Link: reviewed by Dennis Burger on *SoundStage! Access* on June 1, 2022

**General Information**

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

The iFi Audio Zen One Signature was conditioned for 30 min at 0dBFS (2.1Vrms out) into 100k ohms before any measurements were taken.

The Zen One Signature offers four digital inputs: one coaxial S/PDIF (RCA), one optical S/PDIF, one USB, and Bluetooth. There are two line-level outputs (balanced 3.4mm TRRS and unbalanced RCA) and one digital output (coaxial, over the same RCA connector used for the coaxial digital input). Comparisons were made between unbalanced and balanced line-level outputs for a 24/96 0dBFS input, and aside from the 6dB extra voltage over balanced, there were virtually no differences in THD+N and dynamic range. In terms of input types (USB, coaxial, optical), there were no differences in terms of THD+N.

Unless otherwise stated, all measurements are with the coaxial digital input and unbalanced outputs.

**Published specifications vs. our primary measurements**

The table below summarizes the measurements published by iFi Audio for the Zen One Signature compared directly against our own. The published specifications are sourced from iFi’s website, either directly or from the manual available for download or included in the box, 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, the coaxial digital input (24/96 1kHz sine wave at 0dBFS), the unbalanced line-level output into 100k ohms (line-level), using a measurement input bandwidth of 10Hz to 90kHz, and the worst-case measured result between the left and right channels.

Parameter | Manufacturer | SoundStage! Lab |

Output impedance (BAL/UnBAL) | <72/36 ohms | 73/37 ohms |

Output voltage (0dBFS, BAL/UnBAL) | 4/2Vrms | 4.3/2.1Vrms |

Frequency response (24/192) | 5Hz-80kHz ±3dB | 5Hz-80kHz, -0.07/-2.7dB |

Signal-to-noise (A-weighted, 1kHz, 24/96@0dBFS) | 105dB | 106dB |

THD+N (1kHz, 24/48@0dBFS, 10Hz-22.4kHz BW) | <0.002% | <0.0023% |

Our primary measurements revealed the following using the coaxial input and the unbalanced line-level outputs (unless specified, assume a 1kHz sine wave at 0dBFS, 100k ohms loading, 10Hz to 90kHz bandwidth):

