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

Details

Parent Category: Products

Category: Amplifier Measurements

Created: 15 March 2021

Link: reviewed by Evan McCosham on SoundStage! Hi-Fi on March 15, 2021

General information

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

The C 298 was conditioned for 30 minutes at 10W (8 ohms) before any measurements were performed. All measurements were taken with both channels driven, unless otherwise stated, with the C 298 connected to a dedicated 120V/20A circuit.

The C 298 offers one set each of balanced (XLR) and unbalanced (RCA) inputs, as well as a set of fixed line-level unbalanced (RCA) outputs, for daisy-chaining another amplifier or amplifiers. Unless otherwise specified, the balanced input connections were used for all measurements. No differences were seen between unbalanced and balanced input connections in terms of THD+N. There is a toggle switch on the back panel to choose between fixed gain or variable gain, for which a small potentiometer offers adjustments from 8.5 to 28.5dB (fixed gain is 28.7dB). There was a difference between the fixed- and variable-gain modes in terms of THD+N. In fixed mode, THD+N (A-weighted) at 10W into 8 ohms measured approximately 0.0004%, while variable mode yielded 0.0006%. All measurements, with the exception of gain-control accuracy, were performed in fixed-gain mode.

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

Published specifications vs. our primary measurements

The table below summarizes the measurements published by NAD for the C 298 compared directly against our own measurements. The published specifications are sourced from NAD’s website, either directly printed on the site or from the manual available for download. 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.

* the power level at the “knee” from our measured THD ratio vs output power graphs below

** the NAD THD specification on their website is published as “20Hz-20kHz,” however, after discussions with NAD, it was discovered that the company actually measures THD from 20Hz to 6.5kHz, with a 20Hz to 20kHz input bandwidth filter.

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 C 298 was not able to sustain 528W (4 ohms) using an 80Hz tone burst (500ms) without causing the protection circuit to almost immediately shut down the amp. But we were able to achieve 517W into 4 ohms for 500ms, alternating with the same signal at -10dB of the peak (51.7W) for five seconds, for five continuous minutes without inducing a fault or the initiation of a protective circuit. During the test, the top of the C 298 was slightly warm to the touch. This test is meant to simulate sporadic dynamic bass peaks in music and movies.

Frequency response (8-ohm loading)

In our frequency-response chart above, the C 298 is essentially flat within the audioband (20Hz to 20kHz), with the exception of an insignificant 0.02dB rise between 10 and 20kHz, and a 0.02dB dip at 20Hz. NAD’s claim of +/-0.2dB from 20Hz to 20kHz is corroborated by our measurement, as is the -3dB point at 60kHz. The C 298 cannot be considered a high-bandwidth audio device however, due to a steep rolloff starting at about 30kHz. In the graph above and most of the graphs below, only a single trace may be easily 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 8 ohms or 2.83Vrms) as a function of frequency, for the balanced 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. The graph above shows that all four plots are indistinguishable from one another. This is an indication of a high damping factor, or low output impedance.

The graph above shows the same data as the graph above it, but with the vertical axis expanded to show differences. Here we find that there’s a total deviation of only 0.02dB in the flat portion of the curve between 4 ohms and no load. At 20kHz, the spread is a little larger, perhaps 0.03dB. The maximum variation in RMS level when a real speaker was used as a load is extremely small, deviating by less than 0.01dB within the flat portion of the curve. There is a rise around 10kHz, which is due to the C 298’s inherent frequency response and not the output impedance of the C 298 interacting with a load that varies with frequency.

Phase response

Above is the phase-response plot for the C 298 from 20Hz to 20kHz. The C 298 does not invert polarity, and there is virtually no phase shift throughout the audioband, with a worst case of under +10 degrees at 20Hz.

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

The chart above shows THD ratios at the C 298’s output into 8 ohms as a function of frequency (20Hz to 20kHz) for a sinewave stimulus at the balanced line-level inputs. The blue and red plots are for left and right channels at 1W output into 8 ohms, purple/green at 10W, and pink/orange at the rated 185Wpc. There is about a 10dB improvement in THD ratios at 10W versus 1W. At 10W, the THD values are extremely low from 20Hz to 2kHz, hovering above and below 0.0001%. Due to these extremely low values (as low as 0.00007%), it’s important to note that we are approaching the limits of the APx555 analyzer, which inherently measures at approximately 0.00001-0.00002% THD when fed its own signal at these output levels. The rise in THD above 2kHz is steeper at 10W compared to 1W, measuring about 0.01% at 20kHz against the 0.003% value at 20kHz for the 1W data. The general trend at 185W is more of a linear rise in THD as a function of frequency. For the 185W data, at 20Hz, we measured just below 0.0001%, 0.0005% at 1kHz, and rising all the way up to 0.03% at 20kHz, which is still low.

