Link: reviewed by Roger Kanno on *SoundStage! Hi-Fi* on September 1, 2024

**General Information**

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

The NAD Masters M66 was conditioned for 30 minutes 2Vrms in/out into 200k ohms before any measurements were taken.

The M66 offers a multitude of inputs, both digital and analog (balanced and unbalanced), and line-level/subwoofer analog balanced outputs over XLR and unbalanced over RCA. Also offered is a ¼″ TRS headphone output on the front panel. For the purposes of these measurements, unless otherwise stated, the following inputs were evaluated: digital coaxial S/PDIF (RCA), analog balanced (XLR), as well as phono moving magnet (MM) and moving coil (MC). Comparisons were made between unbalanced (RCA) and balanced (XLR) line inputs and outputs, other than the 6dB of extra gain over the balanced outputs, there were no appreciable differences in terms THD+N (FFTs for different configurations can be seen in this report). Unless otherwise stated, the Analog Direct mode was used for the analog inputs.

Most measurements were made with a 2Vrms line-level and 0dBFS digital input with the volume set to achieve 2Vrms at the output. For the phono input, a 6.4mVrms MM level and 0.5mVrms MC level was used to achieve 1Vrms at the output. Of note is the low preamp gain in Analog Direct mode (-3.6/2.7dB RCA/XLR out). Using the MM input over the balanced output, 1Vrms could not be achieved with the volume at maximum with a standard 5mVrms input. Also noteworthy is the early onset of clipping when Analog Direct is turned off (which is required to use DSP functions). The ADC clipped with a 1.92Vrms input (RCA and XLR). The signal-to-noise ratio (SNR) measurements were made with the same input signal values and 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 2Vrms output.

The M66 offers 80 volume steps in 1dB increments, from -76dB to +2.7dB (line-level, XLR in/out). Based on the random and non-repeatable channel deviation values observed below, the M66 utilizes a digitally controlled volume control operating in the analog domain.

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

Volume position | Channel deviation |

-79 | 0.010dB |

-70 | 0.003dB |

-60 | 0.016dB |

-50 | 0.003dB |

-40 | 0.017dB |

-30 | 0.011dB |

-20 | 0.002dB |

-10 | 0.011dB |

0 | 0.002dB |

**Published specifications vs. our primary measurements**

The table below summarizes the measurements published by NAD for the M66 compared directly against our own. The published specifications are sourced from NAD’s website, either directly or from the manual available for download, or a combination thereof. Except for frequency response, where the Audio Precision bandwidth was set at its maximum (DC to 1MHz), unless otherwise stated, assume a 1kHz sinewave at 2Vrms or 0dBFS at the input, 2Vrms at the output into 200k ohms, and a measurement input bandwidth of 10Hz to 22.4kHz, and the worst-case measured result between the left and right channels.

Parameter (Analog Direct off unless specified) | Manufacturer | SoundStage! Lab |

Line-level input | ||

THD (20Hz-20kHz, 2Vrms out) | <0.001% | <0.0002% |

Signal-to-noise ratio (A-wgt, 0.5Vrms out) | >105dB | 111dB |

Channel separation (1kHz) | >116dB | 136dB |

Channel separation (10kHz) | >106dB | 125dB |

Input impedance | 56k ohms | 59k ohms |

Maximum input signal (0.1% THD) | >5.6Vrms | 1.92Vrms |

Maximum input signal (0.1% THD, Analog Direct mode) | >8Vrms | 11.18Vrms |

Output impedance | 22 ohms | 22.9 ohms |

Gain (unbalanced, Analog Direct off) | 8.63dB | 2.6dB |

Gain (balanced, Analog Direct off) | 14.89dB | 8.6dB |

Frequency response (Analog Direct off) | ±0.2dB (20Hz-20kHz) | 0/-0.09dB(20Hz/20kHz) |

Frequency response (Analog Direct on) | ±0.2dB (20Hz-80kHz) | 0/-0.09dB(20Hz/80kHz) |

