Details

Parent Category: Products

Category: Amplifier Measurements

Created: 01 February 2024

Link: reviewed by Dennis Burger on SoundStage! Access on February 1, 2024

General Information

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

The Technics Grand Class SU-GX70 was conditioned for one hour at 1/8th full rated power (~5W 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 SU-GX70 offers two line-level analog inputs (RCA), one moving-magnet (MM) phono input (RCA), one pair of preamp outputs (RCA), one digital coaxial (RCA) and two optical (TosLink) S/PDIF inputs, one USB digital inputs, two pairs of speaker-level outputs and one headphone output over 1/4″ TRS connector. For the purposes of these measurements, the following inputs were evaluated: digital coaxial, analog line-level, as well as phono.

The SU-GX70 is a sophisticated device that digitizes all incoming signals and can apply DSP for various functions. An “initialization” was performed before any measurements were made, to ensure that any room EQ DSP had been cleared. Unless otherwise stated, Pure Amplification was turned on, MQA off, and LAPC off, although comparisons between the on and off effects of these functions can be seen in this report.

Most measurements were made with a 2Vrms line-level analog input, 5mVrms MM input and 0dBFS digital input. The volume control is variable from -99dB to 0dB. The signal-to-noise (SNR) measurements were made with the default input signal values but with the volume set to achieve the rated output power of 40W (8 ohms). For comparison, on the line-level input, a SNR measurement was also made with the volume at maximum.

Based on the high accuracy and repeatability of the left/right volume channel matching (see table below), the SU-GX70 volume control operates in the digital domain. The SU-GX70 offers 1dB volume steps ranging from -99dB to -54dB, then 0.5dB steps from -53.5dB to 0dB. Overall range is -59.3dB to +39.6dB (line-level input, speaker output).

Because the SU-GX70 is a digital amplifier technology that exhibits considerable noise above 20kHz (see FFTs below), our typical input bandwidth filter setting of 10Hz-90kHz was necessarily changed to 10Hz-22.4kHz for all measurements, except for frequency response and for FFTs. In addition, THD versus frequency sweeps were limited to 6kHz to adequately capture the second and third signal harmonics with the restricted bandwidth setting.

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

Published specifications vs. our primary measurements

The table below summarizes the measurements published by Technics for the SU-GX70 compared directly against our own. The published specifications are sourced from Technics’ 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 extended to 250kHz, assume, unless otherwise stated, 10W into 8 ohms and a measurement input bandwidth of 10Hz to 22.4kHz, and the worst-case measured result between the left and right channels.

Our primary measurements revealed the following using the line-level analog input and digital coaxial input (unless specified, assume a 1kHz sinewave at 2Vrms or 0dBFS, 10W output, 8-ohm loading, 10Hz to 22.4kHz bandwidth):

For the continuous dynamic power test, the SU-GX70 was able to sustain 105W into 4 ohms (~6% THD) using an 80Hz tone for 500ms, alternating with a signal at -10dB of the peak (10.5W) 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 SU-GX70 was only slightly warm to the touch.

Our primary measurements revealed the following using the phono-level input, MM configuration (unless specified, assume a 1kHz 5mVrms sinewave input, 10W output, 8-ohm loading, 10Hz to 22.4kHz bandwidth):

Our primary measurements revealed the following using the analog input at the headphone output (unless specified, assume a 1kHz sinewave, 1Vrms output, 300 ohms loading, 10Hz to 22.4kHz bandwidth):

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

In our frequency-response plots above (relative to 1kHz), measured across the speaker outputs at 10W into 8 ohms, the SU-GX70 is nearly flat within the audioband (20Hz to 20kHz). At the extremes the SU-GX70 is -0.1dB at 20Hz and +0.5dB down at 20kHz. There’s a rise in the frequency response above 20kHz, where we see +2.2dB just past 40kHz, which is a result of the digital amplifier and its high output impedance at high frequencies. Into a 4-ohm load (see RMS level vs. frequency vs load impedance graph below), the response is essentially flat at and above 20kHz. The -3dB point was also explored and found to be at roughly 46kHz, exactly where it was measured for a 24-bit/96kHz digital input signal (see “Frequency response vs. input type chart” below). 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, line-level input, bass and treble controls)

Above is a frequency response (relative to 1kHz) plot measured at the speaker-level outputs into 8 ohms, with the bass and treble controls set to maximum (blue/red plots) and minimum (purple/green plots). We see that for the bass and treble controls, roughly +/- 5dB of gain/cut is available at 20Hz/20kHz.