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

The chart above shows THD ratios measured at the output of the C 298 as a function of output power for the balanced line-level inputs, for an 8-ohm load (blue/red for left/right) and a 4-ohm load (purple/green for left/right). At 50mW, THD values are near 0.001% for both 8- and 4-ohm data. At 10W, the 8-ohm THD value is at 0.0001%, and just above 0.0002% at 4 ohms. At the “knees” in the curves, the 8-ohm THD value is at around 0.0004%, with power nearing 200W, while the 4-ohm THD value is at around 0.002%, with power at just over 400W.

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

The chart above shows THD+N ratios measured at the output of the C 298 as a function of output power for the balanced left and right line-level inputs, for an 8-ohm load (blue/red for left/right) and a 4-ohm load (purple/green for left/right). At 50mW, THD+N values are near 0.05% for both 8- and 4-ohm data, dipping down to 0.003% between 50 and 100W (8 ohms), and roughly 0.005% between 50 and 100W (4 ohms).

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

The chart above shows THD+N ratios (A-weighted) measured at the output of the C 298 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). At 50mW, THD+N values are near 0.005% for both 8- and 4-ohm data, dipping down to 0.0003% between 20 and 100W (8 ohms), and roughly 0.0006% between 10 and 50W (4 ohms).

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 C 298 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 increasing levels of THD from 8 to 4 to 2 ohms, with about a 5-10dB difference between 8 and 4 ohms from 20Hz to 3kHz, and a 10-15dB difference between 4 and 2 ohms from 20Hz to 5kHz. There’s a convergence of THD values between all loads at higher frequencies. Overall, even with a 2-ohm load at roughly 80W, THD values ranged from 0.001% 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 output. We see that the second harmonic, at 2kHz, is at a very low -130dBrA (relative to the reference 0dB signal), or 0.00003%, while the third harmonic, at 3kHz, is just below -120dBrA, or 0.0001%. The fourth and fifth harmonics are even lower, below -130dBrA. Below 1kHz, we see no noise artifacts, either at 60Hz or 120Hz, due to power-supply noise.

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 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 second harmonic of the 50Hz signal (100Hz) can just be seen in the left channel at just over -130dBrA, or or 0.00003%, while the third harmonic at 150Hz is at -125 dBrA, or or 0.00006%, for both channels. Here again, no noise artifacts due to power-supply noise can be seen.

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 about -125dBrA for the right channel, -135dBrA for the left, while the third-order modulation products, at 17kHz and 20kHz, are just below -110dBrA. These are all extremely low values.

Square-wave response (10kHz)

The charts above and below are the 10kHz square-wave response of the C 298 into 8 ohms. The chart above is using the Audio Precision’s highest input bandwidth setting of 1MHz, while the chart below . . .

. . . is bandwidth limited to 250kHz. Due to limitations inherent to the Audio Precision APx555 B Series analyzer, these graphs should not be used to infer or extrapolate the C 298’s slew-rate performance. Rather, they should be seen as qualitative representations of the C 298’s limited bandwidth (e.g., slow rise time and mild overshoot in the corners), but also as a representation of the noise artifacts present due to the class-D amp topology that the C 298 uses. 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 in the corners and softening of the edges, which are shown above. In the case of the C 298, however, what dominates the plateaus of the square waves in the first graph is a 500kHz sinewave, the frequency at which the switching oscillator in the class- D amp is operating (see FFT below). With the bandwidth limited to 250kHz (second graph), the 500kHz switching signal is no longer visible.

