Link: reviewed by Phil Gold on *SoundStage! Hi-Fi* on July 15, 2023

**General Information**

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

The Musical Fidelity M6x was conditioned for 30 minutes at 0dBFS (4Vrms out) into 200k ohms before any measurements were taken.

The M6x offers five digital inputs: one coaxial S/PDIF (RCA), two optical S/PDIF (TosLink), one AES/EBU (XLR), and one USB. There are two line-level outputs (balanced XLR and unbalanced RCA) and one headphone output (1/4″ TRS). There is a digital volume control that can be engaged for both the line-level outputs and headphone output, but was left in the Fix (fixed) default setting for all measurements, with the exception of the volume tracking table. Comparisons were made between unbalanced and balanced line-level outputs for a 24/96 0dBFS input, and aside from the 6dB extra voltage using balanced, there were no differences in THD+N. In terms of input types (USB, coaxial, optical), there were no differences in terms of THD+N at 24/96.

There is a button labeled Upsample on the M6x, which, when engaged, upsamples incoming PCM data up to 192kHz to 352.8 or 384kHz (using integer multiples of the incoming sample rate). There are eight filter settings labeled 1 through 8. All measurements below, unless otherwise stated, are for the coaxial digital input, and the balanced output, using filter 1. The eight filters are described as follows in the M6x manual:

- Linear phase fast roll off: most common filter with clean overall suppression and excellent rejection, best for music with large transients. Provides clean, crisp highs.
- Linear phase slow roll off: low group delay – and symmetrical input response. Less ringing than linear-phase fast roll-off (LPFR). Punchier bass than LPFR, with clean highs.
- Minimum phase fast roll off: minimal pre-ringing preferred for imaging and soundstage. No aliasing in frequency domain. Stronger bass than linear phase, clean highs.
- Minimum phase slow roll off: non-symmetrical filter designed to minimize pre-ringing. Strong punchy bass with good transient attacks.
- Apodizing fast roll off: a version of linear-phase fast roll off filter optimized to improve pre-ringing.
- Hybrid fast roll off: a combination of linear-phase and minimum phase. Fast transient attack, strong punchy bass, crisp highs.
- Brick wall: one of the earliest designs, intended for highest suppression possible, with high delay and pre-ringing. Linear phase, crisp clean highs.
- Oversampling bypass: the oversampling FIR filter, used for the 7 above mentioned presets, is bypassed and source data is up-sampled to 352.8/384kHz.

Note: it seemed clear comparing frequency, phase, and impulse responses for 16-bit/44.1kHz input data, as well as wideband noise FFTs, between filter 8 and any other filter with “Upsample” engaged, yielded the exact same results.

The M6x volume control has no indicator on the front panel. The volume control can be engaged by pressing the Output button on the front panel for 2 to 3 seconds to change from Fix (fixed) to Var (variable) output. When headphones are plugged in, Var is automatically selected. For a 0dBFS 1kHz input signal using the full range of the volume control will yield from a minimum of about 0.1mVrms (-90dB) to 4.1Vrms (0dB) in 1dB steps at the balanced line-level outputs, and the headphone outputs. The volume control operates in the digital domain, as every step was exactly 1dB, and the channel-to-channel deviation was exactly 0.105dB at every step, throughout the range.

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

Volume position | Channel deviation |

min | 0.105dB |

7 o’clock | 0.105dB |

9 o’clock | 0.105dB |

12 o’clock | 0.105dB |

3 o’clock | 0.105dB |

4 o’clock | 0.105dB |

max | 0.105dB |

**Published specifications vs. our primary measurements**

The table below summarizes the measurements published by Musical Fidelity for the M6x DAC compared directly against our own. The published specifications are sourced from Musical Fidlelity’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 set at its maximum (DC to 1MHz), assume, unless otherwise stated, the coaxial digital input (24/96 1kHz sinewave at 0dBFS), the balanced line-level or unbalanced headphone outputs into 200k ohms (line-level) and 300 ohms (headphone) 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 (RCA/XLR) | <10 ohms | 1 ohm |

Linearity (16/44.1) | <±0.1dB to -96dB | <±0.1dB to -96dB |

Frequency response (16/44,1, Filter 1) | -0.1dB@10Hz, -0.4dB@20kHz | 0dB@10Hz, -0.35dB@20kHz |

