Musical Fidelity M6si Integrated Amplifier-DAC

Link: reviewed by Dennis Burger on SoundStage! Access on August 1, 2021

General Information

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

The M6si 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 M6si offers several line-level analog inputs (XLR and RCA); one pair of phono RCA inputs, with a switch for moving-magnet (MM) and moving-coil (MC) operation; RCA pre-amp outputs; one USB digital input; and a pair of speaker-level outputs. For the purposes of these measurements, the following inputs were evaluated: digital USB, analog balanced (XLR) and unbalanced (RCA) line-level, as well as phono (RCA, configured both for MM and MC). Comparisons were made between unbalanced (RCA) and balanced line-level inputs, and no differences were seen in terms of THD+N and gain. Most line-level analog input measurements were made using the balanced XLR inputs.

Most measurements, with the exception of signal-to-noise ratio (SNR), or otherwise stated, for the balanced line-level analog input were made with the volume set to unity gain (0dB) on the volume control (roughly 1 o’clock) with respect to the pre-amp outputs (which offers 12dB of gain). At this volume position, to achieve 10W into 8 ohms, 260mVrms was required at the balanced line-level input. For the digital inputs, a volume position of between 10 and 11 o’clock yielded 10W into 8 ohms with a 0dBFS input. For the phono input, configured for MM, with the volume position at unity, 2.8mVrms at 1kHz at the input yielded 10W into 8 ohms. Configured for MC, 0.435mVrms at 1kHz at the input yielded 10W into 8 ohms. The SNR measurements were made with the volume control set to maximum, and the dynamic range measurements were made with the volume set to roughly 3 o’clock, which yielded 1% THD at the output into 8 ohms for a 0dBFS input.

Based on the accuracy of the left/right volume channel matching (see table below), the M6si volume control is likely digitally controlled in the analog domain. The M6si offers 0.5dB volume steps, ranging from -68dB to +42.8dB (analog line-level input).

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

Published specifications vs. our primary measurements

The table below summarizes the measurements published by Musical Fidelity for the M6si compared directly against our own. The published specifications are sourced from Musical Fidelity’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 is set at its maximum (DC to 1MHz), assume, unless otherwise stated, 10W into 8ohms and a measurement input bandwidth of 10Hz to 90kHz, and the worst-case measured result between the left and right channel.

*203W with one channel driven
**103dB with volume at unity gain

Our primary measurements revealed the following using the balanced line-level analog input and digital USB input (unless specified, assume a 1kHz sinewave at 260mVrms or 0dBFS, 10W output, 8-ohm loading, 10Hz to 90kHz bandwidth):

For the continuous dynamic power test, the M6si was able to sustain 190W into 8 ohms using an 80Hz tone for 500ms, alternating with a signal at -10dB of the peak (19W) 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 sides of the M6si were warm to the touch, but did not cause discomfort to the touch.

Our primary measurements revealed the following using the phono-level input, MM configuration (unless specified, assume a 1kHz sinewave, 10W output, 8-ohm loading, 10Hz to 90kHz bandwidth):

Our primary measurements revealed the following using the phono-level input, MC configuration (unless specified, assume a 1kHz sinewave, 10W output, 8-ohm loading, 10Hz to 90kHz bandwidth):

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

In our frequency-response plot above, measured across the speaker outputs at 10W into 8 ohms, the M6si is nearly flat within the audioband (20Hz to 20kHz). At the extremes the M6si is -0.5dB down at 10Hz and at 100kHz. These data only half corroborate Musical Fidelity’s claim of 10Hz to 20kHz (0/-0.1dB). Still, the M6si can be considered a high-bandwidth audio device. 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 optimally matched.

Phase response (8-ohm loading, line-level input)

Above is the phase response plot from 20Hz to 20kHz for the balanced line-level input, measured across the speaker outputs at 10W into 8 ohms. The M6si does not invert polarity and exhibits, at worst, 20 degrees (at 20Hz) of phase shift within the audioband.

Frequency response vs. input type (8-ohm loading, left channel only)

The chart above shows the M6si’s frequency response as a function of input type measured across the speaker outputs at 10W into 8 ohms. The green trace is the same analog input data from the previous graph. The blue trace is for a 24bit/44.1kHz dithered digital input signal from 5Hz to 22kHz using the USB input, and the purple trace is for a 24/96 dithered digital input signal from 5Hz to 48kHz. The M6si USB input does not support a 24/192 input sample rate. The behavior at low frequencies is the same for both digital sample rates: -0.6dB at 20Hz. The behavior at high frequencies for both digital sample rates is, as expected, offering filtering around 22kHz and 48kHz (half the respective sample rate). The 44.1kHz sampled input signal does not exhibit the typical “brick-wall” type behavior found in many DACs, with a -3dB point at 20.7kHz. The -3dB point for the 96kHz sampled data is at 30.6kHz. Curiously, the 24/96 sampled data displays the same early roll-off as the 24/44.1 data between 5kHz and 20kHz (e.g. -1dB at 16kHz).

