Link: reviewed by Roger Kanno on *SoundStage! Hi-Fi* on June 15, 2024

**General information**

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

The MXA-8400 was conditioned for 1 hour at 1/8th full rated power (~25W 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 MXA-8400 has eight balanced inputs (XLR), and eight speaker-level outputs (Neutrik speakON). Each pair of inputs (e.g., 1&2 and 3&4, etc) can be independently configured as a single channel in bridge mode. The MXA-8400 was evaluated as a stereo amplifier using channels 1 and 2, as well as a two-channel bridged amplifier using channels 1/2 (bridged) and 3/4 (bridged). The MXA-8400 also offers a low gain mode (6Vrms sensitivity) and a more typical high gain mode (2Vrms sensitivity). Unless otherwise stated, the high gain mode was used.

Because the MXA-8400 uses a digital amplifier technology that exhibits considerable noise above 20kHz (see FFTs below), our typical input bandwidth filter setting of 10Hz-90kHz was necessarily changed to 10Hz-22.4kHz for all measurements, except for frequency response and for FFTs. In addition, THD versus frequency sweeps were limited to 6kHz to adequately capture the second and third signal harmonics with the restricted bandwidth setting.

**Published specifications vs. our primary measurements**

The table below summarizes the measurements published by Lyngdorf for the MXA-8400 compared directly against our own. The published specifications are sourced from Lyngdorf’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 to 500kHz, assume, unless otherwise stated, 10W into 8ohms 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 | 200W | 269W |

Rated output power into 4 ohms | 400W | 542W |

Rated output power into 8 ohms (mono) | 800W | 950W |

Gain (high sensitivity, 2-channel mode) | 26.1dB | 26.3dB |

Gain (low sensitivity, 2-channel mode) | 16.6dB | 16.9dB |

Gain (high sensitivity, bridge mode) | 31.7dB | 31.9dB |

Gain (low sensitivity, bridge mode) | 22.2dB | 22.5 |

Input sensitivity (for 200W into 8 ohms, high sensitivity) | 2Vrms | 1.94Vrms |

Input sensitivity (for 200W into 8 ohms, low sensitivity) | 6Vrms | 5.74Vrms |

Our primary measurements in two-channel mode revealed the following using the line-level balanced analog input (unless specified, assume a 1kHz sinewave at 440mVrms, 10W output, 8-ohm loading, 10Hz to 22.4kHz bandwidth):

Parameter | Left Channel | Right Channel |

Maximum output power into 8 ohms (1% THD+N, unweighted) | 269W | 269W |

Maximum output power into 4 ohms (1% THD+N, unweighted) | 542W | 542W |

Maximum burst output power (IHF, 8 ohms) | 273W | 273W |

Maximum burst output power (IHF, 4 ohms) | 556W | 556W |

Continuous dynamic power test (5 minutes) | passed | passed |

Damping factor | 739 | 778 |

Clipping no-load output voltage | 45.8Vrms | 45.8Vrms |

DC offset | <1.5mV | <1.2mV |

Gain (high) | 26.3dB | 26.3dB |

Gain (low) | 16.9dB | 16.9dB |

IMD ratio (CCIF, 18kHz + 19kHz stimulus tones, 1:1) | <-95dB | <-95dB |

IMD ratio (SMPTE, 60Hz + 7kHz stimulus tones, 4:1 ) | <-106dB | <-106dB |

Input sensitivity (for full rated power) | 1.94Vrms | 1.94Vrms |

Input impedance | 14.8k ohms | 14.9k ohms |

Noise level (with signal, A-weighted) | <22uVrms | <22uVrms |

Noise level (with signal, 20Hz to 20kHz) | <27uVrms | <27uVrms |

Noise level (no signal, A-weighted) | <22uVrms | <22uVrms |

Noise level (no signal, 20Hz to 20kHz) | <27uVrms | <27uVrms |

Noise level (no signal, A-weighted, low gain) | <14uVrms | <14uVrms |

Noise level (no signal, 20Hz to 20kHz, low gain) | <18uVrms | <18uVrms |

Signal-to-noise ratio (200W, A-weighted) | 125.0dB | 125.0dB |

Signal-to-noise ratio (200W, 20Hz to 20kHz) | 123.1dB | 123.4dB |

THD ratio (unweighted) | <0.00009% | <0.00009% |

THD+N ratio (A-weighted) | <0.00025% | <0.00025% |

THD+N ratio (unweighted) | <0.00035% | <0.00035% |

Minimum observed line AC voltage | 121.6VAC | 121.6VAC |

For the continuous dynamic power test, the MXA-8400 was able to sustain 548W into 4 ohms (~1.2% THD) using an 80Hz tone for 500ms, alternating with a signal at -10dB of the peak (54.8W) for 5 seconds, for 5 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 top of the MXA-8400 was only slightly warm to the touch.

