Details

Parent Category: Products

Category: Phono Preamplifier Measurements

Created: 15 March 2022

Link: reviewed by Jason Thorpe on SoundStage! Hi-Fi on March 15, 2022

General information

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

The Saturn Audio 401 was conditioned for 30 minutes at 2Vrms (1Vrms unbalanced) at the output before any measurements were taken.

The 401 offers two pairs of unbalanced RCA inputs—one pair for a moving-magnet (MM) cartridge, the other for a moving coil (MC) cartridge—selectable by a switch on the rear panel. There are both unbalanced (RCA) and balanced (XLR) outputs.

Besides the extra 6dB in gain the balanced outputs had over the unbalanced outputs, we found no appreciable differences in terms of THD+N. The MM gain is fixed (42dB), while the 401 offers five MC gain settings: 87, 81, 75, 67, and 61dB. Of note, Saturn Audio specifies all of their gain values for the balanced outputs—so, subtract 6dB for each gain value if using the unbalanced outputs. Also included are a grounding post with a ground-lift switch, a rumble filter, and four resistive loading settings for the MC input: 100, 220, 470, and 1000 ohms.

Unless otherwise specified, the balanced outputs were used for all measurements, with the rumble filter off, and with the MC input set to 67dB of gain and a 100-ohm input impedance. The 401 power supply is external—it connects to the main unit using an umbilical terminated with a four-pin XLR connector. Slightly lower noise was achieved with the power supply approximately 3′ away from the main unit, which is what the measurements reflect. Using the default settings above, to achieve the reference output voltage of 2Vrms (1Vrms unbalanced) at 1kHz, 18mVrms was required for the MM input and 1.1mVrms with the MC input.

Published specifications vs. our primary measurements

The table below summarizes the measurements published by Saturn Audio for the 401 compared directly against our own. The published specifications are sourced from Saturn Audio’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), assume, unless otherwise stated, 2Vrms balanced output into 200k ohms (100k ohms unbalanced) and a measurement input bandwidth of 10Hz to 90kHz, and the worst-case measured result between the left and right channels. For the MC gain setting measurements, the input impedance was set to 1k ohm.

Our primary measurements revealed the following using the unbalanced MM input (unless specified, assume a 1kHz sinewave, 2Vrms output into a 200k ohms load, 10Hz to 90kHz bandwidth):

Our primary measurements revealed the following using the unbalanced MC input (unless specified, assume a 1kHz sinewave, 2Vrms output into a 200k ohms load, 10Hz to 90kHz bandwidth):

Frequency response - MM input

In our measured frequency-response plots above for the MM input measured at the balanced output, the blue/red traces are with the rumble filter disengaged, while the purple and green represent the responses with the rumble filter.  An inverse RIAA EQ is applied to the input sweep, so that if a device were to track the RIAA curve perfectly, a flat line would emerge. The 401 is within +/-0.1dB or so of flat from 26Hz to 13kHz, and about +0.5dB up at 20kHz, with a steady rise above the audioband. These data do not corroborate Saturn’s claim of 10Hz to 100kHz +/-0.1dB. With the rumble filter engaged, there is steep attenuation below 18Hz, as advertised. Without the rumble filter, the 401 is at -0.2dB at 20Hz, and about -0.7dB at 10Hz. The worst-case channel to channel deviation is between 2kHz and 20kHz, where the right channel is 0.1dB hotter than the left channel. In the chart above and some of the charts below, we see two visible traces—the left channel (blue or purple lines) and the right channel (red or green lines). On other charts, only one trace may be visible—this is because the left and right channels are tracking extremely closely, so they do not show a difference with the chosen axis scales.

Frequency response - MC input

In our measured frequency-response plot above for the MC input, the 401 yields virtually the same results as with the MM input above. Please note that the rumble-filter plot is not show above, but when measured, was virtually the same as the MM plot.

Phase response - MM input

Above is the phase response of the 401 for the MM input, from 20Hz to 20kHz. The 401 does not invert polarity. Since phono preamplifiers must implement the RIAA equalization curve, which ranges from +19.9dB (20Hz) to -32.6dB (90kHz), phase shift at the output is inevitable. Here we find a worst-case -60 degrees around 200Hz and 5-7kHz.

Phase response - MC input

Above is the phase response of the 401 for the MC input, from 20Hz to 20kHz. The 401 does not invert polarity. Here we find a worst case -60 degrees around 200Hz and 5-7kHz, which mirrors what was found with the MM input.

THD ratio (unweighted) vs. frequency - MM and MC inputs

The chart above shows THD ratios as a function of frequency, where the input sweep is EQ’d with an inverted RIAA curve. The balanced output voltage is maintained at the refrence 2Vrms. The red/blue (left and right channels) traces represent the MM input, and purple/green are the MC input. For the MM input, THD values are very low, ranging from 0.002% at 20Hz, down to 0.00003% at 20kHz. The MC input yielded higher THD ratios, but still admirably low, ranging from 0.01% at 20Hz, down to around 0.0002% at 5kHz, then back up to 0.001% at 20kHz.

THD ratio (unweighted) vs. output voltage at 1kHz - MM and MC inputs

Above we can see a plot of THD+N ratios as a function of output voltage for the balanced output. The red/blue (left and right channels) traces represent the MM input, and purple/green (left and right channels) for the MC input. For the MM input, THD+N values at 100mVrms are at 0.5%, then dip as low as 0.0006% around 10Vrms, then the “knee” just below 14Vrms. For the MC input, THD+N values at 100mVrms are at 0.2%, then dip as low as 0.003% around 10Vrms until the “knee” just below 14Vrms.

THD+N ratio (A-weighted) vs. output voltage at 1kHz - MM and MC inputs

Above we can see a plot of THD+N (A-weighted) ratios as a function of output voltage for the balanced output. The red/blue (left and right channels) traces represent the MM input, and purple/green (left and right channels) for the MC input. For the MM input, THD+N values at 100mVrms are at 0.02%, then dip as low as 0.0002% around 10Vrms. For the MC input, THD+N values at 100mVrms are at 0.1%, then dip as low as 0.0007% around the “knee,” at just below 14Vrms.

FFT spectrum, 1kHz - MM input

Shown above is a fast Fourier Transform (FFT) of a 1kHz input sine-wave stimulus for the MM input, which results in the reference voltage of 2Vrms (0dBrA) at the balanced output. Here we see exceptionally clean results. Signal harmonics are non-existent above the -130 to -150dBrA noise floor. On the left side of the signal peak, there is only a very small 60Hz power-supply fundamental peak at around -105dBrA, or 0.0006%.

FFT spectrum, 1kHz - MC input

Shown above is an FFT of a 1kHz input sine-wave stimulus for the MC input at the balanced output. As there is 25dB more gain with the MC setting, predictably the noise floor is elevated compared to the MM input FFT, although, only by about 10dB. This is also an exceptionally clean FFT for an MC phono preamplifier. We can just barely see the second signal harmonic (2kHz) above the -120dBrA noise floor. The 60Hz power-supply noise peak is more pronounced due to the higher gain, at -80dBrA, or 0.01%. The third noise harmonic (180Hz) can also be seen at -95dBrA, or 0.002%.

FFT spectrum, 50Hz - MM input

Shown above is the FFT for a 50Hz input sine-wave stimulus measured at the balanced output for the MM 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 harmonics from the 50Hz signal (100, 150, 200Hz, etc.) are non-existent above the noise floor, and the two power-supply-related noise peaks can just be seen above the low noise floor of -110 to -120 dBrA, or 0.0003% and 0.0001%, at 60Hz and 180Hz.

FFT spectrum, 50Hz - MC input

Shown above is the FFT for a 50Hz input sine-wave stimulus measured at the balanced output for the MC 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. Only the signal’s second harmonic (100Hz) can be seen just above the noise floor at -100dBrA, or 0.001%. The 60Hz power-supply related noise peak is clearly seen at -80dBrA, or 0.01%, as is the third harmonic (180Hz) at -95dBrA, or 0.002%.

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

Above is an FFT of the IMD products for an 18kHz and 19kHz summed sine-wave stimulus tone for the MM input measured at the balanced output. The input rms values are set so that if summed (for a mean frequency of 18.5kHz), would yield 2Vrms (reference or 0dBRa) at the output. Here we find the second-order modulation product (i.e., the difference signal of 1kHz) at a very low -105dBrA, or 0.0006%. We can also see the third-order modulation products (i.e., 17kHz and 20kHz) sitting at a vanishingly low -125dBrA, or 0.00006%. This is an exceptional IMD result for a phono preamplifier.

Intermodulation distortion FFT (18kHz + 19kHz summed stimulus) - MC

The last chart is an FFT of the IMD products for an 18kHz and 19kHz summed sine-wave stimulus tone for the MC input. Here we find the second-order modulation product (i.e., the difference signal of 1kHz) is at -95dBrA, or 0.002%. We can also see the third-order modulation products (i.e., 17kHz and 20kHz) at roughly the same amplitude as the MM setting, sitting at or just below -120dBRa, or 0.0001%.

Diego Estan
Electronics Measurement Specialist

Details

Parent Category: Products

Category: Phono Preamplifier Measurements

Created: 15 July 2021

Link: reviewed by Thom Moon on SoundStage! Access on July 15, 2021

General information

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

The AT-PEQ30 was conditioned for 30 minutes at 1Vrms at the output before any measurements were taken.

The AT-PEQ30 offers one pair of unbalanced RCA inputs and one pair of unbalanced RCA outputs. The input can be configured for a moving magnet (MM) or moving coil (MC) cartridge by selecting a switch on the front panel. To achieve the reference output voltage of 1Vrms at 1 kHz, 18mVrms was required with the MM setting, and 0.9mVrms with the MC setting.