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

The C 298’s topology relies on a roughly 500kHz modulation frequency in the feedback network of the amplifier. The graph above plots an FFT spectrum of the amplifier’s output at 10W into 8 ohms as it’s fed a 1kHz sine wave, but with a wide 1MHz bandwidth. We can see that the peak at about 500kHz is quite evident, at -40dBrA or 1% relative to the 1kHz signal. There is also a peak at 1MHz, (the second harmonic of the 500kHz peak), which is -70dBrA relative to the 1kHz signal. Those two peaks—the fundamental and its second harmonic—are direct results of the design of the C 298 amp modules. The noise around those very-high-frequency signals is always present within the amplifier, but far above the audioband, and therefore inaudible. The noise is also so high in frequency that any loudspeaker the amplifier is driving should filter it all out anyway, since the output of most tweeters begins to fall off not far above 20kHz.

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

The chart above represents the damping factor (DF for an 8-ohm load) of the C 298 as a function of frequency. Damping factor is calculated by measuring the voltages across an 8-ohm load (V_L) and across no load (V_NL), which, in our tests, is the 200k ohms input impedance of the analyzer, and then applying the formula DF = V_L / (V_NL – V_L). For the C 298, both channels show a general trend of a higher damping factor at lower frequencies and lower damping factor at higher frequencies, varying by almost a factor of 2 between 20Hz and 20kHz. The right channel outperformed the left channel, with a peak value around 2700 between 25 and 60Hz, while the left channel peaked around 2100 at the same frequencies.

Diego Estan
Electronics Measurement Specialist

Details

Parent Category: Products

Category: Amplifier Measurements

Created: 15 March 2021

Link: reviewed by Roger Kanno SoundStage! Access on March 15, 2021

General information

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

The NAD Masters M28 offers seven channels of amplification, each with an unbalanced (RCA) and balanced (XLR) input connector. However, since our measurement setup is optimized for stereo amplifiers, we were only able to test two channels at a time; therefore, we were not able to test the M28 with all seven channels being driven simultaneously.

The M28 was conditioned for one hour at 1/8th power (27W, 8 ohms) before any measurements were taken. All measurements were performed with two channels driven, unless otherwise stated, with the M28 connected to a dedicated 120V/20A circuit.

Unless otherwise specified, balanced input connections were used for all measurements, using channels 2 (as the right channel) and 3 (as the left channel). No important differences were seen between unbalanced and balanced input connections in terms of THD+N. There were small signal-to-noise ratio (SNR) differences highlighted in the table below. We found no differences in maximum output power (1% THD+N) between all seven channels when measured separately; however, significant differences in THD were observed between about 210W and 240W output (8 ohms)—channels 1-3 outperformed channels 5-7 in terms of THD by 20-30dB. Below 200W, and at the 1% THD power level (about 268W, 8 ohms), all channels performed similarly.

Published specifications vs. our primary measurements

The table below summarizes the measurements published by NAD for the M28 compared directly against our own. The published specifications are sourced from NAD’s website, either directly or from the manual available for download, or a combination thereof. With the exception of frequency response, where the Audio Precision bandwidth was 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 one of the two channels driven.

*The NAD THD specification on their website is published as “20Hz-20kHz”; however, after discussions with NAD, it was discovered that the company measures THD from 20Hz to 6.5kHz, with a 20Hz to 20kHz input bandwidth filter.

**The discrepancy in balanced input impedance may be due to NAD specifying this value for the inverting and noninverting pins separately. Our measurement considers both inputs on the balanced connector together. Treated separately, our measurement would be halved, or 39.2k ohms.

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 M28 was able to sustain 500W (1dB over rated output and roughly 1%THD) into 4 ohms using an 80Hz tone for 500ms, alternating with a signal at -10dB of the peak (50W) 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 M28 was just slightly warm to the touch.

Frequency response (8-ohm loading)

In our measured frequency response plot above, the M28 is essentially perfectly flat within the audio band (20Hz to 20kHz), with the exception of an insignificant 0.05dB rise at 20Hz and 20kHz. NAD’s claim of ±0.1dB from 20Hz to 20kHz is corroborated by our measurement, as is the -3dB point at 60kHz (we measured -2.3dB). The M28 cannot be considered a high-bandwidth audio device , due to the steep rolloff that begins at around 30kHz. In the graph above and most of the graphs below, only a single trace may be visible. This is because channel 3 (blue or purple trace) is performing identically to channel 2 (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, channel 3 only)

The charts above and below show RMS level (relative to 0dBrA, which is 1W into 8 ohms or 2.83Vrms) as a function of frequency, for the balanced line-level inputs 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 top graph shows that all four plots are indistinguishable from one another. This is an indication of a high damping factor, or low output impedance. The graph below. . .