Frequency response (8-ohm loading, MM phono input)

The chart above shows the frequency response for the phono input (MM configuration). What is represented 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). We can see small maximum deviations of about -0.5/-0.7dB (20Hz/20kHz) from 20Hz to 20kHz.

Frequency response (8-ohm loading, MC phono input)

The chart above shows the frequency response for the phono input (MC configuration). As with the MM chart, 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). Here we can see maximum deviations of about -1.5/-0.7dB (20Hz/20kHz) from 20Hz to 20kHz.

Phase response (MM input)

Above is the phase response plot from 20Hz to 20kHz for the phono input (MM configuration), measured across the speaker outputs at 10W into 8 ohms. The M6si does not invert polarity. 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 of phase shift at 200Hz and 5kHz.

Phase response (MC input)

Above is the phase response plot from 20Hz to 20kHz for the phono input (MC configuration), measured across the speaker outputs at 10W into 8 ohms. Once again, the M6si does not invert polarity, and as with the MM result above, here we find a worst case of about -60 degrees of phase shift at 200Hz and 5kHz.

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

The chart above shows the results of a linearity test for the USB digital input for both 24/44.1 (blue/red) and 24/96 (purple/green) input data, measured at the line-level output of the M6si. 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. Both digital input sample rates performed similarly, but somewhat poorly by modern DAC standards. They both approached the ideal 0dB relative level at -80 dBFS, then yielding perfect results to 0dBFS. At or near -120dBFS, however, both sample rates overshot by over 10dB.

Impulse response (24/44.1 data)

The graph above shows the impulse responses for a -20dBFS 24/44.1 dithered input stimulus, measured at the line-level output of the M6si, 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. The shape is that of a typical sinc function filter. Typically we show impulse responses generated by the Audio Precision Transfer Function measurement, which applies an inverse FFT to a noise signal applied to the DAC, for both 44.1 and 96kHz sampled data. In this case, noise issues with the USB input of the M6si yielded inconsistent results using the Transfer Function method, and therefore was not used.

J-Test (USB input)

The chart above shows the results of the J-Test test for the USB digital input measured at the line-level output of the M6si. 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 bit). 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 USB input does not show obvious peaks in the audioband; however, the noise floor in the M6si is rather high by modern DAC standards, potentially masking low level peaks in the FFT, making it difficult to conclude whether the M6si DAC would offer good jitter immunity.

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

The chart above shows a fast Fourier transform (FFT) of the M6si’s line-level output with white noise at -4dBFS (blue/red), and a 19.1kHz sinewave at 0dBFS fed to the USB digital input, sampled at 24/44.1. The steep roll-off around 20kHz in the white-noise spectrum shows the behavior of the M6si’s reconstruction filter. There are no aliased images within the audioband; however here again, the high noise floor may be masking low-level peaks. The primary aliasing signal at 25kHz is just below -70dBrA, while the second and third distortion harmonics (38.2, 57.3kHz) of the 19.1kHz tone are at -80 and -70dBrA.

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

The chart above shows RMS level (relative to 0dBrA, which is 1W into 8ohms or 2.83Vrms) as a function of frequency, from 5Hz to 100kHz, for the balanced line-level input. 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. We can see a maximum deviation within the audioband of about 0.5dB from 4 ohms to no load, which is an indication of a relatively low damping factor, or high output impedance. The maximum variation in RMS level when a real speaker was connected was about the same, deviating by about 0.5dB within audioband, with the lowest RMS level, which would correspond to the lowest impedance point for the load, exhibited around 200Hz, and the highest RMS level, which would correspond to the highest impedance point for the load, at around 2kHz.

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 sine-wave stimulus at the balanced line-level input. 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 180W (near full 1% THD power). The power was varied using the volume control. The 10W and 1W data exhibited effectively the same THD values, above 1kHz, between 0.001% and 0.01%. Below 1kHz, the 10W data outperformed the 1W data by about 5dB, with THD values ranging from 0.002% (20Hz) down to 0.0006% (200Hz). At 190W, THD values were the lowest at 20Hz (0.005%), then increased to 0.3% at 5-6kHz. Over most of the audioband, at 180W, THD values ranged from 0.1 to 0.2%.