Published specifications vs. our primary measurements

The table below summarizes the measurements published by Audio-Technica for the AT-PEQ30 compared directly against our own measurements. The published specifications are sourced from Audio-Technica’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 is set at its maximum (DC to 1MHz), assume, unless otherwise stated, 1Vrms output into 100k ohms, a measurement input bandwidth of 10Hz to 90kHz, and the worst-case measured result between the left and right channels.

*Audio-Technica's SNR specifications (100/74dB for MM/MC) were not given with a reference output voltage, therefore, our SNR measurements are shown with both a 1Vrms output, and Audio-Technica's 250mVrms rated output.

Our primary measurements revealed the following using the unbalanced MM setting (unless specified, assume a 1kHz sinewave, 1Vrms output into a 100k ohms load, 10Hz to 90kHz bandwidth):

Our primary measurements revealed the following using the unbalanced MC input (unless specified, assume a 1kHz sinewave, 1Vrms output into a 100k ohms load, 10Hz to 90kHz bandwidth):

Frequency response - MM input

Shown above is our frequency-response plot for the MM setting measured at the unbalanced output. An inverse RIAA EQ is applied to the input sweep, so that if a device were to track the RIAA curve perfectly, a flat line would emerge. The AT-PEQ30 is within +/-0.3dB or so of flat from 20Hz to 20kHz, meeting Audio-Technica’s claim of 20Hz-20kHz (+/-0.5dB). It is about -2dB down at 80kHz, and +1/1.5dB (left/right) at about 55kHz. The worst-case channel-to-channel deviation is between 5kHz and 20kHz, where the right channel is almost 0.5dB hotter than the left. In the graph above and some of the graphs below, we see two visible traces; the left channel (blue or purple) and the right channel (red or green). On other graphs, only one trace may be visible, this is because the left and right channels are tracking extremely closely, so as not to show a difference with the chosen axis scales.

Frequency response - MC input

In our measured frequency-response plot above for the MC setting, the AT-PEQ30 is within +0.5dB/-0dB or so of flat from 20Hz to 20kHz, meeting Audio-Technica’s of claim of 20Hz-20kHz (+/-0.5dB). We can see that the MM configuration offers a more extended low frequency bandwidth, where the MC configuration is at -2dB at 10Hz compared to the MM configuration that measured near 0dB at 10Hz. Channel-to-channel deviations can also be seen with the MC setting from 5kHz to 20kHz, but to a lesser degree compared to the MM setting, with about 0.2dB deviation.

Phase response - MM input

Above is the phase response of the AT-PEQ30 for the MM setting, from 20Hz to 20kHz. The AT-PEQ30 does not invert polarity. Since phono preamplifiers must implement the RIAA equalization curve, which ranges from +19.9dB (20Hz) to -32.6dB (90kHz), phase shift at the output is inevitable. Here we find a worst-case -60 degrees around 200Hz and 5-8kHz.

Phase response - MC input

Above is the phase response of the AT-PEQ30 for the MC setting, from 20Hz to 20kHz. The AT-PEQ30 does not invert polarity. Here we find a worst-case -65 degrees around 200Hz and 7-8kHz.

THD ratio (unweighted) vs. frequency - MM and MC inputs

The chart above shows THD ratios as a function of frequency, where the input sweep is EQ’d with an inverted RIAA curve. The unbalanced output voltage is maintained at the refrence 1Vrms. The red/blue (L/R) traces represent the MM configured input, and purple/green for MC. For the MM configuration, THD values at 20Hz are at 0.004%, then dip as low as 0.00007% around 1-2kHz, then up to 0.003% at 20kHz. For the MC configuration, THD values at 20Hz are at 0.1%, then dip down to 0.0002% around 10kHz, then a climb to 0.0007% at 20kHz.

THD ratio (unweighted) vs. output voltage at 1kHz - MM and MC inputs

The chart above shows THD ratios as a function of output voltage for the unbalanced output. The red/blue (L/R) traces represent the MM configured input, and purple/green for MC. For the MM configuration, THD values at 100mVrms are at 0.0005%, then dip as low as 0.00007% between 0.8 and 1Vrms, then the “knee” around 2.5Vrms, where THD values reach 0.0002%. For the MC configuration, THD values at 100mVrms are at 0.01%, then steadily decrease down to 0.005% at the 2.5Vrms “knee.” The 1% THD values for the both inputs are reached at around 3Vrms at the output. It’s important to mention that anything above 1-2Vrms is not typically required for most line-level preamps or integrated amps.

THD+N ratio (unweighted) vs. output voltage at 1kHz - MM and MC inputs

Above we can see a plot of THD+N ratios as a function of output voltage for the unbalanced output. The red/blue (L/R) traces represent the MM-configured input, and purple/green for the MC-configured input For the MM configuration, THD+N values at 100mVrms are at 0.5%, then dip as low as 0.015% between 1 to 2.5Vrms, then the “knee” around 2.5Vrms. For the MC configuration, THD+N values at 100mVrms are at 3%, then dip as low as 0.15% around the 2.5Vrms “knee.”

THD+N ratio (A-weighted) vs. output voltage at 1kHz - MM and MC inputs

Above we can see a plot of THD+N (A-weighted) ratios as a function of output voltage for the unbalanced output. The red/blue (L/R) traces represent the MM configured input, and purple/green for MC. For the MM configuration, THD+N values at 100mVrms are at 0.006%, then dip as low as 0.0003% at the “knee” around 2.5Vrms. For the MC configuration, THD+N values at 100mVrms are at 0.15%, then dip as low as 0.006% around the 2.5Vrms “knee.”

FFT spectrum, 1kHz - MM input

Shown above is a fast Fourier transform (FFT) of a 1kHz input sine-wave stimulus for the MM setting, which results in the reference voltage of 1Vrms (0dBrA) at the unbalanced output. Here we see exceptionally clean results. Signal harmonics are just below -120dBrA (2kHz), or 0.0001%, and below. The first odd-order harmonic (3kHz) is just barely perceptible above the noise floor at -135dBrA, or 0.00002%. On the left side of the signal peak, there is perhaps just a hint of a 60Hz peak due to power-supply noise at -100dBrA, or 0.001%, and again at the third noise harmonic of 180Hz, at -115dBrA, or 0.0002%.

FFT spectrum, 1kHz - MC input

Shown above is an FFT of a 1kHz input sine-wave stimulus for the MC setting at the unbalanced output. As there is 25dB more gain with the MC setting, predictably, the noise floor is higher, and with this, there are no visible signal or noise harmonic peaks.

FFT spectrum, 50Hz - MM input

Shown above is the FFT for a 50Hz input sine-wave stimulus measured at the unbalanced output for the MM setting. 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 harmonics from the 50Hz signal (100, 150, 200Hz, etc) are nonexistent above the noise floor, as are the power-supply noise peaks.

FFT spectrum, 50Hz - MC input

Shown above is the FFT for a 50Hz input sine-wave stimulus measured at the unbalanced output for the MC setting. 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 harmonics from the 50Hz signal (100, 150, 200Hz, etc.) are nonexistent above the noise floor, as are the power-supply noise peaks.

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

Above is an FFT of the IMD products for an 18kHz and 19kHz summed sine-wave stimulus tone for the MM setting measured at the unbalanced output. The input RMS values are set so that if summed (for a mean frequency of 18.5kHz), would yield 1Vrms (Reference or 0dBRa) at the output. Here we find the second-order modulation product (i.e., the difference signal of 1kHz) at -100dBrA, or 0.001%. We can also see the third-order modulation products (i.e., 17kHz and 20kHz) sitting just above -110dBRa, or 0.0003%. The fourth- and fifth-modulation products are also clearly visible.  

Intermodulation distortion FFT (18kHz + 19kHz summed stimulus) - MC

The last graph is an FFT of the IMD products for an 18kHz and 19kHz summed sinewave stimulus tone for the MC setting. Here we find the second-order modulation product (i.e., the difference signal of 1kHz) is at -95dBrA, or 0.002%. We can also see the third-order modulation products (i.e., 17kHz and 20kHz) at roughly the same amplitude as the MM setting, sitting just above -110dBRa, or 0.0003%. Unlike the MM setting however, with the MC setting, the fourth- and fifth-modulation products are not visible, likely due to the higher noise floor.

Diego Estan
Electronics Measurement Specialist

Details

Parent Category: Products

Category: Phono Preamplifier Measurements

Created: 01 July 2021

Link: reviewed by James Hale on SoundStage! Xperience on July 1, 2021

General information

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

The iFi Audio Zen Phono was conditioned for 30 minutes at 1Vrms at the output before any measurements were taken.

The Zen Phono offers one pair of unbalanced RCA inputs, a pair of unbalanced RCA outputs, and a balanced output via a 4.4mm TRRS connector. The input can be configured for different moving magnet (MM) and moving coil (MC) cartridges via a small DIP switch on the back panel. A small white LED on the front panel indicates the selection: MM, MC High, MC Low, or MC Very Low.  Also included is a subsonic filter, which can be activated using a button on the front panel.

I measured the performance of the Zen Phono for the following input configurations: MM, MC Low, MC Very Low. To achieve the reference output voltage of 1Vrms at 1 kHz, 15mVrms was required with the MM setting, 1mVrms with the MC Low setting, and 0.265mVrms with the MC Very Low setting. Other than the extra 6dB of gain and double the maximum output voltage, we found no differences between the balanced and unbalanced outputs, provided the balanced output was referenced to 2Vrms, and the unbalanced output to 1Vrms (i.e., THD, THD+N and signal-to-noise ratios (SNRs) were identical for the same input voltage).

Published specifications vs. our primary measurements

The table below summarizes the measurements published by iFi Audio for the Zen Phono compared directly against our own. The published specifications are sourced from iFi Audio’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 is set at its maximum (DC to 1MHz), assume, unless otherwise stated, 1Vrms output into 100k ohms (200k ohms balanced) and a measurement input bandwidth of 10Hz to 90kHz, and the worst-case measured result between the left and right channel.