Our primary measurements in bridge mode revealed the following using the line-level balanced analog input (unless specified, assume a 1kHz sinewave at 230mVrms, 10W output, 8-ohm loading, 10Hz to 22.4kHz bandwidth):

Parameter | Left Channel | Right Channel |

Maximum output power into 8 ohms (1% THD+N, unweighted) | 950W | 950W |

Maximum output power into 4 ohms (1% THD+N, unweighted) | 1040W | 1040W |

Maximum burst output power (IHF, 8 ohms) | 980W | 980W |

Maximum burst output power (IHF, 4 ohms) | 1074W | 1074W |

DC offset | <-0.7mV | <-0.5mV |

Gain (high) | 31.9dB | 31.9dB |

Gain (low) | 22.5dB | 22.5dB |

IMD ratio (CCIF, 18kHz + 19kHz stimulus tones, 1:1) | <-97dB | <-97dB |

IMD ratio (SMPTE, 60Hz + 7kHz stimulus tones, 4:1 ) | <--102dB | <-102dB |

Input sensitivity (for full rated 800W) | 2.02Vrms | 2.02Vrms |

Noise level (with signal, A-weighted) | 37uVrms | 37uVrms |

Noise level (with signal, 20Hz to 20kHz) | 47uVrms | 47uVrms |

Noise level (no signal, A-weighted) | 37uVrms | 37uVrms |

Noise level (no signal, 20Hz to 20kHz) | 47uVrms | 47uVrms |

Signal-to-noise ratio (800W, A-weighted) | 126.4dB | 126.4dB |

Signal-to-noise ratio (800W, 20Hz to 20kHz) | 124.4dB | 124.4dB |

THD ratio (unweighted) | <0.00009% | <0.00009% |

THD+N ratio (A-weighted) | <0.00041% | <0.00041% |

THD+N ratio (unweighted) | <0.00056% | <0.00056% |

Minimum observed line AC voltage | 118VAC | 118VAC |

**Frequency response (8-ohm loading)**

In our frequency-response plots above, measured across the speaker outputs at 10W into 8 ohms, the MXA-8400 is essentially flat within the audioband. At the extremes the MXA-8400 is at 0dB at 5Hz and -3dB just past 60kHz. In 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 (8-ohm loading)**

Above are the phase response plots from 20Hz to 20kHz for the line-level input, measured across the speaker outputs at 10W into 8 ohms. The MXA-8400 does not invert polarity and exhibits at worst, about 30 degrees (at 20kHz) of phase shift within the audioband.

**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, in stereo mode. 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. The chart below . . .

. . . is the same but zoomed in to highlight differences. Here we find that maximum deviation between no-load and a 4-ohm load is very small, at around 0.025dB. This is an indication of a very high damping factor, or low output impedance. With a real speaker, the deviations are smaller, at roughly 0.01dB.

**THD ratio (unweighted) vs. frequency vs. output power (two-channel mode)**

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 in stereo mode. The blue and red plots are for left and right channels at 1W output into 8 ohms, purple/green at 10W, and pink/orange at the rated 200W. The 10W data yielded the lowest THD figures, ranging from 0.00004% from 20Hz to 200Hz, then up to 0.0005% at 6kHz. These are extraordinarily low THD ratios, nearing the limits of the APx555 analyzer. At 1W, THD ratios were more constant, from 0.0001% from 20Hz to 1kHz, then up to 0.0003% at 6kHz. A 200W, THD ratios ranged from 0.00006% from 20Hz to 200Hz, then up to 0.0005% at 6kHz.

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

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 in bridge mode. The blue and red plots are for left and right channels at 1W output into 8 ohms, purple/green at 10W, and pink/orange at the rated 450W. The 1W data yielded the most constant results, ranging from 0.0002% from 20Hz to 2kHz, then down to 0.0001% at 3-4kHz. At 10W, THD ratios ranged from 0.00006% from 20Hz to 1kHz, then up to 0.0006% at 6kHz. A 450W, THD ratios ranged from 0.00002/0.00003% from 20Hz to 100Hz, then a steady climb to 0.0004% at 6kHz.

