Bluesound Node Icon Streaming Preamplifier Measurements

Link: reviewed by Roger Kanno on SoundStage! Simplifi on June 1, 2025

General Information

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

The Bluesound Node Icon was conditioned for 30 minutes at 2Vrms in/out into 200k ohms before any measurements were taken.

The Node Icon offers one RCA analog input and four digital inputs—optical S/PDIF, USB-C, HDMI ARC, Bluetooth—plus network streaming. There are two analog outputs (XLR and RCA). Also included are a line-level sub-out (with internal bass management) and a ¼″ TRS headphone output. For the purposes of these measurements, unless otherwise stated, the following inputs were evaluated using the balanced XLR outputs: digital optical S/PDIF, analog (RCA), and the headphone output. Comparisons were made between unbalanced (RCA) and balanced (XLR) outputs, and no appreciable differences were seen other than twice the gain over the balanced outputs (FFTs for different configurations can be seen in this report).

Most measurements were made with a 2Vrms line-level and 0dBFS digital input with the volume set to achieve 2Vrms at the output. The signal-to-noise ratio (SNR) measurements were made with the same input signal values and for comparison, on the line-level input, a SNR measurement was also made with the volume at maximum, but with a lower input voltage to achieve the same 2Vrms output.

Based on the repeatability of the results of the left/right volume channel matching (see table below), the Icon volume control is digitally controlled and operating in the digital domain. The icon automatically digitizes the analog input, which allows it to perform DSP functions such as bass/treble control and bass management. The Icon offers a volume range of -85dB to +3.7dB for the analog input over balanced outputs, in 100 steps. Steps range between 0.5 and 1dB in size.

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

Published specifications vs. our primary measurements

The table below summarizes the measurements published by Bluesound for the Node Icon compared directly against our own. The published specifications are sourced from Bluesound’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 set at its maximum (DC to 1MHz), unless otherwise stated, assume a 1kHz sinewave at 2Vrms or 0dBFS at the input, 2Vrms at the output into 200k ohms, and a measurement input bandwidth of 10Hz to 22.4kHz, 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 optical input (unless specified, assume a 1kHz sinewave at 2Vrms or 0dBFS, 2Vrms output, 200k ohm loading, 10Hz to 22.4kHz bandwidth):

* Due to very low noise of DUT, analyzer self-noise has been removed from measurement to more accurately report value

Our primary measurements revealed the following using the digital optical input (gain was measured using the analog input) at the headphone output (unless specified, assume a 1kHz sinewave, 0dBFS 24/96  input, 2Vrms output, 300 ohms loading, 10Hz to 22.4kHz bandwidth):

* Default is 24/96 0dBFS in, 2Vrms out into 300 ohms

Frequency response (line-level input)

In our measured frequency-response (relative to 1kHz) plot above, the Node Icon is near perfectly flat within the audioband (0dB at 20Hz, -0.2dB at 20kHz). At the extremes, the Node Icon is -0.25dB at 5Hz and -5dB at roughly 23kHz. The Node Icon digitizes incoming analog signals with a 48kHz sample rate. 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 with crossover at 80Hz (line-level input)

Above is the frequency response plot (relative to 20Hz for the sub-out, 1kHz for the line-level XLR out) for the Node Icon with bass management engaged in the BluOS app, applying an 80Hz crossover point. The crossover point is correctly applied, with attenuation at 18dB per octave.

Phase response (line-level analog input)

Above is the phase response plot from 20Hz to 20kHz for the analog line level input. The Node Icon does not invert polarity. Because of the sampling of the input by the ADC, absolute phase is extremely high (-2 million degrees at 20kHz) because it includes timing delays due to digitization. Below is a phase plot which aims to show only excess phase (excluding timing delays).

Phase response (line-level analog input, excess only)

There is essentially no phase shift up to just past 10kHz, then a spike to +160000 degrees just below 20kHz, then down to -100000 degrees at 20kHz.

Frequency response vs. input type

The chart above shows the Node Icon’s frequency response (relative to 1kHz) as a function of input type. The dark green trace is the same analog input data from the previous graph. The blue/red traces are for a 16bit/44.1kHz dithered digital input signal from 5Hz to 22kHz using the optical input, the purple/green traces are for a 24/96 dithered digital input signal from 5Hz to 48kHz, and finally pink/orange at 24/192 from 5Hz to 96kHz. The behavior at low frequencies is the same for all the digital sample rates; perfectly flat (0dB) down to 5Hz, as opposed to the analog signal which is -0.25dB at 5Hz. The behavior at high frequencies for all three digital sample rates differ. We see sharp, but not quite "brick-wall" filtering around 20kHz for the 16/44.1 data, with a -3dB point at 21kHz. The -3dB point for the 24/96 sampled data is at 35kHz, with a slower high-frequency roll-off. The -3dB point for the 24/192 sampled data is at 47kHz, with the slowest high-frequency roll-off. Case-in-point, the 24/192 data shows a -0.1dB at 10kHz response, while all other sample rates are at 0dB at 10kHz. 