Max voltage output (0.1%THD, RCA, Analog Direct off) | 5Vrms | 2.6Vrms |

Max voltage output (0.1%THD, RCA, Analog Direct on) | 10Vrms | 7.6Vrms |

Max voltage output (0.1%THD, XLR, Analog Direct off) | 10Vrms | 5.2Vrms |

Max voltage output (0.1%THD, XLR, Analog Direct on) | 20Vrms | 15.5Vrms |

Digital input | ||

THD (20Hz-20kHz, 2Vrms out) | <0.0005% | <0.0003% |

Channel separation (1kHz) | >126dB | 138dB |

Channel separation (10kHz) | >115dB | 122dB |

Input sensitivity (0.5Vrms out, max volume) | -20.25dBFS | -20.32dBFS |

Subwoofer outputs | ||

THD (20Hz-200Hz, 2Vrms out) | <0.005% | <0.005% |

Signal-to-noise ratio (A-wgt, 0.5Vrms out, RCA) | >84dB | 85dB |

Signal-to-noise ratio (A-wgt, 0.5Vrms out, XLR) | >80dB | 79dB |

Output impedance | 480 ohms | 433 ohms |

Phono input | ||

THD (MM, 20Hz-20kHz, 2Vrms out) | <0.008% | <0.02% |

THD (MC, 20Hz-20kHz, 2Vrms out) | <0.02% | <0.3% |

Signal-to-noise ratio (MM, A-wgt, 0.5Vrms out) | >82dB | 75dB |

Signal-to-noise ratio (MC, A-wgt, 0.5Vrms out) | >75dB | 54dB |

Input impedance (MM) | 56k ohms | 59.9k ohms |

Input impedance (MC) | 100 ohms | 139 ohms |

Input sensitivity (MM, 0.5Vrms out, max volume) | 1.7mVrms | 3.2mVrms (1.6Vrms XLR) |

Input sensitivity (MM, 0.5Vrms out, max volume) | 0.123mVrms | 0.254mVrms (0.127mVrms XLR) |

Frequency response | ±0.2dB (20Hz-20kHz) | 0/+0.2dB(20Hz/20kHz) |

Maximum input signal (MM, 0.1% THD, 1kHz) | >80mVrms | 94mVrms |

Maximum input signal (MM, 0.1% THD, 1kHz) | >7mVrms | 7.4mVrms |

Headphone output (Analog Direct off) | ||

THD (20Hz-20kHz, 1Vrms out) | <0.002% | <0.0003% |

Signal-to-noise ratio (A-wgt, 0.5Vrms out, 32-ohm load) | >98dB | 99dB |

Frequency response | ±0.3dB (20Hz-20kHz) | 0/-0.03dB(20Hz/20kHz) |

Channel separation (1kHz) | >62dB | 105dB |

Output impedance | 4.7 ohms | 5.4 ohms |

Our primary measurements revealed the following using the balanced line-level analog input and digital coaxial input (unless specified, assume a 1kHz sinewave at 2Vrms or 0dBFS, 2Vrms output, 200k-ohm loading, 10Hz to 22.4kHz bandwidth):

Parameter | Left channel | Right channel |

Crosstalk, one channel driven (10kHz) | -126dB | -126dB |

DC offset | <-0.7mV | <-0.6mV |

Gain (RCA in/out) | -3.6dB | -3.6dB |

Gain (XLR in/out) | 2.7dB | 2.7dB |

IMD ratio (CCIF, 18kHz + 19kHz stimulus tones, 1:1) | <-117dB | <-117dB |

IMD ratio (SMPTE, 60Hz + 7kHz stimulus tones, 4:1 ) | <-107dB | <-108dB |

Input impedance (line input, RCA) | 59.0k ohms | 58.3k ohms |

Input impedance (line input, XLR) | 119.0k ohms | 119.1k ohms |

Maximum output voltage (at clipping 1% THD+N) | 15.8Vrms | 15.8Vrms |

Maximum output voltage (at clipping 1% THD+N into 600 ohms) | 14.7Vrms | 14.7Vrms |