. . . shows the same data, but with the vertical and horizontal axis expanded to show differences more clearly. Here we find that there’s a total deviation of only about 0.03dB in the flat portion of the curve between 4 ohms and no load. At 20kHz, the spread is a little larger, perhaps 0.04dB. The maximum variation in RMS level when a real speaker was used as a load is extremely small, deviating by about 0.01dB within the flat portion of the curve. There is a rise at low and high frequencies; however, this is due to the M28’s inherent frequency response, and not the output impedance of the M28 interacting with a load that varies with frequency.

Phase response

Above is the phase response plot for the M28 from 20Hz to 20kHz. The M28 does not invert polarity, and there is virtually no phase shift throughout the audioband, with a worst case of just over +10 degrees at 20Hz.

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

The chart above shows THD ratios at the M28’s output into 8 ohms as a function of frequency (20Hz to 20kHz) for a sine-wave stimulus at the balanced line-level inputs. The blue and red plots are for channels 3 and 2 at 1W output, purple/green at 10W, and pink/orange at the rated 220W, all into 8 ohms. There is about a 5dB improvement in THD ratios at 10W versus 1W throughout most of the audioband. At 10W, the THD values are as low as 0.00007% at 40Hz, and as high as 0.01% at 20kHz. Due to these extremely low values, it’s important to note that we are approaching the limits of the APx555 analyzer, which inherently measures at approximately 0.00001-0.00002% THD when fed its own signal at this level. The THD levels at 220W are significantly higher than at 1W and 10W, ranging from 0.3% at 20Hz down to about 0.002% around 2-3kHz, then back up to 0.04% at 20kHz. These differences are more of an indicator that NAD may not have been conservative in specifying output power for the M28, and that 200W into 8 ohms, or just below, would have been a more appropriate value for the M28 (also see chart detailing THD vs. output power below).

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

The chart above shows THD ratios measured at the output of the M28 as a function of output power for the balanced line level input, for an 8-ohm load (blue/red for channels 3 and 2) and a 4-ohm load (purple/green for channels 3 and 2). At 50mW, THD values are near 0.001% for both 8- and 4-ohm data. At 10W, the 8-ohm THD value is just below 0.0002%, and about 0.0004% at 4 ohms. At the “knees,” the 8-ohm THD value is at around 0.0005%, nearing 200W, while the 4 ohms THD value is at around 0.002% at around 350W. Here is another indication that 200W may be a more appropriate rated output power into 8 ohms for the M28.

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 M28 as a function of output power for the balanced line-level inputs, for an 8-ohm load (blue/red for channels 3 and 2) and a 4-ohm load (purple/green for channels 3 and 2). At 50mW, THD+N values are near 0.03% for both 8- and 4-ohm data, dipping down to near 0.002% between 50 and 150W (8 ohms), and roughly 0.003% between 50 and 300W (4 ohms).

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

The chart above shows THD+N ratios (A-weighted) measured at the output of the M28 as a function of output power for the balanced line-level input, for an 8-ohm load (blue/red for channels 3 and 2) and a 4-ohm load (purple/green for channels 3 and 2). At 50mW, THD+N values are just below (8 ohms) and above (4 ohms) 0.005%, dipping down to near 0.0003% between 30 and 50W (8 ohms), and roughly 0.0004% between 10 and 30W (4 ohms).

THD ratio (unweighted) vs. frequency at 8/4/2 ohms (channel 3 only)

The chart above shows THD ratios measured at the output of the M28 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 channel 3 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 increasing levels of THD from 8 to 4 to 2 ohms, with about a 10db difference between 8, 4, and 2 ohms respectively, from 50Hz to 3kHz. There’s a convergence of THD values between all loads at higher frequencies. Overall, even with a 2-ohm load at roughly 80W, THD values ranged from 0.001% at 20Hz to just above 0.01% at 20kHz.

FFT spectrum – 1kHz

Shown above is a fast Fourier transform (FFT) for a 1kHz input sine-wave stimulus, measured at the output across an 8-ohm load at 10W. We see that the second harmonic, at 2kHz, is at a very low -130dBrA (relative to the reference 0dB signal), or 0.00003%. The third harmonic, at 3kHz, is just above -120dBrA, or 0.0001%. The fifth harmonic is approaching -140dBrA, or 0.00001%. Below 1kHz, we see no noise artifacts, either at 60Hz or 120Hz due to power-supply noise.

