Details

Parent Category: Products

Category: Amplifier Measurements

Created: 15 August 2021

Link: reviewed by Roger Kanno on SoundStage! Hi-Fi on August 15, 2021

General Information

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

The TDAI-1120 was conditioned for 1 hour at 1/8th full rated power (~6W 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 TDAI-1120 offers a multitude of inputs, both digital and analog (unbalanced), a set of configurable line-level analog outputs (fixed or variable, full-range or with crossover), and a pair of speaker-level outputs. For the purposes of these measurements, the following inputs were evaluated: digital coaxial (RCA) and optical (TosLink) S/PDIF, and the line-level and moving-magnet (MM) phono analog unbalanced (RCA) inputs. The TDAI-1120 is a sophisticated integrated amplifier capable of room correction (RoomPerfect) and bass management. As such, a factory reset was performed before measurements were performed, and particular attention was paid to ensure room correction was disengaged, and that both the line-level and speaker level outputs were set to full-range (i.e., no crossovers).

Based on the accuracy and repeatability at various volume levels of the left/right channel matching (see table below), the TDAI-1120 volume control is likely operating in the digital domain. Consequently, all analog signals are digitized at the TDAI-1120’s inputs so the unit may perform volume, bass management, and room correction.

All measurements, with the exception of signal-to-noise ratio (SNR), or otherwise stated, were made with the volume set to or near unity gain (0dB) on the volume control. SNR measurements were made with the volume control set to maximum. At the unity gain volume position, to achieve 10W into 8 ohms, 2Vrms was required at the line-level input and 11mVrms at the phono input. For the digital inputs, 0dBFS required the volume set to -8.5dB to achieve 10W into 8 ohms at the output.

Because the TDAI-1120 is a digital amplifier technology that exhibits considerable noise just 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.

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 Lyngdorf Audio for the TDAI-1120 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 is set at its maximum (DC to 1MHz), assume, unless otherwise stated, a measurement input bandwidth of 10Hz to 22.4kHz, and the worst-case measured result between the left and right channels.

Our primary measurements revealed the following using the line-level inputs (unless specified, assume a 1kHz sine wave, 10W output, 8-ohm loading, 10Hz to 22.4kHz bandwidth):

For the continuous dynamic power test, the TDAI-1120 was able to sustain 130W into 4 ohms using an 80Hz tone for 500 ms, alternating with a signal at -10dB of the peak (13W) 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 TDAI-1120 was just slightly warm to the touch.

Our primary measurements revealed the following using the phono-level input (unless specified, assume a 1kHz sine wave, 10W output, 8-ohm loading, 10Hz to 22.4kHz bandwidth):

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

In our measured frequency-response plot above, the TDAI-1120 is nearly flat within the audioband (20Hz to 20kHz). At the audioband extremes the TDAI-1120 is -1.5dB down at 20Hz and +0.4dB at 20kHz. These data do not quite corroborate Lyngdorf’s claim of 20Hz to 20kHz (+/-0.5dB). The TDAI-1120 cannot be considered a high-bandwidth audio device as the -3dB point is just shy of 50kHz. In the chart above and most of the charts 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 vs. input type (8-ohm loading, left channel only)

The plot above shows the TDAI-1120’s frequency response as a function of input type. The green trace is the same analog input data from the previous graph. The blue trace is for a 16-bit/44.1kHz dithered digital signal from 5Hz to 22kHz, the purple trace is for a 24/96 dithered digital signal from 5Hz to 48kHz, and, finally, pink is 24/192 from 5Hz to 96kHz. The behavior at low frequencies is the same across input types: -1.5dB at 20Hz. The behavior approaching 20kHz for all input types is also identical, in that there is a rise in level beginning around 5kHz. However, the 16/44.1 signal exhibits a sharp, brick-wall-type attenuation right around 20kHz. The 24/96 digital input also exhibits a sharp, brick-wall-type attenuation near the limit of its frequency range (48kHz), peaking around +2.2dB at around 40kHz. The 24/192 digital input frequency response is identical to the 24/96 plot, despite the extended theoretical range up to 96kHz.

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

The plot above shows frequency response for the phono input (MM), and shows the same maximum deviation of -1.5dB at 20Hz and +0.4dB at 20kHz as seen for the line-level analog input. What is shown is the deviation from the RIAA curve, where the input signal sweep is EQd with an inverted RIAA curve supplied by Audio Precision (i.e., zero deviation would yield a flat line at 0dB). In the flat portion of the curve, the worst-case RIAA and channel-to-channel deviations are from 5 to 6kHz, where the left channel is -0.1dB down and the right about -0.2dB down.

Phase response (MM input)

Above is the phase response plot from 20Hz to 20kHz for the phono input from 20Hz to 20kHz. 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 -400 degrees at 200-300Hz. This is an indication that the TDAI-1120 likely inverts polarity on the phono input. If we look at the phase response at 20Hz, the phase shift is 200 degrees. If we assume a polarity inversion (180 degrees), then there would only be 20 degrees of extra phase shift at 20Hz.

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 line-level output of the TDAI-1120. The digital input is swept with a dithered 1kHz 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 types performed similarly, approaching the ideal 0dB relative level from 0dBFS to -90dBFS. At -110dBFS, both channels at 16/44.1 overshot the ideal output signal amplitude by about 1dB, while the left/right channels at 24/96 undershot by 1dB.

Impulse response (16/44.1 and 24/96 data)

The graph above shows the impulse responses for a -20dBFS 16/44.1 dithered input stimulus (red), and 24/96 dithered input stimulus (green), measured at the line-level output of the TDAI-1120. The shape is similar to that of a typical sinc function filter, although with less pre- and post-ringing for the 24/96 input data.

J-Test (coaxial and optical inputs)

The plot above shows the results of the J-Test test for the optical digital input (the coaxial input performed identically) measured at the line-level output of the TDAI-1120. J-Test was developed by Julian Dunn the 1990s. It is a test signal—specifically a -3dBFS undithered 12kHz square wave sampled (in this case) at 48kHz (24 bits). Since even the first odd harmonic (i.e., 36kHz) of the 12kHz square wave is removed by the bandwidth limitation of the sampling rate, we are left with a 12kHz sine wave. In addition, an undithered 250Hz square wave 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 -144dBFS and below.  The test file can also be used in conjunction with artificially injected sine-wave jitter by the Audio Precision, to show how well the DAC rejects jitter.

As mentioned, both the coaxial and optical S/PDIF TDAI-1120 inputs performed identically, showing only spurious peaks in the audioband at -125dBFS and below. When sine-wave jitter was injected at 2kHz, which would manifest as sidebands at 10kHz and 14kHz without any jitter rejection, absolutely no peaks were observed above the noise floor with both inputs on the TDAI-1120, even at the maximum jitter level available of 1592ns, indicating excellent jitter immunity.

Wideband FFT spectrum of white noise and 19.1kHz sine-wave tone (coaxial input)

The plot above shows a fast Fourier transform (FFT) of the TDAI-1120’s line-level output with white noise at -4 dBFS (blue/red), and a 19.1 kHz sine wave at 0dBFS fed to the coaxial digital input, sampled at 16/44.1. The sharp roll-off above 20kHz in the white-noise spectrum shows the implementation of a brick-wall-type reconstruction filter. The aliased image at 25kHz is extremely low at -125dB, and any resultant intermodulated signals (between either the alias or signal harmonics) within the audioband are all very low, below -120dBrA, or 0.0001%. The second, third, and fourth distortion harmonics (38.2, 57.3, 76.4kHz) of the 19.1kHz tone are much higher in amplitude, lying between -80 and -95dBrA, or 0.01% and 0.002%.

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

The plots above show RMS level (relative to 0dBrA, which is 1W into 8 ohms or 2.83Vrms) as a function of frequency, for the analog line-level input swept from 5Hz to 100kHz. The blue plot is into an 8-ohm load, the purple is into a 4-ohm load, the pink is an actual speaker (Focal Chora 806, measurements can found here), and the cyan is no load connected. We find a maximum deviation within the audioband of about 3dB (at 20kHz), which is an indication of a low damping factor, or high output impedance. The maximum variation in RMS level when a real speaker was used as a load is smaller, deviating by a little less than 0.5dB within the flat portion of the curve (30Hz to 20kHz), 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 5kHz.

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

The plot above shows THD ratios at the output into 8 ohms as a function of frequency (20Hz to 6kHz) for a sine-wave stimulus at the 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 the full rated power of 60W. The power was varied using the volume control. All three THD plots are relatively flat. The 10W data exhibited the lowest THD values, between 0.006% and 0.015%, although there is an almost 5dB difference between channels in favor of the right channel. The 1W data show THD values between 0.01 and 0.02%. At the full rated power of 60W, THD values ranged from 0.01 to 0.03%.

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

Next is a THD ratio as a function of frequency plot for the phono input measured across an 8-ohm load at 10W. The input sweep is EQ’d with an inverted RIAA curve. The THD values vary from about 0.006% to just above 0.01%, but are fairly flat from 20Hz to 6kHz. Again, the right channel outperformed the left by as mush as 5dB.

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

The plot above shows THD ratios measured at the output of the TDAI-1120 as a function of output power for the analog line-level-input, for an 8-ohm load (blue/red for left/right channels) and a 4-ohm load (purple/green for left/right channels). Although there are fluctuations before the “knee,” both the 4-ohm and 8-ohm data are roughly the same, ranging from 0.005% to 0.02%. The “knee” in the 8-ohm data occurs just past 50W, hitting the 1% THD mark between 60 and 70W. For the 4-ohm data, the “knee” occurs around 100W, hitting the 1% THD around 130W.

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 TDAI-1120 as a function of output power for the analog line level-input, for an 8-ohm load (blue/red for left/right channels) and a 4-ohm load (purple/green for left/right channels). There’s a distinct 5dB jump in THD+N (also visible in the THD plot above, but to a lesser degree) when the output voltage is around 1.5-1.6Vrms (i.e., 0.3W into 8 ohms, 0.7W into 4 ohms), and then a sharp 10dB decrease in THD+N at around 4-5Vrms at the output (i.e., 2/5W into 4/8 ohms). This behavior was repeatable over multiple measurement trials. Overall, THD+N values before the “knee” ranged from around 0.01% (3 to 10W) to 0.2% (50mW).

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 TDAI-1120 as a function of load (8/4/2 ohms) for a constant input voltage that yielded 5W at the output into 8 ohms (and roughly 10W into 4 ohms, and 20W into 2 ohms) for the analog line-level input. The 8-ohm load is the blue trace, the 4-ohm load the purple trace, and the 2-ohm load the pink trace. We find increasing levels of THD from 8 to 4 to 2 ohms, with about a 5dB increase with each halving of the load. Overall, even with a 2-ohm load at roughly 20W, THD values ranged from 0.015% at around 1kHz to just below 0.04% at 6kHz.