Noise level (with signal, A-weighted)* | <2.6uVrms | <2.6uVrms |

Noise level (with signal, 20Hz to 20kHz)* | <3.3uVrms | <3.3uVrms |

Noise level (with signal, A-weighted, RCA)* | <1.9uVrms | <1.9uVrms |

Noise level (no signal, A-weighted, volume min)* | <1.7uVrms | <1.7uVrms |

Noise level (no signal, 20Hz to 20kHz, volume min)* | <2.1uVrms | <2.1uVrms |

Noise level (no signal, A-weighted, volume min, RCA)* | <1.2uVrms | <1.2uVrms |

Output impedance (RCA) | 22.4 ohms | 22.9 ohms |

Output impedance (XLR) | 44.4 ohms | 44.4 ohms |

Signal-to-noise ratio (2Vrms out, A-weighted, 2Vrms in)* | 118.3dB | 118.1dB |

Signal-to-noise ratio (2Vrms out, 20Hz to 20kHz, 2Vrms in)* | 116.2dB | 116.1dB |

Signal-to-noise ratio (2Vrms out, A-weighted, max volume)* | 118.1dB | 117.9dB |

Signal-to-noise ratio (2Vrms out, A-weighted, 2Vrms in, RCA)* | 122.5dB | 122.5dB |

Dynamic range (2Vrms out, A-weighted, digital 24/96)* | 122.1dB | 121.9dB |

Dynamic range (2Vrms out, A-weighted, digital 16/44.1)* | 96.0dB | 96.0dB |

THD ratio (unweighted) | <0.00006% | <0.00006% |

THD ratio (unweighted, digital 24/96) | <0.00018% | <0.00018% |

THD ratio (unweighted, digital 16/44.1) | <0.0004% | <0.0004% |

THD+N ratio (A-weighted) | <0.00015% | <0.00015% |

THD+N ratio (A-weighted, digital 24/96) | <0.00022% | <0.00026% |

THD+N ratio (A-weighted, digital 16/44.1) | <0.0016% | <0.0016% |

THD+N ratio (unweighted) | <0.0002% | <0.0002% |

*due to very low noise of DUT, analyzer self-noise has been removed from measurement to more accurately report value.

Our primary measurements revealed the following using the phono-level input, MM configuration (unless specified, assume a 1kHz sinewave at 6.4mVrms, 1Vrms output, 200k-ohm loading, 10Hz to 22.4kHz bandwidth):

Parameter | Left channel | Right channel |

Crosstalk, one channel driven (10kHz) | -97.3dB | -96.5dB |

DC offset | <1.1mV | <1.1mV |

Gain (default phono preamplifier) | 41.2dB | 41.2dB |

IMD ratio (18kHz and 19 kHz stimulus tones) | <-92dB | <-92dB |

IMD ratio (3kHz and 4kHz stimulus tones) | <-90dB | <-90dB |

Input impedance | 58.6k ohms | 59.9k ohms |

Input sensitivity (1Vrms out, max volume) | 6.4mVrms | 6.4mVrms |

Noise level (with signal, A-weighted) | <70uVrms | <70uVrms |

Noise level (with signal, 20Hz to 20kHz) | <650uVrms | <650uVrms |

Overload margin (relative 5mVrms input, 1kHz) | 25.7dB | 25.7dB |

Signal-to-noise ratio (1Vrms out, A-weighted) | 81.8dB | 81.6dB |

Signal-to-noise ratio (1Vrms out, 20Hz to 20kHz) | 69.6dB | 69.4dB |

THD (unweighted) | <0.0007% | <0.0007% |

THD+N (A-weighted) | <0.007% | <0.007% |

THD+N (unweighted) | <0.08% | <0.08% |

Our primary measurements revealed the following using the phono-level input, MC configuration (unless specified, assume a 1kHz sinewave at 0.5mVrms, 1Vrms output, 200k-ohm loading, 10Hz to 22.4kHz bandwidth)

