Bluesound Node X DAC-Preamplifier Measurements

Link: reviewed by Roger Kanno on SoundStage! Simplifi on October 15, 2023

General Information

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

The Bluesound Node X was evaluated as a digital-to-analog converter and conditioned for 30 minutes at 0dBFS (2.1Vrms out) into 100k ohms before any measurements were taken.

The Node X offers one combination digital-optical (S/PDIF) and analog 1/8″ TRS input. There is a 1/4″ TRS headphone output on the front of the unit. There is a digital volume control for the headphone and line-level outputs. There are also tone controls and a subwoofer output that can be turned on using the accompanying BluOS app. The app also offers full bass control with adjustable low/high pass filters. For the analog input, our standard 2Vrms level was replaced with 1Vrms, because at 2Vrms, the Node X’s ADC was nearing overload and random excessive noise was observed at the output. This is consistent with the behavior we have noted with other Bluesound products.

The analyzer’s input bandwidth filter was set to 10Hz–22.4kHz for all measurements, except for frequency response (DC to 1 MHz), FFTs (10Hz to 90kHz), and THD vs. frequency (10Hz to 90kHz). The latter to capture the second and third harmonic of the 20kHz output signal.

The Node X digital volume control ranges from -80 to 0dB, in steps ranging from 1 to 4dB. Channel-to-channel deviation proved excellent, at 0.001dB throughout the range.

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

Primary measurements

Our primary measurements revealed the following using the digitall input and the line-level outputs (unless specified, assume a 1kHz sinewave at 0dBFS, 200k ohms loading, 10Hz to 22.4kHz bandwidth):

Our primary measurements revealed the following using the digital input and the headphone output (unless specified, assume a 1kHz sinewave at 0dBFS, 300 ohms loading, 10Hz to 22.4kHz bandwidth):

Frequency response vs. sample rate (16/44.1, 24/96, 24/192, analog)

The plot above shows the Node X’s frequency response (relative to 1kHz) 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 cyan plot is for the analog input. It’s obvious from the response that incoming analog signals are sampled at 44.1kHz. There is also a slight roll-off (-0.3dB) from 5–10Hz that is not present for the digital input. The behavior at low frequencies is the same for all digital sample rates—perfectly flat down to 5Hz. The behavior at high frequencies for all three digital sample rates is as expected, offering sharp filtering around 22, 48, and 96kHz (half the respective sample rate), although the 24/192 data shows softer attenuation around the corner frequency. The -3dB point for each sample rate is roughly 21, 46, and 91.5kHz, 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.

Frequency response (bass and treble, 24/96)

Above are two frequency-response plots (relative to 1kHz) for the digital input (24/96), measured at the analog outputs, with the treble/balance controls set at both minimum and maximum. They show that the Node X will provide a maximum gain/cut of approximately 6dB at 20Hz and 20kHz.

Frequency response (bass management, 24/96)

Above are two frequency-response plots for the digital input (24/96), measured at the subwoofer output and left/right analog outputs, with the crossover set at 120Hz. The Node X crossover uses a slope of 18dB/octave, and the subwoofer output is flat down to 5Hz.

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

The graph above shows the results of a linearity test for the digital input for both 16/44.1 (blue/red) and 24/96 (purple/green) input data, measured at the line-level outputs of the Node X. The digital input was swept with a dithered 1kHz input signal from -120dBFS to 0dBFS, and the output was 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 input data is essentially perfect down to -120dBFS, while the 16/44.1 input data performed well, only over-responding by 3/2dB (left/right) at -120dBFS. The 24/96 data yielded such superb results that we extended the sweep down to . . .

. . . -140dBFS. Above we see that even at -140dBFS, the Node X is only overshooting by 1 to 3dB with 24/96 data. This is an exemplary linearity test result.

Impulse response

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, fed to the digital input, measured at the analog outputs, for the left channel only. We can see that Node X DAC reconstruction filter exhibits symmetrical pre- and post-ringing as seen in a typical sinc function.

J-Test (optical input)

The plot above shows the results of the J-Test test for the coaxial digital input measured at the analog outputs of the Node X. 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 optical digital input shows an average-to-mediocre J-Test result, with several peaks at the -130dBrA level and below clearly visible throughout the audioband. This is an indication that the Node X may be sensitive to jitter.

J-Test (optical input, 2kHz sinewave jitter at 100ns)

The plot above shows the results of the J-Test test for the optical digital input measured at the line-level output, with an additional 100ns of 2kHz sinewave jitter injected by the APx555, which would manifest as sideband peaks at 10kHz and 14kHz, without perfect jitter immunity. The results show no visible sidebands at 10kHz and 14kHz, and essentially the same J-Test result as seen above without the injection of jitter. The Node X DAC lost sync with the signal when roughly 600ns of jitter was added to the test signal.

Wideband FFT spectrum of white noise and 19.1kHz sinewave tone

The plot above shows a fast Fourier transform (FFT) of the Node X’s line-level output with white noise at -4dBFS (blue/red) and a 19.1 kHz sinewave at 0dBFS fed to the optical digital input, sampled at 16/44.1 (purple/green). There is a steep rolloff above 20kHz in the white-noise spectrum, characteristic of a brick-wall-type filter. There are no imaged aliasing artifacts in the audioband above the -135dBrA noise floor, except for a very small peak at roughly 11kHz at -130dBrA from the left channel. The primary aliasing signal at 25kHz is at -80dBrA.