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 analog line-level input. We see that the signal’s second harmonic, at 2kHz, is at -80dBrA (left), or 0.01%, and -95dBrA (right), or 0.002%. The third harmonic is at -85dBrA, or 0.006%; the remaining signal harmonics range from -90dBrA to -120dBrA, or 0.003 and 0.0001%. Just above 20kHz, we see a steep rise in the noise floor, up to -70dBrA, or 0.03%. Below 1kHz, we see no power-supply noise artifacts.

FFT spectrum – 1kHz (digital input, 16/44.1 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 coaxial digital input, sampled at 16/44.1. We see essentially the same signal harmonic profile as with the analog input. The noise floor, however, is reduced here below 100Hz by about 15dB compared to the analog input.

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 coaxial digital input, sampled at 24/96. We see essentially the same signal harmonic profile as with the analog input and the digital coaxial input with a 16/44.1 signal. The noise floor, however, is reduced here above 5kHz by about 5dB compared to the 16/44.1 sampled FFT above.

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 sine-wave stimulus at the coaxial digital input, measured at the output across an 8-ohm load. We only see the 1kHz primary signal peak, at the correct amplitude, with a slightly raised noise floor at low frequencies with respect to the 0dBFS FFT above.

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

Shown above is the FFT for a 1kHz -90dBFS dithered 24/96 input sine-wave stimulus at the coaxial digital input, measured at the output across an 8-ohm load. We only see the 1kHz primary signal peak, at the correct amplitude, with a slightly raised noise floor at low frequencies with respect to the 0dBFS 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 MM phono input. We see essentially the same signal harmonic profile as with the analog line-level and digital inputs. The highest peak from power-supply noise is at the fundamental (60Hz), reaching about -85dBrA, or 0.006%, and the third noise harmonic (180Hz) is just below -90dBrA, or 0.003%.

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 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. Once again, here there are no noise signals to speak of. Instead, the most predominant peaks are that of the signal’s second (100Hz) harmonic at -80/-95dBrA (left/right), or 0.01/0.002%, and third harmonic (150Hz) at about -85dBrA, or 0.006%.

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. The most predominant peaks are that of the signal’s second (100Hz) harmonic at -80/-95dBrA (left/right), or 0.01/0.002%, and third harmonic (150Hz) at about -85dBrA, or 0.006%. Peaks due to power supply noise are visible at 60Hz (-85dBrA or 0.006%) and 180Hz (-90dBrA or 0.003%). 

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

Shown above is an FFT of the intermodulation disortion (IMD) products for an 18kHz + 19kHz summed sine-wave stimulus tone measured at the output across an 8-ohm load at 10W for the analog line-level input. The input RMS values are set at -6.02dBrA so that, if summed for a mean frequency of 18.5kHz, would yield 10W (0dBrA) into 8 ohms at the output. We find that the second-order modulation product (i.e., the difference signal of 1kHz) is at -85/-95dBRA (left/right channels), or 0.006/0.002%, while the third-order modulation products, at 17kHz and 20kHz are higher, at around -80dBrA, or 0.01%.

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 sine-wave stimulus tone measured at the output across an 8-ohm load at 10W for the phono input. Here we find essentially the same result as with the line-level analog input, with the exception of the expected elevated noise floor at low frequencies due to the RIAA equalization, and the absence of a 2kHz peak at -110dBrA, or 0.0003%, for the left channel that was visible in the line-level IMD FFT.

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 TDAI-1120’s slew-rate performance; rather, it should be seen as a qualitative representation of its limited 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 TDAI-1120’s reproduction of the 10kHz square wave is poor, with noticeable overshoot and undershoot, due to its limited bandwidth, and the 400kHz switching-oscillator frequency used in the digital amplifier section clearly visible modulating the waveform.

FFT spectrum of 400kHz switching frequency relative to a 1kHz tone

The TDA-1120’s class-D amplifier relies on a switching oscillator to convert the input signal to a pulse-width modulated (PWM) square-wave (on/off) signal before sending the signal through a low-pass filter to generate an output signal. The TDAI-1120 oscillator switches at a rate of about 400kHz. This chart plots an FFT spectrum of the amplifier’s output at 10W into 8 ohms as it’s fed a 1kHz sine wave. We can see that the 400kHz peak is quite evident, at -25dBrA below the main signal level. There is also a peak at 800kHz and 1200kHz (the second and third harmonics of the 400kHz peak at -50 and -65dBrA). Those three peaks—the fundamental and its second and third harmonics—are direct results of the switching oscillators in the TDAI-1120 amp left- and right-channel modules. The noise around those very-high-frequency signals are in the signal, but all that noise is far above the audioband—and is therefore inaudible—as well as so high in frequency that any loudspeaker the amplifier is driving should filter it all out anyway.

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

The final chart above is the damping factor as a function of frequency. Both channels show a general trend of a higher damping factor at lower frequencies, and lower damping factor at higher frequencies. The right channel outperformed the left with a peak value around 80 from 20Hz to 1kHz, while the left channel achieved a damping factor of around 40 within the same frequency range. Both channels’ damping factors are down to around 10 at 20kHz.

Diego Estan
Electronics Measurement Specialist

Details

Parent Category: Products

Category: Amplifier Measurements

Created: 15 August 2021

Link: reviewed by Dennis Burger on SoundStage! Hi-Fi on September 1, 2021

General Information

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

The M4800 was conditioned for 1 hour at 1/8th full rated power (~12W into 8 ohms) before any measurements were taken. All measurements were taken with two channels driven, using a 120V/20A dedicated circuit, unless otherwise stated.

The M4800 is an eight-channel multi-zone network matrix amplifier, and not the usual type of amp that we would measure at SoundStage! Network. The M4800 offers analog (RCA) or digital (coaxial or optical) inputs for each channel and is fully configurable using an ethernet connection. Speaker output connections are four-pin screw-down-type connectors that only accept bare wire. Each stereo output can be configured for mono (bridged) operation with double the power into 8 ohms. For the purposes of these measurements, the following inputs were evaluated: inputs 7 and 8 over analog (RCA, assigned as left and right respectively in our measurements below) and digital coaxial (RCA).

Based on the accuracy and repeatability at various volume levels of the left/right channel matching (see table below), the M4800 volume control is likely operating in the digital domain. When we consider this and all the EQ and crossover functions available in the M4800 web interface menu, it’s obvious that the M4800 digitizes all incoming analog signals. This was confirmed in our measurements below. The volume control offers a total range from -70dB to +29.8dB, in increments of 2dB to 0.8dB, depending on level.

All measurements were made with the volume set to 100%. At this volume position, to achieve 10W into 8 ohms, 286mVrms was required at the analog line-level input and -17.3dBFS for the digital input.

Input sensitivities can be configured for every input pair on the M4800. If 1Vrms is chosen, then the amplifier will apply about 29dB of gain to achieve 100W into 8ohms. If 2Vrms is chosen, then the amplifier will apply about 23dB of gain to achieve the same 100W into 8ohms. Since 29dB is a very common gain found in home audio amplifiers, this is how the M4800 was evaluated.

Because the M4800 is 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.

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 AudioControl for the M4800 compared directly against our own. The published specifications are sourced from AudioControl’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, a measurement input bandwidth of 10Hz to 22.4kHz, and the worst-case measured result between the left and right channel.

Our primary measurements revealed the following using the analog or digital input (unless specified, assume a 1kHz sinewave at 286mVrms or -17.3dBFS, 10W output, 8-ohm loading, 10Hz to 22.4kHz bandwidth):

For the continuous dynamic power test, the M4800 was able to sustain 207W into 4 ohms using an 80Hz tone for 500ms, alternating with a signal at -10dB of the peak (7.7W) for five seconds, for five continuous minutes without inducing a fault or the initiation of a protective circuit. This test is meant to simulate sporadic dynamic bass peaks in music and movies. During the test, the M4800 stayed slightly warm to the touch. During the high-power peaks, a small low-level buzz could be heard and felt from the chassis.

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

In our measured frequency-response plot above, the M4800 is nearly flat within the audioband (20Hz to 20kHz). At the extremes the M4800 is -0.75dB down at 20Hz and -1.3dB at 20kHz. The M4800 cannot be considered a high-bandwidth audio device as the -3dB point is just past 20kHz. In fact, the M4800 exhibits brick-wall-type filtering just past 20kHz. 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 vs. input type (8-ohm loading, left channel only)

The plot above shows the M4800’s frequency response (left channel only) as a function of input type. The green trace is the same analog input data from the previous 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. The behavior at low frequencies is the same across digital input types: -0.5dB at 20Hz, compared to the -0.75dB at 20Hz for the analog input. The behavior at 20kHz for all digital sample rates is identical (about -0.6dB at 20kHz). The 24/96 and 24/192 sample rates do have slightly higher extension than the 16/44.1 sample rate, with -3dB points at 22.8kHz vs 20.8kHz at 16/44.1. Overall, the digital input offers a slightly more extended frequency response at the extremes of the audioband compared to the analog input.

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

The plot above shows the results of a linearity test for the digital input for both 16/44.1 (blue/red) and 24/96 (purple/green) input data, measured at the speaker outputs of the M4800. The digital input is swept with a dithered 1kHz input signal from -120dBFS to 0dBFS, and the output is analyzed by the APx555 and normalized to the -20dBFS result. The ideal response would be a straight flat line at 0dB. Both digital input types performed similarly, approaching the ideal 0dB relative level at -110dBFS, then yielding perfect results from -100dBFs to -10dBFS. At -120dBFS, both channels at 16/44.1 overshot the ideal output signal amplitude by 1-1.5dB, while the left/right channels at 24/96 undershot by less than 1dB. Above -10dBFS, the M4800 appears to attenuate the incoming digital signal, perhaps in an effort to eliminate digital clipping. At 0dBFS, the output was measured at -6dBFS for both sample rates. For this test, we reduced the M4800 volume control slightly to ensure analog clipping was not introduced at the speaker outputs. We also repeated the measurement at different volume levels, and the same results emerged.

Impulse response (16/44.1 and 24/96 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 speaker outputs (~10W 8-ohm) of the M4800. We can see that the M4800 utilizes a typical sinc function reconstruction filter.

J-Test (coaxial)

The plot above shows the results of the J-Test test for the coaxial digital input measured at the speaker outputs (10W into 8 ohms) of the M4800. J-Test was developed by Julian Dunn the 1990s. It is a test signal—specifically, a -3dBFS, undithered 12kHz square wave sampled (in this case) at 48kHz (24 bit). Since even the first odd harmonic (i.e., 36kHz) of the 12kHz square wave is removed by the bandwidth limitation of the sampling rate, we are left with a 12kHz sinewave. In addition, an undithered 250Hz square wave 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 -144dBFS and below.  The test file can also be used in conjunction with artificially injected sine-wave jitter by the Audio Precision, to show how well the DAC rejects jitter.

