WiiM Ultra Streaming Preamplifier Measurements

Link: reviewed by AJ Wykes on SoundStage! Simplifi on May 1, 2025

General Information

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

The WiiM Ultra was conditioned for 30 minutes at 2Vrms in/out into 100k ohms before any measurements were taken.

The Ultra offers one set of line-level analog inputs (RCA), one set of moving-magnet (MM) phono-level analog inputs (RCA), one S/PDIF digital input (optical), one HDMI (ARC) input, as well as streaming over ethernet or WiFi and Bluetooth inputs. Available outputs are a set of analog line-level (RCA), S/PDIF digital over optical (TosLink) and coaxial (RCA), as well as a single subwoofer output (configurable with internal bass management). There is also a ⅛″ TRS headphone output jack on the front panel. For the purposes of these measurements, unless otherwise stated, the following inputs were evaluated: digital optical S/PDIF (RCA), analog line-level (RCA), as well as phono MM (RCA).

Most measurements were made with a 2Vrms line-level and 0dBFS digital input with the volume set to achieve 2Vrms at the output (volume at maximum). Of note is that the Ultra digitizes all incoming analog signals to perform volume, EQ, and bass management. The ADC’s bit depth and sample rate are user-selectable (unless otherwise stated, 24/192 was used). As a line-level analog preamp, the Ultra does not offer any gain (-0.05dB at max volume), and the ADC will clip with a 2.15Vrms input signal. As a phono preamp, the Ultra was set to maximum gain (51.6dB at max volume), which yielded an output of roughly 1.9Vrms for a 5mVrms input level.

The Ultra also offers seven different reconstruction filters for the DAC, they are:

1. Brick Wall Filter (labelled Filter 1 in this report)
2. Corrected Minimum Phase Fast Roll-Off (labelled Filter 2 in this report)
3. Apodizing Fast Roll-Off (labelled Filter 3 in this report)
4. Minimum Phase Slow Roll-Off (labelled Filter 4 in this report)
5. Minimum Phase Fast Roll-Off (labelled Filter 5 in this report)
6. Linear Phase Slow Roll-Off (labelled Filter 6 in this report)
7. Linear Phase Fast Roll-Off (labelled Filter 7 in this report and the default filter unless otherwise stated)

The Ultra’s volume control operates in the digital domain, evidenced by our left-right channel tracking table below, which shows identical deviations (dB) at each sample volume level. Overall volume ranges from -59.6dB to -0.05dB (analog line-level in/out). Volume steps range from 1.5dB (lower volume levels) to 0.3dB (levels 50 through 100).

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 WiiM for the Ultra compared directly against our own. The published specifications are sourced from WiiM’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 0dBFS (24/96) at the input, 2Vrms at the output into 100k 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 coaxial input (unless specified, assume a 1kHz sinewave at 2Vrms or 0dBFS, 2Vrms output, 100kohm loading, 10Hz to 22.4kHz bandwidth):

Our primary measurements revealed the following using the MM phono-level input (unless specified, assume a 1kHz sinewave at 5mVrms, 1.9Vrms output, 100kohm loading, 10Hz to 22.4kHz bandwidth):

Our primary measurements revealed the following 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):

Frequency response (line-level input)

In our measured frequency-response (relative to 1kHz) plot above, the Ultra is near flat within the audioband (-0.2dB at 20Hz and 20kHz). At the extremes, the Ultra is -2.5dB at 5Hz, and brickwall filtering just below 96kHz (due to the 24/192 ADC sampling). 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 vs. ADC sample rate (line-level analog input)

In our measured frequency-response (relative to 1kHz) plot above, the blue/red traces are sampled at 44.1kHz, purple/green at 48kHz, pink/orange at 96kHz, and finally the two green traces are sampled using the default 192kHz (seen in the first graph above but limited to 80kHz). At high frequencies, there is nothing unusual, as we see brickwall type filtering around half the sample rate for all data. At low frequencies, strangely, we see different low-frequency extensions. The best results are with the 44.1kHz and 48kHz sample rates (-0.5dB at 5Hz), then 96kHz (-1.0dB at 5Hz), and finally the worst performer, the 192kHz sample rate (-2.5dB at 5Hz).

Frequency response with bass management (line-level analog input)

In our measured frequency response (relative to 1kHz for main output, relative to 20Hz for the subout) plot above, the blue trace is the subout, whereas the red trace is the main left output. The crossover frequency was chosen at 80Hz. We see 12dB/octave slopes, and a crossover point at the correct selected frequency.