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

The chart above shows THD ratios at the line-level output into 100k ohms (blue/red) and 600 ohms (purple/green) as a function of frequency for a 24/96 dithered 1kHz signal at the optical input. The 100k and 600 ohms data are extremely close throughout the audioband (3-5dB higher for the 600-ohm load at the frequency extremes), which is an in indication that Node X’s outputs are robust and can handle loads below 1k ohms with no difficultly. THD ratios into 100k ohms ranged from 0.0004% from 20Hz to 300Hz, then up to 0.01% at 20kHz.

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

The chart above shows THD ratios at the line-level output into 100k ohms as a function of frequency for a 16/44.1 (blue/red) and a 24/96 (purple/green) dithered 1kHz signal at the optical input. THD ratios were identical and ranged from 0.0004% from 20Hz to 300Hz, then up to 0.01% at 20kHz.

THD ratio (unweighted) vs. output (16/44.1, 24/96)

The chart above shows THD ratios measured at the line-level output as a function of output voltage for the optical input into 100k ohms from -90dBFS to 0dBFS at 16/44.1 (blue/red) and 24/96 (purple/green). The 24/96 data outperformed the 16/44.1 data at low output voltages by roughly 10dB, with a THD range from 0.5% at 200uVrms to 0.0006-0.001% at 0.5 to 2.1Vrms, while the 16/44.1 ranged from 3% down to the same 0.0006-0.001% at 0.5 to 2.1Vrms. The difference in THD ratios is owed to the lower noise floor with 24/96 data—the analyzer cannot measure/assign a THD ratio below the noise floor.

THD+N ratio (unweighted) vs. output (16/44.1, 24/96)

The chart above shows THD+N ratios measured at the line-level output as a function of output voltage for the optical input into 100k ohms from -90dBFS to 0dBFS at 16/44.1 (blue/red) and 24/96 (purple/green). The 24/96 outperformed the 16/44.1 data throughout by roughly 10dB, with a THD+N range from 6% down to 0.001% at 1–2Vrms, while the 16/44.1 ranged from 20% down to 0.002% at the maximum output voltage of 2.1Vrms.

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 line-level output for 16/44.1 (blue/red) input data and 24/96 input data (purple/green), from -60dBFS to 0dBFS. 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 16/44.1 data yields IMD ratios from 2% down to 0.003% at 0dBFS. The 24/96 data yields IMD ratios from 0.2% down to 0.001% at 0dBFS. The difference here again is likely due to the lower noise floor with 24/96 data.

FFT spectrum – 1kHz (analog input at 1Vrms)

Shown above is the fast Fourier transform (FFT) for a 1kHz input sinewave stimulus, measured at the line-level output into 100k ohm for the analog input, which is resampled by the Node X ADC at 16/44.1. The second (2kHz) harmonic dominates at nearly -90dBra, or 0.003%, while the third (3kHz) harmonic is at -105dBrA, or 0.0006%. There are very low-level power-supply-related noise peaks to the left of the main signal peak around the -130dBrA, or 0.00003%, level. Also visible are the 43.1kHz and 45.1kHz IMD peaks associated with the 44.1kHz sample rate.

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

Shown above is the fast Fourier transform (FFT) for a 1kHz input sinewave stimulus, measured at the line-level output into 100k ohm for the optical digital input, sampled at 16/44.1. The signal harmonic profile is similar but lower in amplitude to the FFT above, which would include artifacts of the Node X’s ADC. The second (2kHz), third (3kHz), and fifth (5kHz) signal harmonics dominate at the -100 to -110dBrA level, or 0.001 to 0.0003%. The noise floor is also lower from 10Hz to 50Hz.

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 100k ohm for the optical digital input, sampled at 24/96. Due to the increased bit-depth, the noise floor is lower compared to the 16/44.1 FFT above, at a very low -150dBrA. We see signal harmonics are essentially the same as the 16/44.1 FFT above. With the lower noise floor, noise-related harmonics are easier to see, and are actually a bit higher than the 16/44.1 FFT above, reaching nearly -120dBrA, or 0.0001%.

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 line-level output into 100k ohm for the optical digital input, sampled at 16/44.1 at -90dBFS. We see the signal peak at the correct amplitude, and no signal 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 line-level output into 100k ohm for the optical digital input, sampled at 24/961 at -90dBFS. We see the signal peak at the correct amplitude, and extremely low-level signal harmonics (2/3/4kHz) at and below -150dBrA, or 0.000003%. The 60Hz power-supply fundamental peak can be seen at -135dBrA, or 0.00002%.

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 line-level output into 100k ohms for the optical 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 2.1Vrms (0dBrA) at the output. We find that the second-order modulation product (i.e., the difference signal of 1kHz), is at -120dBrA, or 0.0001%, and the third-order modulation products, at 17kHz and 20kHz, are at -95dBrA, or 0.002%.

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 line-level output into 100k ohms for the optical 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 2.1Vrms (0dBrA) at the output. We find that the second-order modulation product (i.e., the difference signal of 1kHz), is at -120dBrA, or 0.0001%, and the third-order modulation products, at 17kHz and 20kHz, are at -95dBrA, or 0.002%.

Diego Estan
Electronics Measurement Specialist