THD ratio (unweighted) vs. frequency at 10W (phono input)

The chart above shows THD ratio as a function of frequency plots for the phono input measured across an 8-ohm load at 10W. The MM configuration is shown in blue/red (left/right), and MC in purple/green (left/right). The input sweep is EQ’d with an inverted RIAA curve. For both data sets, the left channel outperformed the right by up to 10dB from 20Hz to about 5kHz. The THD values for the MM configuration (left channel) vary from around 0.02% (20Hz) down to 0.002% (200-300Hz), then up to 0.03% (20kHz). The MC THD values were higher, ranging from around 0.05% (20Hz) down to 0.007% (150Hz), then up to 0.15% (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 M6si as a function of output power for the balanced 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 data outperformed the 4-ohm data by 4-6dB, with the 8-ohm data ranging from 0.01% down to 0.001% (10W), then up to 0.002% at the “knee” (about 150W). The 4-ohm data “knee” occurs at around 200W. The 1% THD mark for the 8-ohm data is at 186W, and 265W for the 4-ohm data.

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 M6si as a function of output power for the balanced 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). Overall, THD+N values for the 8-ohm load before the “knee” ranged from around 0.1% (50mW) down to about 0.003%. The 4-ohm data was similar, but 2-4 dB worse.

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 M6si as a function of load (8/4/2 ohms) for a constant input voltage that yields 10W at the output into 8 ohms (and roughly 20W into 4 ohms, and 40W into 2 ohms) for the balanced 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 increasing levels of THD from 8 to 4 to 2 ohms, with about a 5dB increase between 8 and 4 ohms, and as much as 8-10dB between 4 and 2 ohms. Overall, even with a 2-ohm load at roughly 40W, THD values range from as low as 0.002% (50 to 200Hz) up to 0.03% at 20kHz.

FFT spectrum – 1kHz (line-level input)

Shown above is the fast Fourier transform (FFT) for a 1kHz input sine-wave stimulus, measured at the output across an 8-ohm load at 10W for the balanced line-level input. We see that the signal’s second harmonic, at 2kHz, is at -100dBrA, or 0.001%, and around -110dBrA, or 0.0003%, at the odd third (3kHz) and fifth (5kHz) harmonic. Below 1kHz, we see peaks from power supply noise artifacts at 60Hz (just below -100dBrA, or 0.001%), and then the odd harmonics (180Hz, 300Hz) dominating at -100dBrA, or 0.001%.

FFT spectrum – 1kHz (digital input, 24/44.1 data at 0dBFS)

Shown above is the fast Fourier transform (FFT) for a 1kHz input sine-wave stimulus, measured at the output across an 8-ohm load at 10W for the USB digital input, sampled at 24/44.1. We see that the signal’s second harmonic, at 2kHz, is at -95/-100dBrA (left/right), or 0.002/0.001%, and around -85dBrA, or 0.006%, at the odd third harmonic (3kHz). Below 1kHz, we see small peaks from power-supply noise artifacts at 60Hz and 120Hz (near -110dBrA, or 0.001%, for the left channel), and lower-level odd power-supply harmonics. It’s important to note here that despite the lower noise peak levels with the digital USB input as compared to the analog balanced input FFT above, overall, the USB input is significantly noisier than the analog input for the M6si. In order to achieve 10W with a 0dBFS input signal, the volume control had to be set to 10-11 o’clock, compared to the unity position (1 o’clock) used for the analog FFT above. The volume position causes significant changes in noise levels with the M6si. With the volume position at the same level, more noise is measured at the output with the USB input selected. This is reflected in the M6si’s rather poor SNR (90dB, A-weighted) and dynamic range (83dB, A-weighted) measurements in our primary table, where the volume position is by default set to maximum for SNR (analog inputs), and about at the 3 o’clock position (0dBFS for full power 1%THD into 8 ohms) for dynamic range (digital input). Had the volume position ended up at or near maximum to achieve maximum power with a 0dBFS digital input, the dynamic range measurement would have been even worse.

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

Shown above is the fast Fourier transform (FFT) for a 1kHz input sine-wave stimulus, measured at the output across an 8-ohm load at 10W for the USB digital input, sampled at 24/96. We see effectively the same signal and power supply related noise peaks as with the 24/44.1 input FFT above.

FFT spectrum – 1kHz (digital input, 24/44.1 data at -90dBFS)

Shown above is the FFT for a 1kHz -90dBFS dithered 24/44.1 input sine-wave stimulus at the USB digital input, measured at the output across an 8-ohm load. We see the 1kHz primary signal peak, at a slightly lower than 90dBrA amplitude (see the Digital Linearity test above), a low-level second-harmonic (2kHz) at near -110dBrA, or 0.0003%, along with just a hint of a 60Hz power-supply peak (below -110dBrA, or 0.0003%) above the 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 USB digital input, measured at the output across an 8-ohm load. We see effectively the same signal- and power-supply-related noise peaks as with the 24/44.1 input FFT above.