Our primary measurements revealed the following using the unbalanced MM setting (unless specified, assume a 1kHz sinewave, 1Vrms output into a 100k ohms load, 10Hz to 90kHz bandwidth):

Our primary measurements revealed the following using the unbalanced MC Low setting (unless specified, assume a 1kHz sinewave, 1Vrms output into a 100k ohms load, 10Hz to 90kHz bandwidth):

Our primary measurements revealed the following using the unbalanced MC Very Low setting (unless specified, assume a 1kHz sinewave, 1Vrms output into a 100k ohms load, 10Hz to 90kHz bandwidth):

Frequency response RIAA - MM setting

In our measured frequency-response plot above for the MM setting measured at the unbalanced output, the Zen is within +/-0.1dB or so of flat from 5Hz to 50kHz, and about -2dB down at 80kHz, meeting iFi audio’s first claim of 20Hz-20kHz (+/-0.15dB), but not meeting the second claim of 10Hz-100kHz (+/-0.4dB). An inverse RIAA EQ is applied to the input sweep, so that if a device were to track the RIAA curve perfectly, a flat line would emerge. In our results, the Zen Phono has essentially perfect RIAA accuracy from 5Hz to 50kHz. The purple/green (left/right) traces represent the frequency response with the subsonic filter engaged, where we see -3dB at around 5Hz. In the graph above and some of the graphs below, we see two visible traces: the left channel (blue, purple or pink trace) and the right channel (red, green or orange trace). On other graphs, only one trace may be visible, this is because the left and right channels are tracking extremely closely, so as not to show a difference with the chosen axis scales.

Frequency response RIAA - MC Low and MC Very Low settings

In our measured frequency-response plot above for the MC Low/MC Very Low settings (they performed identically) measured at the unbalanced output, the Zen is within +/-0.1dB or so of flat from 20Hz to 20kHz, meeting iFi Audio’s first claim of 20Hz-20kHz (+/-0.15dB). We can see that the MM configuration offers a more extended bandwidth, where these MC configurations are at -1.5dB at 5Hz and -0.7dB at 50kHz. The purple/green (left/right) traces represent the frequency response with the subsonic filter engaged, where see a -3dB point at around 7Hz.

Phase response - MM, MC Low, and MC Very Low settings

Above is the phase response of the Zen for three input settings (they measured effectively identically) measured at the unbalanced output, from 20Hz to 20kHz. The purple/green traces represent the measured phase shift with the subsonic filter engaged. The Zen does not invert polarity. Since phono preamplifiers must implement the RIAA equalization curve, which ranges from +19.9dB (20Hz) to -32.6dB (90kHz), phase shift at the output is inevitable. Here we find a worst-case -60 degrees around 200Hz and -55 degrees at 5kHz. With the subsonic filter engaged, there’s a less than a 20 degree difference in phase shift at 20Hz.

THD ratio (unweighted) vs. frequency - MM, MC Low, and MC Very Low settings

The chart above shows THD ratio as a function of frequency, where the input sweep is EQ’d with an inverted RIAA curve. The unbalanced output voltage is maintained at the refrence 1Vrms. The red/blue (left/right) traces represent the MM configured input, purple/green MC Low, and pink/orange MC Very Low. For the MM configuration, THD values at 20Hz are at 0.005%, then dip as low as 0.0004% around 1kHz, then up to 0.003% at 20kHz. For the MC Low configuration, THD values at 20Hz are at 0.01%, then dip as low as 0.003% around 100Hz, then a steady climb to 0.3% at 20kHz. For the MC Very Low configuration, THD values at 20Hz are at around 0.04%, then dip as low as 0.004% between 500Hz and 1kHz, then up to 0.025% at 20kHz.

THD ratio (unweighted) vs. output voltage at 1kHz - MM, MC Low, MC Very settings

Above we can see a plot of THD ratios as a function of output voltage for the unbalanced output. The red/blue (left/right) traces represent the MM configured input, purple/green MC Low, and pink/orange MC Very Low. For the MM configuration, THD values at 100mVrms are at 0.003%, then dip as low as 0.0003% between 1 and 3Vrms, then the “knee” around 8-9Vrms, where THD values reach 0.003%. For the MC Low configuration, THD values at 100mVrms are at 0.005%, then steadily increase up to the 1% mark at 10Vrms, where THD vs. output values for all input configurations meet. For the MC Very Low configuration, THD values at 100mVrms are near 0.02%, then dip as low as 0.003% around 1Vrms, then the “knee” around 8-9Vrms, where THD values reach 0.03%. It’s important to mention that anything above 1-2Vrms is not typically required for most line-level preamps or integrated amps.

THD+N ratio (unweighted) vs. output voltage at 1kHz - MM, MC Low, and MC Very Low settings

Above we can see a plot of THD+N ratios as a function of output voltage for the unbalanced output. The red/blue (L/R) traces represent the MM configured input, purple/green MC Low, and pink/orange MC Very Low. For the MM configuration, THD+N values at 100mVrms are at 0.15%, then dip as low as 0.002% between 6 to 8Vrms, then the “knee” around 9Vrms. For the MC Low configuration, THD+N values at 100mVrms are at 0.2%, then dip as low as 0.03% around 1Vrms, then rise steadily to the “knee” near 10Vrms, where the 1%THD+N values can be seen. For the MC Very Low configuration, THD+N values at 100mVrms are at around 0.7%, then dip as low as 0.02% between 3 to 5Vrms, then the “knee” around 9Vrms, where THD+N sits at 0.03%.

FFT spectrum, 1kHz - MM setting

Shown above is a fast Fourier Transform (FFT) of a 1kHz input sinewave stimulus for the MM setting, which results in the reference voltage of 1Vrms at the unbalanced output. Here we see exceptionally clean results. Signal harmonics are at -120dBrA or 0.0001% and below. The odd order harmonics (i.e., 3kHz, 5kHz) are slightly higher in amplitude than the 2kHz even order second harmonic. On the left side of the signal peak, there are no peaks due to power supply noise above the noise floor, which ranges from -90dBrA, at 20Hz to -120dBrA at 1kHz.

FFT spectrum, 1kHz - MC Low setting

Shown above is an FFT of a 1kHz input sinewave stimulus for the MC Low setting at the unbalanced output. The 2kHz second-order signal harmonic peak is at -75dBrA, or 0.02%%, while the 3kHz third-order harmonic is at -85dBrA, or 0.006%. On the left side of the signal peak, we see the power-supply noise fundamental peak (60Hz) at -90/-85dBrA (L/R) or 0.003/0.006%, and just barely above the noise floor, we see the second- and third-harmonic noise peaks (120Hz, 180Hz), just above -100dBrA, or 0.001%.

FFT spectrum, 1kHz - MC Very Low setting

Shown above is an FFT of a 1kHz input sinewave stimulus for the MC Very Low setting at the unbalanced output. Despite the extra gain compared to the MC Low setting, this setting yields far less distortion, although predictably, with a higher noise floor. The 2kHz second-order signal harmonic peak is at -90dBrA, or 0.003%, while the 3kHz third-order harmonic is practically nonexistent. On the left side of the signal peak, we see the power-supply noise fundamental peak (60Hz) at -75/-70dBrA (left/right), or 0.02/0.03%, and just barely above the noise floor, we see the second and third harmonic noise peaks (120Hz, 180Hz), right around -90dBrA, or 0.003%.

FFT spectrum, 50Hz - MM setting

Shown above is the FFT for a 50Hz input sinewave stimulus measured at the unbalanced output for the MM setting. 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 harmonics from the 50Hz signal (100, 150, 200Hz, etc) are nonexistent above the noise floor, as are the power-supply noise peaks.

FFT spectrum, 50Hz - MC Low setting

The chart above is the FFT for a 50Hz input sinewave stimulus measured at the unbalanced output for the MC Low setting. The second harmonic from the 50Hz signal (100Hz) is at -90dBrA, or 0.003%, so right around the same level as the 60Hz noise peak (left channel). Subsequent signal harmonics cannot be seen above the noise floor.

FFT spectrum, 50Hz - MC Very Low setting

Shown above is the FFT for a 50Hz input sinewave stimulus measured at the unbalanced output for the MC Very Low setting. The harmonics from the 50Hz signal cannot be seen above the noise floor, while the 60Hz noise peak is again visible at -75/-70dBrA (left/right), or 0.02/0.03%.

Intermodulation distortion FFT (18kHz + 19kHz summed stimulus) - MM setting

Above is an FFT of the IMD products for an 18kHz and 19kHz summed sinewave stimulus tone for the MM setting measured at the unbalanced output. The input rms values are set so that if summed (for a mean frequency of 18.5kHz), would yield 1Vrms (Reference or 0dBRa) at the output. Here we find the second-order modulation product (i.e., the difference signal of 1kHz) at -85dBrA, or 0.006%. We can also see the third-order modulation products (i.e., 17kHz and 20kHz) sitting at -110/-105dBRa (left/right), or 0.0003/0.0006%. The fourth and fifth modulation products are also clearly visible.  

Intermodulation distortion FFT (18kHz + 19kHz summed stimulus) - MC Low setting

This chart is an FFT of the IMD products for an 18kHz and 19kHz summed sinewave stimulus tone for the MC Low setting. Here we find the second-order modulation product (i.e., the difference signal of 1kHz) is quite high at -40dBrA, or 1%. We can also see the third-order modulation products (i.e., 17kHz and 20kHz) are also high, at -50dBRa, or 0.3%. These IMD FFTs are reflected in our simplified IMD results (which only account for the sum of the second- and third-order modulation products) in our primary measurement table, where the MM setting measured a respectable -76dB, or 0.015%, but the MC Low setting yielded only -33dB, or 0.02%.