Channel separation (10kHz, 24/96@0dBFS) | <-130dB | -153dB |

Signal-to-noise ratio (A-weighted, 1kHz, 24/96@0dBFS) | >120dB | 128dB |

THD (1kHz, 24/96@0dBFS) | <0.0005% | 0.00024% |

Headphone maximun output power (1%THD, 32 ohms) | 1.5W | 0.1W |

Headphone output impedance | <5 ohms | 39.8 ohms |

Headphone THD (1kHz, 24/96@0dBFS) | <0.005% | 0.00043% |

Headphone signal-to-noise ratio (1kHz, 24/96@0dBFS) | >115dB | 122.1dB |

Headphone frequency response (16/44.1, Filter 1) | +0.1dB@20Hz, -0.4dB@20kHz | 0dB@20Hz, -0.35dB@20kHz |

Our primary measurements revealed the following using the coaxial input and the balanced line-level outputs (unless specified, assume a 1kHz sinewave at 0dBFS, 200k ohms loading, 10Hz to 90kHz bandwidth):

Parameter | Left channel | Right channel |

Crosstalk, one channel driven (10kHz, 16/44.1) | -118.0dB | -117.7dB |

Crosstalk, one channel driven (10kHz, 24/96) | -152.7dB | -155.8dB |

DC offset | <1.3mV | <0.18mV |

Dynamic range (A-weighted, 16/44.1) | 96.0dB | 96.0dB |

Dynamic range (unweighted, 16/44.1) | 93.7dB | 93.6dB |

Dynamic range (A-weighted, 24/96) | 128.3dB | 128.1dB |

Dynamic range (unweighted, 24/96) | 118.1dB | 118.0dB |

IMD ratio (CCIF, 18kHz + 19kHz stimulus tones, 1:1, 16/44.1) | <-105dB | <-105dB |

IMD ratio (CCIF, 18kHz + 19kHz stimulus tones, 24/96) | <-110dB | <-112dB |

IMD ratio (SMPTE, 60Hz + 7kHz stimulus tones, 4:1, 16/44.1) | <-92dB | <-92dB |

IMD ratio (SMPTE, 60Hz + 7kHz stimulus tones, 4:1, 24/96 ) | <-101dB | <-103dB |

Maximum output voltage (0dBFS) | 4.176Vrms | 4.126Vrms |

Output impedance (XLR) | 1.0 ohm | 0.9 ohm |

Output impedance (RCA) | 0.9 ohm | 0.9 ohm |

Noise level (A-weighted, 16/44.1) | <65uVmrs | <65uVmrs |

Noise level (unweighted, 16/44.1) | <85uVmrs | <85uVmrs |

Noise level (A-weighted, 24/96) | <2.9uVrms | <2.9uVrms |

Noise level (unweighted, 24/96) | <8.7uVrms | <8.6uVrms |

THD ratio (unweighted, 16/44.1) | <0.00044% | <0.00044% |

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

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

THD ratio (unweighted, 24/96) | <0.00024% | <0.00024% |

THD+N ratio (A-weighted, 24/96) | <0.00027% | <0.00027% |

THD+N ratio (unweighted, 24/96) | <0.00032% | <0.00032% |

Our primary measurements revealed the following using the coaxial input and the headphone output (unless specified, assume a 1kHz sinewave at 0dBFS input, 2Vrms into 300 ohms, 10Hz to 90kHz bandwidth):

