Link: reviewed by Jason Thorpe on *SoundStage! Ultra* on October 1, 2023

**General Information**

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

The Fezz Audio Lybra 300B was conditioned for one hour at 1/8th full rated power (~2W into 8 ohms) before any measurements were taken. All measurements were taken with both channels driven, using a 120V/20A dedicated circuit, unless otherwise stated.

The Lybra offers three unbalanced (RCA) inputs and two pairs of speaker level outputs (one for 8 ohms, and one for 4 ohms). For the purposes of these measurements, the following input was evaluated: analog line-level input. Unless otherwise stated, when a measurement was made into an 8-ohm load the, 8-ohm speaker output was used, and for a 4-ohm (or 2-ohm) load, the 4-ohm speaker output was used. By default (if no load is mentioned), an 8-ohm load was used with the 8-ohm speaker outputs.

Most measurements were made with a 2Vrms line-level analog input. The signal-to-noise ratio (SNR) measurements were made with the same input signal values but with the volume set to achieve the rated output power of 15W (8 ohms). For comparison, on the line-level input, a SNR measurement was also made with the volume at maximum, but with a lower input voltage to achieve the same 15W output.

Based on the accuracy and non-repeatable results at various volume levels of the left/right channel matching (see table below), the Lybra volume control is a potentiometer operating in the analog domain. The volume control offers a total range from -75dB to +27.9dB for the 8-ohm output and 24.6dB for the 4-ohm output.

The analyzer’s input bandwidth filter was set to 10Hz–22.4kHz for all measurements, except for frequency response (DC to 1MHz), FFTs (10Hz to 90kHz), and THD vs. frequency (10Hz to 90kHz). The latter to capture the second and third harmonic of the 20kHz output signal. Since the Lybra is not a class-D amp, there was no issue with excessive noise above 20kHz.

**Volume-control accuracy (measured at speaker outputs): left-right channel tracking**

Volume position | Channel deviation |

min | 15dB |

7 o'clock | 1.77dB |

9 o'clock | 1.11dB |

10 o'clock | 0.633dB |

12 o'clock | 0.325dB |

1 o'clock | 0.308dB |

3 o'clock | 0.537dB |

4 o'clock | 0.518dB |

max | 0.294dB |

**Published specifications vs. our primary measurements**

The table below summarizes the measurements published by Fezz Audio for the Lybra 300B compared directly against our own. The published specifications are sourced from Fezz 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 extended from DC to 1MHz, assume, unless otherwise stated, 10W into 8 ohms and a measurement input bandwidth of 10Hz to 22.4kHz, and the worst-case measured result between the left and right channels.

Parameter | Manufacturer | SoundStage! Lab |

Rated output power into 8 ohms (10% THD) | 15W | 14.7W |

Frequency response (-3dB) | 12Hz–60kHz | 11Hz–61kHz |

THD (1W, 8 ohms) | <0.25% | 0.3% |

Our primary measurements revealed the following using the line-level analog input (unless specified, assume a 1kHz sinewave at 2Vrms, 10W output, 8-ohm loading, 10Hz to 22.4kHz bandwidth):

Parameter | Left channel | Right channel |

Maximum output power into 8 ohms (10% THD+N, unweighted) | 14.7W | 15.4W |

Maximum output power into 4 ohms (10% THD+N, unweighted) | 15.0W | 15.5W |

Maximum burst output power (IHF, 8 ohms) | 15W | 15W |

Maximum burst output power (IHF, 4 ohms) | 15W | 15W |

Continuous dynamic power test (5 minutes, both channels driven) | passed | passed |