Intermodulation distortion FFT (18kHz + 19kHz summed stimulus) - MC Very Low setting

This chart is an FFT of the IMD products for an 18kHz and 19kHz summed sinewave stimulus tone for the MC Very Low setting. Here we find the second-order modulation product (i.e., the difference signal of 1kHz) at around -55dBrA, or 0.2%. We can also see the third-order modulation products (i.e., 17kHz and 20kHz) are much lower, at -75dBRa, or 0.02%.

Diego Estan
Electronics Measurement Specialist

Details

Parent Category: Products

Category: Phono Preamplifier Measurements

Created: 15 May 2021

Link: reviewed by Jason Thorpe on SoundStage! Hi-Fi on May 15, 2021

General information

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

The iPhono3 was conditioned for 30 minutes at 1Vrms at the output before any measurements were taken.

The iPhono3 has one switch and one unbalanced pair of RCA outputs on one end, while the other end has two pairs of unbalanced RCA inputs for connection of a moving-magnet (MM) or moving-coil (MC) cartridge. The switch allows the user to select between RIAA, Columbia, and Decca EQ settings. There are a number of DIP switches underneath, allowing the user to alter MC resistive loading, MM capacitive loading, gain, and variations on EQ (i.e., enhanced RIAA and standard with and without subsonic filter).

For the MM input, capacitive loading was set to 100pF and the gain set to 36dB, which required a 16.2mVrms 1kHz sinewave to achieve the reference output voltage. For the MC input, resistive loading was set to 100 ohms and the gain set to 60dB, which required a 1.18mVrms 1kHz sinewave to achieve the reference output voltage

Published specifications vs. our primary measurements

The table below summarizes the measurements published by iFi Audio for the iPhono3 Black Label compared directly against our own. The published specifications are sourced from iFi Audio’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 is set at its maximum (DC to 1MHz), assume, unless otherwise stated, 1Vrms output into 100k 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 unbalanced MM input (unless specified, assume a 1kHz sinewave, 1Vrms output in 100k ohms, 10Hz to 90kHz bandwidth):

Our primary measurements revealed the following using the unbalanced MC input (unless specified, assume a 1kHz sinewave, 1Vrms output into 100k ohms, 10Hz to 90kHz bandwidth):

Frequency response RIAA - MM input

In our measured frequency-response plot above for the MM input, the iPhono3 is at worst +0.7dB (left channel) or so of flat from 20Hz to 20kHz, not quite meeting iFi’s claim of +/-0.2dB. An inverse RIAA EQ is applied to the input sweep, so that if a device were to track the RIAA curve perfectly, a flat line would emerge. The claim of +/-0.3dB from 10Hz to 100kHz was also not corroborated by our measurements, where at 80kHz, the MM input was at about +2dB. In the graph above and some of the graphs below, we see two visible traces; the left channel (blue or purple trace) and the right channel (red or green trace). On other graphs, only one trace may be visible, which is because the left and right channels are tracking extremely closely, so as not to show a difference with the chosen axis scales.

Frequency response RIAA - MC input

In our measured frequency-response plot above for the MC input, the iPhono3 is at worst +0.7dB (left channel) or so of flat from 20Hz to 20kHz, not quite meeting iFi’s claim of +/-0.2dB. The claim of +/-0.3dB from 10Hz to 100kHz was also not corroborated by our measurements, where at 80kHz, the MC input was at about -2.5dB.

Frequency response for RIAA, Columbia, and Decca settings - MM input (no EQ applied to input sweep)

Above is the raw (no EQ applied in the Audio Precision generator) frequency response of the iPhono3 using the RIAA (blue), Columbia (green) and Decca (burgundy) EQ settings for the MM input with a 0.5mVrms sinewave input swept from 10Hz to 80kHz (the results for the MC input were effectively identical). We find deviations of about 5 dB at 20Hz, and 3dB at 20kHz between all three EQ settings.

Phase response - MM and MC inputs

Above is the phase response of the iPhono3 for both the MM and MC inputs (they measured effectively identically), from 20Hz to 20kHz. Since phono preamplifiers must implement the RIAA equalization curve, which ranges from +19.9dB (20Hz) to -32.6dB (90kHz), phase shift at the output is inevitable. Here we find a worst-case -60 degrees around 200Hz and 6kHz. Because the worst case is -60 degrees, that indicates that the iPhono3 does not invert polarity. 

THD ratio (unweighted) vs. frequency - MM input

Above is the THD ratio as a function of frequency chart for the MM input, where the input sweep is EQ’d with an inverted RIAA curve. The output voltage is maintained at the reference 1Vrms. Since iFi provides specs for 600-ohm loading, here we show data for a typical 100k ohms load (blue/red), and for a 600 ohms load (purple/green). The iPhono3 performed identically with either load. The THD values vary from 0.01% at 20Hz, down to below 0.001% from 100Hz to 200Hz, then back up just above 0.02% from 2kHz to 20kHz.

THD ratio (unweighted) vs. frequency - MC input

Above is the THD ratio as a function of frequency chart for the MC input. The output voltage is maintained at the refrence 1Vrms. Since iFi provides specs for 600 ohm loading, here we show data for a typical 100k ohms load (blue/red), and for a 600 ohms load (purple/green). The THD values vary from 0.1% at 20Hz, down to 0.004% at around 100Hz, then a steady near linear climb to 0.3% at 20kHz.

THD ratio (unweighted) vs. output voltage at 1kHz (input voltage from 1mVrms to 100mVrms) - MM input

Above is the chart of THD ratio as a function of output voltage for the MM input. We can see very low THD ratio values, ranging from as low as 0.002% at 150mVrms, up to about 0.06% at 4Vrms. Beyond this point there is sharp rise in THD. The 1% THD ratio value is reached at 6Vrms at the output.

THD+N ratio (unweighted) vs. output voltage at 1kHz (input voltage from 1mVrms to 100mVrms) - MM input

Above we can see the plot of THD+N ratio as a function of output voltage for the MM input. We can see THD+N ratio values ranging from 0.2% at around 50mVrms, down to about 0.02% at 1Vrms.

THD ratio (unweighted) vs. output voltage at 1kHz (input voltage from 0.1mVrms to 12mVrms) - MC input

Above is the THD ratio as a function of output voltage for the MC input. The MC input behaved almost identically to the MM input, with the lowest THD values recorded also around 150mVrms, at around 0.004%.

THD+N ratio (unweighted) vs. output voltage at 1kHz (input voltage from 0.1mVrms to 12mVrms) - MC input

Above we can see the plot of THD+N ratio as a function of output voltage for the MC input. We can see THD+N ratio values ranging from 1.5% at around 50mVrms, down to about 0.05% between 2 and 3Vrms.

FFT spectrum, 1kHz - MM input

Shown above is a fast Fourier Transform (FFT) of a 1kHz input sinewave stimulus for the MM input, which results in the reference output voltage of 1Vrms. Here we see that the third signal harmonic at 3kHz is close to -75dB below the reference signal, or at -75dBrA, equivalent to 0.02%. The second harmonic at 2kHz is non-existent. The frequency components on the left side of the 1kHz peak are mostly due to power supply noise, where the typical 60Hz and 120Hz peaks can be seen. The worst noise peak (60Hz) is at around -80dBrA, or 0.01%, with the subsequent harmonic (120Hz) at around -90dBrA, or 0.003%. The third (180Hz) and fourth (240Hz) harmonics are just above -100dBrA, or 0.001%.

FFT spectrum, 1kHz - MC input

Shown above is the FFT of a 1kHz input sinewave stimulus for the MC input. Here we see that the second and third signal harmonics (2 and 3kHz) are close to -75dBrA, or 0.02%. The 60Hz peak is at -60dBrA, or 0.1%; the second noise harmonic (120Hz) is close to -70dBrA, or 0.03%; and the third (180Hz) and fourth (240Hz) harmonics are just above -80dBrA, or 0.01%.

FFT spectrum, 50Hz - MM input

Shown above is the FFTs for a 50Hz input sinewave stimulus measured at the output for the MM 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 second harmonic of the 50Hz signal (100Hz) is non-existent. What dominate are the power supply noise peaks, which were described in the 1kHz FFT above.

FFT spectrum, 50Hz - MC input

Shown above is the FFTs for a 50Hz input sinewave stimulus for the MC input. The second harmonic of the 50Hz signal (100Hz) is at -90dBrA, or 0.003%. What dominate are the power supply noise peaks, which were described in the 1kHz FFT above.

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

Shown above is an FFT of the IMD products for an 18kHz and 19kHz summed sinewave stimulus tone for the MM input. The input RMS values are set so that if summed (for a mean frequency of 18.5kHz), would yield 1Vrms (Reference or 0dBRa) at the output. Here we find the second-order modulation product (i.e., the difference signal of 1kHz) at almost -100dBrA, or 0.001%. We can also see the third-order modulation products (i.e., 17kHz and 20kHz) measure at around -85dBrA, or 0.006%.

Intermodulation distortion FFT (18kHz + 19kHz summed stimulus) - MC input

This chart shows an FFT of the IMD products for an 18kHz and 19kHz summed sinewave stimulus tone for the MC input. Here we find the second-order modulation product (i.e., the difference signal of 1kHz) at around -40dBrA, or 1%. We can also see the third-order modulation products (i.e., 17kHz and 20kHz) are also considerably different between the MM and MC inputs, where the MM peaks measure at around -85dBrA, or 0.006%, and the MC peaks are just under -50dBrA, or 0.3%. These differences are reflected in our simplified IMD results (which only account for the sum of the second- and third-order modulation products) in our primary measurement table, where the MM input outperformed the MC input by 36dB.

Diego Estan
Electronics Measurement Specialist

Details

Parent Category: Products

Category: Phono Preamplifier Measurements

Created: 15 January 2021

Link: reviewed by Jason Thorpe on SoundStage! Hi-Fi on January 15, 2021

General information

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

The V10 was conditioned for 30 minutes at 1Vrms at the output before any measurements were taken.