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

Above are the phase-response plots from 20Hz to 20kHz for the line-level input, measured across the speaker outputs at 10W into 8 ohms. The SU-GX70 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 SU-GX70’s frequency response (relative to 1kHz) as a function of input type measured across the speaker outputs at 10W into 8 ohms. The green trace (overlapping the purple trace) is the same analog input data from the previous graph. The blue trace is for a 16-bit/44.1kHz dithered digital input signal from 5Hz to 22kHz using the coaxial input, and the purple trace is for a 24/96 dithered digital input signal from 5Hz to 48kHz, while the pink trace is for a 24/192 dithered digital input signal from 5Hz to 96kHz. The 16/44.1 data exhibits brickwall-type filtering, with a -3dB at 21.1kHz. The 24/96 (and analog input) and 24/192 kHz data yielded -3dB points at 46.8kHz and 92.9kHz respectively. The analog data looks nearly identical to the 24/96 digital data, which is evidence for the SU-GX70 sampling incoming analog signals at 96kHz. 

Frequency response vs. MQA (16/44.1)

The chart above shows the SU-GX70’s frequency response (relative to 1kHz) for a 16/44.1 dithered digital input signal from 5Hz to 22kHz using the coaxial input, with MQA turned on. We find no difference in the measured frequency response for 16/44.1 data input whether MQA is turned on or off.

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

The chart above shows the frequency response (relative to 1kHz) for the MM phono input without (blue/red) and with (purple/green) the subsonic filter enabled. 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). We see a maximum deviation of about +0.5/-0.2dB (150Hz and 20kHz/20Hz) from 20Hz to 20kHz. With the subsonic filter engaged, we find the -3dB point at 20Hz.

Phase response (MM input)

Above is the phase response plot from 20Hz to 20kHz for the MM phono input without (blue/red) and with (purple/green) the subsonic filter enabled, measured across the speaker outputs at 10W into 8 ohms. The SU-GX70 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 +80 degrees at 20Hz without the subsonic filter and +160 degrees with the filter.

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

The chart above shows the results of a linearity test for the coaxial digital input for both 16/44.1 (blue/red) and 24/96 (purple/green) input data, measured at the line-level output of the SU-GX70. 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 data were essentially perfect as of -100dBFS down to 0dBFS. At -120dBFS, the 16/44.1 data were only +2dB (left) and +4dB (right) above reference, while the 24/96 data were within +1dBFS.

Impulse response (24/44.1 data)

The graph above shows the impulse response for the SU-GX70 with MQA turned off (blue) and MQA turned on (purple), fed to the coaxial digital input, measured at the line level output, 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. We find a reconstruction filter that adheres to a typical symmetrical sinc function. There appears to be no difference in the impulse response with MQA on or off through the coaxial input.

J-Test (coaxial, MQA off)

The chart above shows the results of the J-Test test for the coaxial digital input measured at the line-level output of the SU-GX70 with MQA turned off. J-Test was developed by Julian Dunn the 1990s. It is a test signal—specifically, a -3dBFS undithered 12kHz squarewave sampled (in this case) at 48kHz (24 bits). Since even the first odd harmonic (i.e., 36kHz) of the 12kHz squarewave is removed by the bandwidth limitation of the sampling rate, we are left with a 12kHz sinewave (the main peak). In addition, an undithered 250Hz squarewave at -144dBFS is mixed with the signal. This test file causes the 22 least significant bits to constantly toggle, which produces strong jitter spectral components at the 250Hz rate and its odd harmonics. The test file shows how susceptible the DAC and delivery interface are to jitter, which would manifest as peaks above the noise floor at 500Hz intervals (e.g., 250Hz, 750Hz, 1250Hz, etc.). Note that the alternating peaks are in the test file itself, but at levels of -144dBrA and below.  The test file can also be used in conjunction with artificially injected sinewave jitter by the Audio Precision, to show how well the DAC rejects jitter.

The coaxial input exhibits low-level rises (-135dBrA) in the noise floor within the audioband at 6.5kHz and 13kHz. This is a good J-Test result, indicating that SU-GX70 DAC should yield good jitter immunity.

J-Test (optical, MQA off)

The chart above shows the results of the J-Test test for the optical digital input measured at the line-level output of the SU-GX70. The optical input yielded essentially the same result compared to the coaxial input.

J-Test (coaxial, MQA on)

The chart above shows the results of the J-Test test for the coaxial digital input measured at the line-level output of the SU-GX70 with MQA turned on. The result is similar to the one with MQA turned off, only slightly improved, with the rises in the noise floor no longer visible.