Crosstalk, one channel driven (10kHz) | -57.5dB | -49.5dB |

Damping factor | 2.9 | 2.9 |

Clipping no-load output voltage (10% THD) | 22Vrms | 22Vrms |

DC offset | <-0.5mV | <-0.5mV |

Gain (maximum volume) | 27.9dB | 27.6dB |

IMD ratio (CCIF, 18kHz + 19kHz stimulus tones, 1:1) | <-31dB | <-31dB |

IMD ratio (SMPTE, 60Hz + 7kHz stimulus tones, 4:1 ) | <-19dB | <-19dB |

Input impedance (line input, RCA) | 52.8k ohms | 52.1k ohms |

Input sensitivity (for rated power, maximum volume) | 515mVrms | 520mVrms |

Noise level (no signal, A-weighted, volume min) | <73uVrms | <71uVrms |

Noise level (no signal, 20Hz to 20kHz, volume min) | <450uVrms | <380uVrms |

Signal-to-noise ratio (15W, A-weighted, 2Vrms in) | 101.6dB | 101.2dB |

Signal-to-noise ratio (15W, 20Hz to 20kHz, 2Vrms in) | 87.0dB | 87.8dB |

Signal-to-noise ratio (15W, A-weighted, max volume) | 97.2dB | 95.9dB |

THD ratio (unweighted) | <2.77% | <2.88% |

THD+N ratio (A-weighted) | <3.17% | <3.30% |

THD+N ratio (unweighted) | <2.77% | <2.88% |

Minimum observed line AC voltage | 122VAC | 122VAC |

For the continuous dynamic power test, the Lybra 300B was able to sustain 15W into 4 ohms (~10% THD) using an 80Hz tone for 500ms, alternating with a signal at -10dB of the peak (50.8W) for five seconds, for five continuous minutes without inducing a fault or the initiation of a protective circuit. This test is meant to simulate sporadic dynamic bass peaks in music and movies. During the test, the Lybra was very warm; however, this is the normal operating condition of the amplifier.

**Frequency response (8-ohm loading, line-level input, relative level)**

In our measured frequency response (relative to 1kHz) chart above, the blue and red plots show the speaker-level outputs (relative to 1kHz, at 10W into 8 ohms). The Lybra’s speaker outputs are near flat within the audioband (about -0.5dB at 20Hz and -0.2dB at 20kHz), and exhibit an average bandwidth (-3dB at 61kHz). The small ~0.2dB blip at around 650Hz is real, was repeatable, and was also observed with constant signals in the analyzer’s bench mode. With the graph above and most of the graphs below, only a single trace may be visible. This is because the left channel (blue or purple trace) is performing identically to the right channel (red or green trace), and so they perfectly overlap, indicating that the two channels are ideally matched.

**Phase response (line-level input)**

Above is the phase response plot from 20Hz to 20kHz for the line-level input, measured across the speaker outputs at 10W into 8 ohms. The Lybra does not invert polarity. Here we find +70 degrees at 20Hz, and -20 degrees at 20kHz.

**RMS level vs. frequency vs. load impedance (1W, left channel only)**

The chart above shows RMS level (relative to 0dBrA, which is 1W into 8 ohms or 2.83Vrms) as a function of frequency, for the analog line-level input swept from 5Hz to 100kHz. The blue plot is into an 8-ohm load, the purple is into a 4-ohm load, the pink plot is an actual speaker (Focal Chora 806, measurements can be found here), and the cyan plot is no load connected. All sweeps were performed using the Lybra’s 8-ohm speaker outputs, to show the effects of load on frequency response. Here we can see significant deviations of about 4dB from 4 ohms to no load through the flat part of the audioband (100Hz to 10kHz), reaching as high as about 6.5dB at 20Hz. This is an indication of a very low damping factor, or high output impedance. The variation in RMS level when a real speaker was used is about 3.5dB through most of the audioband.

To expand on the Lybra’s frequency response when using a real speaker, the chart below . . .

. . . shows the frequency response (relative to 1kHz) using a continuous sweep for the Focal Chora 806. Again we see deviations of up to 3.5dB within the audioband. It’s important to mention that deviations of this magnitude would be clearly audible, giving the Lybra amplifier a “sound” that would change based on the characteristic impedance curve of the speaker it’s connected to.

**THD ratio (unweighted) vs. frequency vs. output power**

The chart above shows THD ratios at the output into 8 ohms as a function of frequency for a sinewave stimulus at the analog line-level input. The blue and red plots are for left and right channels at 0.1W output into 8 ohms, purple/green at 1W, and pink/orange at 10W. The power was varied using the volume control. At 0.1W, THD ratios hovered between from 0.2 an 0.1% from 20Hz to 20kHz. At 1W, THD ratios hovered between from 0.7 to 0.3% from 20Hz to 20kHz. At 10W, THD ratios hovered between from 5 and 2% from 20Hz to 20kHz.