The V10 offers one pair of unbalanced RCA outputs and one pair of balanced XLR outputs, as well as two pairs of unbalanced RCA inputs, one pair for moving-magnet (MM) and the other for a moving-coil (MC) cartridge. There are a number of DIP switches on the rear panel of the V10, allowing the user to alter MC resistive loading, MM capacitive loading, and gain, as well to turn on or off a subsonic filter. Other than the 6dB difference in gain between the balanced and unbalanced outputs (+6dB for balanced), there were no mentionable differences between both outputs types in terms of THD+N, as long as the gain was kept constant. Settings were left at the manufacturer’s default positions (see specs below). To achieve the reference output voltage of 1Vrms at 1kHz at the balanced output, 10mVrms was required at the MM input and 1mVrms at the MC input.

Published specifications vs. our primary measurements

The tables below summarize our primary measurements performed on the V10. Here we can compare directly against Hegel’s own published specifications for the V10, which are stated as follows:

• Gain (XLR): MM, 40/45/50/52dB; MC, 60/65/70/72dB (default = 40/60dB for MM/MC)
• Gain (RCA): MM, 34/39/44/46dB; MC, 54/59/64/66dB (default = 34/54dB for MM/MC)
• MC load impedance: 100/300/47k ohms (default = 100 ohms)
• MM load capacitance: 100/147/200/220/247/320/420pF at 47k ohms (default = 100pF)
• Subsonic filter: on, -3dB at 20Hz; off, -18dB/octave
• RIAA accuracy: +/-0.1dB 20Hz to 20kHz
• Output noise: -84dB (MM, A-weighted, ref 1Vrms), -81dB (MC, A-weighted, ref 1Vrms)
• Output impedance: XLR and RCA, 200 ohms
• Channel crosstalk: -84dB @ 1kHz (ref 1Vrms)
• Frequency response: 2Hz to 20kHz
• THD: MM, <0.005%; MC, <0.009% at 1kHz ref 1Vrms
• Maximum output voltage: 23Vrms

Our primary measurements revealed the following using the unbalanced MM input (unless specified, assume a 1kHz sinewave, 1Vrms at the balanced output into a 200k ohms load, 10Hz to 90kHz bandwidth):

Our primary measurements revealed the following using the unbalanced MC input (unless specified, assume a 1kHz sinewave, 1Vrms at the balanced output into a 200k ohms load, 10Hz to 90kHz bandwidth):

Gain was measured at 40 and 60dB (Balanced, MM/MC), the same as Hegel’s spec. The MC load input impedance was measured at 124 ohms, closely matching Hegel’s spec and setting of 100 ohms. The input impedance for the MM input was measured at 47 and 46k ohms (L/R), very close to the industry standard 47k ohms.

Output noise (A-weighted) was measured at -85dB (MM) and -79dB (MC), either exceeding or approaching Hegel’s specs of -84/-81dB (MM/MC). Our measured output impedance of 200 ohms confirmed Hegel’s spec of the same value.

Our measured crosstalk values at 10kHz were close to Hegel’s spec of -84dB (at 1kHz), where we measured -83dB for both channels for the MM input and -88/-95dB (left and right channels) for the MC input. For a direct comparison, we also measured crosstalk at 1kHz, and found -98/-100dB (L/R channels, MM input), and -88/-90dB (L/R channels, MC input), bettering the -84dB Hegel spec.

Hegel’s claim of better than 0.005% (MM) and 0.009% (MC) THD was also verified. There are many ways to measure THD, and manufacturers are often shy on showing all of the parameters used for the measurement. Here, we assume that Hegel is referring to THD (not THD+N), unweighted, against a 1Vrms 1kHz output signal. The bandwidth for Hegel’s measurement is unknown, but we use 10Hz to 90kHz for our measurements. Under those conditions, we found the V10’s THD to be below 0.0008% (MM) and 0.001% (MC). Even if Hegel’s THD spec is actually THD+N (A-weighted), they still meet spec for the MM input, as we measured 0.005%, and come very close for the MC input at 0.01%.

Hegel’s maximum output voltage (1% THD+N) claim of 23Vrms (balanced) was also confirmed, where we measured 24Vrms.

Frequency response - MM input

The blue (left channel) and red (right channel) traces represent the frequency response without the subsonic filter turned on. In our measured frequency-response plot above for the MM input, the V10 is within +/-0.2dB or so of flat from 20Hz to 20kHz, just about meeting Hegel’s RIAA accuracy claim of +/-0.1dB. An inverse RIAA EQ is applied to the input sweep, so that if a device were to track the RIAA curve perfectly, a flat line would emerge—the V10 has nearly perfect RIAA accuracy from 5Hz to 80kHz. The purple (left channel) and green (right channel) traces represent the frequency response with the subsonic filter engaged, where Hegel’s claim of -3dB at 20Hz is confirmed. In the graph above and some of the graphs below, we see two visible traces; the left channel (blue or purple trace) and the right channel (red or green trace). On other graphs, only one trace may be visible, which is because the left and right channels are tracking extremely closely, so as not to show a difference with the chosen axis scales.

Frequency response - MC input

In our measured frequency-response plot above for the MC input shown with the blue and red traces (left and right channels), the V10 is within +/-0.2dB or so of flat from 20Hz to 20kHz, just about meeting Hegel’s RIAA accuracy claim of +/-0.1dB. The worst-case deviation was seen at around 8Hz, were the V10 over-responded by 0.5dB. The purple and green trace (left and right channels) represent the frequency response with the sub-sonic filter engaged, where Hegel’s claim of -3dB at 20Hz is confirmed.

Phase response - MM and MC inputs

Above is the phase response of the V10 for both the MM and MC inputs (they measured effectively identically), from 20Hz to 20kHz. The V10 does not invert polarity. Since phono preamplifiers must implement the RIAA equalization curve, which ranges from +19.9dB (20Hz) to -32.6dB (90kHz), phase shift at the output is inevitable. Here we find a worst case -60 degrees around 200Hz and -55 degrees at 5kHz.

THD ratio (unweighted) vs. frequency - MM input

The chart above shows THD ratio as a function of frequency for the MM input, where the input sweep is EQ’d with an inverted RIAA curve. The output voltage is maintained at the refrence 1Vrms. The THD values vary from 0.01% at 20Hz, down to below 0.0001% at 10kHz, then back up just above 0.0003% at 20kHz.

THD ratio (unweighted) vs. frequency - MC input

The chart above shows THD ratio as a function of frequency for the MM input, where the input sweep is EQ’d with an inverted RIAA curve. The THD values vary from 0.04% at 20Hz, down to around 0.0004% at 5kHz, then a climb to 0.001% at 20kHz.

THD ratio (unweighted) vs. output voltage at 1kHz (input voltage from 1mVrms to 30mVrms) - MM input

Above we can see a plot of THD ratios as a function of output voltage for the MM input. We can see very low THD ratio values, ranging from as low as 0.0001% at 5Vrms, up to about 0.0006% at the “knee”, just past 20Vrms, and about 0.005% at the lowest output voltage of 100mVrms. Beyond the “knee” there is sharp rise in THD as the V10 reaches its maximum output voltage. It’s important to mention that anything above 1-2Vrms is not typically required for most line-level preamps or integrated amps.

THD+N ratio (unweighted) vs. output voltage at 1kHz (input voltage from 1mVrms to 100mVrms) - MM input

Above we can see a plot of THD+N ratios as a function of output voltage for the MM input. We can see THD+N ratio values, ranging from 0.2% at 100Vrms, down to about 0.0015% between 15 and 20Vrms.

THD ratio (unweighted) vs. output voltage at 1kHz (input voltage from 0.1mVrms to 30mVrms) - MC input

Above we can see a plot of THD ratios as a function of output voltage for the MC input. The MC input behaved similarly to the MM input, with 0.007% THD at 100mVrms, dipping to the lowest THD value of 0.0005% at 2Vrms, then up just past 0.003% at the knee at 20Vrms. The 1% THD ratio value is reached at 24Vrms at the output.

THD+N ratio (unweighted) vs. output voltage at 1kHz (input voltage from 0.1mVrms to 30mVrms) - MC input

Above we can see a plot of THD+N ratios as a function of output voltage for the MC input. We can see THD+N ratio values ranging from around 0.5% at 100Vrms, down to about 0.005% between 15 and 20Vrms.

FFT spectrum, 1kHz - MM input

Shown above is a fast Fourier transform (FFT) of a 1kHz input sinewave stimulus for the MM input, which results in the reference output voltage of 1Vrms. Here we see exceptionally clean results. Signal harmonics are non-existent (i.e., cannot be seen above the -120dB noise floor at 2kHz). The frequency components on the left side of the 1kHz peak are mostly due to power-supply noise, where the typical 60Hz and 120Hz peaks can be seen. The worst noise peak (60Hz) is below -80dBrA, with the subsequent harmonic (120Hz) at around -90dBrA. The third harmonic (180Hz) is just below -90dBrA, and the subsequent harmonics are below -100dBrA.

FFT spectrum, 1kHz - MC input

Shown above is the FFT of a 1kHz input sinewave stimulus for the MC input. Signal harmonics are virtually non-existent, only a hint of a peak can be seen below -110dB at 2kHz. The 60Hz and 120Hz peaks are just below -70dBrA, and the third and fourth noise harmonics are just below -80dBrA.

FFT spectrum, 50Hz - MM input

Shown above is the FFT for a 50Hz input sinewave stimulus measured at the output for the MM 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 harmonics from the 50Hz signal (100, 150, 200Hz, etc) are non-existent. The power supply noise peaks can clearly be seen, which were described in the 1kHz FFT chart above.

FFT spectrum, 50Hz - MC input

Shown above is the FFT for a 50Hz input sinewave stimulus measured at the output for the MC input. Like the MM FFT, here again, the harmonics from the 50Hz signal (100, 150, 200Hz, etc.) are non-existent. The power-supply noise peaks can clearly be seen, which were described in the 1kHz FFT above.

