotel Diamond Series RA-6000 was conditioned for 1 hour at 1/8th full rated power (~25W into 8 ohms) before any measurements were taken. All measurements were taken with both channels driven, using a 120V/20A dedicated circuit, unless otherwise stated.

The Rotel RA-6000 offers three line-level analog inputs (RCA), one balanced line-level analog input (XLR), one phono input for moving magnet (MM) cartridges (RCA), pre-outs and sub-outs that operate identically (both RCA), three S/PDIF coaxial inputs (RCA), two S/PDIF optical inputs (TosLink), one USB input, one Bluetooth input, two separate (A and B) speaker-level outputs, and one headphone output (1/8″ TRS).

For the purposes of these measurements, the following inputs were evaluated: S/PDIF digital coaxial (RCA), analog line-level (XLR), and MM phono (RCA). Unless otherwise stated, the tone controls were bypassed. The only difference between the analog RCA and XLR inputs is 6dB less gain over the balanced XLR inputs. That is to say, to achieve the same output power at the speaker outputs using a 2Vrms input over then RCA input, 4Vrms is required over the balanced inputs.

Most measurements were made with a 4Vrms line-level signal at the analog input, 5mVrms at the MM input, and 0dBFS at the digital input. The volume display is variable between 0 and 96. To achieve 10W at the speaker outputs with the input values stipulated, the volume position required for the XLR input was 67, but 65 for the digital input and 70 for the phono input. The signal-to-noise ratio (SNR) measurements were made with the same input signal values but with the volume set to achieve the rated output power of 200W into 8 ohms. For comparison, on the line-level input, a SNR measurement was also made with the volume at maximum, where only 0.219Vrms was required to achieve 70W into 8 ohms.

Based on the high accuracy and low variability of the left/right volume channel matching (see table below), the RA-6000 volume control is likely digitally controlled but operating in the analog domain. The RA-6000 offers between 5 and 2dB volume steps between volume positions 1 and 7, and then 1dB steps up to position 76, then 0.5dB stepss up to the maximum level (96). Overall range is -52dB to +27.9dB (line-level RCA input, speaker output). Note that throughout the volume range, many volume steps were unused. For example, step 7 and step 8 had the same level of attenuation.

Volume-control accuracy (measured at speaker outputs): left-right channel tracking

Published specifications vs. our primary measurements

The table below summarizes the measurements published by Rotel for the RA-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 a combination thereof. With the exception of frequency response, where the Audio Precision bandwidth was extended to 500kHz, assume, unless otherwise stated, 10W into 8 ohms and a measurement input bandwidth of 10Hz to 90kHz, and the worst-case measured result between the left and right channels.

Our primary measurements revealed the following using the balanced line-level analog input and digital coaxial input (unless specified, assume a 1kHz sinewave at 2Vrms or 0dBFS, 10W output, 8-ohm loading, 10Hz to 90kHz bandwidth):

For the continuous dynamic power test, the RA-6000 was able to sustain 400W into 4 ohms (~3% THD) using an 80Hz tone for 500ms, alternating with a signal at -10dB of the peak (40W) for 5 seconds, for 5 continuous minutes without inducing a fault or the initiation of a protective circuit. This test is meant to simulate sporadic dynamic bass peaks in music and movies. During the test, the top of the RA-6000 was hot the touch, causing discomfort.

Our primary measurements revealed the following using the phono-level input, MM configuration (unless specified, assume a 1kHz 5mVrms sinewave input, 10W output, 8-ohm loading, 10Hz to 90kHz bandwidth):

Our primary measurements revealed the following using the analog input at the headphone output (unless specified, assume a 1kHz 2Vrms sinewave input (RCA), 2Vrms output, 300 ohms loading, 10Hz to 90kHz bandwidth):

Frequency response (8-ohm loading, line-level input)

In our frequency response plots above, measured across the speaker outputs at 10W into 8 ohms, the RA-6000 is near perfectly flat within the audioband (-0.2/0dB, 20Hz/20kHz). At the extremes, the RA-6000 is at -1.5dB at 5Hz and +0.7dB at 200kHz, making “wide bandwidth audio amplifier” an apt descriptor for the RA-6000. Rotel’s claim of 10Hz-100kHz, ±0.5dB, is not quite corroborated, as we measured -0.5dB at 10Hz and about +0.2dB at 100kHz. In the graph above and most of the graphs below, only a single trace may be visible. This is because the left channel (blue or purple trace) is performing identically to the right channel (red or green trace), and so they perfectly overlap, indicating that the two channels are ideally matched.

