Saturn Audio 401 Phono Preamplifier Measurements

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