We see a few spurious peaks in the audioband below -110dBFS. This is a reasonably clean J-Test result, and an indication that the M4800 DAC has good jitter immunity. When sine-wave jitter was injected at 2kHz, which would manifest as sidebands at 10kHz and 14kHz without any jitter rejection, no further peaks, or worsening of existing peaks, were observed up to 1000ns of jitter level.

Wideband FFT spectrum of white noise and 19.1kHz sine-wave tone (coaxial input)

The chart above shows a fast Fourier transform (FFT) of the M4800’s speaker outputs (1W into 8-ohms) with white noise at -20.3dBFS (blue/red), and a 19.1 kHz sinewave at -17.3dBFS fed to the coaxial digital input, sampled at 16/44.1. The sharp roll-off above 20kHz in the white-noise spectrum shows the implementation of a brick-wall-type reconstruction filter. There are no aliased image peaks in the audioband visible above the noise floor. There is a strong 28.9kHz alias peak at -80dBrA (as opposed to a peak at 25kHz), indicating that the M4800 likely resamples incoming 16/44.1 data to 48kHz. There is also a small 48kHz peak at -120dBrA to support this hypothesis. The second, third, and fourth distortion harmonics (38.2, 57.3, 76.4kHz) of the 19.1kHz tone are between -65dBrA and -110dBrA.

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, for the analog input swept from 10Hz to 100kHz. The blue plot is into an 8-ohm load, the purple is into a 4-ohm load, the pink plot is an actual speaker (Focal Chora 806, measurements can be found here), and the cyan plot is no load connected. The chart below . . .

. . . is the same but zoomed in to highlight differences. Zoomed in, we can see a maximum deviation within the audioband of only about 0.1dB from 4 ohms to no load, which is an indication of a high damping factor, or low output impedance. The maximum variation in RMS level when a real speaker was used as a load was a little less, deviating by about 0.09dB within the flat portion of the curve (100Hz to 5kHz), 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 plot above shows THD ratios at the output into 8 ohms as a function of frequency (20Hz to 6kHz) for a sine-wave stimulus at the analog input. The blue and red plots are for the left and right channels at 1W output into 8 ohms, purple/green at 10W, and pink/orange at 100W (full rated power). The 1W data exhibited the lowest THD values, with a distinct rise at low frequencies (0.003% at 60Hz and up to 0.03% at 20Hz), then relatively flat around 0.002-0.003% to 1kHz, then up to 0.008% at 6kHz. The 10W data followed a near identical trend but with almost 10dB higher THD ratios. The 100W data was at worst at 0.3% at 25Hz, and fairly constant from 200Hz to 6kHz at 0.05-0.06%.

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

The plot above shows THD ratios measured at the output of the M4800 as a function of output power for the analog line-level input, for an 8-ohm load (blue/red for left/right channels) and a 4-ohm load (purple/green for left/right channels). At very low output power, the 8-ohm data does offer lower THD values (0.002%) compared to the 4-ohm data (0.005%). At higher output levels (above 10W), before the “knee,” the 4-ohm data slightly outperformed the 8-ohm data by about 2-5dB. For example, at 50W, the 4-ohm THD ratios are just below 0.02%, while the 8-ohm ratios are at 0.03%. The “knee” in the 8-ohm data occurs just past 100W, hitting the 1% THD mark at 108W. For the 4-ohm data, the “knee” occurs just past 200W, hitting the 1% THD at 214W.

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

The plot above shows THD+N ratios measured at the output of the M4800 as a function of output power for the analog line-level input, for an 8-ohm load (blue/red for left/right channels) and a 4-ohm load (purple/green for left/right channels). There’s a distinct 5dB jump in THD+N when the output voltage is around 1Vrms (i.e., 0.15W into 8 ohms, 0.25W into 4 ohms). This behavior was repeatable over multiple measurement trials. Overall, THD+N values before the “knee” ranged from around 0.01% to 0.05%.

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 M4800 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 analog line-level input. The 8-ohm load is the blue trace, the 4-ohm load the purple trace, and the 2-ohm load the pink trace. We find increasing levels of THD from 8 to 4 to 2 ohms between 80Hz and 3kHz, with about a 2-3dB increase when halving of the load. At the frequency extremes, THD ratios are near identical for all loads, and at their worst at 20Hz (0.1%). Overall, even with a 2-ohm load at roughly 40W, THD values ranged from 0.1% at around 20Hz to at or near 0.01% from 60Hz to 6kHz. One thing to take note of is that distortion does rise in the low-bass region for 8-, 4-, and 2-ohm loading. 

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 analog line-level input. We see that the signal’s second harmonic, at 2kHz, is at -95/-90dBrA (left/right channels), or 0.002/0.003%, while the third harmonic is predominant at -85dBrA, or 0.006%. The remaining signal harmonics are at or below -110dBrA. Below 1kHz, we see no power-supply noise-related peaks. There is a peak at 48kHz, and stronger peaks (-75dBrA) at 47kHz and 49kHz. These peaks are a clear indication that the 1kHz analog input signal was sampled by the M4800 at 48kHz, resulting in obvious intermodulation distortion (IMD) products (i.e., 48kHz - 1kHz and 48kHz + 1kHz).

FFT spectrum – 1kHz (digital input, 16/44.1 data at -17.3dBFS)

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 coaxial digital input, sampled at 16/44.1. We see close to the same signal harmonic profile as with the analog input, but with any lower-level peaks obscured by the higher noise floor. The same peaks at 47/48/49kHz are visible, indicating that the incoming digital input has been resampled to 48kHz.

FFT spectrum – 1kHz (digital input, 24/96 data at -17.3dBFS)

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 coaxial digital input, sampled at 24/96. We see essentially the same signal harmonic profile as with the 16/44.1 FFT above, but with lower-level peaks visible due to the lower noise floor of the 24-bit data. The same peaks at 47/48/49kHz are visible, indicating that the incoming digital input has been re-sampled to 48kHz.

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 sine-wave stimulus at the coaxial digital input, measured at the output across an 8-ohm load. We see the 1kHz primary signal peak at the correct amplitude. Since a -17.3dBFS input signal yielded 10W at the output, which is defined here as 0dBrA, then a -90dBFS digital input is 72.7dBrA below reference. The second harmonic is predominant at -125/-130dBrA (left/right channels), or 0.001/0.0003%. One thing to note is that the right-channel noise floor (red) is about 10dB lower than the left-channel noise floor (blue).

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

Shown above is the FFT for a 1kHz, -90dBFS dithered 24/96 input sine-wave stimulus at the coaxial digital input, measured at the output across an 8-ohm load. We see the 1kHz primary signal peak at the correct amplitude. Since a -17.3dBFS input signal yielded 10W at the output, which is defined here as 0dBrA, then a -90dBFS digital input is 72.7dBrA below reference. The second harmonic is predominant at around -130dBrA, or 0.0003%. Below 1kHz, the left-channel noise floor (blue) is about 10-20dB lower than the right-channel noise floor (red).

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 analog 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) peak is the signal third harmonic (150Hz) at -80dBrA, or 0.01%, while the second signal harmonic (100Hz) is seen at -90/-95dBrA (left/right channels), or 0.003/0.002%.

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 sine-wave stimulus tone measured at the output across an 8-ohm load at 10W for the analog 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 -100/-95dBrA (left/right channels), or 0.001/0.002%, while the third-order modulation products, at 17kHz and 20kHz, are higher, at around -75dBrA, or 0.02%. We also see the main aliased peaks at 29kHz and 30kHz around -90dBrA, again indicating that the M4800 ADC is digitizing the incoming analog signal at 48kHz (i.e., 48kHz-19kHz = 29kHz).

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

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 digital coaxial input at 16/44.1. We find that the second-order modulation product (i.e., the difference signal of 1kHz) is just below -90dBRA, or about 0.003%, while the third-order modulation products, at 17kHz and 20kHz, are just below -70dBrA, or 0.03%. We also see the main aliased peaks at 29kHz and 30kHz around -90dBrA, again indicating that the M4800 is resampling to 48kHz.

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

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 digital coaxial input at 24/96. We find that the second-order modulation product (i.e., the difference signal of 1kHz) is just below -90dBRA, or about 0.003%, while the third-order modulation products, at 17kHz and 20kHz, are just below -70dBrA, or 0.03%. We also see the main aliased peaks at 29kHz and 30kHz around -90dBrA, once again indicating that the M4800 is resampling all inputs (digital and analog) to 48kHz.

Square-wave response (10kHz)

Above is the 10kHz square-wave response using the analog 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 M4800’s slew-rate performance. Rather, it should be seen as a qualitative representation of its very limited 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, as well as softening of the edges. Due to the M4800’s very limited bandwidth, only the square wave’s fundamental (10kHz) sine wave is reproduced here. In addition, we can see the 400kHz switching oscillator frequency used in the digital amplifier section clearly visible modulating the waveform.

Square-wave response (1kHz, bandwidth limited to 250kHz)

Above is the 1kHz square-wave response using the analog input, at roughly 10W into 8 ohms. The Audio Precision’s input bandwidth filter has been lowered to 250kHz to filter out the switching oscillator frequency. Due to the M4800’s very limited bandwidth, even with a 1kHz fundamental frequency, we see clear over and undershoot at the corners of the square wave.

FFT spectrum of 400kHz switching frequency relative to a 1kHz tone

The M4800’s class-D amplifier relies on a switching oscillator to convert the input signal to a pulse-width modulated (PWM) square-wave (on/off) signal before sending the signal through a low-pass filter to generate an output signal. The M4800 oscillator switches at a rate of about 400kHz, 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 sine wave.

We can see that the 400kHz peak is quite evident, and at -45dBrA. There is also a peak at 800kHz and 1200kHz (the second and third harmonic of the 400kHz peak) at -70 and -115dBrA. Those three peaks—the fundamental and its second/third harmonics—are direct results of the switching oscillators in the M4800 amp modules. Also obvious are the 47/48/49kHz peaks due to the ADC sampling the incoming signal to 48kHz. The noise around those very-high-frequency signals are in the signal, but all that noise is far above the audioband—and therefore inaudible—as well as so high in frequency that any loudspeaker the amplifier is driving should filter it all out anyway.

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

The final chart above is the damping factor as a function of frequency. Both channels show a relatively constant damping factor from 20Hz to 20kHz. The damping factor is at around 200/230 (left/right channels) from 20Hz to 6kHz, then down to 180/190 (left/right channels) at 20kHz. These values represent high damping factors.