Frequency response (8-ohm loading, line-level input, bass and treble controls)

Above is a frequency response plot measured at the speaker-level outputs into 8 ohms, with the bass and treble controls set to maximum (blue/red plots) and minimum (purple/green plots). We see that for the bass and treble controls, roughly +/-12dB of gain/cut is available at 20Hz and roughly +/-9dB of gain/cut at 20kHz.

Phase response (8-ohm loading, line-level input)

Above are the phase response plots from 20Hz to 20kHz for the line-level input, measured across the speaker outputs at 10W into 8 ohms. The RA-6000 does not invert polarity and exhibits, at worst, about 30 degrees (at 20kHz) of phase shift within the audioband.

Frequency response vs. input type (8-ohm loading, left channel only)

The chart above shows the RA-6000’s frequency response as a function of input type measured across the speaker outputs at 10W into 8 ohms. The green trace is the same analog input data from the previous graph (but limited to 80kHz). The blue trace is for a 16-bit/44.1kHz dithered digital input signal from 5Hz to 22kHz using the coaxial input, the purple trace is for a 24/96 dithered digital input signal from 5Hz to 48kHz, while the pink trace is for a 24/192 dithered digital input signal from 5Hz to 96kHz. At low frequencies, the analog input exhibits a more extended response, measuring -0.5dB at 10Hz, whereas the digital inputs are at -1.7dB at 10Hz. At high frequencies, the 16/44.1 data exhibits brickwall-type filtering, with a -3dB point at 21.0kHz. The 24/96 and 24/192 kHz data yielded -3dB points at 46.3kHz and 92kHz, respectively.

Frequency response (8-ohm loading, MM phono input)

The chart above shows the frequency response for the phono input (MM). What is shown is the deviation from the RIAA curve, where the input signal sweep is EQ’d with an inverted RIAA curve supplied by Audio Precision (i.e., zero deviation would yield a flat line at 0dB). We see a maximum deviation within the audioband of about +0.5dB at 40kHz.

Phase response (MM input)

Above is the phase response plot from 20Hz to 20kHz for the phono input (MM), measured across the speaker outputs at 10W into 8 ohms. The RA-6000 does not invert polarity. For the phono input, since the RIAA equalization curve must be implemented, which ranges from +19.9dB (20Hz) to -32.6dB (90kHz), phase shift at the output is inevitable. Here we find a worst case of about -60 degrees at 200Hz and 5kHz.

Digital linearity (16/44.1 and 24/96 data)

The chart above shows the results of a linearity test for the coaxial digital input for both 16/44.1 (blue/red) and 24/96 (purple/green) input data, measured at the line-level output of the RA-6000. 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 would be a straight flat line at 0dB. Both data were essentially perfect as of -90dBFS down to 0dBFS. At -120dBFS, the 16/44.1 data were +4dB above reference, while the 24/96 data were +1/+7.5dB (left/right) above reference. This is a relatively poor linearity test result.

Impulse response (24/44.1 data)

The graph above shows the impulse responses for the RA-6000, fed to the coaxial digital input, measured at the line-level output, 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. We find a symmetrical sinc function type of impulse response.

J-Test (coaxial)

The chart above shows the results of the J-Test test for the coaxial digital input measured at the line-level output of the RA-6000. J-Test was developed by Julian Dunn in 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 and below.  The test file can also be used in conjunction with artificially injected sinewave jitter by the Audio Precision, to show how well the DAC rejects jitter.

The coaxial input exhibits obvious peaks throughout the audioband, as high as -95dBrA on both sides of the 12kHz fundamental. This is a relatively poor J-Test result, indicating that RA-6000 DAC may be susceptible to jitter through this input.

J-Test (optical)

The chart above shows the results of the J-Test test for the optical digital input measured at the line-level output of the RA-6000. The result is much cleaner than for the coaxial input. The highest peaks are just below -110dBrA, on both sides of the 12kHz fundamental. This input may be less susceptible to jitter.

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

The coaxial input was also tested for jitter immunity by injecting artificial sinewave jitter at 2kHz, which would manifest as sidebands at 10kHz and 14kHz without any jitter rejection. Jitter immunity proved poor, as predicted because of the above J-Test result, with visible sidebands at only the 10ns jitter level, at -70dBrA.

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

The coaxial input was also tested for jitter immunity by injecting artificial sinewave jitter at 2kHz, which would manifest as sidebands at 10kHz and 14kHz without any jitter rejection. Jitter immunity proved poor, as predicted because of the other results, with visible sidebands at the 100ns jitter level, at -50dBrA. The optical input (not shown) performed similarly with the same 100ns jitter level injected.