**THD ratio (unweighted) vs. output power at 1kHz into 4 and 8 ohms**

The chart above shows THD ratios measured at the output of the Lybra as a function of output power for the analog line-level input for an 8-ohm load (blue/red for left/right channels) and a 4-ohm load (purple/green for left/right channels). The 8-ohm and 4-ohm data tracked closely. THD ratios started at 0.03% at 10mW, with a steady rise to 2-3% at just over 10W, then a shaper rise to 10% THD at 15W.

**THD+N ratio (unweighted) vs. output power at 1kHz into 4 and 8 ohms**

The chart above shows THD+N ratios measured at the output of the Lybra as a function of output power for the analog line-level-input for an 8-ohm load (blue/red for left/right channels) and a 4-ohm load (purple/green for left/right channels). The 8-ohm and 4-ohm data tracked closely. THD+N ratios started at 0.1 to 0.2% from 10mW to 200mW, followed by a steady rise to 2 to 3% at just over 10W, then a shaper rise to 10% THD at 15W.

**THD ratio (unweighted) vs. frequency at 8, 4, and 2 ohms (left channel only)**

The chart above shows THD ratios measured at the output of the Lybra as a function of frequency into three different loads (8/4/2 ohms) for a constant input voltage of 2.83Vrms that yields 1W at the output into 8 ohms using the 8-ohm speaker output, and roughly 2/4W into 4/2 ohms using the 4-ohm speaker outputs, for the analog line-level input. The 8-ohm load is the blue trace, the 4-ohm load the purple trace, and the 2-ohm load the pink trace. We find very similar THD ratios for the 8- and 4-ohm data (the 8-ohm data is about 2-3dB lower than the 4-ohm data), ranging from around 1% down to 0.3% from 20Hz to 20kHz. For the 2-ohm load, THD ratios were higher, hovering around the 2% mark through most of the audioband.

**THD ratio (unweighted) vs. frequency into 8 ohms and real speakers (left channel only)**

The chart above shows THD ratios measured at the output of the Lybra as a function of frequency into an 8-ohm load and two different speakers for a constant output voltage of 2.83Vrms (1W into 8 ohms) for the analog line-level input. The 8-ohm load is the blue trace, the purple plot is a two-way speaker (Focal Chora 806, measurements can be found here), and the pink plot is a three-way speaker (Paradigm Founder Series 100F, measurements can be found here). Both speaker THD plots showed signficant variations in THD ratios, both above and below the 0.6–0.4% line for the 8-ohm dummy load. The two-way speaker ranged from 5% THD at 20Hz to a low of 0.15% at 2–3kHz. The three-way speaker ranged from 2% at 100Hz to a low of 0.06% at 3kHz. This shows that THD ratios can vary signficanlty for the Lybra, depending on the speaker’s impedance at a given frequency.

**IMD ratio (CCIF) vs. frequency into 8 ohms and real speakers (left channel only)**

The chart above shows intermodulation distortion (IMD) ratios measured at the output of the Lybra as a function of frequency into an 8-ohm load and two different speakers for a constant output voltage of 2.83Vrms (1W into 8 ohms) for the analog line-level input. Here the CCIF IMD method was used, where the primary frequency is swept from 20kHz (F1) down to 2.5kHz, and the secondary frequency (F2) is always 1kHz lower than the primary, with a 1:1 ratio. The CCIF IMD analysis results are the sum of the second (F1-F2 or 1kHz) and third modulation products (F1+1kHz, F2-1kHz). The 8-ohm load is the blue trace, the purple plot is a two-way speaker (Focal Chora 806, measurements can be found here), and the pink plot is a three-way speaker (Paradigm Founder Series 100F, measurements can be found here). The results for the dummy load were fairly consistent, hovering around 0.4% throughout the sweep. We find that both speakers yielded IMD ratios that were lower (by 10dB) compared to the dummy load between 2.5kHz and 5kHz. From the 6kHz to 20kHz, IMD ratios were almost 10dB higher for the three-way speaker compared to the dummy load, while the two-way speaker yielded IMD ratios lower than the dummy load throughout the sweep.