Diego Estan
Electronics Measurement Specialist

Details

Parent Category: Products

Category: Amplifier Measurements

Created: 01 August 2021

Link: reviewed by Roger Kanno on SoundStage! Hi-Fi on August 1, 2021

General Information

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

The Michi X5 was conditioned for 1 hour at 1/8th full rated power (~43W 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 X5 offers a multitude of inputs, both digital and analog, line-level analog preamp outputs, subwoofer line-level outputs, two pairs of speaker-level outputs (A and B), and a 1/8" TRS headphone output. For the purposes of these measurements, the following inputs were evaluated: digital coaxial 1 (RCA), optical 1 (TosLink), USB, analog balanced (XLR) and unbalanced (RCA) line-level, and moving-magnet (MM) as well as moving-coil (MC) phono. Comparisons were made between unbalanced (RCA) and balanced (XLR) line-level inputs, and no differences were seen in terms of THD+N; however, the balanced inputs offer 3.85dB less gain than the unbalanced inputs. Comparisons were made between digital coaxial, optical, and USB inputs, and no differences were seen in terms of THD+N. Bluetooth is also offered, but our APx555 does not currently have a Bluetooth board installed.

Most measurements, with the exception of signal-to-noise ratio (SNR) or otherwise stated, for the line-level analog inputs, were made with the volume set to unity gain (0dB) on the volume control (position 86 for the balanced XLR input, and 78 for single-ended RCA) with respect to the preamp outputs (which offers 6.6dB of gain with the unbalanced input, 2.75dB with the balanced input). At the unity-gain volume position, to achieve 10W into 8 ohms, 325mVrms was required at the balanced line-level input. For the digital inputs, a volume position of 55 yielded 10W into 8 ohms with a 0dBFS input. For the phono input, configured for MM, a volume position of 78 yielded 10W into 8 ohms with a 1kHz 5.1mVrms input. Configured for MC, a volume position of 78 yielded 10W into 8 ohms with a 1kHz 0.48mVrms input. The SNR and dynamic range measurements were made with the volume control set to maximum.

Based on the accuracy of the left/right volume channel matching (see table below), the X5 volume control is likely digitally controlled in the analog domain. The X5 offers 96 volume steps. Between steps 1 and 5, step increases or decreases are 2dB; steps 6 to 18 are 1.5dB; steps 19 to 66 are 1 dB; 66 to 86 are 0.5dB; and 87 to 96 are 0.25dB.

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 Rotel for the Michi X5 compared directly against our own. The published specifications are sourced from Rotel’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.

Our primary measurements revealed the following using the balanced line-level analog input and digital coaxial input (unless specified, assume a 1kHz sinewave at 325mVrms or 0dBFS, 10W output, 8-ohm loading, 10Hz to 90kHz bandwidth):

For the continuous dynamic power test, the X5 was able to sustain 630W into 4 ohms using an 80Hz tone for 500ms, alternating with a signal at -10dB of the peak (63W) 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 X5 were quite warm to the touch, causing discomfort and pain after about 10 seconds.

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):

Our primary measurements revealed the following using the balanced line-level inputs at the headphone output (unless specified, assume a 1kHz sinewave, 2Vrms output, 300 ohms loading, 10Hz to 90kHz bandwidth):

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

In our measured frequency response plot above, the X5 is nearly flat within the audioband (20Hz to 20kHz). At the extremes the X5 is -0.5dB down at 10Hz and at 100kHz. These data only half corroborate Rotel’s claim of 10Hz to 100kHz (0/-0.6dB). The X5 can be considered a high-bandwidth audio device. In the chart above and most of the charts 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, line-level input)

Above is the phase response plot from 20Hz to 20kHz for the balanced line-level input, measured across the speakers outputs at 10W into 8 ohms. The X5 does not invert polarity and exhibits, at worst, 20 degrees (at 20Hz) of phase shift within the audioband.

Frequency response (treble/bass at minimum and maximum settings, 8-ohm loading, line-level input)

Above are two frequency-response plots for the balanced line-level input, measured at 10W (8-ohm) at the speaker outputs, with the treble and bass controls set at both minimum and maximum. They show that the X5 will provide a maximum gain/cut of approximately 12dB at 20Hz, and a maximum gain/cut of approximately 9dB at 20kHz.

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

The chart above shows the X5’s frequency response as a function of input type. The green trace is the same analog input data from the previous graph. The blue trace is for a 16bit/44.1kHz dithered digital input signal from 5Hz to 22kHz using the coaxial input, 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. The behavior at low frequencies is the same for all the digital sample rates: -0.6dB at 20Hz. The behavior at high frequencies for all three digital sample rates is as expected, offering filtering around 22kHz, 48kHz, and 96kHz (half the respective sample rate). The 44.1kHz sampled input signal does not exhibit the typical “brick-wall”-type behavior found in most DACs, with a -3dB point at 18kHz. The -3dB point for the 96kHz sampled data is at 38kHz, and 71kHz for the 192kHz sampled data.

Frequency response (8-ohm loading, MM and MC phono inputs)

The chart above shows the frequency responses for the MM and MC phono inputs. The responses represent deviations 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 MC and MM inputs perform almost identically, and, as you can see, adhere very closely to the ideal RIAA curve. Both traces show very small maximum deviations of about -0.2/-0.15dB (20Hz/20kHz) and +0.1dB (100Hz) from 20Hz to 20kHz.

Phase response (MM and MC phono inputs)

Above is the phase response plot from 20Hz to 20kHz for the phono input for both the MM and MC configuration—they behaved identically—measured across the speaker outputs at 10W into 8 ohms. 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 5kHz.

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 line-level output of the X5. 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 types performed similarly, approaching the ideal 0dB relative level at -100dBFS, then yielding perfect results to 0dBFS. At or near -120dBFS, the 16/44.1 data overshot by 4dB (left) and undershot by 2dB (right) the ideal output signal amplitude, while the left/right channels at 24/96 overshot by 2dB (left) and 1dB (right).

Impulse response (16/44.1 and 24/96 data)

The graph above shows the impulse responses for a -20dBFS 16/44.1 dithered input stimulus (red), and -20dBFS 24/96 dithered input stimulus (green), measured at the line-level output of the X5. The shape is similar to that of a typical sinc function filter, although with less 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 output of the X5. The J-Test was developed by Julian Dunn the 1990s. It is a test signal, specifically a -3dBFS, undithered 12kHz square wave sampled (in this case) at 48kHz (24 bit). Since even the first odd harmonic (i.e., 36kHz) of the 12kHz square wave 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 sine-wave jitter by the Audio Precision, to show how well the DAC rejects jitter.

The coaxial S/PDIF X5 input shows obvious peaks in the audioband from -110dBrA to -140dBrA. This is an indication that the X5’s DAC may be susceptible to jitter.

J-Test (optical input)

The chart above shows the results of the J-Test test for the optical digital input measured at the line-level output of the X5. The results here are worse than the coaxial input, with the highest peaks above -100dBrA, so this input is likely more susceptible to jitter.

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 X5, with an additional 10ns of 2kHz sinewave jitter injected by the APx555. The results are very clear, as we see the sidebands at 10kHz and 14kHz (12kHz main signal +/-2kHz jitter signal) manifest at near -70dBrA. This is a clear indication that the DAC in the X5 has poor jitter immunity. For this test, the optical input yielded effectively the same results.

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

The chart above shows a fast Fourier transform (FFT) of the X5’s line-level output with white noise at -4dBFS (blue/red), and a 19.1 kHz sinewave at 0dBFS fed to the coaxial digital input, sampled at 16/44.1. The shallow roll-off around 20kHz in the white-noise spectrum shows that the X5 does not use a brick-wall-type reconstruction filter. As a result, there are obvious aliased images (and/or resultant inter-modulated signals between either the alias or signal harmonics) within the audioband, reaching -90dBrA around 13kHz. The primary aliasing signal at 25kHz is just below -10dBrA, while the second- and third-distortion harmonics (38.2, 57.3kHz) of the 19.1kHz tone are at -90 and -80dBrA.

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 balanced analog line-level input swept from 5Hz to 100kHz. The blue plot is into an 8-ohm load, the purple is into a 4-ohm load, the pink plot is an actual speaker (Focal Chora 806, measurements can be found here), and the cyan plot is no load connected. The chart below . . .

. . . is the same but zoomed in to highlight differences. Zoomed in, we can see a maximum deviation within the audioband of only about 0.15dB (at 20kHz) from 4 ohms to no load, and much less (0.05dB) within the flatter portion of the curve, which is an indication of a high damping factor, or low output impedance. The maximum variation in RMS level when a real speaker was used was about the same, deviating by about 0.05dB within the flat portion of the curve (100Hz to 5kHz), 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 3-5kHz. The significant deviation in RMS at 10kHz to 20kHz between no load and 4 ohms is an indication of a strong dip in damping factor in this frequency range. This can be seen in our damping factor graph (see the last chart in this report).

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 at 1W output into 8 ohms, purple/green at 10W, and pink/orange at the full rated power of 350W. The power was varied using the volume control. The 10W and 1W data exhibited effectively the same THD values, and remained commendably flat with the entire audioband, between 0.005% and 0.01%. At the full rated power of 350W, THD values were the lowest at 20Hz (0.007%), then increase to near 0.05% from 100Hz to 2kHz, then up again to 0.1% at 10kHz-20kHz.

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

The graph above shows THD ratios as a function of frequency plots for the phono input measured across an 8 ohms 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. The THD values for the MM configuration vary from around 0.01% (20Hz and 20kHz) down to just above and below 0.005% (150Hz to 10kHz).  The MC THD values were higher, ranging from 0.3% (20Hz, left channel), down to 0.004% (2kHz, left channel). Between 1kHz and 3kHz, the left channel outperformed the right channel for the MC configuration by as much as 10dB.

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 X5 as a function of output power for the balanced line-level input, for an 8-ohm load (blue/red for left/right) and a 4-ohm load (purple/green for left/right). The 8-ohm data outperformed the 4-ohm data by about 5-6dB, and both data sets show fairly constant THD values across measured output power levels until the “knees” at 250W (8 ohm loading) and near 500W (4 ohm loading). THD levels for the 8-ohm data are around 0.005-0.007%, and 0.01-0.015% for the 4-ohm data. The 1% THD mark for the 8-ohm data is at 390W, and 646W 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 X5 as a function of output power for the balanced line-level-input, for an 8 ohms load (blue/red for left/right) and a 4 ohms load (purple/green for left/right). Overall, THD+N values for the 8-ohm load before the “knee” ranged from around 0.1% (50mW) down to about 0.006%. 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 X5 as a function of load (8/4/2 ohms) for a constant input voltage that yielded 20W at the output into 8 ohms (and roughly 40W into 4 ohms, and 80W 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 each halving of the load. Overall, even with a 2-ohm load at roughly 80W, THD values ranged from as low as 0.02% through most of the audioband 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 -85dBrA, or 0.005%; and around -110dBrA, or 0.0003%, at the odd third harmonic (3kHz); and -100dBrA, or 0.001%, for the fifth harmonic (5kHz). Below 1kHz, we see peaks from power-supply noise artifacts at 60Hz (around -100dBrA or 0.001%), and then the odd harmonics (180Hz, 300Hz, 420Hz) dominating at between -90dBrA, or 0.003%, and -100dBrA, or 0.001%.