Wideband FFT spectrum of white noise and 19.1kHz sine-wave tone (coaxial input)

The chart above shows a fast Fourier transform (FFT) of the RA-6000’s line-level output with white noise at -4dBFS (blue/red) and a 19.1kHz sinewave at 0dBFS fed to the coaxial digital input, sampled at 16/44.1. The steep roll-off around 20kHz in the white-noise spectrum shows the behavior of the RA-6000’s reconstruction filter. There are no aliased image peaks within the audioband visible above the -120dBrA noise floor. The primary aliasing signal at 25kHz is at -60dBrA, while the second and third distortion harmonics (38.2, 57.3kHz) of the 19.1kHz tone are at -75dBrA and below.

RMS level vs. frequency vs. load impedance (1W, left channel only)

The chart above shows RMS level (relative to 0dBrA, which is 1W into 8ohms or 2.83Vrms) as a function of frequency, for the analog line-level input swept from 5Hz to 100kHz. The blue plot is into an 8-ohm load, the purple is into a 4-ohm load, the pink plot is an actual speaker (Focal Chora 806, measurements can be found here), and the cyan plot is no load connected. The chart below . . .

. . . is the same but zoomed in to highlight differences. Here we find that the maximum deviation between no-load and a 4-ohm load is very small, at just below 0.04dB. This is an indication of a very high damping factor, or low output impedance. With a real speaker, maximum deviations in RMS level were slightly smaller.

THD ratio (unweighted) vs. frequency vs. output power

The chart above shows THD ratios at the output into 8 ohms as a function of frequency for a sinewave stimulus at the analog line-level input. The blue and red plots are for left and right channels at 1W output into 8 ohms, purple/green at 10W, and pink/orange at 200W. Power was varied using the volume control. We find exceptionally consistent THD ratios at all power levels. THD ratios ranged from 0.005-0.01% at 20Hz, down to 0.002% between 100 and 500Hz, then up to 0.02-0.03% at 20Hz. Between all three power levels, a worst case of about 5dB of THD difference was observed.

THD ratio (unweighted) vs. frequency at 10W (MM input)

The chart above shows THD ratios as a function of frequency plots for the phono input measured across an 8-ohm load at 10W. The input sweep is EQ’d with an inverted RIAA curve. THD values vary from around 0.05% (20Hz), down to 0.001% (1kHz), then up to 0.03% (20kHz).

THD ratio (unweighted) vs. output power at 1kHz into 4 and 8 ohms

The chart above shows THD ratios measured at the output of the RA-6000 as a function of output power for the analog line-level input, for an 8-ohm load (blue/red for left/right channels) and a 4-ohm load (purple/green for left/right channels). The 8-ohm data ranged from about 0.01% at 50mW, down to 0.0015% at 50W, to the “knee,” which is just shy of 200W. The 4-ohm data yielded THD ratios roughly 5dB higher, and hit the “knee” at just over 300W. The 1% THD marks were hit at 225W (8 ohms) and 374W (4 ohms).

THD+N ratio (unweighted) vs. output power at 1kHz into 4 and 8 ohms

The chart above shows THD+N ratios measured at the output of the RA-6000 as a function of output power for the analog line-level input, for an 8-ohm load (blue/red for left/right channels) and a 4-ohm load (purple/green for left/right channels). THD+N values for the 8-ohm data ranged from 0.015%, down to 0.003% at the “knee.” The 8-ohm data outperformed the 4-ohm data by 3-4dB.

THD ratio (unweighted) vs. frequency at 8, 4, and 2 ohms (left channel only)

The chart above shows THD ratios measured at the output of the RA-6000 as a function of frequency into three different loads (8/4/2 ohms) for a constant input voltage that yields 20W at the output into 8 ohms (and roughly 40W into 4 ohms, and 80W into 2 ohms) for the analog line-level input. The 8-ohm load is the blue trace, the 4-ohm load the purple trace, and the 2-ohm load the pink trace. We see increasing levels (5-10dB) of THD from 8 to 4 to 2 ohms. Even into 2 ohms, however, THD ratios ranged from 0.05% down to 0.007%. Overall, this is a good result and shows how stable the RA-6000 is into low impedances.