Parameter | Left channel | Right channel |

Crosstalk, one channel driven (10kHz) | -94.2dB | -91.3dB |

DC offset | <-1.3mV | <-1.3mV |

Gain (default phono preamplifier) | 63.3dB | 63.3dB |

IMD ratio (18kHz and 19 kHz stimulus tones) | <74dB | <74dB |

IMD ratio (3kHz and 4kHz stimulus tones) | <72dB | <72dB |

Input impedance | 139 ohms | 139 ohms |

Input sensitivity (1Vrms out, max volume) | 0.5mVrms | 0.5mVrms |

Noise level (with signal, A-weighted) | <700uVrms | <700uVrms |

Noise level (with signal, 20Hz to 20kHz) | <7mVrms | <7mVrms |

Overload margin (relative 0.5mVrms input, 1kHz) | 23.6dB | 23.6dB |

Signal-to-noise ratio (1Vrms out, A-weighted) | 62.3dB | 61.7dB |

Signal-to-noise ratio (1Vrms out, 20Hz to 20kHz) | 50.4dB | 49.9dB |

THD (unweighted) | <0.005% | <0.005% |

THD+N (A-weighted) | <0.07% | <0.07% |

THD+N (unweighted) | <0.8% | <0.8% |

Our primary measurements revealed the following using the analog input at the headphone output (unless specified, assume a 1kHz sinewave, 2Vrms input/output, 300-ohm loading, 10Hz to 22.4kHz bandwidth, Analog Direct Mode on):

Parameter | Left and right channels |

Maximum gain | 6.2dB |

Maximum output power into 600 ohms | 180mW |

Maximum output power into 300 ohms | 347mW |

Maximum output power into 32 ohms | 1.75W |

Output impedance | 5.4 ohms |

Maximum output voltage (100k ohm load) | 10.6Vrms |

Noise level (with signal, A-weighted) | <6.0uVrms |

Noise level (with signal, 20Hz to 20kHz) | <7.3uVrms |

Noise level (no signal, A-weighted, volume min) | <5.6uVrms |

Noise level (no signal, 20Hz to 20kHz, volume min) | <6.8uVrms |

Signal-to-noise ratio (A-weighted, 1% THD, 10.2Vrms out) | 124.3dB |

Signal-to-noise ratio (20Hz - 20kHz, 1% THD, 10.2Vrms out) | 122.7dB |

THD ratio (unweighted) | <0.00008% |

THD+N ratio (A-weighted) | <0.0003% |

THD+N ratio (unweighted) | <0.0004% |

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

In our measured frequency-response (relative to 1kHz) chart above, the M66 is essentially perfectly flat within the audioband (0dB at 20Hz and 20kHz). At the extremes, the M66 is 0dB at 5Hz, and -0.2dB at 200kHz. These data corroborate NAD’s claim of ±0.2dB (20Hz-80kHz). With Analog Direct mode, the M66 can be considered an extremely wide-bandwidth audio device. The M66 also appears to be DC-coupled, as there is no attenuation at low frequencies, even at 5Hz. 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.

**Frequency response (line-level input, subwoofers active)**

Above are the frequency-response (relative to 1kHz and 20Hz) plots for the M66 balanced line-level outputs (red/blue) with two subwoofers (purple/green) engaged and the crossover set to 120Hz. In order to implement bass management, Analog Direct must be turned off to enable DSP functions. The line-level outputs show sharp attenuation just past 30kHz, with a -3dB point just past 40kHz, suggesting that the M66 samples incoming analog signals at 96kHz. The high- and low-pass slopes appear to be second-order (12dB/octave), with the crossover point at -6dB. The subwoofer outputs are perfectly flat down to 5Hz.

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

Above is the phase response plot from 20Hz to 20kHz for the balanced line-level input. The M66 does not invert polarity and exhibits, at worst, less than -5 degrees (at 20kHz) of phase shift within the audioband.

**Frequency response vs. input type (left channel only)**

The chart above shows the M66’s frequency response (relative to 1kHz) as a function of input type. The green trace is the same (but limited to 80kHz) analog input data from the first frequency- response graph. The blue trace is for a 16-bit/44.1kHz dithered digital input signal from 5Hz to 22kHz, the purple trace is for a 24/96 dithered digital input signal from 5Hz to 48kHz, and finally pink is 24/192 from 5Hz to 96kHz—all using the coaxial input. The behavior at low frequencies is the same for all the digital sample rates, as well as the analog input: 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). All three digital data plots show “brick-wall”-type behavior, with-3dB points at 21.8kHz (16/44.1), 47kHz (24/96), and 94kHz (24/192).