Phase response (line-level analog input)

Above is the phase-response plot from 20Hz to 20kHz for the line-level input. The Ultra does not invert polarity, but because the signal is digitized, there is a significant amount of phase shift (350000 degrees of shift at 20kHz) due to the delay involved in performing the sampling.

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

Above is the phase response plot from 20Hz to 20kHz for the line-level input, only showing excess phase shift (beyond the timing delays due to ADC sampling). We see just shy of +20 degrees at 20Hz, and no phase shift at 20kHz.

Frequency response vs. input type

The chart above shows the Ultra’s frequency response (relative to 1kHz) as a function of input type. The green trace is the same (but limited to 80kHz) analog input data from the previous graph (sampled at 24/192). 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, and finally pink is 24/192 from 5Hz to 96kHz. The behavior at low frequencies is the same for all the digital sample rates, perfectly flat down to 5Hz, as opposed to the -2.5dB at 5Hz response from the sampled analog input. The behavior at high frequencies for all three digital sample rates is as expected, offering brickwall type filtering around 22, 48, and 96kHz (half the respective sample rate). Also as expected, the analog input (sampled at 24/192), follows the exact same high frequency response as the 24/192 input data.

Frequency response vs. digital filter type (16/44.1)

The chart above shows the Ultra’s frequency response (relative to 1kHz) for three filter types with a 16/44.1 input signal. The blue trace is the Brick Wall filter, red is Corrected Minimum Phase Fast Roll-Off, and green is Apodizing Fast Roll-Off. The Brick Wall filter has a -3dB point at 19.9kHz and is near 0dB at 19kHz. The Corrected Minimum Phase Fast Roll-Off filter has a -3dB point at 19kHz and is at -0.5dB at 18kHz. The Apodizing Fast Roll-Off filter has a -3dB point just past 20kHz and is near 0dB at 20kHz, though shows a ripple response (+/-0.2dB) from 10kHz to 20kHz.

The chart above shows the Ultra’s frequency response (relative to 1kHz) for four filter types with a 16/44.1 input signal. The pink trace is the Minimum Phase Slow Roll-Off filter, blue is Minimum Phase Fast Roll-Off, purple is Linear Phase Slow Roll-Off, and orange is Linear Phase Fast Roll-Off (the default filter). The Minimum Phase Slow Roll-Off filter has a -3dB point at 19.1kHz and is at -0.5dB at 17kHz. The Minimum Phase Fast Roll-Off filter has a -3dB point at 21.1kHz and is near 0dB at 19kHz. The Linear Phase Slow Roll-Off filter has a -3dB point just past 19.7kHz and is at -0.5dB at 17.3kHz. The Linear Phase Fast Roll-Off filter has a -3dB point at 21.1kHz and is near 0dB at 19kHz, essentially identical to the Minimum Phase Fast Roll-Off filter.

Phase response vs filter type (16/44.1 input, excess only)

Above is the phase-response plot (excess) from 20Hz to 20kHz for the first three filter type with a 16/44.1 input signal. The blue trace is the Brick Wall filter, red is Corrected Minimum Phase Fast Roll-Off, and green is Apodizing Fast Roll-Off. The Brick Wall and Apodizing Fast Roll-Off filters have identical behaviour at -45 degrees at 20kHz. The Corrected Minimum Phase Fast Roll-Off filter shows a bump from 10-15kHz (-20 to -10 degrees), then a steep phase roll-off (-180 degrees at 20kHz).

Above is the phase response plot (excess) from 20Hz to 20kHz for the next four filter types with a 16/44.1 input signal. The pink trace is the Minimum Phase Slow Roll-Off filter, blue is Minimum Phase Fast Roll-Off, purple is Linear Phase Slow Roll-Off, and orange is Linear Phase Fast Roll-Off (the default filter). The Minimum Phase Slow Roll-Off and Minimum Phase Fast Roll-Off filters have very close behaviours, right around -180 degrees at 20kHz. The Linear Phase Slow Roll-Off and Linear Phase Fast Roll-Off filters are identical at -45 degrees at 20kHz.

Frequency response (MM input)

The chart above shows the frequency response (relative to 1 kHz) for the MM phono input. What is shown is the deviation from the RIAA curve, where the input signal sweep is EQd with an inverted RIAA curve supplied by Audio Precision (i.e., zero deviation would yield a flat line at 0dB). The result shows relatively small maximum deviations within the audioband: about +0.2dB at 50kHz and +0.5dB at 20kHz. At low frequencies we see -1dB at 20Hz, and the -3dB point is at 15Hz.