THD ratio (unweighted) vs. frequency into 8 ohms and real speakers (left channel only)

The chart above shows THD ratios measured at the output of the RA-6000 as a function of frequency into an 8-ohm load and two different speakers for a constant output voltage of 2.83Vrms (1W into 8 ohms) for the balanced analog line-level input. The 8-ohm load is the blue trace, the purple plot is a two-way speaker (Focal Chora 806, measurements can be found here), and the pink plot is a three-way speaker (Paradigm Founder Series 100F, measurements can be found here). There are deviations in THD ratios above 6kHz, where the three-way speaker yielded the highest results (0.02% at 20Hz), the two-way speaker the lowest results (0.005% at 20kHz), and the 8-ohm dummy load in between (0.01% at 20kHz). For the rest of the audioband, all three THD results are fairly close to another, ranging from 0.01-0.02% at 20Hz, down to 0.002% around 1kHz. This is a testament to the RA-6000’s robust power supply and high damping factor.

IMD ratio (CCIF) vs. frequency into 8 ohms and real speakers (left channel only)

The chart above shows intermodulation distortion (IMD) ratios measured at the output of the RA-6000 as a function of frequency into an 8-ohm load and two different speakers for a constant output voltage of 2.83Vrms (1W into 8 ohms) for the analog line-level input. Here the CCIF IMD method is used, where the primary frequency is swept from 20kHz (F1) down to 2.5kHz, and the secondary frequency (F2) is always 1kHz lower than the primary, with a 1:1 ratio. The CCIF IMD analysis results are the sum of the second (F1-F2 or 1kHz) and third modulation products (F1+1kHz, F2-1kHz). The 8-ohm load is the blue trace, the purple plot is a two-way speaker (Focal Chora 806, measurements can be found here), and the pink plot is a three-way speaker (Paradigm Founder Series 100F, measurements can be found here). All three data sets track fairly closely, with the expection of the three-way speaker showing higher IMD values above 10kHz, differing by about 5dB at 20kHz. Overall, IMD ratios were hovering around 0.002 to 0.003%.

IMD ratio (SMPTE) vs. frequency into 8 ohms and real speakers (left channel only)

The chart above shows IMD ratios measured at the output of the RA-6000 as a function of frequency into an 8-ohm load and two different speakers for a constant output voltage of 2.83Vrms (1W into 8 ohms) for the analog line-level input. Here, the SMPTE IMD method is used, where the primary frequency (F1) is swept from 250Hz down to 40Hz, and the secondary frequency (F2) is held at 7kHz with a 4:1 ratio. The SMPTE IMD analysis results consider the second (F2 ± F1) through the fifth (F2 ± 4xF1) modulation products. The 8-ohm load is the blue trace, the purple plot is a two-way speaker (Focal Chora 806, measurements can be found here), and the pink plot is a three-way speaker (Paradigm Founder Series 100F, measurements can be found here). All three data sets are close enough to be judged as identical, hovering just below the 0.032% mark.

FFT spectrum – 1kHz (line-level input)

Shown above is the fast Fourier transform (FFT) for a 1kHz input sinewave stimulus, measured at the output across an 8-ohm load at 10W for the analog line-level input. We see that the signal’s second harmonic, at 2kHz, is at -95dBrA, or 0.002%, while all subsequent harmonics are below the -105dBrA level, or 0.0006% mark. Power-supply-related noise at the fundamental frequency (60Hz), as well as both even and odd harmonics, are evident, at -100dBrA, or 0.001%, levels and below.

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 output across an 8-ohm load at 10W for the coaxial digital input, sampled at 16/44.1. Signal harmonics and power-supply-related noise peaks are very similar in level to the analog input FFT above, with the exception of the large peaks just below 50 and 100kHz, which are IMD products of the 44.1kHz sample rate and signal (43.1kHz and 45.1kHz), and their resultant second harmonics.

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 output across an 8-ohm load at 10W for the coaxial digital input, sampled at 24/96. Signal harmonics and power supply related noise peaks are very similar in level to the analog input FFT above, except here the IMD products of the 96kHz sample rate and signal (95kHz and 97kHz) are visible.

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

Shown above is the FFT for a 1kHz -90dBFS dithered 16/44.1 input sinewave stimulus at the coaxial digital input, measured at the output across an 8-ohm load. We see the 1kHz primary signal peak, at the correct amplitude, and low-level power-supply-related noise harmonics at -110dBrA, or 0.0003%, and below.

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

Shown above is the FFT for a 1kHz -90dBFS dithered 24/96 input sinewave stimulus at the coaxial digital input, measured at the output across an 8-ohm load. We see the 1kHz primary signal peak, at the correct amplitude, and power-supply-related noise harmonics at -110dBrA, or 0.0003%, and below.