FFT spectrum – 1kHz (digital input, 16/44.1 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 coaxial digital input, sampled at 16/44.1. Both the signal and noise harmonic peaks are very similar or identical to the analog input FFT above. The notable exception is the third signal harmonic (3kHz), here at -100dBrA, or 0.001%, which is 10dB higher than for the analog input.

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 coaxial digital input, sampled at 24/96. We see essentially the same signal and noise harmonic profile within the audioband as with the 16/44.1 sampled input.

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 sine-wave stimulus at the coaxial digital input, measured at the output across an 8-ohm load. We only see the 1kHz primary signal peak, at the correct amplitude, along with the 60Hz power supply peak (-100dBrA) with a multitude of subsequent harmonics.

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

Shown above is the FFT for a 1kHz -90dBFS dithered 24/96 input sine-wave stimulus at the coaxial digital input, measured at the output across an 8-ohm load. We only see the 1kHz primary signal peak, at the correct amplitude, along with the 60Hz power-supply peak (-100dBrA) with a multitude of subsequent harmonics.

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 signal harmonic profile is similar to the line-level balanced input, with the second harmonic dominating at -85dBrA, or 0.005%. The noise peaks also follow the same pattern, with the odd harmonics dominating, but at higher levels (up to -85dBrA, or 0.005%, at 180Hz) due to the increased gain.

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. The main signal harmonic is again the second harmonic (2kHz) at around -80dBrA, or 0.01%. What dominates the FFT are the noise peaks, which due to the very high gain required for an MC cartridge, are as high as almost -55dBrA, or around 0.2% at 180Hz and 300Hz.

FFT spectrum – 50Hz (line-level input)

Shown above is the FFT for a 50Hz input sinewave stimulus measured at the output across an 8-ohm load at 10W for the 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 -85dBrA, or 0.005%; and the third power-supply-noise harmonic (180Hz) at just below -90dBrA, or 0.003%.

FFT spectrum – 50Hz (MM phono input)

Shown above is the FFT for a 50Hz input sinewave stimulus measured at the output across an 8-ohm load at 10W for the phono input configured for MM. The most predominant (non-signal) peaks are that of the signal’s second (100Hz) harmonic at -85dBrA, or 0.005%; and the primary and third power-supply-noise harmonics (60Hz, 180Hz) at the same level.

FFT spectrum – 50Hz (MC phono input)

Shown above is the FFT for a 50Hz input sinewave stimulus measured at the output across an 8-ohm load at 10W for the phono input configured for MC. The most predominant (non-signal) peaks are that of primary, third and fifth power supply noise harmonic (60, 180Hz, 300Hz) at just above (left) and below (right) -60dBrA or 0.1%. There are no clear signal harmonics above the higher noise floor.

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 -90dBRA, or 0.003%, while the third-order modulation products, at 17kHz and 20kHz, are lower, at just below -100dBrA, or 0.001%.

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 sine-wave stimulus tone measured at the output across an 8-ohm load at 10W for the digital coaxial input at 16/44.1. We find that the second-order modulation product (i.e., the difference signal of 1kHz) is at -95dBRA, or about 0.002%, while the third-order modulation products, at 17kHz and 20kHz, are just below -100dBrA, or 0.001%. We also see the main aliased peaks at 25.1kHz and 26.1kHz around -20dBrA and their IMD products.

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 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 -90dBRA, or about 0.003%, while the third-order modulation products, at 17kHz and 20kHz, are at -95dBrA, or 0.002%.

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 sine-wave stimulus tone measured at the output across an 8-ohm load at 10W for the phono input configured for MM. Here we find close to the same result as with the balanced line-level analog input. The second order 1kHz peak is at -90dBrA, or 0.003%, while the third order peaks are at -105dBrA, or 0.0006%.

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

Shown above is an FFT of the intermodulation (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 MC. The second-order 1kHz peak is at -90/85dBrA (left/right), or 0.003%/0.006%, while the third-order peaks are at -95/105dBrA (left/right), or 0.002%/0.0006%.

Square-wave response (10kHz)

Above is the 10kHz squarewave response using the analog line-level input, at roughly 10W into 8 ohms. Due to limitations inherent to the Audio Precision APx555 B Series analyzer, this graph should not be used to infer or extrapolate the X5’s slew-rate performance; rather, it should be seen as a qualitative representation 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 X5’s reproduction of the 10kHz square wave is very clean, with only very mild softening in the edges.

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

The final graph above is the damping factor as a function of frequency. Both channels show relatively steady decline in damping factors from low to high frequencies, with the right channel slightly outperforming the left. The right channel measured from around 590 down to 128, while the left measured from about 570 down to 118.

Diego Estan
Electronics Measurement Specialist

Details

Parent Category: Products

Category: Amplifier Measurements

Created: 01 August 2021

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

Details

Parent Category: Products

Category: Amplifier Measurements

Created: 15 July 2021

Link: reviewed by Aron Garrecht on SoundStage! Ultra on July 15, 2021

General information

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

The SPL Performer m1000 was conditioned for 1 hour at 1/8th power (52W, 8 ohms) before any measurements were taken. Unless otherwise stated, the m1000 was connected to a dedicated 120V/20A circuit.

The m1000 offers a balanced (XLR) input connector, as well as a trim pot to attenuate the input signal between 0 and -5.5dB in 0.5dB increments. This knob was left at the 0dB position for all measurements.

Published specifications vs. our primary measurements

The table below summarizes the measurements published by SPL for the m1000 compared directly against our own. The published specifications are sourced from SPL’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, a measurement input bandwidth of 10Hz to 90kHz.

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

For the continuous dynamic power test, the m1000 was able to sustain 464W (0.43dB over rated output and roughly 1% THD) into 8 ohms using an 80Hz tone for 500ms, alternating with a signal at -10dB of the peak (46W) 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 and sides of the m1000 were slightly warm to the touch.

Frequency response (8-ohm loading)

In our measured frequency response plot above, the m1000 is essentially flat within the audioband (20Hz to 20kHz), and only -0.4dB down at 20kHz. SPL’s claim of 10Hz-80kHz -3dB is only half corroborated, as we found the amplifier -3dB point at 60kHz (SPL claims 80kHz). The m1000 should not be considered a high-bandwidth audio device.

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, swept from 5Hz to 100kHz. The blue plot is into an 8 ohms load, the purple is into a 4 ohms load, the pink is an actual speaker (Focal Chora 806, measurements can found here), and the cyan is no load connected. The chart below . . .

. . . is the same but zoomed in to highlight differences. Here we find that there’s a total deviation of less than 0.2dB in the flat portion of the curve, which is an indication of a relatively high damping factor, or low output impedance. At 20kHz, the spread is larger, at just over 0.3dB. The maximum variation in RMS level when a real speaker was used as a load is very small, deviating by about 0.1dB within the flat portion of the curve (20Hz to 10kHz), 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 3-5kHz.

Phase response

Above is the phase response plot for the m1000 from 20Hz to 20kHz. The SPL does not invert polarity, and there is virtually no phase shift throughout the audioband, with a worst case of just under +20 degrees at 20kHz.

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

The chart above shows THD ratios at the m1000’s output into 8 ohms as a function of frequency (20Hz to 20kHz) for a sine wave stimulus at the balanced line-level input. The blue plot is at 1W output into 8 ohms, purple at 10W, and pink at the rated 420W. Between 20Hz and 5kHz, the lowest THD ratios were achieved at 420W, and the highest THD values at 1W, with a near 10dB difference. The 10W data lies in the middle. Like most amplifiers, THD ratios rose as a function of frequency above 2kHz or so. At 420W, THD values ranged from 0.02% from 20Hz to 1kHz, then up to 6% at 20kHz. At 10W, THD values ranged from 0.04% to 0.4%, and at 1W, from 0.05% to 0.9%.

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

The chart above shows THD ratios measured at the output of the m1000 as a function of output power for the balanced line-level input, for an 8-ohm load (blue) and a 4-ohm load (purple). As is typical, the 8-ohm data yielded slightly lower THD values (about 5dB). At 50mW, THD values are around 0.15/0.2% (8/4 ohms), and at 200W, around 0.02/0.04% (8/4 ohms). The “knee” occurs in the 8-ohm data around 400W, hitting the 1%THD mark at 495W. Into 4-ohms, the “knee” occurs around 800W, hitting the 1% THD mark at 850W.

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

The plot above shows THD+N ratios measured at the output of the m1000 as a function of output power for the balanced line-level input, for an 8-ohm load (blue) and a 4-ohm load (purple). Once again, the 8-ohm data outperformed the 4-ohm data by about 5dB. At 50mW, THD+N values are just above (4-ohm) and below (8-ohm) 0.2%, dipping down to around 0.02% (8-ohm) and 0.04% (4-ohm) at 200W.

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

The chart above shows THD ratios measured at the output of the m1000 as a function of load (8/4/2 ohms) for a constant input voltage that yields 20W at the output into 8 ohms (and roughly 40W into 4 ohms, and 80W into 2 ohms) for the balanced 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 a small 2-3dB degradation in THD performance from 8 to 4 ohms and then from 4 to 2 ohms across most of the audioband, where values ranged from 0.03 to 0.04 to 0.05%. At 20kHz, all three data sets show THD near 0.3%. This graphs shows that the m1000 is stable and a solid performer when presented with a 2-ohm load.

FFT spectrum – 1kHz

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. We see that the second harmonic, at 2kHz, is at -70dBrA, or 0.03%, while the third and fourth harmonics, at 3kHz and 4kHz, are at -80 dBrA, or 0.01%. Higher-order signal harmonics are also visible but at lower amplitudes. Below 1kHz, we see noise artifacts, with the worst-case peak at 120Hz (second harmonic of the 60Hz fundamental) at -100dBrA, or 0.001%. Other multiples of the fundamental noise harmonic (e.g., 60Hz, 180Hz, 240Hz, etc), as well as IMD products, are visible below -100dBrA.

FFT spectrum – 50Hz

Shown above is the FFT for a 50Hz input sine-wave stimulus measured at the output across an 8-ohm load at 10W. 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 second (100Hz) and third harmonic (150Hz) of the 50Hz signal are at -70dBrA and -80dBrA respectively, or 0.03% and 0.01%. The peaks from noise harmonics are between -130dBrA and -100dBrA, or 0.00003% and 0.001%.

Intermodulation distortion FFT (18kHz + 19kHz summed stimulus)

Shown above is the FFT of the intermodulation (IMD) products for an 18kHz + 19kHz summed sinewave stimulus tone at the balanced 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 -75dBrA, or 0.02%, while the third-order modulation products, at 17kHz and 20kHz, are near -85dBrA, or 0.006%.