**Frequency response (MM input)**

The chart above shows the frequency response (relative to 1 kHz) for the MM phono input. What is shown is the deviation from the RIAA curve, where the input signal sweep is EQ’d with an inverted RIAA curve supplied by Audio Precision (i.e., zero deviation would yield a flat line at 0dB). The chart shows extremely small maximum deviations within the audioband: 0dB at 20Hz and about +0.2dB at 20kHz. This is an example of exceptional RIAA tracking implemented in the analog domain (Analog Direct is turned on). At the extremes, there is sharp attenuation at low frequencies (-3dB at 5Hz) and a rise at high frequencies (+0.5dB at 50kHz).

**Frequency response (MC input)**

The chart above shows the frequency response for the MC phono input. We see essentially the same result as with the MM configuration.

**Phase response (MM and MC phono inputs)**

Above is the phase-response plot from 20Hz to 20kHz for the phono input (MM and MC behaved identically). For the phono input, since the RIAA equalization curve must be implemented, which ranges from +19.9dB (20Hz) to -32.6dB (90kHz), phase shift at the output is inevitable. Here we find a worst case of about -60 degrees at 200Hz and -90 degrees at 20kHz.

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

The chart 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 balanced outputs of the M66. 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 would be a straight flat line at 0dB. At -120dBFS, the 16/44.1 data overshot by only 1-2dB, while the 24/96 remained perfect. To verify how well the 24/96 data would perform down to -140dBFS, we extended the sweep in the chart below.

Here we can see that the 24/96 data only undershot the mark by 2dB (left/right) at -140/-130dBFS. This is an exceptional digital linearity-test result.

**Impulse response (24/48 data)**

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, measured at the balanced outputs of the M66. We can see that the M66 utilizes a reconstruction filter that favors no pre-ringing.

**J-Test (coaxial input)**

The chart above shows the results of the J-Test test for the coaxial digital input measured at the line-level pre-outputs of the M66 where 0dBFS is set to 1Vrms. J-Test was developed by Julian Dunn the 1990s. It is a test signal—specifically, a -3dBFS undithered 12kHz squarewave sampled (in this case) at 48kHz (24 bits). Since even the first odd harmonic (i.e., 36kHz) of the 12kHz squarewave 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 squarewave 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 and below. The test file can also be used in conjunction with artificially injected sinewave jitter by the Audio Precision, to show how well the DAC rejects jitter.

The coaxial S/PDIF input of the M66 shows a strong but not perfect J-Test result, with small peaks in the audioband at a low -135dBrA and below level.

**J-Test (optical input)**

The chart above shows the results of the J-Test test for the optical digital input measured at the balanced outputs of the M66. The results here are similar but not quite as strong as the coaxial input above. Here, the peaks reach the -125dBrA level.

**J-Test (AES-EBU input)**

The chart above shows the results of the J-Test test for the AES-EBU balanced digital input measured at the balanced outputs of the M66. The results here are very similar to those found on the coaxial input.

**J-Test (coaxial, 2kHz sinewave jitter at 10ns)**

The chart above shows the results of the J-Test test for the coaxial digital input measured at the line-level output of the M66, with an additional 10ns of 2kHz sinewave jitter injected by the APx555. The results are close to the same as the J-Test result without additional jitter. The same was true for the optical input. No peaks can be seen at the 10kHz and 14kHz positions.

**J-Test (coaxial, 2kHz sinewave jitter at 100ns)**

The chart above shows the results of the J-Test test for the coaxial digital input measured at the line-level output of the M66, with an additional 100ns of 2kHz sinewave jitter injected by the APx555. The tell-tale peaks at 10kHz and 14kHz still cannot be seen. This is a strong result showing that the M66 DAC has very good jitter immunity.

**Wideband FFT spectrum of white noise and 19.1kHz sinewave tone (coaxial input)**

The chart above shows a fast Fourier transform (FFT) of the M66’s balanced outputs with white noise at -4 dBFS (blue/red) and a 19.1 kHz sinewave at 0dBFS fed to the coaxial digital input, sampled at 16/44.1. The steep roll-off around 20kHz in the white-noise spectrum shows that the M66 uses a brick-wall-type reconstruction filter. There are no obvious aliased images within the audioband. The primary aliasing signal at 25kHz is highly suppressed at -70dBrA, while the second and third distortion harmonics (38.2, 57.3kHz) of the 19.1kHz tone are even lower.