**IMD ratio (SMPTE) vs. frequency into 8 ohms and real speakers (left channel only)**

The chart above shows IMD ratios measured at the output of the Lybra as a function of frequency into an 8-ohm load and two different speakers for a constant output voltage of 2.83Vrms (1W into 8 ohms) for the analog line-level input. Here, the SMPTE IMD method was used, where the primary frequency (F1) is swept from 250Hz down to 40Hz, and the secondary frequency (F2) is held at 7kHz with a 4:1 ratio. The SMPTE IMD analysis results consider the second (F2 ± F1) through the fifth (F2 ± 4xF1) modulation products. The 8-ohm load is the blue trace, the purple plot is a two-way speaker (Focal Chora 806, measurements can be found here), and the pink plot is a three-way speaker (Paradigm Founder Series 100F, measurements can be found here). All three results are similar but still varied by as much as +5dB and -10dB relative to the constant 1.5% measured across the resistive dummy load. The lowest IMD level for the 2-way speaker was found at 80Hz at 0.3%, while the two-way speaker yielded 0.7% around 60Hz.

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

Shown above is the fast Fourier transform (FFT) for a 1kHz input sinewave stimulus, measured at the output across an 8-ohm load at 10W for the analog line-level input. We see that the signal’s second (2kHz) and third (3kHz) harmonic dominate at around -35dBrA, or 2%. The other signal harmonics are below -50dBrA, or 0.3%. There are clearly visible power-supply-related noise peaks at even and odd harmonics (60Hz, 120Hz, 180Hz, 240Hz, etc.) on the left side of the main 1kHz peak, with the 60Hz peak dominating at -90dBrA, or 0.003%.

**FFT spectrum – 50Hz (line-level input)**

Shown above is the FFT for a 50Hz input sinewave stimulus measured at the output across an 8-ohm load at 10W for the analog line-level 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. We see that the signal’s second (2kHz) and third (3kHz) harmonic dominate at around -35dBrA, or 2%. The other signal harmonics are below -50dBrA, or 0.3%. There are clearly visible power-supply-related noise peaks at even and odd harmonics (60Hz, 120Hz, 180Hz, 240Hz, etc.) on the left side of the main 1kHz peak, with the 60Hz peak dominating at -90dBrA, or 0.003%.

**Intermodulation distortion FFT (18kHz + 19kHz summed stimulus, line-level input)**

Shown above is an FFT of the intermodulation distortion (IMD) products for an 18kHz + 19kHz summed sinewave stimulus tone measured at the output across an 8-ohm load at 10W for the analog line-level input. The input RMS values are set at -6.02dBrA so that, if summed for a mean frequency of 18.5kHz, would yield 10W (0dBrA) into 8 ohms at the output. We find that the second-order modulation product (*i.e.*, the difference signal of 1kHz) is at -45dBrA, or 0.6%. The third-order modulation products, at 17kHz and 20kHz, are at around the same level.

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

Shown above is the FFT of the speaker-level output of the Lybra with the APx 32-tone signal applied to the input. The combined amplitude of the 32 tones is the 0dBrA reference, and corresponds to 10W into 8 ohms. The intermodulation products—*i.e.*, the "grass" between the test tones—are distortion products from the amplifier that are below the -70dBrA, or 0.03%, level.

**Square-wave response (10kHz)**

Above is the 10kHz squarewave response using the analog line-level input, at roughly 10W into 8 ohms. Due to limitations inherent to the Audio Precision APx555 B Series analyzer, this graph should not be used to infer or extrapolate the Lybra’s slew-rate performance. Rather, it should be seen as a qualitative representation of the Lybra’s average bandwidth. An ideal squarewave can be represented as the sum of a sinewave and an infinite series of its odd-order harmonics (*e.g.*, 10kHz + 30kHz + 50kHz + 70kHz . . .). A limited bandwidth will show only the sum of the lower-order harmonics, which may result in noticeable undershoot and/or overshoot, and softening of the edges. Here we can see an average squarewave reproduction, with some softening and over/under-shoot in the corners.

**Damping factor vs. frequency (20Hz to 20kHz)**

The graph above is the damping factor as a function of frequency with an 8-ohm load connected to the 8-ohm speaker output. Both channels track closely, with a very low damping factor of between 2 (20Hz) and 3 (100Hz to 20kHz). This very high output impedance (2.67 ohms at 1kHz) explains the significant variations measured in frequency response with a real speaker load.

The graph above is the damping factor as a function of frequency with a 4-ohm load connected to the 4-ohm speaker output. It is effectively identical to the 8-ohm damping factor graph. Since damping factor is defined as the ratio of the refrence load impedance over the output impedance, this means that the output impedance on the 4-ohm tap is half that of the output impedance on the 8-ohm tap.

*Diego Estan*

Electronics Measurement Specialist