Square-wave response (10kHz)

Above is the 10kHz square-wave response of the m1000 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 m1000’s slew-rate performance. Rather, it should be seen as a qualitative representation of the m1000’s somewhat limited 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 and corners. The m1000’s reproduction of the square wave is clean, but with some softening of the edges and corners.

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

The final plot above is the damping factor (8 ohms) of the m2000 as a function of frequency. Between 20Hz and 2kHz, the damping factor is fairly constant at near 140. Above 2kHz, the damping factor dips down to just below 60 at 20kHz.

Diego Estan
Electronics Measurement Specialist

Details

Parent Category: Products

Category: Amplifier Measurements

Created: 15 June 2021

Link: reviewed by Philip Beaudette on SoundStage! Hi-Fi on June 15, 2021

General Information

The B135³ (Cubed) was conditioned for one hour at 1/8th full rated power (~17W 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 B135³ offers six sets of line-level unbalanced (RCA) inputs; a set of fixed, line-level unbalanced (RCA) outputs; a set of variable unbalanced (RCA) pre-outs; a set of unbalanced (RCA) main-ins; and a pair of speaker outputs. Based on the accuracy of the left/right channel matching (see table below), and 0.5dB volume-step resolution throughout its range, the B135³ volume knob is not a potentiometer in the signal path, but, rather, provides digital control (analog domain) over a proprietary or integrated volume circuit.

All measurements, with the exception of signal-to-noise (SNR) or as otherwise stated, were made with the volume set to unity gain for the preamplifier (about 2 o’clock) as measured at the pre-outputs. Signal-to-noise ratio (SNR) measurements were made with the volume control set to maximum. At the unity gain volume position, to achieve 10W into 8 ohms, 310mVrms was required at the RCA 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 Bryston for the B135³ compared directly against our own. The published specifications are sourced from Bryston’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, a measurement input bandwidth of 10Hz to 90kHz, and the worst-case measured result between the left and right channel.

Our primary measurements for the B135³ integrated amplifier as a whole revealed the following using the RCA line-level inputs (unless specified, assume a 1kHz sinewave, 10W output, 8-ohm loading, 10Hz to 90kHz bandwidth):

For the continuous dynamic power test, the Bryston able to sustain 159W into 8 ohms using an 80Hz tone for 500ms, alternating with a signal at -10dB of the peak (15.7W) for five seconds, for five continuous minutes without inducing a fault or the initiation of a protective circuit. Although the peak power level for the test was just below the 1% THD+N level, the Clip indicator never light up during the high power bursts during the five-minute measurement. This test is meant to simulate sporadic dynamic bass peaks in music and movies. During the test, the B135³ heatsinks were quite warm to the touch, where touching for more than five seconds would induce pain.

Our primary measurements revealed the following using the balanced line-level inputs at the headphone output (unless specified, assume a 1kHz sinewave, 2Vrms output, 300 ohms loading, 10Hz to 90kHz bandwidth):

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

In our measured frequency response plot above, the B135³ is perfectly flat within the audioband (20Hz to 20kHz) and beyond. These data come very close to corroborating Bryston’s claim of 20Hz to 20kHz +/-0.05dB (although Bryston claims this for the preamp section, while the graph above is for the integrated amp as a whole). The B135³ is -0.02dB at 5Hz, -0.14dB at 20kHz, and -3dB at about 70kHz. The B135³ should not be considered a high-bandwidth audio device, as the -3dB point is below 100kHz. In the chart above and most of the charts 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.

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, swept from 5Hz to 100kHz. The blue plot is into an 8-ohm load, the purple is into a 4-ohm load, the pink is an actual speaker (Focal Chora 806, measurements can found here), and the cyan is no load connected. The chart below . . .

. . . is the same but zoomed in to highlight differences. Here we find that there’s a total deviation of less than 0.1dB in the flat portion of the curve, which is an indication of a high damping factor, or low output impedance. At 20kHz, the spread is larger, at about 0.15dB. The maximum variation in RMS level when a real speaker was used as a load is very small, deviating by less than 0.1dB within the flat portion of the curve (20Hz to 10kHz), 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 5kHz.

Phase response

Above is the phase response plot from 20Hz to 20kHz. The B135³ does not invert polarity, and the plot shows very little phase shift, with a worst case of under +10 degrees at 20kHz.

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 (20Hz to 20kHz) for a sinewave input stimulus. The blue and red plots are for left and right at 1W output into 8 ohms, purple/green at 10W, and pink/orange at full rated power (135W). The power was varied using the volume control. At all frequencies and power levels, THD ratios varied between about 0.0005% and 0.005%. As is typical, there is a rise in THD values at high frequencies, however, the differences (about 5dB increase from 10kHz to 20kHz) are small.  The right channel also generally outperforms the left, especially at lower frequencies, by as much as 5dB. The lowest THD ratio values were found at 135W for the right channel between 100 and 200Hz, at 0.0005%. The highest THD values were also found at high power, at about 0.007% at 20kHz also for the right channel. At 1kHz, the 10W and 135W data measured the same at just below 0.002% THD, while the 1W data is worse at 0.003/0.002% (left/right channels).

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 B135³ as a function of output power for an 8-ohm load (blue/red for left/right) and a 4-ohm load (purple/green for left/right), with a 1kHz input sinewave. The 4-ohm data shows consistently slightly higher THD values compared to the 8-ohm data (about a 2-3dB difference). At the 50mW level, THD values measured around 0.2/0.4% (8/4 ohms), dipping down to around 0.001% at 30W to 100W for the 8-ohm data, and 0.0015% for the 4-ohm data from 50W to 150W. The “knee” in the 8-ohm data occurs at around 115W, hitting the 1% THD mark at 159W. For the 4-ohm data, the “knee” occurs near 155W, hitting the 1% THD mark at 250W.

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 B135³ as a function of output power for an 8-ohm load (blue/red for left/right) and a 4-ohm load (purple/green for left/right), with a 1kHz input sinewave. The 4-ohm data shows consistently slightly higher THD+N values compared to the 8-ohm data (about a 2-3dB difference). At the 50mW level, THD+N values measured around 0.2/0.4% (8/4 ohms), dipping down to around 0.003% at 100W for the 8-ohm data, and 0.004% for the 4-ohm data at around 150W.

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 B135³ as a function of frequency and 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). 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 fairly consistent THD ratio values across all loads up to 1kHz (between 0.001 and 0.003%).  Above 1kHz, the spread between 8/4/2 ohms is obvious, with an increase in THD of about 5dB each time the impedance is reduced. Overall, even with a 2-ohm load at roughly 40W, THD values ranged from 0.003% at 20Hz to just above 0.01% at 20kHz.

FFT spectrum – 1kHz

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 line-level input. We see that the signal’s second harmonic, at 2kHz, is at -105/-100dBrA (left/right channels), or 0.0005/0.001%, while the third harmonic, at 3 kHz, is at -100dBrA for the left channel but indistinguishable for the right channel. Odd order harmonics are obvious in the left channel, but not in the right channel, however, these are all below -100dBrA, or 0.001%.  The fourth harmonic at 4kHz is at -110dBrA, or 0.0003%. Below 1kHz, we see noise artifacts, with the 60Hz peak due to power supply noise at -95dBrA, or about 0.002%, and the 180Hz (third harmonic) peak at -100dBrA, or 0.001%. The second noise harmonic (120Hz) is only visible from the left channel, but is low at -105dBrA, or 0.0005%.

FFT spectrum – 50Hz

Shown above is the FFT for a 50Hz input sinewave stimulus measured at the output across an 8-ohm load at 10W output. 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 signal harmonic peak is that of the 3rd harmonic (150Hz) at about -100dBrA, or 0.001%, but only on the left channel, while the signal’s second harmonic peak (100Hz) is visible at -105dBrA, or 0.0005%, for both channels. The most predominant noise peak is at the fundamental (60Hz) at -95dBrA, or 0.005%, then at -105dBrA from the left channel only at the second harmonic (120Hz).

Intermodulation distortion FFT (18kHz + 19kHz summed stimulus)

Shown above is an FFT of the intermodulation (IMD) products for an 18kHz + 19kHz summed sinewave stimulus tone measured at the output across an 8-ohm load at 10W. 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 just above -110dBrA, or 0.00003%, while the third-order modulation products, at 17kHz and 20kHz are around -105dBrA, or 0.0005%. Other modulation product peaks can be seen above the -100dBrA, or 0.001%, threshold at 8, 9, and 10kHz.

Square-wave response (10kHz)

Above is the 10kHz square-wave response 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 Bryston’s slew-rate performance. Rather, it should be seen as a qualitative representation of its relatively extended bandwidth. An ideal square wave 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 B135³ reproduction of the 10kHz squarewave can be considered clean, with slightly rounded edges devoid of undershoot and overshoot.

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

The final chart above is the damping factor as a function of frequency. Both channels show a general trend of a higher damping factor at lower frequencies, and lower damping factor at higher frequencies, with roughly a factor of 2x difference at 30Hz compared to 20kHz. Both channels’ damping factors tracked fairly closely, with a peak value of 274/309 (L/R) at around 40Hz, and a low value at 125/112 (L/R) at 20kHz.

Diego Estan
Electronics Measurement Specialist

Details

Parent Category: Products

Category: Amplifier Measurements

Created: 01 June 2021

Link: reviewed by Roger Kanno on SoundStage! Hi-Fi on June 1, 2021

General Information

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

The RA-1572MKII was conditioned for one hour at 1/8th full rated power (~15W 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 RA-1572MKII offers a multitude of inputs, both digital and analog, line-level analog preamp outputs, subwoofer line-level outputs, and two pairs of speaker level outputs (A and B). For the purposes of these measurements, the following inputs were evaluated: digital coaxial 1 (RCA) and optical 1 (TosLink) S/PDIF, analog balanced line-level (XLR), and phono (moving magnet, MM). Comparisons were made between unbalanced (RCA) and balanced line-level inputs, and no differences were seen in terms of THD+N; however, the balanced input offers 4dB less gain than the unbalanced inputs. The RA-1572MKII also offers a USB input, however, I was unable to successfully recognize the RA-1572MKII using Rotel’s USB driver for Windows. Bluetooth is also offered, but our APx555 does not currently have a Bluetooth board, so that could not be tested.

Most measurements, with the exception of signal-to-noise (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 (position 92) with respect to the preamp outputs (which offers 1dB of gain with the unbalanced input, and -3dB with the balanced input). At this volume position, to achieve 10W into 8 ohms, 660mVrms was required at the balanced line-level input. For the digital inputs, a volume position of 53 yielded 10W into 8 ohms with a 0dBFS input. For the phono input, a volume position of 77 yielded 10W into 8 ohms with a 1kHz 5mVrms input. The SNR measurements were made with the volume control set to maximum.