**THD ratio (unweighted) vs. frequency vs. load (analog)**

The chart above shows THD ratios at the balanced line-level output into 200k ohms (blue/red) and 600 ohms (purple/green) as a function of frequency for the analog balanced inputs. The 200k-ohm THD data are lower than the 600-ohm data by 5-10dB from 20Hz to 200Hz. The 200k-ohm data range from 0.00015% at 20Hz, down to an astonishingly low 0.00003% at 200Hz, then a steady climb to 0.002% near 20kHz. It’s important to note that these THD values are only about twice as high as the AP’s signal generator, thereby pushing the limits of the analyzer’s capabilities.

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

The chart above shows THD ratios at the balanced line-level output into 200k ohms for a 16/44.1 (blue/red) dithered 1kHz signal at the coaxial input and a 24/96 (purple/green) signal, as a function of frequency. The 16/44.1 THD ratios were slightly higher (0.0003% to 0.0002%) than the 24/96 THD ratios, which were more of a constant 0.0002% across most of the audioband.

**THD ratio (unweighted) vs. frequency (phono input, MM and MC)**

The graph above shows THD ratio as a function of frequency plots for the phono input. The MM input is shown in blue/red (left/right channels) and MC in purple/green (left/right channels). The input sweep is EQ’d with an inverted RIAA curve. The THD values for the MM configuration vary from around 0.02% (20Hz) down to around 0.0003% (2kHz to 3kHz), then up to 0.0005% at 20kHz. The MC THD values were higher, although these are limited by the higher noise floor (the analyzer cannot assign a THD ratio for harmonic peaks it cannot see above the noise floor). THD ratios ranged from 0.3% (20Hz), down to 0.002% (2kHz to 20kHz). It should be pointed out that the higher THD ratios at low frequencies are also likely limited and driven by the higher noise floor.

**THD ratio (unweighted) vs. output (analog)**

The chart above shows THD ratios measured at the balanced outputs of the M66 as a function of output voltage for the balanced line-level input, with the volume control at maximum. THD values start at 0.03% at 1mVrms, down to a low of 0.00004% at 2-3Vrms, then a steep rise past 10Vrms to the 1% THD mark at 15.5Vrms.

**THD+N ratio (unweighted) vs. output (analog)**

The chart above shows THD+N ratios measured at the balanced outputs of the M66 as a function of output voltage for the balanced line level-input, with the volume control at maximum. THD+N values start at 0.4% at 1mVrms, down to a low of 0.00015% at 4-5Vrms, then a steep rise past 10Vrms to the 1% THD mark at 15.5Vrms.

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

The chart above shows THD ratios measured at the balanced outputs of the M66 as a function of output voltage for the digital coaxial S/PDIF input, swept from -90dBFS to 0dBFS, with the volume control at maximum. Blue/red traces are for 16/44.1 data, and purple/green for 24/96. For the 16/44.1 data, THD values start at 2%, and predictably, reach their low at the maximum output voltage of about 5Vrms, at 0.0003%. For the 24/96 data, THD ratios ranged from 0.1% down to 0.0001% near the maximum output voltage.

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

The chart above shows THD+N ratios measured at the balanced outputs of the M66 as a function of output voltage for the digital coaxial S/PDIF input, swept from -90dBFS to 0dBFS, with the volume control at maximum. Blue/red traces are for 16/44.1 data, and purple/green for 24/96. For the 16/44.1 data, THD+N values start at 20% and reach their low at the maximum output voltage of about 5Vrms, at 0.002%. For the 24/96 data, THD ratios ranged from 0.5% down to 0.0002% near the maximum output voltage.

**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 balanced output for 16/44.1 (blue/red) input data and 24/96 input data (purple/green), from -60dBFS to 0dBFS. Here, the SMPTE IMD method was 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 16/44.1 data yields IMD ratios from 2% down to 0.003% at 0dBFS. The 24/96 data yields IMD ratios from 0.1% down to 0.0005% at 0dBFS.