Phase response (MM phono input)

Above is the phase response plot from 20Hz to 20kHz for the MM phono input. The Ultra does not invert polarity, but because the signal is digitized, there is a significant amount of phase shift (400000 degrees of shift at 20kHz) due to the delay involved in performing the sampling.

Phase response (MM phono input, excess only)

Above is the phase response plot from 20Hz to 20kHz for the phono MM input only showing excess phase shift (beyond the timing delays due to ADC sampling). 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. We see typical phase shifts associated with the implementation of RIAA filters: +100 degrees at 20Hz, 0 degrees at 200Hz and 5-6kHz, +20 degrees at 1kHz.

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 Ultra. 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 missed the mark by +1/-1dB (left/right) at -140dBFS. This is an exceptional 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 outputs of the Ultra. The blue trace is the Brick Wall filter, red is Corrected Minimum Phase Fast Roll-Off, and green is Apodizing Fast Roll-Off. Both the Brick Wall and Apodizing Fast Roll-Off filters show typical symmetrical sinc functions with pre/post ringing. The Corrected Minimum Phase Fast Roll-Off filter minimizes pre-ringing but shows extensive post-ringing.

This chart shows the remaining four filters. The pink trace is the Minimum Phase Slow Roll-Off filter, blue is Minimum Phase Fast Roll-Off, purple is Linear Phase Slow Roll-Off, and orange is Linear Phase Fast Roll-Off (the default filter). The Minimum Phase Slow Roll-Off filter shows no pre-ringing and minimum post-ringing. The Minimum Phase Fast Roll-Off filter shows no pre-ringing but significant post-ringing. The Linear Phase Slow Roll-Off filter is closest to an idealized impulse response: symmetrical with very little pre- and post-ringing. The Linear Phase Fast Roll-Off filter is another example of a typical symmetrical Sinc function with pre/post ringing.

J-Test (optical input)

The chart above shows the results of the J-Test test for the optical digital input measured at the line-level output of the Ultra. TheJ-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 SPDIF input of the Ultra shows a strong J-Test result, with spurious peaks at the -130dBrA and below level.

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

The chart above shows the results of the J-Test test for the optical digital input measured at the line-level output of the Ultra, but 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, with no visible peaks at the 10kHz and 14kHz positions. Further evidence of the Ultra’s strong jitter immunity.

Wideband FFT spectrum of white noise and 19.1kHz sinewave tone (optical input, Filter 1)

The chart above shows a fast Fourier transform (FFT) of the Ultra’s outputs with white noise at -4 dBFS (blue/red) and a 19.1 kHz sinewave at 0dBFS fed to the optical digital input, sampled at 16/44.1, using the Brick Wall filter. The steep roll-off around 20kHz in the white-noise spectrum shows the use of a brickwall-type reconstruction filter. There are no obvious aliased images within the audioband. The primary aliasing signal at 25kHz is highly suppressed at -100dBrA.

Wideband FFT spectrum of white noise and 19.1kHz sinewave tone (optical input, Filter 2)

The chart above shows a fast Fourier transform (FFT) of the Ultra’s outputs with white noise at -4 dBFS (blue/red) and a 19.1 kHz sinewave at 0dBFS fed to the optical digital input, sampled at 16/44.1, using the Corrected Minimum Phase Fast Roll-Off filter. The steep roll-off around 20kHz in the white-noise spectrum shows the use of a brickwall-type reconstruction filter. There are no obvious aliased images within the audioband. The primary aliasing signal at 25kHz is highly suppressed at -110dBrA.

Wideband FFT spectrum of white noise and 19.1kHz sinewave tone (optical input, Filter 3)

The chart above shows a fast Fourier transform (FFT) of the Ultra’s outputs with white noise at -4 dBFS (blue/red) and a 19.1 kHz sinewave at 0dBFS fed to the optical digital input, sampled at 16/44.1, using the Apodizing Fast Roll-Off filter. The steep roll-off around 20kHz in the white-noise spectrum shows the use of a brickwall-type reconstruction filter. There are no obvious aliased images within the audioband. The primary aliasing signal at 25kHz is highly suppressed at -100dBrA.