Based on the high accuracy of the left-right volume channel matching (see table below), the RA-1572MKII volume control is likely in the analog domain but digitally controlled. The RA-1572MKII offers 100 volume steps. Between steps 1 and 10, step increases range from 7dB to 2dB. Steps 10 and 11 offer 1.5dB volume increments, steps 11 through 20 offer 1dB, then from 21 to 100 each volume step is 0.5dB.

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 Rotel for the RA-1572MKII compared directly against our own. The published specifications are sourced from Rotel’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 channels.

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

For the continuous dynamic power test, the RA-1572MKII was able to sustain 230W into 4 ohms using an 80Hz tone for 500ms, alternating with a signal at -10dB of the peak (23W) for five seconds, for five continuous minutes without inducing a fault or the initiation of a protective circuit. This test is meant to simulate sporadic dynamic bass peaks in music and movies. During the test, the top of the RA-1572MKII was warm to the touch, but did not cause discomfort to the touch.

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

Our primary measurements revealed the following using the balanced line-level inputs at the headphone output (unless specified, assume a 1kHz sinewave, 2Vrms output, 300 ohms loading, 10Hz to 90kHz bandwidth):

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

In our measured frequency-response plot above, the RA-1572MKII is nearly flat within the audioband (20Hz to 20kHz). At the extremes the RA-1572MKII is 0.5dB down at 10Hz and 0.3dB up at 100kHz. These data corroborate Rotel’s claim of 10Hz to 100kHz (+/-0.5dB). The RA-1572MKII can be considered a high-bandwidth audio device. In the chart above and most of the charts 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 (treble/bass at minimum and maximum settings, 8-ohm loading, line-level input)

Above are two frequency-response plots for the balanced line-level input, measured at 10W (8-ohm) at the speaker outputs, with the treble and bass controls set at both minimum and maximum. They show that the RA-1572MKII will provide a maximum gain or cut of approximately 12dB at 20Hz, and a maximum gain or cut of approximately 9dB at 20kHz.

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

The chart above shows the RA-1572MKII’s frequency response as a function of input type. The green trace is the same analog input data from the previous chart. 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 pink is 24/192 from 5Hz to 96kHz. The behavior at low frequencies is the same for the digital input for all sampling frequencies: -0.5dB at 20Hz. The behavior at high frequencies for all three digital sample frequencies is as expected, offering sharp filtering around 22k, 48k, and 96kHz (half the respective sample rate). It is also obvious from the plots above that the 44.1kHz sampled input signal offers the most “brick-wall”-type behavior, while the attenuation of the 96kHz and 192kHz sampled input signals approaching the corner frequencies (48kHz and 96kHz) is more gentle.

Frequency response (8-ohm loading, phono input)

What’s displayed in the chart above is the RA-1572MKII’s frequency-response deviation from the standard RIAA curve frequency response. To display that, the input signal sweep is EQ’d with an inverted RIAA curve supplied by Audio Precision. Therefore, zero deviation would yield a flat line at 0dB. The plots above show a very small maximum deviation of about -0.2dB (200Hz) from 20Hz to 20kHz, which is typical of many high-quality phono stages we have measured.

Phase response (phono input)

Above is the phase response plot from 20Hz to 20kHz for the phono input, measured across the speaker outputs at 10W into 8 ohms. 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 5kHz.

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

The plot 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 line-level output of the RA-1572MKII. 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). Both digital input types performed similarly, approaching the ideal 0dB relative level at -100 dBFS, then yielding perfect results to 0dBFS. At -120dBFS, both channels at 16/44.1 overshot the ideal output signal amplitude by 1dB (right channel) and 2.5dB (left channel), while the left and right channels at 24/96 overshot by 1dB (left channel) and just above 0dB (right channel).

Impulse response (16/44.1 and 24/96 data)

The graph above shows the impulse responses for a -20dBFS 16/44.1 (blue/red) and 24/96 (purple/green) dithered input stimulus, measured at the line level output of the RA-1572MKII. We see symmetrical pre and post ringing that is typical of “fast” or “sharp” linear-phase reconstruction filters.

J-Test (coaxial input)

The plot above shows the results of the J-Test test for the coaxial digital input, measured at the line-level output of the RA-1572MKII. The J-Test was developed by Julian Dunn in the 1990s. It is a test signal comprised of, specifically, a -3dBFS, 24-bit, undithered 12kHz square wave sampled (in this case) at 48kHz. 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 SPDIF RA-1572MKII input shows obvious peaks in the audioband from -95dBrA to just below -120dBrA. This is an indication that the RA-1572MKII’s DAC section may be susceptible to jitter, which the jitter-injected tests on the coaxial input below demonstrate.

J-Test (optical input)

The chart above shows the results of the J-Test test for the optical digital input, measured at the line level output of the RA-1572MKII. The results here are very similar to the coaxial input, but slightly worse, with the highest peaks nearing -90dBrA, indicating that the optical input may be slightly more susceptible to jitter.

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

The plot above shows the results of the J-Test test for the coaxial digital input, measured at the line-level output of the RA-1572MKII, with an additional 10ns of 2kHz sinewave jitter injected by the APx555. The results are very clear, as we see the sidebands at 10kHz and 14kHz (12kHz main signal +/-2kHz jitter signal) manifest at -70dBrA. This is a clear indication that the DAC in the RA-1572MKII has poor jitter immunity. For this test, the optical input yielded effectively the same results, so we chose to show only the coaxial result.

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 RA-1572MKII, with an additional 100ns of 2kHz sinewave jitter injected by the APx555. The results are very clear again, as we see the sidebands at 10kHz and 14kHz (12kHz main signal +/-2kHz jitter signal) manifest at -50dBrA—the RA-1572MKII’s digital section degrades further as more jitter is introduced. For this test, the optical input yielded effectively the same results, so, again, we chose to only show the coaxial input result.

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

The chart above shows a fast Fourier transform (FFT) of the RA-1572MKII’s line-level output with white noise at -4dBFS (blue/red) and a 19.1kHz sinewave at 0dBFS fed to the coaxial digital input, dithered and sampled at 16/44.1. The sharp roll-off above 20kHz in the white noise spectrum shows the implementation of a brick-wall type reconstruction filter. There are obvious aliased images (or resultant intermodulated signals between either the alias or signal harmonics) within the audioband reaching -85dBrA around 10kHz. The primary aliasing signal at 25kHz is at -60dBrA, while the second, third, and fourth distortion harmonics (38.2, 57.3, 76.4kHz) of the 19.1kHz tone are much higher in amplitude, lying between -60 and -35dBrA.

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, for the analog line-level input swept from 5Hz to 100kHz. The blue plot is into an 8-ohm load, the purple is into a 4-ohm load, the pink plot is an actual speaker (Focal Chora 806, measurements can be found here), and the cyan plot is no load connected. The chart below . . .

. . . is the same but zoomed in to highlight differences. Zoomed in, we can see a maximum deviation within the audioband of only about 0.05dB (at 20kHz) from 4 ohms to no load, which is an indication of a high damping factor, or low output impedance. The maximum variation in RMS level when a real speaker was used as a load is smaller still, deviating by a little less than 0.04dB within the flat portion of the curve (100Hz to 20kHz), 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 sinewave stimulus at the balanced line-level input. The blue and red plots are for left and right at 1W output into 8 ohms, purple/green at 10W, and pink/orange at the full rated power of 120W. The power output was varied using the volume control. The 10W data exhibited the lowest THD values (but very close to the 1W data), from just above 0.001% to 0.01%. At the full rated power of 120W, THD values track the 1 and 10W data below 300Hz, however, above this frequency, THD values steadily increase from around 0.002% to about 0.04%.

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

Above are THD ratio plots as a function of frequency for the phono input measured across an 8-ohm load at 10W output. The input sweep is EQ’d with an inverted RIAA curve. The THD values vary from just above 0.001% (500Hz to 1kHz) to 0.01% (20Hz and 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 RA-1572MKII as a function of output power for the balanced line-level input, for an 8-ohm load (blue/red for left/right), and a 4-ohm load (purple/green for left/right). The 8-ohm data outperformed the 4-ohm data by only 2-3dB and range from 0.005/0.01% at 50mW, respectively (8/4 ohms), down to just above 0.001% between 50W and 100W for both loads. The “knee” in the 8-ohm data occurs just past 100W, hitting the 1% THD mark at 143W. For the 4-ohm data, the “knee” occurs around 150W, hitting 1% THD around 226W.

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 RA-1572MKII as a function of output power for the balanced line-level input, for an 8-ohm load (blue/red for left/right) and a 4-ohm load (purple/green for left/right). 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 2-3 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 RA-1572MKII as a function of load (8, 4, and 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 20dB worse from 4 ohms to 2 ohms above 5kHz. Overall, even with a 2-ohm load at roughly 40W, THD values ranged from as low as 0.001% at around 500Hz to 0.04% at 20kHz.

FFT spectrum – 1kHz (line-level input)

Shown above is the fast Fourier transform (FFT) for a 1kHz input sinewave stimulus, measured at the output across an 8-ohm load at 10W for the balanced line-level input. We see that the signal’s second harmonic, at 2kHz, is very low at -110/-120dBrA (left/right), or 0.0003/0.0001%, and around -105dBrA, or 0.0005%, at the odd third (3kHz) and fifth (5kHz) harmonics. Below 1kHz, we see peaks from power supply noise artifacts at 60Hz (just below -110dBrA, or 0.0003%), 180Hz (just above -110dBrA), with the subsequent harmonics falling below this level.

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 output across an 8-ohm load at 10W for the coaxial digital input, sampled at 16/44.1 resolution. We see signal harmonics at higher levels compared to the analog input, reaching -90/-100dBrA, or 0.003/0.001% (left/right), at 2kHz, and exceeding -90dBrA at 3kHz.

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 output across an 8-ohm load at 10W for the coaxial digital input, sampled at 24/96. We see essentially the same signal-harmonic profile as with the 16/44.1 sampled input. The noise floor, however, is reduced by a small margin compared to the 16/44.1 sampled FFT above.

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 output across an 8-ohm load. We only see the 1kHz primary signal peak, at the correct amplitude, along with the 60Hz power-supply peak (-110dBrA, or 0.0003%) with subsequent lower-level harmonics (i.e., 120/180Hz) visible.

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 output across an 8-ohm load. As with the 16/44.1 chart above, we only see the 1kHz primary signal peak, again at the correct amplitude, and the 60Hz power-supply peak (-110dBrA, or 0.0003%) with the subsequent lower-level harmonics (i.e., 120/180Hz) visible.

FFT spectrum – 1kHz (phono input)

Shown above is the FFT for a 1kHz input sinewave stimulus, measured at the output across an 8-ohm load at 10W for the phono input. We see the signal harmonic profile is similar to the line-level balanced input–the second, third, and fifth harmonics are all below -100dBrA, or 0.001%.