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

The chart above shows THD ratios measured at the output of the MXA-8400 as a function of output power for the analog line level-input in two-channel mode, 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 data ranged from about 0.0003% at 50mW, down to 0.00007% from 10 to 50W, then up to the “knee” right around 200W. The 4-ohm data ranged from about 0.0006% at 50mW, down to 0.00007% from 5 to 100W, then up to the “knee” around 350W. The 1% THD marks were hit at 278W (8-ohm loading) and 540W (4-ohm loading).

**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 MXA-8400 as a function of output power for the analog line level-input in stereo mode, for an 8-ohm load (blue/red for left/right) and a 4-ohm load (purple/green for left/right channels). THD+N values for the 8-ohm data ranged from 0.003% down to just below 0.0002% at 100-200W. The 4-ohm data yielded THD+N values 3-4dB higher.

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

The chart above shows THD ratios measured at the output of the MXA-8400 as a function of output power for the analog line-level-input in bridge mode for a 4-ohm load (blue/red for left/right channels). THD ratios were not measured into an 8-ohm load because our dummy-load configuration allows for a power handling of only 500W (but 1000W for 4 ohms). Maximum (1% THD) power into 8 ohms was measured in bridge mode with very short 1-second iterative measurements where a staggering 950W with two bridged channels driven was observed. The 4-ohm data ranged from about 0.002% at 300mW, down to 0.0001-0.0002% from 1.5 to 500W, then up to the “knee” around 700-800W. The 1% THD mark was hit at 1040W.

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

The chart above shows THD ratios measured at the output of the MXA-8400 as a function of frequency into three different loads (8/4/2 ohms) for a constant input voltage that yields 50W at the output into 8 ohms (and roughly 100W into 4 ohms, and 200W into 2 ohms) for the balanced analog line-level input in two-channel mode. The 8-ohm load is the blue trace, the 4-ohm load the purple trace, and the 2-ohm load the pink trace. The 8 and 4-ohm THD data are nearly identical, ranging from 0.00002-0.00003% from 20Hz to 100Hz, then up to 0.0004% at 6kHz. The 2-ohm data ranged from 0.0001% to 0.003% across the audioband. It’s clear that these amplifier modules are optimized for 4- and 8-ohm loads. Nonetheless, they are stable into 2 ohms, yielding admirably low THD ratios.

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

The chart above shows THD ratios measured at the output of the MXA-8400 as a function of frequency into two different loads (8/4 ohms) for a constant input voltage that yields 200W at the output into 8 ohms (and roughly 400W into 4 ohms) for the analog line-level input in bridged mode. THD ratios were essentially identical, ranging from 0.00004% at low frequencies, then up to 0.0004% at 4kHz.

**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 MXA-8400 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 balanced analog line-level input in stereo mode. 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 8-ohm plot is fairly flat and between 0.0001% and 0.0003% from 20Hz to 6kHz. Between 1kHz and 6kHz, the THD ratios when real speakers were used as loads are identical to the dummy load. The two-way speaker THD results were as high as 0.02% at 20Hz. Between 40Hz and 200Hz, the speaker THD results were roughly 10-20dB higher than that of the dummy load. While THD ratios remain low and below the threshold of audibility into real-world speaker loads, the NAD M23, using similar amplifier modules, performed better in this regard than the MX-8400.

**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 MXA-8400 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 is 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). All three IMD plots are within 10-15dB of one another, hovering between 0.0002% and 0.0008%. The IMD results for the real-world speaker loads can be seen both above an below the resistive dummy load results, depending on frequency.

**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 MXA-8400 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 in stereo mode. 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 data sets are close enough to be judged as identical, hovering around the 0.001% level.

**FFT spectrum – 1kHz (line-level input, two-channel mode)**

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 in two-channel mode. We see that the signal’s third (3kHz) and fifth (5kHz) harmonics are at -125dBrA, or 0.00006%, and -130dBrA, or 0.00003%, respectively. The remaining visible signal harmonics are below the -135dBrA, or 0.00002%, levels. These are extraordinarily low THD levels. The power-supply-related noise peak at the fundamental (60Hz) frequency is barely seen at just above the -150dBrA, or 0.000003%, level; however, this peak is inherent to the AP’s signal generator. A rise in the noise floor can be seen above 20kHz, indicative of this type of digital amplifier technology. This is an exceptionally clean FFT result.