Wideband FFT spectrum of white noise and 19.1kHz sinewave tone (optical input, Filter 4)

The chart above shows a fast Fourier transform (FFT) of the Ultra’s outputs with white noise at -4 dBFS (blue/red) and a 19.1 kHz sinewave at 0dBFS fed to the optical digital input, sampled at 16/44.1, using the Minimum Phase Slow Roll-Off  filter. The slow roll-off around 20kHz in the white-noise spectrum matches the description of the reconstruction filter. There are no obvious aliased images within the audioband. The primary aliasing signal at 25kHz is barely suppressed at -30dBrA.

Wideband FFT spectrum of white noise and 19.1kHz sinewave tone (optical input, Filter 5)

The chart above shows a fast Fourier transform (FFT) of the Ultra’s outputs with white noise at -4 dBFS (blue/red) and a 19.1 kHz sinewave at 0dBFS fed to the optical digital input, sampled at 16/44.1, using the Minimum Phase Fast Roll-Off filter. The steep roll-off around 20kHz in the white-noise spectrum shows the use of a brickwall-type reconstruction filter. There are no obvious aliased images within the audioband. The primary aliasing signal at 25kHz is highly suppressed at -110dBrA.

Wideband FFT spectrum of white noise and 19.1kHz sinewave tone (optical input, Filter 6)

The chart above shows a fast Fourier transform (FFT) of the Ultra’s outputs with white noise at -4 dBFS (blue/red) and a 19.1 kHz sinewave at 0dBFS fed to the optical digital input, sampled at 16/44.1, using the Linear Phase Slow Roll-Off filter. The slow roll-off around 20kHz in the white-noise spectrum matches the description of the reconstruction filter. There are no obvious aliased images within the audioband. The primary aliasing signal at 25kHz is barely suppressed at -30dBrA.

Wideband FFT spectrum of white noise and 19.1kHz sinewave tone (optical input, Filter 7)

The chart above shows a fast Fourier transform (FFT) of the Ultra’s outputs with white noise at -4 dBFS (blue/red) and a 19.1 kHz sinewave at 0dBFS fed to the optical digital input, sampled at 16/44.1, using the Linear Phase Fast Roll-Off filter. The steep roll-off around 20kHz in the white-noise spectrum shows the use of a brick-wall type reconstruction filter. There are no obvious aliased images within the audioband. The primary aliasing signal at 25kHz is highly suppressed at -110dBrA.

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

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 the analog inputs. The 100k-ohm THD data are identical to the 600-ohm data, an indication that the Ultra will have no issues with sub-1k ohm amplifier input impedances. THD data ranged from 0.05% at 20Hz, down to 0.003% from 200Hz to 4kHz, then up to 0.006% at 20kHz.

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

The chart above shows THD ratios at the line-level output into 100k ohms for a 16/44.1 (blue/red) dithered 1kHz 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 (around 0.0002%) 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). 24/96 THD ratios were lower (by about 5dB) for the right channel from 30Hz to 1kHz, ranging from 0.00003% to 0.00005%. At 20kHz, all THD ratios measured 0.0002%.

THD ratio (unweighted) vs. frequency (MM phono input)

The graph above shows THD ratio as a function of frequency plot for the phono input. The input sweep is EQ’d with an inverted RIAA curve. The THD values ranged from around 0.3% (20Hz) down to around 0.003% (200Hz to 300Hz), then up to 0.06% at 20kHz.

THD ratio (unweighted) vs. output (analog)

The chart above shows THD ratios measured at the outputs of the Ultra as a function of output voltage for the analog line-level input, with the volume control set to maximum. THD values start at 0.2% at 1mVrms, down to a low of 0.001% at 0.2-0.5Vrms, then a rise to 0.003% nearing 2Vrms, then the 1% THD mark, due to ADC clipping, at 2.15Vrms.

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

The chart above shows THD+N ratios measured at the outputs of the Ultra as a function of output voltage for the analog line-level input, with the volume control at maximum. THD+N values start at 3% at 1mVrms, down to a low of 0.003% at 1-1.9Vrms, then a steep rise to the 1% THD mark, due to ADC clipping, at 2.15Vrms.

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

The chart above shows THD ratios measured at the outputs of the Ultra as a function of output voltage for the digital optical S/PDIF 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 2%, and predictably, reach their low at the maximum output voltage of just over 2Vrms, at 0.0002%. For the 24/96 data, THD ratios ranged from 0.2% down to 0.00005% 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 outputs of the Ultra as a function of output voltage for the digital optical S/PDIF 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 20% and reach their low at the maximum output voltage of just over 2Vrms, at 0.002%. For the 24/96 data, THD+N ratios ranged from 1.5% down to 0.0002% 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 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 0.15% down to 0.0005% at 0dBFS.