FFT spectrum – 50Hz (line-level input)

Shown above is the FFT for a 50Hz input sinewave stimulus measured at the output across an 8-ohm load at 10W for the 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 signal’s second (100Hz) harmonic at -110dBrA, or 0.0003%, and the second power-supply-related noise harmonic (120Hz) at just above -110dBrA.

FFT spectrum – 50Hz (phono input)

Shown above is the FFT for a 50Hz input sinewave stimulus measured at the output across an 8-ohm load at 10W for the phono input. The most predominant (non-signal) peaks are that of the primary (60Hz) and third (180Hz) power-supply-related noise harmonics at or near -90dBrA, or 0.003%. The second signal harmonic (100Hz) cannot be seen above the noise floor.

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, at just above and below -100dBrA, or 0.001%.

Intermodulation distortion FFT (18kHz + 19kHz summed stimulus, 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. Here we find close to the same result as with the balanced line-level analog input. The second-order 1kHz peak is at -100dBrA, or 0.001%, while the third-order peaks are the same as with the balanced line-level input above.

Square-wave response (10kHz)

Above is the 10kHz square-wave 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 RA-1572MKII’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 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 RA-1572MKII’s reproduction of the 10kHz squarewave is very clean, with only very mild overshoot and undershoot in the edges.

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

The final chart shown above is the RA-1572MKII’s damping factor as a function of frequency. Both channels show relatively constant damping factors across the audioband, with the left channel slightly outperforming the right channel. The right channel measured from around 250 to 260, while the left measured from about 280 to 295.

Diego Estan
Electronics Measurement Specialist

Details

Parent Category: Products

Category: Amplifier Measurements

Created: 01 April 2021

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

General information

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

The Rotel A11 Tribute was conditioned for 1 hour at 1/8th full rated power (~6W 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 A11 Tribute offers four sets of line-level unbalanced inputs (RCA), one moving magnet (MM) phono input (RCA), a set of variable line-level preamp outputs (RCA), and two pairs of speaker outputs. Also available is Bluetooth input (untested) and a headphone output via a 3.5mm TRS jack on the front panel. Based on the accuracy of the left/right channel matching (see table below), the A11’s volume knob is not a potentiometer in the signal path, but rather provides digital control over a proprietary or integrated, analog-domain volume circuit. The volume control offers 1dB increments from -79dB to +14.6dB as measured at the preamp outputs using a line-level input.

All measurements, with the exception of signal-to-noise ratio (SNR) or otherwise stated, were made with the volume set to unity gain for the preamplifier (position 82) as measured at the preamp outputs. SNR measurements were made with the volume control set to maximum. At the unity gain volume position, to achieve 10W into 8 ohms, 383mVrms was required at the line level input and 5mVrms at the phono 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 Rotel for the A11 Tribute compared directly against our own. The published specifications are sourced from Rotel’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, a measurement input bandwidth of 10Hz to 90kHz, and the worst-case measured result between the left and right channels.

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

Our clipping headroom result was 1.7dB for the A11 Tribute, defined as the ratio of max power over rated power into 8 ohms. The A11 Tribute was also able to sustain 75W (1.7dB over rated output) into 8 ohms using an 80Hz tone for 500 ms, alternating with a signal at -10dB of the peak (7.5 W) 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 A11 was warm to the touch, but not quite hot enough to induce pain.

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

Our primary measurements revealed the following using the unbalanced line-level inputs at the headphone output (unless specified, assume a 1kHz sinewave, 2Vrms output, 300-ohm loading, 10Hz to 90kHz bandwidth):

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

In our measured frequency-response plot above, the A11 Tribute is essentially flat within the audioband (20Hz to 20kHz) for the line-level input. These data corroborate Rotel’s claim of 10Hz to 100kHz (+/-0.5dB), since the worst-case deviation is at 10kHz where the response is about -0.2dB. The A11 can be considered a high-bandwidth audio device as the response at 100kHz is approximately 0dB. In the chart above and most of the charts 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 (treble/bass at minimum and maximum settings, 8-ohm loading, line-level input)

Above are two frequency response plots for the line-level input, measured at 10W (8-ohm) at the speaker outputs, with the treble/bass controls set at both minimum and maximum. They show that the A11 Tribute will provide a maximum gain/cut of approximately 13dB at 20Hz, and a maximum gain/cut of approximately 9dB at 20kHz.

Frequency response (8-ohm loading, phono input)

The chart above shows frequency response for the phono input, and shows a maximum deviation of about +4dB at 50Hz from flat within the audio band. 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., no deviation from the RIAA reference would yield a flat line at 0dB).

Phase response (phono input)

Above is the phase response plot from 20Hz to 20kHz for the phono input. 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 -60/-50 degrees at 200/5000Hz and +20 degrees at 20Hz.

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 line-level input swept from 5Hz to 100kHz. The blue plot is into an 8-ohm load, the purple is into a 4-ohm load, the pink is an actual speaker (Focal Chora 806, measurements can be found here), and the cyan is no load connected. The chart below . . .

. . . is zoomed in to highlight differences. Here we find that there’s a total deviation of about 0.1dB throughout the audioband, which is an indication of a high damping factor, or low output impedance. The maximum variation in RMS level when a real speaker was used as a load is very small, deviating by just under 0.1dB within the audioband (20Hz to 20kHz), 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 1-2kHz. 

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 (20Hz to 20kHz) for a sine-wave stimulus at the 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 the full rated power of 50W. The power was varied using the volume control. All three THD plots exhibit a rise in THD above a “knee” at roughly 1-3kHz. The graphs are a little difficult to follow because above 1kHz, there is an up-to-10dB channel deviation in THD at all power levels, where the right channel outperformed the left one. At all power levels, THD values were remarkably close to one another. The biggest deviations were between 200 and 500Hz, where the 1W and 10W data show THD values around 0.004%, while at 50W we see 0.005%. At 20Hz, all THD values are around 0.02%. At 20 kHz, between 0.03% (left) and 0.02% (right).

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

This chart shows THD ratios as a function of frequency for the phono input measured across an 8-ohm load at 10W output. The input sweep is EQ’d with an inverted RIAA curve. The THD values vary from near 0.1% at 20 and 30Hz, down to about 0.002% (right channel) at 2.5kHz and 0.005% (left channel) at 500Hz, then up to about 0.02/0.04% (right/left channels) at 20 kHz. As with the chart above, we again see significant channel-to-channel THD deviations, with as much as a 15dB difference in favor of the right channel over the left at 3kHz.

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 A11 Tribute as a function of output power for the line level-input, for an 8-ohm load (blue/red for left/right channel) and a 4-ohm load (purple/green for left/right channel). The 4-ohm data shows consistently slightly higher THD values compared to the 8-ohm data (about a 6-7dB difference), and the right channel outperforms the left by about 2dB in both plots. At the 50mW level, THD values measured around 0.01% and stay roughly around this level for the 4-ohm data until the “knee” around 70W, then hitting the 1% THD mark at 101W. The 8-ohm data improves on this with THD levels around 0.003/0.004% from 0.5 to 10W. The “knee” in the 8-ohm data occurs at 50W, then hitting the 1% THD mark at 74W.

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 A11 Tribute as a function of output power for the line level-input, for an 8-ohm load (blue/red for left/right channel) and a 4-ohm load (purple/green for left/right channel). The 4-ohm data shows consistently slightly higher THD+N values compared to the 8-ohm data (about a 5dB difference). At the 50mW level, THD+N values measured around 0.1% (8 ohms) and 0.2% (4 ohms), dipping down to around 0.01% at 20W to 50W for the 8-ohm data, and 0.02% for the 4-ohm data from 20 to 70W.

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 A11 as a function of load (8, 4, and 2 ohms) for a constant input voltage that yields 5W at the output into 8 ohms (and roughly 10W into 4 ohms, and 20W into 2 ohms) for the 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. However, in the case of the A11, the 2-ohm trace is not available because the protection circuit was immediately engaged when trying to drive this load, which likely means that this amplifier was not design to drive a load that demanding. Thankfully, it protects itself. We find increasing levels of THD from 8 to 4 ohms, with about a 5dB difference from 20Hz to 20kHz. Overall, even with a 4-ohm load at roughly 10W, THD values ranged from 0.02% at 20Hz to just below 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 line-level input. As shown in the results above, we see that the right channel outperforms the left with slightly lower peaks. We see that the signal’s second and third harmonics, at 2 and 3kHz, are at around -90/100dBrA, while the remaining harmonics are below -100dBrA (with the exception of the left channel at 4kHz sitting at -100dBrA). Below 1kHz, we see noise artifacts, with the 60Hz peak due to power-supply noise just below -110dBrA, and the 120Hz peak at -80dBrA, and the fourth harmonic at 240Hz at just below -90dBrA.

FFT spectrum – 1kHz (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. The second signal harmonic at 2kHz is at -85/-95dBrA (right/left channel), with subsequent harmonics below this level. Here again, the right channel clearly outperformed the left in terms of signal distortion, for example, at the fourth signal harmonic, the peak for the left channel is at -95dBrA, while the right channel is below -110dBrA. The highest peak from power supply noise is at the fundamental (60Hz), reaching just below -60dBrA, and the second noise harmonic (120Hz) is at -80dBrA.

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 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 peak aside from the fundamental is that of the noise signal’s second harmonic (120Hz) at about -80dBrA. The second most significant peak is from the signal second harmonic (100Hz) at around -85dBrA.

FFT spectrum – 50Hz (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. The most predominant peak aside from the signal fundamental is the noise signal’s fundamental (60Hz) at just below -60dBrA. The most predominant signal harmonic peak is the second harmonic (100Hz) at -85dBrA.

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

Shown above is an FFT of the intermodulation (IMD) products for an 18kHz + 19kHz summed sinewave stimulus tone measured at the output across an 8-ohm load at 10W for the 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 very low at -115/-105dBrA (right/left channel), or 0.0002%/0.0005%, while the third-order modulation products, at 17kHz and 20kHz, are at around -105dBrA, or 0.0005%.

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

Shown above is an FFT of the intermodulation (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. Here we find that the second-order modulation product (i.e., the difference signal of 1kHz) is just above -100dBrA, or 0.001%, for both channels. The third-order modulation products, at 17kHz and 20kHz, are at around -105dBrA.

Square-wave response (10kHz)

Above is the 10kHz square-wave response using the 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 Rotel’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 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 A11’s reproduction of the 10kHz square-wave can be considered very clean, with sharp edges and no overshoot and/or undershoot.

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

This final chart is the damping factor as a function of frequency. Both channels show a trend of a higher damping factor at lower frequencies, and lower damping factor at higher frequencies (about 15% lower at 20kHz compared to 20Hz). The right channel outperformed the left with a peak value around 150, while the left channel achieved a peak damping factor of around 140.

Diego Estan
Electronics Measurement Specialist