Parameter | Left channel | Right channel |

Maximum Vrms/0dBFS | 4.171Vrms | 4.121Vrms |

Maximum output power into 600 ohms | 25.5mW | 24.9mW |

Maximum output power into 300 ohms | 45.2mW | 44.1mW |

Maximum output power into 32 ohms | 102.0mW | 100.3mW |

Output impedance | 39.8 ohms | 39.8 ohms |

Noise level (A-weighted, 16/44.1) | <32uVrms | <32uVrms |

Noise level (unweighted, 16/44.1) | <43uVrms | <43uVrms |

Noise level (A-weighted, 24/96) | <3.5uVrms | <3.5uVrms |

Noise level (unweighted, 24/96) | <9.8uVrms | <9.6uVrms |

Dynamic range (A-weighted, 16/44.1, max volume) | 96.2dB | 96.2dB |

Dynamic range (A-weighted, 24/96, max volume) | 122.5dB | 122.4dB |

THD ratio (unweighted, 16/44.1) | <0.00055% | <0.00055% |

THD+N ratio (A-weighted, 16/44.1) | <0.0017% | <0.0017% |

THD+N ratio (unweighted, 16/44.1) | <0.0022% | <0.0022% |

THD ratio (unweighted, 24/96) | <0.00043% | <0.00043% |

THD+N ratio (A-weighted, 24/96) | <0.00050% | <0.00050% |

THD+N ratio (unweighted, 24/96) | <0.00063% | <0.00063% |

**Frequency response vs. sample rate (16/44.1, 24/96, 24/192; filter 1)**

The plot above shows the M6x frequency response (relative to 1kHz) 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 digital input—perfectly flat down to 5Hz. The behavior at high frequencies for all three digital sample rates is as expected, offering sharp filtering around 22, 48, and 96kHz (half the respective sample rate for each). The -3dB point for each sample rate is roughly 21, 45.7, and 70.7kHz, respectively. It is also obvious from the plots above that the 44.1kHz sampled input signal offers the most brickwall-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.

**Frequency response vs. filter type (16/44.1, filters 1 to 4)**

The plots above show frequency-response for a 16/44.1 input, for filters 1 through 4, into a 200k ohm-load, for the left channel only. Filter 1 is in red, filter 2 in purple, filter 3 in green, and filter 4 in blue. The graph is zoomed in from 1kHz to 22kH, to highlight the various responses of the filters around the “knee” of the response. At 20kHz, filter 1 is at -0.35dB, filter 2 is at -3.78dB, filter 3 is at -0.38dB, and filter 4 is at -5.33dB.

Note: the filter characteristics are described under the General information section above. Our measured frequency responses match the descriptions provided by Musical Fidelity.

**Phase response vs. filter type (16/44.1, filters 5 to 8)**

The plots above show frequency-response for a 16/44.1 input, for filters 5 through 8, into a 200k ohm-load, for the left channel only. Filter 5 is in red, filter 6 in purple, filter 7 in green, and filter 8 in blue. The graph is zoomed in from 1kHz to 22kHz to highlight the various responses of the filters around the “knee” of the response. At 20kHz, filter 5 is at -0.15dB, filter 6 is at -12.60dB, filter 7 is at 4.31dB, and filter 8, which is 16/44.1 input data up-sampled to 352.8kHz and the ESS DAC oversampling filter disabled, is at -0.59dB. Of note, filter 5 yields up/down deviations in the frequency response by almost 0.5dB nearing 20kHz.

Note: the filter characteristics are described under General Information section above. Our measured frequency responses generally match the descriptions provided by Musical Fidelity.

**Phase response vs. sample rate (16/44.1, 24/96, 24/192; filter 1)**

Above are the absolute phase response plots (including group delay) from 20Hz to 20kHz for the coaxial input, measured at the balanced output for the left channel only. The blue/red traces are for a dithered 16/44.1 input at -20dBFS, the purple/green for 24/96, and the orange/pink for 24/192. We can see that the M6x does not invert polarity, with a worst-case phase shift of just under 80 degrees at 20kHz for the 16/44.1, and phase shift just above and below 20 degrees at 20kHz for the 24/96 and 24/192 input data, respectively.

**Phase response vs. filter type (16/44.1, filters 1 to 4)**

Above are the absolute phase response plots (including group delay) plots from 20Hz to 20kHz, for a 16/44.1 signal at the coaxial input, measured at the balanced output, for filters 1 through 4, into a 200k ohm-load, for the left channel only. Filter 1 is in blue, filter 2 in purple, filter 3 in orange, and filter 4 in green. We find the general trend of better or less severe phase shift for filters that have poorer frequency response (*e.g.*, more attenuation at 20kHz), and vice-versa.

**Phase response vs. filter type (16/44.1, filters 5 to 8)**

Above are the absolute phase response plots (including group delay) from 20Hz to 20kHz for a 16/44.1 signal at the coaxial input, measured at the balanced output, for filters 5 through 8, into a 200k ohm-load, for the left channel only. Filter 5 is in blue, filter 6 in purple, filter 7 in orange, and filter 8 in green. We find the general trend of better or less severe phase shift for filters that have poorer frequency response (*e.g.*, more attenuation at 20kHz), and vice-versa.