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

The chart 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 outputs of the Node Icon. For this test, 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. At -120dBFS, the 16/44.1 data overshot by only 1-2dB, while the 24/96 remained perfect. To verify how well the 24/96 data would perform down to -140dBFS, we extended the sweep in the chart below.

Here we can see that the 24/96 data only overshot the mark by +1dB (left) at -135 to -140dBFS. This is an outstanding digital-linearity test result.

Impulse response (24/48 data)

The graph above shows the impulse response 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, measured at the balanced outputs of the Node Icon. We can see that the Node Icon utilizes a reconstruction filter that favors no pre-ringing.

J-Test (optical input)

The chart above shows the results of the J-Test test for the optical digital input measured at the balanced line-level output of the Node Icon. 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 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 optical input of the Node Icon shows a strong J-Test result, with several spurious peaks but at and below the low level of -140dBrA.

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

The chart above shows the results of the J-Ttest test for the optical digital input measured at the line-level output of the Node Icon, with an additional 100ns of 2kHz sinewave jitter injected by the APx555. The results are essentially the same as the J-Test result without additional jitter. More evidence of strong jitter rejection.

Wideband FFT spectrum of white noise and 19.1kHz sinewave tone

The chart above shows a fast Fourier transform (FFT) of the Node Icon’s balanced outputs with white noise at -4 dBFS (blue/red) and a 19.1 kHz sinewave at -2dBFS (0dB caused clipping) fed to the optical digital input, sampled at 16/44.1. The shallow roll-off around 20kHz in the white-noise spectrum shows that the Node Icon seems to use a reconstruction filter (for 16/44.1 content) with a high-frequency roll-off somewhere between slow and fast. There are no obvious aliased images within the audioband. The primary aliasing signal at 25kHz is barely suppressed at -20dBrA.

THD ratio (unweighted) vs. frequency vs. load (analog)

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 the analog inputs. THD data at the two different loads are extremely close, except beyond 10kHz, where the 600-ohm load results are roughly 10dB higher. Because the analog input is sampled at 48kHz and bandwidth is limited beyond 20kHz, THD results above 6kHz are not reliable as the signal harmonics above this point are highly suppressed by the ADC process. THD ratios are very low and slightly lower (3-4dB) for the right channel, hovering between 0.0004% and 0.0006% from 20Hz to 5kHz.

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 for a 16/44.1 (blue/red) dithered signal at the optical input and a 24/96 (purple/green) signal, as a function of frequency. The 16/44.1 THD ratios were higher than the 24/96 THD ratios due to the increased noise floor from the lower bit-depth (the analyzer cannot assign a THD ratio for harmonic peaks it cannot see above the noise floor). From 20Hz to 1.5kHz, the 16/44.1 THD ratios are stable at 0.0002-0.0003%, while the 24/96 THD data for the right channel is extremely low at 0.00004%. The left channel at 24/96 was stable at 0.0001% from 20Hz to 1.5kHz. Just shy of and beyond 2kHz, there is a steep rise in all THD data, peaking beyond 1% at 20kHz for the 16/44.1 data. This is not a purely frequency dependent phenomenon, but rather an issue with distortion with an input signal magnitude of 0dBFS. We reached out to Lenbrook Industries (Bluesound's parent company) to ask if they were aware of this issue. They responded that they were aware, that overload at 0dBFS from steady-state signals (sinewaves) is a conseqeunce of their special filters (though overload with musical signals should not occur), and finally that the sofware team was looking at making some adjustments to fix the issue. Below are the same plots, but with a -2dBFS input signal. We see . . .

. . . very different results beyond 2kHz, with THD ratios never exceeding 0.0002%.

THD ratio (unweighted) vs. output (analog)

The chart above shows THD ratios measured at the balanced outputs of the Node Icon as a function of output voltage for the analog line level-input, with the volume at maximum. THD values start at 0.15% at 1mVrms, down to a low of 0.0002% at 1-2Vrms, then a steep rise past 3Vrms to the 1% THD mark at 3.8Vrms. For the Node Icon, clipping is occurring at the input of the ADC, not at the analog output stage.

THD+N ratio (unweighted) vs. output (analog)

The chart above shows THD+N ratios measured at the balanced outputs of the Node Icon as a function of output voltage for the balanced line-level -input, with the volume control at maximum. THD+N values start at 1.5% at 1mVrms, down to a low of 0.0006% at 3Vrms, then a steep rise past 3Vrms to the 1% THD mark at 3.8Vrms.

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

The chart above shows THD ratios measured at the balanced outputs of the Node Icon as a function of output voltage for the digital optical input, swept from -90dBFS to 0dBFS, with the volume control at maximum. Blue/red traces are for 16/44.1 data and purple/green for 24/96. For the 16/44.1 data, THD values start at 3%, and predictably, reach their low at the maximum output voltage of about 3.8Vrms, at 0.0002%. For the 24/96 data, THD ratios ranged from 0.05% down to 0.00005% (right channel) at the maximum output voltage.