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

Shown above is the fast Fourier transform (FFT) for a 1kHz input sinewave stimulus, measured at the balanced outputs for the balanced line-level input. We see that the signal’s third harmonic, at 3kHz, is at -130dBrA, or 0.00003%. There are a multitude of high-order signal harmonics, but they are all at the extremely low -140dBrA, or 0.00001%, level. Below 1kHz, we see two small peaks from power-supply noise artifacts at 60Hz and 120Hz. These are extraordinarily low at around -150dBrA, or 0.000003%, and it must be highlighted that they are actually from the Audio Precision’s sinewave generator, and not inherent to the M66. We can say this with confidence for two reasons: these same peaks can be seen at roughly the same amplitudes when the Audio Precision’s sinewave generator is connected directly to the inputs of its analyzer (loopback), and these peaks are non-existent in the digital 24/96 FFT below, where the Audio Precision’s DAC generator is used instead of its analog sine wave generator. It should also be stressed how extraordinarily low the M66’s noise floor is. The M66 does not seem to have any correlated power-supply-related (60Hz and harmonics) noise (what we would describe as “hum”). The residual A-weighted noise from the M66 (volume control set to minimum with no signal) due to non-correlated thermal noise (what we would describe as “hiss”) was measured at 1.7uVrms, compared to the analyzer’s self-noise of 0.66 uVrms. FFTs don’t get much cleaner than this!

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

Shown above is the fast Fourier transform (FFT) for a 1kHz input sinewave stimulus, measured at the unbalanced outputs for the unbalanced line-level input. We see essentially the same result as with the balanced input FFT above.

**FFT spectrum – 1kHz (XLR line-level input, Analog Direct off)**

Shown above is the fast Fourier transform (FFT) for a 1kHz input sinewave stimulus, measured at the balanced outputs for the balanced line-level input, with Analog Direct turned off. Here we see signal harmonics at higher levels compared to when the signal path is purely analog. There are a multitude of signal harmonics ranging from -120dBrA, or 0.0001%, down to -140dBrA, or 0.00001%. We also see the IMD peaks between the 1kHz signal and the ADC’s 96kHz sample rate at 95 and 97kHz.

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

Shown above is the fast Fourier transform (FFT) for a 1kHz input sinewave stimulus, measured at the balanced outputs for the coaxial digital input, sampled at 16/44.1. We see the third (3kHz) signal harmonic dominate at roughly -120dBrA, or 0.0001%.

**FFT spectrum – 1kHz (digital input, 24/96 data at 0dBFS)**

Shown above is the fast Fourier transform (FFT) for a 1kHz input sinewave stimulus, measured at the balanced outputs for the coaxial digital input, sampled at 24/96. We see no power-supply-related noise peaks above the -155dBrA noise floor, and the third (3kHz) signal harmonic dominates at roughly -120dBrA, or 0.0001%. A multitude of signal harmonics can also be seen below this level down to -140dBrA, or 0.00001%.

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

Shown above is the FFT for a 1kHz -90dBFS dithered 16/44.1 input sinewave stimulus at the coaxial digital input, measured at the balanced outputs. We see the 1kHz primary signal peak, at the correct amplitude, and no signal or noise-related harmonic peaks above the -140dBrA noise floor.

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

Shown above is the FFT for a 1kHz -90dBFS dithered 24/96 input sinewave stimulus at the coaxial digital input, measured at the balanced outputs. We see the 1kHz primary signal peak, at the correct amplitude, and signal related harmonic peaks at 2kHz and 4kHz at and below -140dBrA, or 0.00001%.

**FFT spectrum – 1kHz (MM phono input)**

Shown above is the FFT for a 1kHz input sinewave stimulus, measured at the balanced outputs, for the MM phono input. It is very difficult to see any signal-related harmonics above the noise floor and the myriad of noise-related peaks, at -80dBrA, or 0.01%, and below. Given how clean the M66’s line-level analog FFT is, the MM phono FFT is not the cleanest and somewhat disappointing by comparison.

**FFT spectrum – 1kHz (MC phono input)**

Shown above is the FFT for a 1kHz input sinewave stimulus, measured at the balanced outputs, for the MC phono input. Again, it is very difficult to see any signal-related harmonics above the noise floor and the myriad of noise-related peaks, at -60dBrA, or 0.1%, and below. Once again, given how clean the M66’s line-level analog FFT is, the MC phono FFT is not the cleanest and is disappointing by comparison.