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

Shown above is the fast Fourier transform (FFT) for a 1kHz input sinewave stimulus, measured at the outputs for the analog line-level input. We see that the signal’s second (2kHz), third (3kHz) and fifth (5kHz) harmonics dominate, at -90/-100/-115dBrA, or 0.003/0.001/0.0002%. Subsequent signal harmonics can be seen at and below the -120dBrA, or 0.0001%, level. Below 1kHz, we see two small peaks from power-supply noise artifacts at 60Hz and 180Hz, around the -130dBrA, or 0.00003%, level.

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 outputs for the optical digital input, sampled at 16/44.1. We see the third (3kHz) signal harmonic just above the noise floor at -130dBrA, or 0.00003%. The noise floor is much higher due to the 16-bit depth limitation.

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 outputs for the optical digital input, sampled at 24/96. We see the second (2kHz), third (3kHz) and fifth (5kHz) signal harmonics at the very low levels of -140/-130/-140dBrA, or 0.00003% to 0.00001%. Power-supply-related noise peaks can be seen at and below the very low level of -135dBrA, or 0.00002% at 60/180/300Hz.

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 analog outputs. We see the 1kHz primary signal peak, at the correct amplitude, and no signal- or noise-related harmonic peaks above the -135dBrA 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 analog outputs. We see the 1kHz primary signal peak, at the correct amplitude, and signal- and noise-related peaks at around and below -140dBrA, or 0.00001%.

FFT spectrum – 1kHz (digital input, 24/96 data at -0dBFS by putting volume control at minimum)

Shown above is the FFT for a 1kHz -0dBFS dithered 24/96 input sinewave stimulus at the optical digital input, measured at the analog outputs, with the volume set to minimum, which yields a signal at -60dBrA. This FFT can be compared to the FFT below, which shows . . .

FFT spectrum – 1kHz (digital input, 24/96 data at -60dBFS by reducing analyzer level but with volume control at maximum)

. . . a 1kHz -60dBFS dithered 24/96 input sinewave stimulus but wth the volume set to maximum. We find that both FFTs are essentially the same, indicating that the digital volume control, even at the lowest levels, is transparent.

FFT spectrum – 1kHz (MM phono input)

Shown above is the FFT for a 1kHz input sinewave stimulus, measured at the analog outputs for the MM phono input. The dominant signal related harmonics can be seen at 2/3/4/5/6/7/10kHz, at -80dBrA to -100dBrA, or 0.01% to 0.001%. The main noise-related peak can be seen around 80Hz at -70dBrA, or 0.03%.

FFT spectrum – 50Hz (line-level input)

Shown above is the FFT for a 50Hz input sinewave stimulus measured at the outputs for the line-level 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) peaks are that of the signal’s second (100Hz) harmonic at -80dBrA, or 0.01%, and the third signal harmonic (150Hz) at -90dBrA, or 0.003%. A small power-supply-related noise peak can be seen at 180Hz at -130dBrA, or 0.00003%.

FFT spectrum – 50Hz (MM phono input)

Shown above is the FFT for a 50Hz input sinewave stimulus measured at the outputs for the MM phono input. The most predominant (non-signal) peaks are that of the signal’s second (100Hz) and third (150Hz) harmonics, as well as two noise-related peaks clustered just past 80Hz. All these peaks are around the -70dBrA level, or 0.03%.

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 outputs 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, it 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 -110dBRA, or 0.0003%, while the third-order modulation products, at 17kHz and 20kHz, are at the same level.

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

Shown above is the FFT of the outputs of the Ultra with the APx 32-tone signal applied to the analog 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 -100dBrA, or 0.001%, level from 20Hz to 100Hz, and below the -120dBrA, or 0.0001%, level from 300Hz to 20kHz.

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 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) is difficult to distinguish 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 -120dBrA.

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 coaxial input at 24/96. We find that the second-order modulation product (i.e., the difference signal of 1kHz) is just below -130dBrA, or 0.00003%, while the third-order modulation products, at 17kHz and 20kHz, are at -125dBrA, or 0.00006%.

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 outputs for the phono MM 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 at -60dBrA, or 0.1%.

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 Ultra’s slew-rate performance. Rather, it should be seen as a qualitative representation of its limited bandwidth to the 24/912 digital sampling. 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 Ultra’s reproduction of the 10kHz square wave is relatively clean for a digitized input, but due to the limited bandwidth, rippling can be seen in the plateaus of the squarewave.

Diego Estan
Electronics Measurement Specialist