**FFT spectrum – 1kHz (line-level input, two-channel mode, low gain)**

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 in two-channel mode, this time with the gain set to low. We see that the signal’s second (2kHz) and third (3kHz) harmonic, are nearing the absurdly low -140dBrA, or 0.00001%, level. As a point of comparison, below is a 1kHz FFT with the AP analyzer in loopback mode (generator internally feeds the analyzer) with the same 9Vrms signal amplitude. The only differences are that the overall noise floor, from uncorrelated thermal noise, is roughly 10dB higher with the MXA-8400 in the signal path (in the audioband), and the 2kHz signal harmonic peak is just below (instead of above) of -140dBrA, and the 3kHz peak is at -150dBrA instead of -140dBrA. In low-gain mode, in can be said that the MXA-8400 adds only a very small amount of uncorrelated noise (hiss), and from a THD perspective, is essentially perfectly transparent. In other words, the very definition of a “straight wire with gain.”

**FFT spectrum – 1kHz (loopback, 9Vrms)**

Shown above is the fast Fourier transform (FFT) for a 9Vrms 1kHz input sinewave stimulus, measured with the AP analyzer in loopback mode, for comparison to the above MXA-8400 FFT charts. The overall noise floor is around the -165dBrA level, and the only visible peaks are at 60Hz (-150dBrA), 120/240Hz (-155dBrA), 2kHz (-140dBrA), 3kHz (-150dBrA), and 4/6kHz (-160dBrA).

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

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 in bridged mode (high-gain setting). We see that the signal’s third (3kHz) harmonic is at -130dBrA, or 0.00003%, while the second (2kHz) and fifth harmonics (5kHz) are even lower at -135dBrA, or 0.00002%. These THD levels are slightly lower than the already extraordinarily low levels in two-channel mode. The overall noise floor (from uncorrelated thermal noise) is a few dB higher in bridge mode (slightly above versus slightly below -150dBrA), however. This is an unavoidable consequence of using two amplifier modules instead of just one to drive the load.

**FFT spectrum – 50Hz (line-level input, two-channel mode)**

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 in two-channel mode. 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 most dominant (non-signal) peaks are the second (100Hz) and third (150Hz) signal harmonics at roughly -145dBrA, or 0.000006%, and -140dBrA, or 0.00001%. THD levels are even lower at 50Hz than the already ultra-low levels at 1kHz. The power-supply-related noise peak at the fundamental (60Hz) frequency is evident at the extremely low -140dBrA, or 0.00001%, level. Another near-perfect FFT.

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

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 in two-channel mode. 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 -125dBrA, or 0.00006%, and the third-order modulation products, at 17kHz and 20kHz, are at roughly -110dBrA, or 0.0003%.

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

Shown above is the FFT of the speaker-level output of the MXA-8400 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 and are below the very low -140dBrA, or 0.00001%, level. This is an another ultra-clean IMD FFT.

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

Above is the 10kHz squarewave response using the balanced 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 MXA-8400’s slew-rate performance. Rather, it should be seen as a qualitative representation of the MXA-8400’s mid-tier 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. In the case of the MXA-8400, however, what cn be seen in the plateaus of the squarewave in the top graph is a 500 kHz sinewave, the frequency at which the switching oscillator in the Class D amp is operating (see FFT below).

**Square-wave response (10kHz)—250kHz bandwidth**

Above is the 10kHz squarewave response using the analog input, at roughly 10W into 8 ohms, with a 250kHz restricted bandwidth to remove the modulation from the 500kHz oscillator. Here we find a relatively clean squarewave, with some overshoot in the corners.

**FFT spectrum of 500kHz switching frequency relative to a 1kHz tone**

The MXA-8400’s amplifier relies on a switching oscillator to convert the input signal to a pulse-width-modulated (PWM) squarewave (on/off) signal before sending the signal through a low-pass filter to generate an output signal. The MXA-8400 oscillator switches at a rate of about 500kHz, and this graph plots a wide bandwidth FFT spectrum of the amplifier’s output at 10W into 8 ohms as it’s fed a 1kHz sinewave. We can see that the 500kHz peak is quite evident, and at -40dBrA. There is also a peak at 1MHz (the second harmonic of the 500kHz peak), at -70dBrA. Those three peaks—the fundamental and its second/third harmonics—are direct results of the switching oscillators in the MXA-8400 amp modules. The noise around those very-high-frequency signals are in the signal, but all that noise is far above the audioband—and therefore inaudible—and so high in frequency that any loudspeaker the amplifier is driving should filter it all out anyway.

**Damping factor vs. frequency (20Hz to 20kHz, two-channel mode)**

The graph above is the damping factor as a function of frequency in two-channel mode. We see both channels with very high and constant damping factor, around 650 to 780 across the audioband.

*Diego Estan*

Electronics Measurement Specialist