**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 balanced 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 perfectly linear down to -120dBFS, while the 16/44.1 data was perfect down to -100dBFS, and only +3dB (left) and +1dB (right) at -120dBFS. Because of this DAC’s exceptional linearity performance, a second measurement was performed extending down to -140dBFS . . .

. . . that shows the 24/96 data remained within 0.4dB of flat, which is a phenomenal result. The 16/44.1 data results are as expected, showing significant deviations below -120dBFS. It is worth highlighting that a linearity result that is flat down to -100dBFS for 16-bit input data is also exceptional.

**Impulse response vs. filter type (24/44.1, filters 1 to 4)**

The graph above shows the impulse responses for the first four filter types, 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. Filter 1 is in blue, filter 2 in purple, filter 3 in red, and filter 4 in green.

Note: the filter characteristics are described under General information above. Our measured impulse responses generally match the descriptions provided by Musical Fidelity.

**Impulse response vs. filter type (24/44.1, filters 5 to 8)**

The graph above shows the impulse responses for the first 4 filter types, 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. Filter 5 is in blue, filter 6 in purple, filter 7 in red, and filter 8 in green.

Note: the filter characteristics are described under General Information above. Our measured impulse responses generally match the descriptions provided by Musical Fidelity.

**J-Test (coaxial input)**

The plot above shows the results of the J-Test test for the coaxial digital input measured at the balanced line-level output of the M6x. 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 (fundamental at 250Hz) down to -170dBrA for the odd harmonics. The test file can also be used in conjunction with artificially injected sinewave jitter by the Audio Precision, to also show how well the DAC rejects jitter.

The coaxial SPDIF input shows some of the alternating 500Hz peaks in the audioband but at very low levels, below -140dBrA, with only a few other peaks visible near -150dBrA. This is an indication that the M6x 200M should not be sensitive to jitter.

**J-Test (optical input)**

The optical S/PDIF input shows essentially the same result as the coax input. This is an indication that the M6x should not be sensitive to jitter. Both the coaxial and optical inputs were also tested for jitter immunity by injecting artificial sinewave jitter at 2kHz on top of the J-Test 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 sinewave tone (filter 1)**

The plot above shows a fast Fourier transform (FFT) of the M6x balanced line-level output with white noise at -4dBFS (blue/red), and a 19.1kHz sinewave at 0dBFS fed to the coaxial digital input, sampled at 16/44.1 (purple/green), using filter 1. The sharp roll-off above 20kHz in the white-noise spectrum shows the implementation of the brickwall-type reconstruction filter. There are absolutely no imaged aliasing artifacts in the audioband above the -135dBrA noise floor. The primary aliasing signal at 25kHz is at -115dBrA, with subsequent harmonics of the 25kHz peak at or below this level.

Note: the filter characteristics are described under General Information above. Our measured wideband FFT spectrum of white noise and 19.1kHz sinewave tone, generally match the descriptions provided by Musical Fidelity.

**Wideband FFT spectrum of white noise and 19.1kHz sinewave tone (filter 2)**

The plot above shows a fast Fourier transform (FFT) of the M6x balanced line-level output with white noise at -4dBFS (blue/red), and a 19.1kHz sinewave at 0dBFS fed to the coaxial digital input, sampled at 16/44.1 (purple/green), using filter 2. We see a slower roll-off in the white-noise spectrum compared to filter 1, and slight attenuation of the 19.1kHz signal peak. There are absolutely no imaged aliasing artifacts in the audioband above the -135dBrA noise floor. The primary aliasing signal at 25kHz is at -30dBrA, with subsequent harmonics of the 25kHz peak at or below this level.

Note: the filter characteristics are described under General Information above. Our measured wideband FFT spectrum of white noise and 19.1kHz sinewave tone, generally match the descriptions provided by Musical Fidelity.

**Wideband FFT spectrum of white noise and 19.1kHz sinewave tone (filter 3)**

The plot above shows a fast Fourier transform (FFT) of the M6x balanced line-level output with white noise at -4dBFS (blue/red), and a 19.1kHz sinewave at 0dBFS fed to the coaxial digital input, sampled at 16/44.1 (purple/green), using filter 3. The sharp roll-off above 20kHz in the white-noise spectrum shows the implementation of the brickwall-type reconstruction filter. There are absolutely no imaged aliasing artifacts in the audio band above the -135dBrA noise floor. The primary aliasing signal at 25kHz is at -110dBrA, with subsequent harmonics of the 25kHz peak at Ir below this level.