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 harmonic of the 50Hz signal (100Hz) can be seen at around -130dBrA, or 0.00003%. The third harmonic at 150Hz is at -125 dBrA, or 0.00006%. Here again, no noise artifacts due to power-supply noise can be seen.

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

Shown above is the FFT of the intermodulation distortion (IMD) products for an 18kHz + 19kHz summed sine-wave 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 about -125dBrA, or 0.00006%, while the third-order modulation products, at 17kHz and 20kHz are just above -110dBrA, or 0.0003%.

Square-wave response (10kHz)

Above and below are the 10kHz square-wave responses of the M28 into an 8-ohm load. The top chart is using the Audio Precision’s highest input bandwidth setting of 1MHz, while the chart below . . .

. . . is bandwidth limited to 250kHz. Due to limitations inherent to the Audio Precision APx555 B Series analyzer, these charts should not be used to infer or extrapolate the M28’s slew-rate performance. Rather, they should be seen as a qualitative representation of the M28’s limited bandwidth (e.g., slow rise time and mild overshoot in the corners), but also as a representation of the noise artifacts present due to the class-D amp topology that the M28 relies on. An ideal squarewave can be represented as the sum of a sinewave and an infinite series of its odd-order harmonics (e.g., 10kHz + 30kHz + 50kHz + 70kHz . . .). A limited bandwidth will show only the sum of the lower-order harmonics, which may result in noticeable undershoot and/or overshoot, and softening of the edges. In the case of the M28, however, what dominates the plateaus of the square wave in the top graph is a 500kHz (approximate) sine wave, the frequency at which the switching oscillator in the class-D amp is operating (see FFT below). With the bandwidth limited to 250kHz (bottom graph), the 500kHz switching signal is no longer visible.

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

The M28’s topology relies on a roughly 500kHz modulation frequency in the feedback network of the amplifier. The graph above plots an FFT spectrum of the amplifier’s output at 10W into 8 ohms as it’s fed a 1kHz sine wave, but with a wide 1MHz bandwidth. We can see that the peak at about 500kHz is quite evident, at -40dBrA or 1% relative to the 1kHz signal. There is also a peak at 1MHz, (the second harmonic of the 500kHz peak), which is -70dBrA relative to the 1kHz signal. Those two peaks—the fundamental and its second harmonic—are direct results of the design of the C 298 amp modules. The noise around those very-high-frequency signals is always present within the amplifier, but far above the audioband, and therefore inaudible. The noise is also so high in frequency that any loudspeaker the amplifier is driving should filter it all out anyway, since the output of most tweeters begins to fall off not far above 20kHz.

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

The chart above represents the damping factor (DF for an 8-ohm load) of the M28 as a function of frequency. Damping factor is calculated by measuring the voltages across an 8-ohm load (V_L) and across no load (V_NL), which, in our tests, is the 200k ohms input impedance of the analyzer, and then applying the formula DF = V_L / (V_NL – V_L). Both channels show a general trend of a higher damping factor at lower frequencies, and lower damping factor at higher frequencies, varying by almost a factor of 2 between 20Hz and 20kHz. The left channel just barely outperformed the right (although they are very closely matched) with a peak value around 1270 at 30Hz, down to about 800 at 20kHz.

Diego Estan
Electronics Measurement Specialist

Details

Parent Category: Products

Category: Amplifier Measurements

Created: 01 February 2021

Link: reviewed by James Hale on SoundStage! Xperience on February 1, 2021

General information

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

The Music hall a15.3 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 a15.3 offers five sets of line-level unbalanced (RCA) inputs, one moving-magnet (MM) phono input, a set of variable line-level RCA pre-outputs, and one pair of speaker outputs. Also available is a headphone output via a 3.5mm TRS jack on the front panel. Based on the high accuracy of the left/right channel matching (see table below), the a15.3’s volume knob is not a potentiometer in the signal path, which is typically less accurate, but rather provides digital control over a proprietary or integrated analog-domain volume circuit. The a15.3’s volume control offers between 1 and 2dB step increments throughout most of the volume range. At the lowest end of the range, the first 3 steps offer 4dB increments, and the next 3 steps 3dB.