FFT spectrum – 1kHz (MM phono input)

Shown above is the FFT for a 1kHz input sinewave stimulus, measured at the output across an 8-ohm load at 10W for the MM phono input. We see the second (2kHz) signal harmonics at -100/105dBrA (left/right), or 0.001% and 0.0006%. Higher-order signal harmonics are below the -110dBrA level, or 0.0003%, and difficult to distinguish amongst the high-order power-supply-related noise peaks. Power-supply-related peaks can be seen throughout, with the highest at the third harmonic (180Hz) at -85/-90dBrA (left/right channels), or 0.006/0.003%.

FFT spectrum – 50Hz (line-level input)

Shown above is the FFT for a 50Hz input sinewave stimulus measured at the output across an 8-ohm load at 10W for the analog line-level input. The X axis is zoomed in from 40Hz to 1kHz, so that peaks from noise artifacts can be directly compared against peaks from the harmonics of the signal. Power-supply-related and signal-related peaks can be seen at and below -100dBrA, or 0.001%.

FFT spectrum – 50Hz (MM phono input)

Shown above is the FFT for a 50Hz input sinewave stimulus measured at the output across an 8-ohm load at 10W for the MM phono input. The X axis is zoomed in from 40 Hz to 1kHz, so that peaks from noise artifacts can be directly compared against peaks from the harmonics of the signal. The most predominant (non-signal) peak is an IMD product at 70Hz at -75dBrA, or 0.02%, between the signal and a power-supply noise peak. The highest signal harmonic is at 100Hz, at -90dBrA, or 0.0003%.

Intermodulation distortion FFT (18kHz + 19kHz summed stimulus, line-level input)

Shown above is an FFT of the intermodulation distortion (IMD) products for an 18kHz + 19kHz summed sinewave stimulus tone measured at the output across an 8-ohm load at 10W for the analog line-level input. The input RMS values are set at -6.02dBrA so that, if summed for a mean frequency of 18.5kHz, would yield 10W (0dBrA) into 8 ohms at the output. We find that the second-order modulation product (i.e., the difference signal of 1kHz) is at -100dBrA, or 0.001%, while the third-order modulation products, at 17kHz and 20kHz are just below -110dBrA, or 0.0003%.

Intermodulation distortion FFT (18kHz + 19kHz summed stimulus, coaxial digital input, 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 output across an 8-ohm load at 10W for the digital coaxial input at 16/44.1. We find that the second-order modulation product (i.e., the difference signal of 1kHz) is at -100/-95dBrA, or 0.001/0.002%, while the third-order modulation products, at 17kHz and 20kHz, are at -85dBrA, or 0.006%.

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

Shown above is an FFT of the intermodulation distortion (IMD) products for an 18kHz + 19kHz summed sinewave stimulus tone measured at the output across an 8-ohm load at 10W for the digital coaxial input at 24/96. We find that the second-order modulation product (i.e., the difference signal of 1kHz) is at -100/-95dBrA, or 0.001/0.002%, while the third-order modulation products, at 17kHz and 20kHz, are at -85dBrA, or 0.006%.

Intermodulation distortion FFT (18kHz + 19kHz summed stimulus, MM phono input)

Shown above is an FFT of the intermodulation distortion (IMD) products for an 18kHz + 19kHz summed sinewave stimulus tone measured at the output across an 8-ohm load at 10W for the MM phono input. We find that the second-order modulation product (i.e., the difference signal of 1kHz) is at -95dBrA, or 0.002%, while the third-order modulation products, at 17kHz and 20kHz, are right around the -120dBrA noise floor.

Square-wave response (10kHz)

Above is the 10kHz squarewave response using the balanced analog line-level input, at roughly 10W into 8 ohms. Due to limitations inherent to the Audio Precision APx555 B Series analyzer, this graph should not be used to infer or extrapolate the RA-6000’s slew-rate performance. Rather, it should be seen as a qualitative representation of the RA-6000’s extended bandwidth. An ideal squarewave can be represented as the sum of a sinewave and an infinite series of its odd-order harmonics (e.g., 10kHz + 30kHz + 50kHz + 70kHz . . .). A limited bandwidth will show only the sum of the lower-order harmonics, which may result in noticeable undershoot and/or overshoot, and softening of the edges. The RA-6000’s squarewave response is superb, showing only slight over/undershoot, or ringing near the sharp corners.

Damping factor vs. frequency (20Hz to 20kHz)

The final graph above is the damping factor as a function of frequency. For the left channel, we see a damping factor as high as 800 (20Hz), down to 630 from 30Hz to 4kHz, then down to 440 at 20kHz. For the right channel, we see a damping factor as high as 1000 (20Hz), down to 750 from 30Hz to 2kHz, then down to 400 at 20kHz. These are very high damping factors for an integrated amplifier.

Diego Estan
Electronics Measurement Specialist