Note: the filter characteristics are described under General information above. Our measured wideband FFT spectrum of white noise and 19.1kHz sinewave tone, generally match the descriptions provided by Musical Fidelity.

**Wideband FFT spectrum of white noise and 19.1kHz sinewave tone (filter 4)**

The plot above shows a fast Fourier transform (FFT) of the M6x balanced line-level output with white noise at -4dBFS (blue/red), and a 19.1kHz sinewave at 0dBFS fed to the coaxial digital input, sampled at 16/44.1 (purple/green), using filter 4. We see a slower roll-off in the white noise spectrum compared to filter 1, and slight attenuation of the 19.1kHz signal peak. There are absolutely no imaged aliasing artifacts in the audioband above the -135dBrA noise floor. The primary aliasing signal at 25kHz is at -30dBrA, with subsequent harmonics of the 25kHz peak at or below this level.

Note: the filter characteristics are described under General Information above. Our measured wideband FFT spectrum of white noise and 19.1kHz sinewave tone, generally match the descriptions provided by Musical Fidelity.

**Wideband FFT spectrum of white noise and 19.1kHz sinewave tone (filter 5)**

The plot above shows a fast Fourier transform (FFT) of the M6x balanced line-level output with white noise at -4dBFS (blue/red), and a 19.1kHz sinewave at 0dBFS fed to the coaxial digital input, sampled at 16/44.1 (purple/green), using filter 5. The sharp roll-off above 20kHz in the white-noise spectrum shows the implementation of the brickwall-type reconstruction filter. There are absolutely no imaged aliasing artifacts in the audio band above the -135dBrA noise floor. The primary aliasing signal at 25kHz is at -100dBrA, with subsequent harmonics of the 25kHz peak at or below this level.

**Wideband FFT spectrum of white noise and 19.1kHz sinewave tone (filter 6)**

The plot above shows a fast Fourier transform (FFT) of the M6x balanced line-level output with white noise at -4dBFS (blue/red), and a 19.1kHz sinewave at 0dBFS fed to the coaxial digital input, sampled at 16/44.1 (purple/green), using filter 6. The sharp roll-off above 20kHz in the white-noise spectrum shows the implementation of the brick-wall type reconstruction filter. There are absolutely no imaged aliasing artifacts in the audioband above the -135dBrA noise floor. The primary aliasing signal at 25kHz is at -110dBrA, with subsequent harmonics of the 25kHz peak at or below this level.

**Wideband FFT spectrum of white noise and 19.1kHz sinewave tone (filter 7)**

The plot above shows a fast Fourier transform (FFT) of the M6x balanced line-level output with white noise at -4dBFS (blue/red), and a 19.1kHz sinewave at 0dBFS fed to the coaxial digital input, sampled at 16/44.1 (purple/green), using filter 7. The sharp roll-off above 20kHz in the white noise spectrum shows the implementation of the brickwall-type reconstruction filter. There are absolutely no imaged aliasing artifacts in the audio band above the -135dBrA noise floor. The primary aliasing signal at 25kHz is at -105dBrA, with subsequent harmonics of the 25kHz peak at or below this level.

**Wideband FFT spectrum of white noise and 19.1kHz sinewave tone (filter 8)**

The plot above shows a fast Fourier transform (FFT) of the M6x balanced line-level output with white noise at -4dBFS (blue/red), and a 19.1kHz sinewave at 0dBFS fed to the coaxial digital input, sampled at 16/44.1 (purple/green), using filter 8, which bypasses the ESS DAC over-sampling filter and engages up-sampling at 352.8kHz. With the up-sampling engaged, high frequency signals exhibit digital clipping at 0dBFS, which explains the all of the harmonics seen in the plot above.

**Wideband FFT spectrum of white noise and 19.1kHz sinewave tone (filter 8, -2dBFS)**

The plot above shows a fast Fourier transform (FFT) of the M6x balanced line level output with white noise at -4dBFS (blue/red), and a 19.1kHz sinewave at -2dBFS (to avoid digital clipping) fed to the coaxial digital input, sampled at 16/44.1 (purple/green), using filter 8, which bypasses the ESS DAC over-sampling filter and engages up-sampling at 352.8kHz. The very slow roll-off above 20kHz in the white-noise spectrum shows the lack of a reconstruction oversampling filter. There are nonetheless, absolutely no imaged aliasing artifacts in the audio band above the -135dBrA noise floor. The primary aliasing signal at 25kHz is at -20dBrA, with subsequent harmonics of the 25kHz peak at or below this level.