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

The chart above is an FFT of the IMD products for an 18kHz and 19kHz summed sinewave stimulus tone for the MM input. The input rms values are set so that if summed (for a mean frequency of 18.5kHz), would yield 1Vrms (reference or 0dBRa) at the output. Here we find the second-order modulation product (i.e., the difference signal of 1kHz) just above -100dBrA. We can also see the third-order modulation products (i.e., 17kHz and 20kHz) are extremely low, below -120dBRa.

Intermodulation distortion FFT (18kHz + 19kHz summed stimulus) - MC input

The next chart is an FFT of the IMD products for an 18kHz and 19kHz summed sinewave stimulus tone for the MC input. Here we find the second-order modulation product (i.e., the difference signal of 1kHz) just at -80dBrA, which is very low. We can also see the third-order modulation products (i.e., 17kHz and 20kHz) are extremely low, below -120dBRa. These clean IMD FFTs are reflected in our simplified IMD results (which only account for the sum of the second- and third-order modulation products) in our primary measurement table, where the MM input measured at -92dB for both channels and the MC input at -76/-73dB, for, respectively, the left and right channels. Incidentally, the 3dB difference between left and right channels for the MC input can be seen in the 1kHz peak in the FFT chart above. The fourth-order modulation products are non-existent in this FFT and the one for the MM input.

Diego Estan
Electronics Measurement Specialist

Details

Parent Category: Products

Category: Phono Preamplifier Measurements

Created: 01 January 2021

Link: reviewed by Doug Schneider on SoundStage! Hi-Fi on January 1, 2021

General information

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

The Bellari VP549 was conditioned for 30 minutes at 1Vrms at the output before any measurements were taken.

The VP549 is designed for moving-magnet (MM) cartridges only and has one set of single-ended RCA inputs and outputs. There are two switches and one knob on the front panel of the VP549. One switch selects between 120pF, 220pF, and 330pF cartridge-loading capacitance. The other switch, Rumble Filter, enables a low-frequency filter that provides attenuation below 20Hz. The knob controls the preamp’s overall gain. The center position is labelled “0,” the minimum position “-10dB,” and the maximum position “+4dB.” Unless otherwise stated, measurements were performed with the cartridge load capacitance set to 220pF, the Rumble Filter switch disengaged, and the gain set to the default “0” position. An 8mVrms 1kHz sinewave was required to achieve the reference output voltage of 1Vrms.

Published specifications vs. our primary measurements

The table below summarizes our primary measurements performed on the VP549. Here we can compare directly against Bellari’s own published specifications for the VP549, which are stated as follows:

• Gain: 41.51dB gain @ 1kHz at the "0" setting, 31.64dB of gain at the -10 setting, 45.66dB of gain at the +4 setting
• Input Impedance: 47k ohms
• Output Impedance: 470 ohms
• THD: 0.005% @ 1kHz
• S/N Ratio: >94dB unweighted
• Equalization: RIAA +/-1dB, 14Hz to 23kHz

Our primary measurements revealed the following (unless specified, assume a 1kHz sinewave, 1Vrms output into 100k ohms, 10Hz to 90kHz bandwidth, gain set to 0):

Our measured input (47.8/46.9k ohms, L/R) and output (460.5/460.8 ohms, L/R) impedances match very closely Bellari’s specs of 47k ohms and 470 ohms. Our measured gain values at all three settings also corroborate the published specifications, with maximum deviations in the order of 2-3%.

For the remainder of the specifications, as is often the case with manufacturer supplied measurements, not enough information is given to directly reproduce their results. With respect to Bellari’s THD of 0.005%, the company has not specified a reference output voltage, gain setting, weighting, or bandwidth filter. Our THD measurement shows an even lower <0.0012% (unweighted) against a 1Vrms output voltage, whereas our THD+N figure (A-weighted) is <0.0063% for both channels.

Bellari’s signal-to-noise ratio (SNR) of >94dB (unweighted) seems generous, however, again, we have very little information provided with respect to measurement parameters. The most charitable SNR measurement we could achieve was with the gain setting at -10dB and the output voltage at 1.35 Vrms (1 kHz), where we measured 95dB (A-weighted). Our reference SNR measurement (1Vrms output, gain set to 0) is 83dB (A-weighted), or 60dB (unweighted, input bandwidth filter set from 20Hz to 20kHz).

Frequency response

The chart above shows our frequency response measurement. An inverse RIAA EQ is applied to the input sweep, so that if a device were to track the RIAA curve perfectly, a flat line would emerge. The blue and red curves are the left and right channel without the Rumble Filter switch engaged, the green and purple curves with the Rumble Filter switch engaged. The measurement was also made with the gain set to its minimum and maximum values, and with every cartridge load capacitance setting, which had no effect on the frequency-response results. We can see here that Bellari’s claim of +/-1dB is valid at the top end of the spectrum (up to 23kHz as stated), but the +/-1dB claim down to 14Hz was not corroborated by our measurement. We find a -5.3dB dip at 30Hz, and at 14Hz, down -5.5dB again. In the graph above and some of the graphs below, we see two visible traces; the left channel (blue or purple trace) and the right channel (red or green trace). On other graphs, only one trace may be visible, this is because the left and right channels are tracking extremely closely, so as not to show a difference with the chosen axis scales.

Phase response

Shown above is the phase response from 20Hz to 20kHz. The VP549 does not invert polarity. Since phono preamplifiers must implement the RIAA equalization curve, which ranges from +19.9dB (20Hz) to -32.6dB (90kHz), phase shift at the output is inevitable. Here we find a worst case +20 degrees at 20Hz, and just below -60 degrees between 5 and 10kHz.

THD ratio (unweighted) vs. frequency

The chart above is the THD ratio as a function of frequency, where the input sweep is EQ’d with an inverted RIAA curve. The output voltage is maintained at the refrence 1Vrms. The THD values vary from 0.05% at 20Hz, down to below 0.001% at 1kHz, then back up to 0.007% at 20kHz.

THD ratio (unweighted) vs. output voltage at 1kHz (input voltage from 1mVrms to 100mVrms)

Above we can see a plot of THD ratio as a function of output voltage. We can see very low THD ratio values, ranging from 0.005% at 100mVrms down to below 0.001% at around 1Vrms at the output, then a sharp rise in THD at the “knee” at around 3.5Vrms at the output. Beyond this output voltage, the VP549 distortion begins to rise exponentially. The 1% THD ratio value is reached at around 4Vrms at the output. It’s important to mention that anything above 1-2Vrms is not typically required for most line-level preamps or integrated amps.

THD+N ratio (unweighted) vs. output voltage at 1kHz (input voltage from 1mVrms to 100mVrms)

Above we can see a plot of THD+N ratio as a function of output voltage. At 100mVrms, THD+N values are at around 3%, then down to between 0.1 and 0.2% at 3Vrms. There is a considerable difference in the THD+N vs. THD plots for the VP549, where it’s clear that distortion products are low, but noise, as shown in this chart, is relatively high.

FFT spectrum, 1kHz

Shown above is a fast Fourier transform (FFT) of a 1kHz 8mVrms input sinewave stimulus, which results in the reference output voltage of 1Vrms with the gain set to 0. Here we see that the second harmonic at 2kHz is <-100dB below the reference signal (dBrA). The frequency components on the left side of the 1kHz peak are mostly due to power supply noise. Of note is the peak at 60Hz at a level near -70dBrA.

FFT spectrum, 50Hz

The chart above is an FFT of a 50Hz input sinewave stimulus, which results in the reference output voltage of 1Vrms with the gain set to 0. 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. We find no perceptible signal harmonic peaks (e.g., 100Hz, 150Hz) above the relatively high noise floor. The highest noise peak is from the primary source (60Hz) at about -75dBrA, while the next highest peak is from the third harmonic noise peak (180Hz) at about -85dBrA.

Intermodulation distortion FFT (18kHz + 19kHz summed stimulus)

This final chart shows an FFT of the IMD products for an 18kHz and 19kHz summed sinewave stimulus tone. The input rms values are set so that if summed (for a mean frequency of 18.5kHz) would yield 1Vrms (Reference or 0dBRa) at the output. Although it appears that the second-order modulation product (i.e., the difference signal of 1kHz) is sitting at -105dBrA, the peak is actually at 1020Hz, which is the 17th harmonic from the 60Hz power-supply noise. The 1kHz IMD peak is imperceptible above the noise floor. The third-order modulation products (i.e., 17kHz and 20kHz) are just above -110dBra, and the subsequent modulation products are not noticeable.

Diego Estan
Electronics Measurement Specialist

Details

Parent Category: Products

Category: Phono Preamplifier Measurements

Created: 01 January 2021

Links: reviewed by Doug Schneider on SoundStage! Hi-Fi on January 1, 2021 and by James Hale on SoundStage! Xperience on January 1, 2021

General information

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

The NAD PP 2e was conditioned for 30 minutes at 1Vrms at the output before any measurements were taken.

On the PP 2e’s back panel are one switch, to switch between moving-magnet (MC) and moving-coil (MC) operation, and two sets of single-ended RCA inputs, with one set labeled MC and the other set MM. The rear panel also has a pair of single-ended RCA outputs. For the MM input, a 15mVrms 1kHz sinewave was required to achieve the reference output voltage of 1Vrms, while a 1.2mVrms 1kHz sinewave was required at the MC input.