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

Shown above is the FFT for a 50Hz input sinewave stimulus measured at the balanced outputs for the balanced line-level input. The X axis is zoomed in from 40 Hz to 1kHz, so that peaks from noise artifacts can be directly compared against peaks from the harmonics of the signal. The most predominant (non-signal) peaks are that of the signal’s second (100Hz) harmonic at -120dBrA, or 0.0001%, and the third signal harmonic (150Hz) at -130dBrA or 0.00003%. Power-supply-related peaks can be seen at 60Hz (-140dBrA, or 0.00001%) and 120Hz (-150dBrA, or 0.000003%), but as discussed above, these are inherent to the Audio Precision sinewave generator.

**FFT spectrum – 50Hz (MM phono input)**

Shown above is the FFT for a 50Hz input sinewave stimulus measured at the balanced outputs for the MM phono input. There are a myriad of noise-related peaks at -80dBrA, or 0.01%, and below.

**FFT spectrum – 50Hz (MC phono input)**

Shown above is the FFT for a 50Hz input sinewave stimulus measured at the balanced outputs for the MC phono input. There are, again, numerious noise-related peaks at -60dBrA, or 0.01%, and below.

**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 balanced outputs for the balanced 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 -125dBRA, or 0.00006%, while the third-order modulation products, at 17kHz and 20kHz are at roughly the same level. This is a very clean IMD result.

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

Shown above is the FFT of the balanced outputs of the M66 with the APx 32-tone signal applied to the input. The combined amplitude of the 32 tones is the 0dBrA reference and corresponds to 2Vrms. The intermodulation products—i.e., the “grass” between the test tones—are distortion products from the preamplifier and are around the extremely low -150dBrA, or 0.000003%, level.

**Intermodulation distortion FFT (18kHz + 19kHz summed stimulus, coaxial digital input, 16/44.1)**

Shown above is an FFT of the intermodulation distortion (IMD) products for an 18kHz + 19kHz summed sinewave stimulus tone measured at the balanced outputs for the digital coaxial input at 16/44.1. We find that the second-order modulation product (i.e., the difference signal of 1kHz) cannot be seen above the -135dBrA noise floor, while the third-order modulation products, at 17kHz and 20kHz, are at -125dBrA, or 0.00006%. We also see the main aliased peaks at 25.1kHz and 26.1kHz at around -80dBrA.

**Intermodulation distortion FFT (18kHz + 19kHz summed stimulus, coaxial digital input, 24/96)**

Shown above is an FFT of the intermodulation distortion (IMD) products for an 18kHz + 19kHz summed sinewave stimulus tone measured at the balanced outputs for the digital coaxial input at 24/96. We find that the second-order modulation product (i.e., the difference signal of 1kHz) is at -135dBrA, or 0.00002%, while the third-order modulation products, at 17kHz and 20kHz, are at -125dBrA or 0.00006%.

**Intermodulation distortion FFT (18kHz + 19kHz summed stimulus, MM phono input)**

Shown above is an FFT of the intermodulation distortion (IMD) products for an 18kHz + 19kHz summed sinewave stimulus tone measured at the balanced outputs across for the MM phono input. We find it difficult to distinguish the second-order and third-order IMD peaks amongst all the noise-related peaks.

**Intermodulation distortion FFT (18kHz + 19kHz summed stimulus, MC phono input)**

Shown above is an FFT of the intermodulation distortion (IMD) products for an 18kHz + 19kHz summed sinewave stimulus tone measured at the balanced outputs across for the MC phono input. Once again, we find it difficult to distinguish the second-order and third-order IMD peaks amongst all the noise-related peaks.

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

Above is the 10kHz squarewave response using the analog line-level input, at roughly 2Vrms. Due to limitations inherent to the Audio Precision APx555 B Series analyzer, this graph should not be used to infer or extrapolate the M66’s slew-rate performance. Rather, it should be seen as a qualitative representation of its extended 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. The M66 reproduction of the 10kHz square wave is very clean, with no softening in the edges.

*Diego Estan*

Electronics Measurement Specialist