Parameter | Left channel | Right channel |

Crosstalk, one channel driven (10kHz, 16/44.1) | -98.7dB | -87.6dB |

Crosstalk, one channel driven (10kHz, 24/96) | -98.8dB | -87.7dB |

DC offset | <-0.18mV | <-0.06mV |

Dynamic range (A-weighted, 16/44.1) | 95.7dB | 95.5dB |

Dynamic range (unweighted, 16/44.1) | 91.9dB | 92.0dB |

Dynamic range (A-weighted, 24/96) | 106.4dB | 106.6dB |

Dynamic range (unweighted, 24/96) | 97.5dB | 97.8dB |

IMD ratio (CCIF, 18kHz + 19kHz stimulus tones, 1:1, 16/44.1) | <-85dB | <-85dB |

IMD ratio (CCIF, 18kHz + 19kHz stimulus tones, 1:1, 24/96) | <-73dB | <-74dB |

IMD ratio (SMPTE, 60Hz + 7kHz stimulus tones, 4:1, 16/44.1) | <-88dB | <-83dB |

IMD ratio (SMPTE, 60Hz + 7kHz stimulus tones, 4:1, 24/96 ) | <-88dB | <-79dB |

Maximum output voltage (0dBFS) | 2.13Vrms | 2.12Vrms |

Output impedance (BAL) | 72.9 ohms | 73.1 ohms |

Output impedance (UnBAL) | 36.9 ohms | 37.6 ohms |

Noise level (A-weighted, 16/44.1) | <38uVrms | <37uVrms |

Noise level (unweighted, 16/44.1) | <60uVrms | <60uVrms |

Noise level (A-weighted, 24/96) | <17uVrms | <17uVrms |

Noise level (unweighted, 24/96) | <40uVrms | <40uVrms |

THD ratio (unweighted, 16/44.1) | <0.0016% | <0.0021% |

THD+N ratio (A-weighted, 16/44.1) | <0.0025% | <0.0029% |

THD+N ratio (unweighted, 16/44.1) | <0.0033% | <0.0035% |

THD ratio (unweighted, 24/96) | <0.0016% | <0.0032% |

THD+N ratio (A-weighted, 24/96) | <0.0020% | <0.0037% |

THD+N ratio (unweighted, 24/96) | <0.0025% | <0.0037% |

**Frequency response (16/44.1, 24/96, 24/192)**

The plot above shows the Zen One Signature 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 three sample rates—essentially perfectly flat down to 5Hz. The behavior at high frequencies for all three digital sample rates is as expected, offering filtering around 22k, 48k, and 96kHz (half the respective sample rate). The -3dB point for each sample rate is roughly 21.1, 46.2, and 83kHz, respectively. It is also obvious from the plots above that the 44.1kHz sampled input signal offers the most “brick-wall”-type behavior, while the attenuation of the 96kHz and 192kHz sampled input signals approaching the corner frequencies (48kHz and 96kHz) is more gentle. 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.

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

The graph above shows the results of a linearity test for the coaxial digital input (the optical input performed identically) for both 16/44.1 (blue/red) and 24/96 (purple/green) input data, measured at the unbalanced line-level output. The digital input is swept with a dithered 1kHz input signal from -120dBFS to 0dBFS, and the output is analyzed by the APx555. The ideal response 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 data is only -1dB at -120dBFS, while the 16/44.1 data was perfect down to -100dBFS, and only +2.5dB (left) at -120dBFS. This is an excellent result.

**Impulse response**

The graph above shows the impulse response for a looped 24/44.1 test file that moves from digital silence to full 0dBFS (all “1”s) for one sample period then back to digital silence. We see a typical sinc function response.

**J-Test (coaxial input)**

The plot above shows the results of the J-Test test for the coaxial digital input measured at the unbalanced line-level output of the Zen One Signature. The J-Test was developed by Julian Dunn the 1990s. It is a test signal, specifically a -3dBFS undithered 12kHz square wave sampled (in this case) at 48kHz (24 bits). Since even the first odd harmonic (*i.e.*, 36kHz) of the 12kHz square wave 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 square wave 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 sine-wave jitter by the Audio Precision, to also show how well the DAC rejects jitter.

The coaxial S/PDIF input shows worst case peaks at -130dBrA. This is an indication that the Zen One Signature should not be sensitive to jitter.

**J-Test (optical input)**

The optical S/PDIF input shows essentially the same result as with the coaxial input. Both the coaxial and optical inputs were also tested for jitter immunity by injecting artificial sine-wave jitter at 2kHz on top of the J-Test file, which would manifest as sidebands at 10kHz and 14kHz without any jitter rejection. Jitter immunity proved exceptional, with no visible sidebands at even 1000ns of jitter level.

**Wideband FFT spectrum of white noise and 19.1kHz sine-wave tone**

The plot above shows a fast Fourier transform (FFT) of the Zen One Signature unbalanced line-level output with white noise at -4 dBFS (blue/red), and a 19.1 kHz sine wave at 0dBFS fed to the coaxial digital input, sampled at 16/44.1 (purple/green). The sharp roll-off above 20kHz in the white-noise spectrum shows the implementation of the brick-wall-type reconstruction filter. There are a few imaged aliasing artifacts in the audio band, the most predominant at 17kHz at -110dBrA. The primary aliasing signal at 25kHz is at -80dBrA, with subsequent harmonics of the 25kHz peak slightly above this level.

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

The chart above shows THD ratios at the unbalanced line-level output into 100k ohms (blue/red) and 600 ohms (purple/green) as a function of frequency for a 24/96 dithered 1kHz signal at the coaxial input. There’s a left/right THD imbalance into 100k ohms from 30Hz to 5kHz or so, with the left channel (blue) outperforming the right channel (red) by as much as almost 10dB. Into 600 ohms, the difference in THD between left and right was much smaller, about 3dB. In general, higher THD ratios were observed into 600 ohms, ranging from roughly 0.005% at low frequencies to 0.04% at 20kHz. This compared to the 100k ohm data, which ranged from as low as 0.001% (left channel from 100-300 Hz), to 0.04% at 20kHz.

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

The chart above shows THD ratios at the unbalanced line-level output into 100k 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. All data tracked closely, with THD ratios ranging from 0.002% at 20Hz, down to 0.001% from 100-300Hz, then up to 0.04% at 20kHz. The exception is the right channel at 24/96, which yielded THD values almost 10dB higher from 100Hz to 500Hz or so.

