iFi Audio Zen Phono Preamplifier Measurements

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