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

The chart above shows THD+N ratios measured at the balanced outputs of the Node Icon as a function of output voltage for the digital optical input, swept from -90dBFS to 0dBFS, with the volume control at maximum. Blue/red traces are for 16/44.1 data and purple/green for 24/96. For the 16/44.1 data, THD+N values start at 30% and reach their low at the maximum output voltage of about 3.8Vrms, at 0.002%. For the 24/96 data, THD+N ratios ranged from 0.5% down to just above 0.0001% at the maximum output voltage.

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 balanced 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 0.002% at 0dBFS. The 24/96 data yields IMD ratios from 0.1% down to 0.0005% (right channel) at 0dBFS.

FFT spectrum – 1kHz (analog line-level input, XLR output)

Shown above is the fast Fourier transform (FFT) for a 1kHz input sinewave stimulus, measured at the balanced outputs for the line-level input. We see that the signal’s second (2kHz) harmonic is at -120dBrA, or 0.0001%, the third (3kHz) harmonic is at -110dBrA, or 0.0003%, and the fifth (5kHz) harmonic is at -125dBrA, or 0.00006%. Other signal harmonics can be seen, but around the very low -140dBrA, or 0.00001%, level. Below 1kHz, there are essentially no peaks above the -150dBrA noise floor. Also evident are peaks at 47Kz and 49kHz, proof of the 48kHz sampling of the signal.

FFT spectrum – 1kHz (analog line-level input, RCA output)

Shown above is the fast Fourier transform (FFT) for a 1kHz input sinewave stimulus, measured at the unbalanced outputs for the line-level input. In terms of signal harmonics, this FFT is identical to the FFT above for balanced outputs. Noise artifacts are evident here (unlike the FFT above for the balanced outputs), but at very low levels (at and below -130dBrA, or 0.00003%).

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 outputs for the optical digital input, sampled at 16/44.1. We only see one peak, the second (2kHz) signal harmonic for the left channel at -120dBrA, or 0.0001%.

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 outputs for the optical digital input, sampled at 24/96. We see several spurious noise related noise peaks above the -160dBrA noise floor but at very low levels (-140 to -150dBrA). The second (2kHz) and fifth (5kHz) signal harmonic is at -135dBrA, or 0.00002%, while the third (3kHz) harmonic is at -120/-140dBrA (left/right), or 0.0001/0.00001%.

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 optical digital input, measured at the balanced outputs. We see the 1kHz primary signal peak, at the correct amplitude, and no signal or noise related harmonic peaks above the -140dBrA noise floor.

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 optical digital input, measured at the balanced outputs. We see the 1kHz primary signal peak, at the correct amplitude, and a signal related harmonic peak (left channel only) at 5kHz at around -145dBrA, or 0.000006%. Other noise-related peaks can be seen but at the extremely low -150dBrA, or 0.000003%, level.

FFT spectrum – 50Hz (line-level input)

Shown above is the FFT for a 50Hz input sinewave stimulus measured at the balanced outputs for the 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. The most predominant (non-signal) peaks are that of the signal’s third (150Hz) harmonic at -110dBrA, or 0.0003%. Other signal harmonic peaks can be seen at and below -120dBrA, or 0.0001%.

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 balanced outputs for the 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 -120dBrA or 0.0001%, while the third-order modulation products, at 17kHz and 20kHz are at -110dBrA, or 0.0003%. This is a very clean IMD result.

Intermodulation distortion FFT (line-level input, APx 32 tone)

Shown above is the FFT of the balanced outputs of the Node Icon with the APx 32-tone signal applied to the input. The combined amplitude of the 32 tones is the 0dBrA reference, and corresponds to 2Vrms. The intermodulation products—i.e., the “grass” between the test tones—are distortion products from the preamplifier and are around the -130dBrA, or 0.00003%, level.

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 balanced outputs for the digital optical input at 16/44.1. We find that the second-order modulation product (i.e., the difference signal of 1kHz) cannot be seen above the -135dBrA noise floor, while the third-order modulation products, at 17kHz and 20kHz, are at -125dBrA, or 0.00006%. We also see the main aliased peaks at 25.1kHz and 26.1kHz at around -30dBrA.

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 balanced outputs for the digital optical input at 24/96. We find that the second-order modulation product (i.e., the difference signal of 1kHz) is at -140/-150dBrA (left/right), or 0.00001/0.000003%, while the third-order modulation products, at 17kHz and 20kHz, are at -125dBrA or 0.00006%.

Square-wave response (10kHz)

Above is the 10kHz squarewave response using the analog line-level input, at roughly 2Vrms. Due to limitations inherent to the Audio Precision APx555 B Series analyzer, this graph should not be used to infer or extrapolate the Node Icon’s slew-rate performance. Rather, it should be seen as a qualitative representation of its very limited  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. Here, only the fundamental 10kHz frequency can be seen, as a sinewave.

Square-wave response (1kHz)

Above is the 1kHz squarewave response using the analog line-level input, at roughly 2Vrms. Due to the very limited bandwidth of the Node Icon due to its 48kHz sampling of analog signals, the 1kHz squarewave reproduction is poor, with significant ringing in the plateaus.

Diego Estan
Electronics Measurement Specialist