Published specifications vs. our primary measurements

The tables below summarize our primary measurements performed on the PP 2e. Here we can compare directly against NAD’s own published specifications for the PP 2e, which are stated as follows:

• Input impedance: MM, 47k ohms; MC, 100 ohms
• Gain at 1kHz: MM, 35dB; MC, 60dB
• Input sensitivity (ref. 200mV output): MM, 2.5mV; MC, 0.3mV
• Signal-to-noise ratio (A-weighted): MM, 80dB; MC, 78dB
• Input overload (20Hz/1kHz/20kHz): MM, 10/102/950mV; MC, 0.8/9/84 mV
• Rated distortion (THD 20Hz to 20kHz): MM, <0.03%; MC, <0.03%
• RIAA response accuracy: MM, +/-0.3dB; MC, +/-0.3dB
• Output impedance: 100 ohms
• Maximum output level: 5.3V

Our primary measurements below revealed the following using the unbalanced MM input (unless specified, assume a 1kHz sinewave, 1Vrms output into 100k ohms load, 10Hz to 90kHz bandwidth):

Our primary measurements below revealed the following using the unbalanced MC input (unless specified, assume a 1kHz sinewave, 1Vrms output into 100k ohms load, 10Hz to 90kHz bandwidth):

Our measured input impedances for both the MM (47.65/47.81k ohms) and MC inputs (117.6/118.6 ohms) match very closely NAD’s specs of 47k ohms and 100 ohms.

Our measured gain values for both inputs (MM: 37dB, MC: 59dB) also corroborate the published specifications of 35dB and 60dB. Input sensitivity is another way to express gain (in volts per volts). Interestingly, NAD’s input sensitivity specs when converted to dB are 38dB for the MM input and 56.5dB for the MC input, which are slightly different than their specs for gain. Nevertheless, all published gain values are very close to our measured values.

NAD’s signal-to-noise ratio (SNR) specs of 80dB and 78dB (MM and MC, A-weighted) are difficult to compare to our measured values, because we do not know what NAD used as a reference output voltage. Nevertheless, our measured values of 93dB and 84dB (MM and MC, A weighted, with a 1Vrms reference) exceed those published by NAD.

NAD’s maximum output voltage value of 5.3mVrms was corroborated by our measurements, where we saw 1% THD+N at 5.32Vrms at the output (1kHz).

Our input overload values are expressed in dB, as overload margin, with a reference 5mVrms and 0.5mVrms input signal for the MM and MC inputs respectively. We measured the input signals required at 20Hz, 1kHz, and 20kHz to achieve 5.32Vrms at the output for both input types. We expressed those values in dB as a ratio over the reference values (5 and 0.5Vrms), but we also show the actual input voltages in parentheses so as to directly compare against the NAD values. We measured lower input overload values; with some rounding, our values compare to NADs as follows: 8/75/711 vs. 10/102/950mVrms for the MM input, and 0.7/6/57 vs. 0.9/8/84Vrms for the MC input.

NAD’s published rated distortion values are <0.03% for both input types; unfortunately, we only know these were measured with a 20Hz to 20kHz filter. We don’t know if NAD measured THD or THD+N, and what they used as a reference output voltage. Our measured THD+N values of about 0.003% and 0.006% (MM and MC) show better performance than the published values, but these are A-weighted. We also measured THD+N ratios with a 20Hz to 20kHz bandwidth (instead of A-weighting), using the fourth-order low-/high-pass Butterworth filters in the APx555, and measured less than 0.008% for the MM input, and less than 0.03% for the MC input, corroborating NAD’s claim.

In terms of our measured output impedance, there is a significant discrepancy, where we measured about 522 ohms vs. NAD’s published 100 ohms. While 500 ohms is on the high side for a line level device output impedance, it should pose no issues for any typical active preamp or integrated amp input.

Frequency response - MM and MC inputs

In the measured frequency-response plot (above), the PP 2e is within 0.3dB of flat from 20Hz to 20kHz, corroborating the NAD claim. We found the same measured response with both the MM and MC input. An inverse RIAA EQ is applied to the input sweep, so that if a device were to track the RIAA curve perfectly, a flat line would emerge. In the graph above and some of the graphs below, we see two visible traces; the left channel (blue or purple trace) and the right channel (red or green trace). On other graphs, only one trace may be visible, this is because the left and right channels are tracking extremely closely, so as not to show a difference with the chosen axis scales.

Phase response - MM and MC inputs

Above is the phase response of the PP 2e for both the MM and MC inputs (they measured effectively identically), from 20Hz to 20kHz. The PP 2e does not invert polarity. Since phono preamplifiers must implement the RIAA equalization curve, which ranges from +19.9dB (20Hz) to -32.6dB (90kHz), phase shift at the output is inevitable. Here we find a worst case +28 degrees at 20Hz, and just above -60 degrees between 200 and 300Hz, and right around -60 degrees between 5 and 8kHz.

THD ratio (unweighted) vs. frequency - MM input

This is the THD ratio as a function of frequency plot for the MM input, where the input sweep is EQ’d with an inverted RIAA curve. The output voltage is maintained at the refrence 1Vrms. The THD values vary from 0.005% at 20Hz, down to below 0.001% from 300Hz to 500Hz, then back up just above 0.01% at 20kHz. One curiosity is the deviation in THD between left and right channels, where we find up to 5dB difference in favor of the right channel, which is most obvious between 1 and 5kHz.

THD ratio (unweighted) vs. frequency - MC input

Above is the THD ratio as a function of frequency for the MC input. The THD values vary from 0.01% at 20Hz, down to just above 0.001% at around 500Hz, then back up just above 0.01% at 20kHz. Here again, we see a deviation in THD between left and right channels.

THD ratio (unweighted) vs. output voltage at 1kHz (input voltage from 1mVrms to 100mVrms) - MM input

Above is the THD ratio as a function of output voltage plot for the MM input. We can see very low THD ratio values ranging from just above 0.01% down to 0.001% at around 2Vrms at the output, then to a sharp rise in THD at the “knee” at just shy of 5Vrms at the output. The deviation in THD performance (5-10dB) in favor of the right channel can be seen with output voltages between 1 and 5Vrms. The 1% THD ratio value is reached at 5.32Vrms at the output. It’s important to mention that anything above 1-2Vrms is not typically required for most line-level preamps or integrated amps.

THD+N ratio (unweighted) vs. output voltage at 1kHz (input voltage from 1mVrms to 100mVrms) - MM input

Above is the THD+N ratio as a function of output voltage plot for the MM input. We can see THD+N ratio values ranging from just above 0.1% below 100mVrms, down to 0.002% (R) and 0.005% (L) at around 3-4Vrms at the output.

THD ratio (unweighted) vs. output voltage at 1kHz (input voltage from 1mVrms to 100mVrms) - MC input

Above is the THD ratio as a function of output voltage plot for the MC input. THD ratio values range from about 0.015% at 100mVrms to 0.001% at around 2Vrms at the output, then to a sharp rise in THD at the “knee” at just shy of 5Vrms at the output. Beyond this output voltage, the DUT distortion begins to rise exponentially. The 1% THD ratio value is reached at 5.32Vrms at the output.

THD+N ratio (unweighted) vs. output voltage at 1kHz (input voltage from 1mVrms to 100mVrms) - MC input

The chart above is the THD+N ratio as a function of output voltage plot for the MC input. We can see THD+N ratio values ranging from just above 0.2% below 100mVrms, down to between 0.005% and 0.01% at around 3-4Vrms at the output.

FFT spectrum, 1kHz - MM input

Shown above is a fast Fourier transform (FFT) of a 1kHz 15mVrms input sinewave stimulus for the MM input, which results in the reference output voltage of 1Vrms. Here we see that the second signal harmonic at 2kHz is at -100dBrA (L) and -105dBRa (R). At 3kHz, or the third signal harmonic, we see at 10dB difference between the left (-100dBrA) and right (-110dBrA) channels. The worst noise peak is at 120Hz (second harmonic of 60Hz) and can be seen at around -90dBrA (left) and -100dBrA (right).

FFT spectrum, 1kHz - MC input

Above is a fast Fourier transform (FFT) of a 1kHz 1.2mVrms input sinewave stimulus for the MC input. The second (2kHz) and third (3kHz) signal harmonics are roughly the same here as with the MM input above. The worst noise peaks are at around -90dBrA at 180Hz (third harmonic of 60Hz) and just below -80dBrA at 60Hz. Of note, is that the 120Hz predominates on the MM input, while on the MC input, it’s the 60Hz peak.

FFT spectrum, 50Hz - MM input

The chart above depicts a fast Fourier transform (FFT) of a 50Hz input sinewave stimulus for the MM 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. Here we see that the second noise harmonic at 120Hz dominates at -90dBrA (L) and -100dBRa (R). The second signal harmonic (100Hz) is at -100dBrA.

FFT spectrum, 50Hz - MC input

Above is a fast Fourier transform (FFT) of a 50Hz input sinewave stimulus for the MC input. Here we see that the thrid noise harmonic at 180Hz dominates at -80dBrA (L) and -90dBRa (R). The second signal harmonic (100Hz) is barely perceptible above the noise floor at -95dBrA.

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

The chart above represents an FFT of the IMD products for an 18kHz and 19kHz summed sinewave stimulus tone for the MM input. The input rms values are set so that if summed (for a mean frequency of 18.5kHz), would yield 1Vrms (reference or 0dBRa) at the output. Here we find the second order modulation product (i.e., the difference signal of 1kHz) at -95dBrA, which puts them a little below the primary power supply noise products. The third-order modulation products (i.e., 17kHz and 20kHz) are considerably different between the right and the left channels. This is also reflected in our simplified IMD results (which only accounts for the sum of the second and third order modulation products) in our primary measurement table, where the right channel outperformed the left by 15dB on the MM input, and by 5dB on the MC input. The worst-case third-order modulation product peaks are at about -85dBrA for the left channel, but closer to -105dBrA for the right channel.

Intermodulation distortion FFT (18kHz + 19kHz summed stimulus) - MC input

Above is an FFT of the IMD products for an 18kHz and 19kHz summed sinewave stimulus tone for the MC input. Here we find the second-order modulation product at -85dBrA. The third-order modulation products (i.e., 17kHz and 20kHz) are at about -85dBrA for the left channel, but closer to -105dBrA for the right channel.