FFT spectrum – 1kHz (MM phono input)

Shown above is the FFT for a 1kHz input sine-wave stimulus, measured at the output across an 8-ohm load at 10W for the phono input, configured for MM. We see the second signal harmonic (2kHz) dominating at -90/-80dBrA (left/right), or 0.003/0.01%. The noise peaks are dominated by the primary (60Hz) power supply signal at -80/-70dBrA (left/right), or 0.01/0.03%, and then it’s odd harmonics (180, 300Hz, etc.) at -80dBRa, or 0.01%, and below.

FFT spectrum – 1kHz (MC phono input)

Shown above is the FFT for a 1kHz input sine-wave stimulus, measured at the output across an 8-ohm load at 10W for the phono input, configured for MC. We see the second signal harmonic (2kHz) dominating at -75/-70dBrA (left/right), or 0.02/0.03%. The noise peaks are dominated by the primary (60Hz) power-supply signal at -70/-60dBrA (left/right), or 0.03/0.1%, and then it’s odd harmonics (180, 300Hz, etc.) at -65dBRa, or 0.06%, and below.

FFT spectrum – 50Hz (line-level input)

Shown above is the FFT for a 50Hz input sine-wave stimulus measured at the output across an 8-ohm load at 10W for the balanced 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. The most predominant (non-signal) peaks are that of the power supply’s third (180Hz) and fifth (300Hz) harmonic at -100dBrA, or 0.001%. The signal second (100Hz) and third (150Hz) harmonics are at -110dBrA, or 0.0003%.

FFT spectrum – 50Hz (MM phono input)

Shown above is the FFT for a 50Hz input sine-wave stimulus measured at the output across an 8-ohm load at 10W for the phono input (MM configuration). 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 predominant (non-signal) peaks are that of the power supply’s primary (60Hz), third (180Hz), and fifth (300Hz) harmonics at -70dBrA to -80dBrA, or 0.03% to 0.01%. The signal harmonics are barely perceptible above the noise floor, with the second (100Hz) harmonic (right channel) at -90dBrA, or 0.003%.

FFT spectrum – 50Hz (MC phono input)

Shown above is the FFT for a 50Hz input sine-wave stimulus measured at the output across an 8-ohm load at 10W for the phono input (MC configuration). 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 predominant (non-signal) peaks are that of the power supply’s primary (60Hz), third (180Hz), and fifth (300Hz) harmonics at -60dBrA to -75dBrA, or 0.1% to 0.02%. The second (100Hz) signal harmonic (right channel) is at -80dBrA, or 0.01%.

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 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 -110dBRA, or 0.0003%, while the third-order modulation products, at 17kHz and 20kHz are higher, are approaching -100dBrA, or 0.001%.

Intermodulation distortion FFT (18kHz + 19kHz summed stimulus, USB input, 24/44.1)

Shown above is an FFT of the intermodulation distortion (IMD) products for an 18kHz + 19kHz summed sine-wave stimulus tone measured at the output across an 8-ohm load at 10W for the USB input at 24/44.1. We find that the second-order modulation product (i.e., the difference signal of 1kHz) is at -90dBRA, or about 0.003%, while the third-order modulation products, at 17kHz and 20kHz, are near -65dBrA, or 0.06%. We also see the main aliased peaks at 25.1kHz and 26.1kHz around -60dBrA and their IMD products.

Intermodulation distortion FFT (18kHz + 19kHz summed stimulus, USB 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 output across an 8-ohm load at 10W 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 -100dBRA, or about 0.001%, while the third-order modulation products, at 17kHz and 20kHz, are at -65dBrA, or 0.06%.

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 output across an 8-ohm load at 10W for the phono input configured for MM. The second-order 1kHz peak is just below -50dBrA, or 0.3%, while the third-order peaks are at -100dBrA, or 0.001%.

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 sine-wave stimulus tone measured at the output across an 8-ohm load at 10W for the phono input configured for MC. The second-order 1kHz peak is just below -35dBrA, or 2%, while the third-order peaks are at -95dBrA, or 0.002%.

Square-wave response (10kHz)

Above is the 10kHz square-wave 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 M6si’s slew-rate performance. Rather, it should be seen as a qualitative representation of its extended bandwidth. An ideal square wave can be represented as the sum of a sine wave 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 M6si’s reproduction of the 10kHz square wave is very clean, with sharp corners and very little softening.

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

The chart above is the damping factor as a function of frequency. Both channels show relatively constant damping factors across the audioband, with the right channel slightly outperforming the left. The right channel measured around 44, while the left measured around 37. For solid-state amplifiers, these damping-factor figures are low. They’re also indicative of the amp’s relatively high output impedance.

Diego Estan
Electronics Measurement Specialist