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

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 a 24/96 dithered 1kHz signal at the coaxial input. The 200k and 600 ohms data are nearly identical from 100Hz to 5kHz, hovering around a very low 0.0003%. At the frequency extremes, THD increased into 600 ohms vs 200k ohms, where we see 0.0005% vs. 0.0003% at 20Hz, and 0.001% vs. 0.0003% at around 20kHz.

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

The chart above shows THD ratios at the balanced line-level output into 200k 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. The 24/96 data consistently outperformed the 16/44.1 data by about 3-4dB, up to 2kHz, above which, both data sets performed identically. THD values for the 24/96 data were either just above, or just below, the very low threshold of 0.0002%.

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

The chart above shows THD ratios measured at the balanced output as a function of output voltage for the coaxial input into 200k ohms from -90dBFS to 0dBFS with 16/44.1 (blue/red) and 24/96 (purple/green) dithered input data. Once again, the 24/96 outperformed the 16/44.1 data, with a THD range from 0.2% to nearly 0.0001% at just over 1Vrms, while the 16/44.1 ranged from 5% down to 0.0004% at 4.1Vrms. 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).

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

The chart above shows THD+N ratios measured at the balanced output as a function of output voltage for the coaxial input into 200k 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 4% down to 0.0004%, while the 16/44.1 ranged from 50% down to 0.002% at the maximum output voltage of 4.1Vrms.

**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 balanced output as a function of generator input level for the coaxial input into 200k 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 0.05% down to 0.0005% between -10 and -5dBFS, then up to about 0.0007% at 0dBFS, while the 16/44.1 ranged from 2% down to 0.002% at the maximum output voltage of 4.1Vrms at 0dBFS.

**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 output into 200k ohm for the coaxial digital input, sampled at 16/44.1. We see the third signal harmonic (3kHz) at -115dBrA, or 0.0002%, and subsequent odd harmonics (3, 5, 7, 9kHz) at levels below -120dBrA, or 0.0001%. No even signal harmonics are visible in the audioband above the -135dBrA noise floor. There are also 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 sinewave stimulus, measured at the balanced output into 200k ohm for the coaxial digital input, sampled at 24/96. Due to the increased bit-depth, the noise floor is much lower compared to the 16/44.1 FFT at a very low -160dBrA. We see signal harmonics ranging from -115dBrA, or 0.0002%, at 3kHz, down to -150dBrA. With the lower noise floor, we can see even order harmonics, for example at 2kHz where the peaks (left/right) are just above and below -140dBrA, or 0.00001%. Here also, 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 sinewave stimulus, measured at the balanced output into 200k 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 audioband.

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

Shown above is the fast Fourier transform (FFT) for a 1kHz input sinewave stimulus, measured at the balanced output into 200k ohm for the coaxial digital input, sampled at 24/96 at -90dBFS. We see the main peak at the correct amplitude, and hint of signal harmonic peaks within the audioband at a vanishingly low -160dBrA, or 0.000001%.

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

Shown above is an FFT of the intermodulation distortion (IMD) products for a 18kHz + 19kHz summed sinewave stimulus tone measured at the balanced output into 200k 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 4.1Vrms (0dBrA) at the output. We find that the second-order modulation product (*i.e.*, the difference signal of 1kHz) is at -130dBRA, or 0.00003%, just barely peaking above the noise floor, while the third-order modulation products, at 17kHz and 20kHz, are slightly higher, reaching -125dBrA, or 0.00006%, for the left channel (the right channel peaks are barely perceptible above the -135dBrA noise floor).

**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 sinewave stimulus tone measured at the balanced output into 200k 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 4.1Vrms (0dBrA) at the output. We find that the second-order modulation product (*i.e.*, the difference signal of 1kHz) is just below -130dBRA, or 0.00003%, while the third-order modulation products, at 17kHz and 20kHz are slightly higher, right around -130dBrA.

*Diego Estan*

Electronics Measurement Specialist