Diego Estan
Electronics Measurement Specialist

Details

Parent Category: Products

Category: Digital-to-Analog Converter Measurements

Created: 15 October 2024

Link: reviewed by Matt Bonaccio on SoundStage! Hi-Fi on October 15, 2024

General Information

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

The Orchard Audio PecanPi+ Streamer Premium was evaluated as a DAC and conditioned for 30 min at 0dBFS (volume set to 2Vrms out) into 200k ohms before any measurements were taken.

The Orchard Audio PecanPi+ Premium is marketed as a network streamer, but does offer one coaxial S/PDIF (RCA) digital input. There are two line-level outputs (balanced XLR and unbalanced RCA) and one headphone output (1/4″ TRS). There is a digital volume control for the headphone and line-level outputs via a potentiometer on the front panel. Comparisons were made between unbalanced and balanced line-level outputs and no appreciable differences were seen in terms of THD and noise, but 1kHz FFTs are provided for both balanced and unbalanced outputs.

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

The PecanPi+ digital volume control offers no visual on-screen feedback to the user in terms of level. Volume can be adjusted in 0.5dB steps. Channel-to-channel deviation was good, at around 0.013dB 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 coaxial input and the balanced line-level outputs (unless specified, assume a 1kHz sinewave at 0dBFS, 200k ohms 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 coaxial 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)

The plot above shows the PecanPi+’s frequency response as a function of sample rate. The blue/red traces are for a 16-bit/44.1kHz dithered digital input signal from 5Hz to 22kHz, the purple/green traces are for a 24/96 dithered digital input signal from 5Hz to 48kHz, and finally orange/pink represents 24/192 from 5Hz to 96kHz. The behavior at low frequencies is the same for the digital input—perfectly flat down to 5Hz. The behavior at high frequencies for all three digital sample rates is soft filtering below 20, 30, and 50kHz (less than half the respective sample rate), The -3dB point for each sample rate is roughly 16, 35, and 70kHz, respectively. With 16/44.1 data, the -1dB point is at 13.1kHz. The PecanPi+ appears to utilize a reconstruction filter that prioritizes a clean impulse response (no pre-/post-ringing behaviour) with virtually no phase shift at the expense of more high-frequency attenuation in the frequency domain. Evidence for this can be seen in other graphs in this report. In the graph above and most of the graphs below, only a single trace may be visible. This is because the left channel (blue, purple or orange trace) is performing identically to the right channel (red, green or pink trace), and so they perfectly overlap, indicating that the two channels are ideally matched.

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

Above are the phase response plots from 20Hz to 20kHz for a 0dBFS input signal as a function of sample rate. The blue/red traces are for a 16-bit/44.1kHz dithered digital, the purple/green traces are for a 24/96 dithered digital input signal, and finally orange/pink represents 24/192 from 5Hz to 96kHz.  There is essentially no phase shift in the audioband, even for the 16/44.1 data.

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

The graph 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 balanced line-level output of the PecanPi+. 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 PecanPi+ is only over/undershooting by 1 dB between -140 and -130dBFS. This is an excellent 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 coaxial digital input, measured at the balanced outputs, into a 200k ohm-load for the left channel only. We can see that PecanPi+ yields an impulse response with essentially no pre- or post-ringing behaviour, or one that emulates a non-oversampling DAC.

J-Test (coaxial input)

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

The coaxial input shows a strong J-Test result, with peaks visible, but only below the -140dBrA level.

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

The plot above shows the results of the J-Test test for the coaxial digital input measured at the balanced 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. This is further evidence of the PecanPi+s strong jitter immunity.

Wideband FFT spectrum of white noise and 19.1kHz sinewave tone

The plot above shows a fast Fourier transform (FFT) of the PecanPi+’s balanced-line level output with white noise at -4 dBFS (blue/red) and a 19.1 kHz sinewave at 0dBFS fed to the coaxial digital input, sampled at 16/44.1 (purple/green). There is a soft roll-off above 20kHz in the white-noise spectrum. There are absolutely no imaged aliasing artifacts in the audioband above the -135dBrA noise floor. The primary aliasing signal at 25kHz is barely suppressed at -20dBrA.

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

The chart above shows THD ratios at the balanced line-level output into 200k ohms (blue/red) and 600 ohms (purple/green) as a function of frequency for a 24/96 dithered 1kHz signal at the coaxial input. The 200k and 600 ohms data are identical throughout the audioband, which is an indication that the PecanPi+’s outputs are robust and can handle loads below 1k ohms with no difficulty. There was an evident difference in THD ratios between the left and right channels, with the right channel outperforming the left by about 10-15dB from 20Hz to 1kHz. THD ratios (right channel) ranged from 0.00003-0.00005% from 20Hz to 2kHz, then up to 0.0005% at 20kHz. Despite the left/right THD ratio discrepancy, these values are extremely low and nearing the limits of what the APx555 can measure.

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

The chart above shows THD ratios at the balanced line-level output into 200k ohms as a function of frequency for a 16/44.1 (blue/red) and a 24/96 (purple/green) dithered 1kHz signal at the coaxial input. The 24/96 data (right channel) consistently outperformed the 16/44.1 data by 10-15dB from 20Hz to about 1kHz due to the inherently higher noise floor at 16 bits (i.e., the analyzer cannot assign a THD value below the noise floor). THD ratios with 16/44.1 data ranged from 0.0001 to 0.0005% throughput the audioband. 24/96 THD ratios (left channel) were essentially the same as the 16/44.1 data.

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

The chart above shows THD ratios measured at the balanced output as a function of output voltage for the coaxial input into 200k ohms from -90dBFS to 0dBFS at 16/44.1 (blue/red) and 24/96 (purple/green), with the volume set to maximum. Once again, the 24/96 outperformed the 16/44.1 data, with a THD range from 0.2% at 300uVrms to 0.00005% at 2Vrms, while the 16/44.1 ranged from 3% down to 0.0005% at the maximum output voltage of 5.19Vrms.

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

The chart above shows THD+N ratios measured at the balanced output as a function of output voltage for the coaxial input into 200k ohms from -90dBFS to 0dBFS at 16/44.1 (blue/red) and 24/96 (purple/green), with the volume set to maximum. The 24/96 outperformed the 16/44.1 data, with a THD+N range from 2% down to 0.0002% at 3-5Vrms, while the 16/44.1 ranged from 30% down to 0.002% at the maximum output voltage of 5.19Vrms.

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% from -10 to 0dBFS.

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

Shown above is the fast Fourier transform (FFT) for a 1kHz input sinewave stimulus, measured at the balanced output into 200k ohm for the coaxial digital input, sampled at 16/44.1. We see signal harmonics at 2 and 3kHz. The second (2kHz) harmonic for the left channel (right channel cannot be seen above the noise floor) is at -120dBrA, or 0.0001%, and the third harmonic (3kHz) is at -130dBRa, or 0.00003%. There are also no power-supply noise peaks to speak of to the left of the main signal peak.

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

Shown above is the fast Fourier transform (FFT) for a 1kHz input sinewave stimulus, measured at the balanced output into 200k ohm for the coaxial digital input, sampled at 24/96. Due to the increased bit-depth, the noise floor is much lower compared to the 16/44.1 FFT at a very low -160dBrA. We see signal harmonics ranging from -120dBrA to -150dBrA, or 0.0001% to 0.000003%, all the way to 20kHz (and beyond). The second (2kHz) signal harmonic shows the significant discrepancy between the left (-120dBrA) and right (-150dBrA) channels. Here also, there are no powersupply noise peaks to speak of to the left of the main signal peak.

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

Shown above is the fast Fourier transform (FFT) for a 1kHz input sinewave stimulus, measured at the unbalanced output into 100k ohms for the coaxial digital input, sampled at 24/96. The main difference with the balanced output is at the second (2kHz) signal harmonic for the right channel, where here we find a level of -125dBrA, or 0.00006%, instead of the -150dBrA for the balanced outputs. The third (3kHz) harmonic is also higher here, at -120dBrA, or 0.0001%, instead of -130dBrA for the balanced outputs.

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

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

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

Shown above is the fast Fourier transform (FFT) for a 1kHz input sinewave stimulus, measured at the balanced output into 200k ohm for the coaxial digital input, sampled at 24/961 at -90dBFS. We see the signal peak at the correct amplitude, and essentially no signal harmonics or noise peaks.

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

Shown above is an FFT of the intermodulation distortion (IMD) products for an 18kHz + 19kHz summed sinewave stimulus tone measured at the balanced output into 200k ohms for the coaxial input at 16/44.1. The input dBFS values are set at -6.02dBFS so that, if summed for a mean frequency of 18.5kHz, would yield 2Vrms (0dBrA) at the output. We find that the second-order modulation product (i.e., the difference signal of 1kHz) is perhaps barely visible above the noise floor at -130dBrA, or 0.00003%, and the third-order modulation products, at 17kHz and 20kHz, are at -135dBrA, or 0.00002% (left channel).

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

Shown above is an FFT of the intermodulation distortion (IMD) products for an 18kHz + 19kHz summed sinewave stimulus tone measured at the balanced output into 200k ohms for the coaxial input at 24/96. The input dBFS values are set at -6.02dBFS so that, if summed for a mean frequency of 18.5kHz, would yield 2Vrms (0dBrA) at the output. We find that the second-order modulation product (i.e., the difference signal of 1kHz) is at -130dBrA, or 0.00003%, and the third-order modulation products, at 17kHz and 20kHz, are at the same level.

Intermodulation distortion FFT (coaxial input, APx 32 tone, 24/192)

Shown above is the FFT of the balanced line-level output of the PecanPi+ with the APx 32-tone signal applied to the input. The combined amplitude of the 32 tones is the 0dBFS reference, and it corresponds to 2Vrms into 200k ohms. The intermodulation products—i.e., the “grass” between the test tones—are distortion products from the amplifier and below the -140dBrA, or 0.00001%, level. This is a very clean IMD result.

Diego Estan
Electronics Measurement Specialist