**THD ratio (unweighted) vs. output (16/44.1 and 24/96) at 1kHz**

The chart above shows THD ratios measured at the unbalanced output as a function of output voltage for the coaxial input into 100k ohms from -90dBFS to 0dBFS with 16/44.1 (blue/red) and 24/96 (purple/green) dithered input data. The 24/96 outperformed the 16/44.1 data at lower output levels, with a THD range from 1.5% to nearly 0.0005% at 0.2-0.3Vrms, while the 16/44.1 data ranged from 4% down to 0.001% at 1Vrms. It’s important to highlight that large discrepancies in THD ratios at lower input levels are due to the inherently lower 24-bit noise floor of the 24/96 data (*i.e.*, when no signal harmonics are measurable in an FFT, a THD value can only be assigned as low as the noise floor permits). At output voltages above 0.4Vrms or so, again we see the right channel with higher THD values than the left.

**THD+N ratio (unweighted) vs. output (16/44.1 and 24/96) at 1kHz**

The chart above shows THD+N ratios measured at the unbalanced output as a function of output voltage for the coaxial input into 100k ohms from -90dBFS to 0dBFS with 16/44.1 (blue/red) and 24/96 (purple/green) dithered input data. The 24/96 outperformed the 16/44.1 data, with a THD+N range from 30% down to 0.003%, while the 16/44.1 ranged from 40% down to 0.004% at the maximum output voltage of 2.1Vrms.

**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 the unbalanced output as a function of generator input level for the coaxial input into 100k ohms from -60dBFS to 0dBFS with 16/44.1 (blue/red) and 24/96 (purple/green) dithered input data. Here, the SMPTE IMD method is 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 24/96 outperformed the 16/44.1 data, with an IMD range from 0.5% down to 0.003% between -15 and 0dBFS, although, again, the right channel performed worse (almost 10dB) at these higher generator levels. The 16/44.1 data ranged from 2% down to roughly 0.005% at the maximum output voltage of 2.1Vrms at 0dBFS, with the left channel slightly outperforming the right above -10dBFS.

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

Shown above is the fast Fourier transform (FFT) for a 1kHz input sine-wave stimulus, measured at the unbalanced output into 100k ohms for the coaxial digital input, sampled at 16/44.1. We see the second signal harmonic (2kHz) at -95dBrA, or 0.002%, and subsequent harmonics (3, 4, 5, 6, 7kHz, etc.) at descending lower levels from -100dBrA, or 0.001%, down to below -120dBrA, or 0.0001%. There are 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 sine-wave stimulus, measured at the unbalanced output into 100k ohms for the coaxial digital input, sampled at 24/96. We consistently see the right channel signal harmonic peaks, 5-10dB higher than the left peaks. The left channel signal harmonic peaks essentially match what was measured at 16/44.1 (shown above). There are no power-supply noise peaks to speak of to the left of the main signal peak.

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

Shown above is the fast Fourier transform (FFT) for a 1kHz input sine-wave stimulus, measured at the unbalanced output into 100k ohm for the coaxial digital input, sampled at 16/44.1 at -90dBFS. We see the main peak at the correct amplitude, and no signal harmonics above the noise floor within the audio band.

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

Shown above is the fast Fourier transform (FFT) for a 1kHz input sine-wave stimulus, measured at the unbalanced output into 100k ohm for the coaxial digital input, sampled at 24/96 at -90dBFS. We see the main peak at the correct amplitude, and a hint of signal harmonic peak within the audio band at 3kHz at a very low -140dBrA, or 0.00001%.

**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 sine-wave stimulus tone measured at the unbalanced output into 100k 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 2.1Vrms (0dBrA) at the output. We find that the second-order modulation product (*i.e.*, the difference signal of 1kHz) is just below -90dBRA, or 0.0003%, while the third-order modulation products, at 17kHz and 20kHz, are slightly higher, reaching -85dBrA, or 0.0006%.

**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 sine-wave stimulus tone measured at the unbalanced output into 100k 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 2.1Vrms (0dBrA) at the output. Here again, we find higher distortion peaks for the right channel compared to the left, by about 5dB. Otherwise, the FFT looks essentially the same at the 16/44.1 IMD FFT above.

*Diego Estan*

Electronics Measurement Specialist