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 (about 3 o’clock) as measured at the pre-outputs. SNR measurements were made with the volume control set to maximum. At the unity-gain volume position, to achieve 10W into 8 ohms, 275mVrms was required at the line-level input and 2.65mVrms at the phono input.

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

Published specifications vs. our primary measurements

The tables below summarize our primary measurements performed on the a15.3. Here we can compare our results against Music Hall’s own published specifications for the a15.3, which are stated as follows:

• Rated output power: 50W into 8 ohms, 75W into 4 ohms
• Input sensitivity: 330mV (RCA), 1mV (phono)
• SNR: >90dB
• THD+N: <0.02% (20Hz-20kHz)
• Frequency response: RCA: -0.5dB (20Hz-20kHz), PHONO: RIAA compliant

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 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):

Music Hall’s power output claims of 50 and 75WPc into 8- and 4-ohm loads were corroborated with our maximum (1% THD+N) measurements of 63Wpc and 86Wpc into 8 and 4 ohms, during which time our line AC voltage never dipped below 121VAC.

Our clipping headroom result was 1dB for the a15.3, defined as the ratio of max power over rated power into 8 ohms. The Music Hall a15.3 was also able to sustain 86Wpc (1dB over rated output) into 4 ohms using an 80Hz tone for 500ms, alternating with a signal at -10dB of the peak (8.6Wpc) 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 a15.3 was very warm to the touch, causing some discomfort on the skin after 10 seconds of direct contact.

Music Hall’s input sensitivity claims (volume at maximum) were verified, where we measured 308mVrms and 3mVrms respectively for the single-end RCA and phono inputs, which are close to the 330mVrms and 1mVrms specs.

Music Hall’s 90dB SNR claim was corroborated by our own measurements, where we measured 98.5dB (A-weighted) and 96.2dB (20Hz-20kHz) for the left and right channels.

Music Hall’s THD+N claim of <0.02% (20Hz to 20kHz) was corroborated at 1KHz in our primary table; however, as our THD versus frequency graph below shows, THD ratios were above 0.02% from 6kHz to 20kHz at 10W into 8ohms.

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

Music Hall does not supply any headphone-output specifications, so there there was nothing for us to compare to.

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

In our measured frequency response plot above, the a15.3 is nearly flat within the audio band (20Hz to 20kHz) for the line-level input. These data corroborate Music Hall’s claim of 20Hz to 20kHz (-0.5dB) as the worst-case deviation is at 20kHz, where the response is about -0.2dB. At 20Hz, the response is at 0dB, and -0.5dB at 5Hz. The a15.3 can be considered a high-bandwidth audio device since the response at 100kHz is approximately +0.1dB. 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 (8-ohm loading, phono input)

The plot above shows frequency response for the phono input, and shows a maximum deviation of -1dB (20Hz, right channel) from flat within the audioband. 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, horizontal line at 0dB). Between 100Hz and 500Hz, there is a 0.2dB deviation between channels, as well as at 30Hz and below, indicating small channel-to-channel differences in the RIAA curve implementaion.

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

The plots above shows RMS level (relative to 0dBrA, which is 1W into 8ohms 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 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 line is no load connected. 

This chart is the same test as above, but the chart has been zoomed in to highlight differences. Here we find that there’s a total deviation of about 0.1 dB 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 (in this case, a Focal Chora 806, with measurements available through this link) as a load is very small, deviating by a little over 0.05dB within the flat portion of the curve (20Hz to 1kHz), 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 1kHz.

Phase response (line-level input)

Above is the phase response plot from 20Hz to 20kHz for the line-level input. The a15.3 does not invert polarity on the line-level input, and the plot shows very little phase shift, with a worst case of just under +30 degrees at 20kHz.

Phase response (phono 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 -60 degrees at 200Hz and 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 20kHz) for a sinewave stimulus at the 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 50W. The power was varied using the volume control. All three THD plots exhibit a rise in THD above a “knee” at roughly 1.5kHz. At 1W, the THD values in the flat portion (20Hz to 1kHz) are hovering between 0.005-0.006%, then rise up to about 0.07% at 20kHz. At 10W, the flat portion shows THD ratios around 0.003%, then up to 0.06% at 20Hz. The 50W data shows the flat portion around 0.004%, rising up to just above 0.1% at 20kHz. At 50W, the right channel outperforms the left throughout the sweep by about 2dB.

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

The chart above shows the THD ratio as a function of frequency plot 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 about 0.2% at 20Hz and 30Hz, down to about 0.003/0.005% (left/right channels) at 1kHz, then up to about 0.07/0.06% (left/right channels) at 20kHz.

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

The above chart shows THD ratios measured at the output of the a15.3 as a function of output power for the line level-input, for an 8-ohm load (blue/red lines for left/right chanels) and a 4-ohm load (purple/green for left/right). The 4-ohm data shows consistently slightly higher THD values compared to the 8-ohm data (about a 5dB difference), and the right channel outperforms the left in both plots by about 2dB. At the 50mW level, THD values measured around 0.01% (8 ohms) and 0.02% (4 ohms), dipping down to around 0.004/0.003% at 10W to 30W for the 8-ohm data, and 0.005% for the 4-ohm data from 10W to 50W. The “knee” in the 8-ohm data occurs near 50W, hitting the 1% THD mark at 63W. For the 4-ohm data, the “knee” occurs near 70W, hitting the 1% THD mark at 86W.

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

This chart shows THD+N ratios measured at the output of the a15.3 as a function of output power for the line level-input, for an 8-ohm load (blue/red lines for left/right channels) and a 4-ohm load (purple/green for left/right). 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 0.15% (4 ohms), dipping down to around 0.005% at 20W to 40W for the 8-ohm data, and 0.006% for the 4-ohm data from 30 to 50W.

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 a15.3 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 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 difference from 20Hz to 1kHz, with a smaller degradation in THD performance (1-2dB) between 4 and 2 ohms from 1kHz to 20kHz. Overall, even with a 2-ohm load at roughly 40W, THD values ranged from 0.005% at 20Hz to just below 0.2% at 20 kHz.

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 line-level input. As seen in the plots above, we see that the right channel (red) outperforms the left channel (blue) with slightly lower peaks. We see that the signal’s second and third harmonic, at 2kHz and 3kHz, are both at around -95dBrA, while the remaining harmonics are below -100dBRA. Below 1kHz, we see noise artifacts, with the 60Hz peak due to power-supply noise just above -110dBrA, and the 120Hz peak at -105/110dBrA (left/right channels).

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. The second signal harmonic at 2kHz is at -90/-100dBRA (left/right channels), with subsequent harmonics below this level. The highest peak from power supply noise is at the fundamental (60Hz), reaching almost -55dBrA, and the third noise harmonic (180Hz) is near -75dBrA.

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 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 peak is that of the signal’s third harmonic (150Hz) at about -95dBrA. The signal second harmonic (100Hz) is at around -100dBRA, while the noise second harmonic (120Hz) is at around -105/-110dBrA (left/right channels).

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 peak is the noise signal’s fundamental (60Hz) at near -55dBRA. The most predominant signal harmonic peak is the third harmonic (150Hz) at -90dBrA.

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 near -105/-110dBRA (left/right channels), while the third-order modulation products, at 17kHz and 20kHz, are higher, at around -100dBrA.

Intermodulation distortion FFT (18kHz + 19kHz summed stimulus, 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. Here we find that the second-order modulation product (i.e., the difference signal of 1kHz) is at -95/-100dBrA (left/right channels), and smaller in magnitude than some of the harmonic peaks from the noise signal surrounding it. The third-order modulation products, at 17kHz and 20kHz, are at around -100dBrA.

Square-wave response (10kHz)

Above is the 10kHz squarewave 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 Music Hall’s slew-rate performance. Rather, it should be seen as a qualitative representation of it’s extended bandwidth. An ideal squarewave can be represented as the sum of a sinewave and an infinite series of its odd-order harmonics (e.g., 10kHz + 30kHz + 50kHz + 70kHz . . .). A limited bandwidth will show only the sum of the lower-order harmonics, which may result in noticeable undershoot and/or overshoot, and softening of the edges. The a15.3’s reproduction of the 10kHz squarewave can be considered clean, with sharp edges and very mild overshoot.

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

The final plot 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 (about 20% lower at 20kHz compared to 20Hz). The right channel outperformed the left with a peak value around 190 from 20Hz to 400Hz, while the left channel achieved a damping factor of around 160 within the same frequency range.

Diego Estan
Electronics Measurement Specialist