J-Test with 100ns of injected jitter (coaxial, MQA off)

Both the coaxial and optical inputs were also tested for jitter immunity by injecting artificial sinewave jitter at 2kHz, which would manifest as sidebands at 10kHz and 14kHz without any jitter rejection. Jitter immunity proved exceptional, with no visible sidebands at the 100ns jitter level, and only a spurious peak at 2kHz at the -135dBrA level. The coaxial input is shown, but both performed the same.

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

The chart above shows a fast Fourier transform (FFT) of the SU-GX70’s line-level output with white noise at -4dBFS (blue/red) and a 19.1 kHz sinewave at 0dBFS fed to the coaxial digital input, sampled at 16/44.1, with MQA turned off. The steep roll-off around 20kHz in the white-noise spectrum shows the behavior of the SU-GX70’s reconstruction filter. There are low-level aliased image peaks within the audioband at around 2kHz and 13kHz, at or near -120dBrA. The primary aliasing signal at 25kHz is at -95dBrA, while the second and third distortion harmonics (38.2, 57.3kHz) of the 19.1kHz tone are at -80 and -60dBrA.

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

The chart above shows a fast Fourier transform (FFT) of the SU-GX70’s line-level output with white noise at -4dBFS (blue/red) and a 19.1 kHz sinewave at 0dBFS fed to the coaxial digital input, sampled at 16/44.1, with MQA turned on. The steep roll-off around 20kHz in the white-noise spectrum shows the behavior of the SU-GX70’s reconstruction filter. There are low-level aliased image peaks within the audioband at around 2kHz and 7kHz, at -120dBrA.

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. Here we find that between 20Hz and 6kHz, the deviations between no load and 4 ohms are around 0.45dB, but at high frequencies, the differences are larger, at about 1dB at 20kHz. This is a relatively poor result, and an indication of a relatively high output impedance, or low damping factor. When a real speaker is used, deviations are within around 0.4dB throughout the audioband.

RMS level vs. frequency (1W, left channel only, real speaker, LPAC on and off)

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 20Hz to 20kHz. Both plots are for the Focal Chora 806 speaker, with (purple) and without (blue) LAPC enbaled. The SU-GX70 provides a feature called Load Adaptive Phase Calibration (LAPC). This feature measures the outputs of the amplifier while the speakers are connected using test tones to establish a correction curve to deal with the amplifier’s inherently high output impedance at high frequencies. The theoretical goal is to achieve a flat frequency response for the user’s speakers when LAPC is enabled. We can see here that the purple trace is not flat, but closer to ideal compared to when LAPC is disabled. When LAPC is disabled, deviations reach about 0.35dB, while only 0.15dB with LAPC enabled.

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 analog 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 just under 40W. The power was varied using the volume control. At 1W, THD ratios are fairly constant and range from 0.02% at 20Hz, down to 0.01% from 40Hz to 6kHz. At 10W, THD ratios are as high as 0.3% at 20Hz, with a steady decline to 0.01% at 6kHz. At nearly 40W, THD ratios are as high as 0.6% at 20Hz, with a steady decline to 0.02% at 6kHz.

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

The chart above shows THD ratio as a function of frequency plots for the MM phono input measured across an 8-ohm load at 10W. The input sweep is EQ’d with an inverted RIAA curve. The THD values vary from around 0.3% at 20Hz, then a steady decline down to 0.015% at 6kHz.

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 SU-GX70 as a function of output power for the analog line-level input, for an 8-ohm load (blue/red for left/right channels) and a 4-ohm load (purple/green for left/right channels). Both data sets track closely except for maximum power, with the 4-ohm data slightly outperforming the 8-ohm data at lower power. THD ratios range from as low as 0.0025% at 0.5-1W, up to 0.07% (8-ohm) and 0.2% (4-ohm) at the “knees” at just below 50W and 90W, respectively. The 1% THD marks were reached at just past 50W (8 ohms) and just shy of 100W (4 ohms).

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 SU-GX70 as a function of output power for the 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). Both data sets track closely except for maximum power, with the 4-ohm data slightly outperforming the 8-ohm data at lower power. Overall, THD+N values for both loads ranged from 0.05% at 50mW, down to near 0.01% at 3-5W, then up to the “knees,” as described in the caption for the chart directly above.

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 SU-GX70 as a function of frequency into three different loads (8/4/2 ohms) for a constant input voltage that yields 10W at the output into 8 ohms (and roughly 20W into 4 ohms, and 40W into 2 ohms) for the analog line-level input. The 8-ohm load is the blue trace, the 4-ohm load the purple trace, and the 2-ohm load the pink trace. We find roughly the same THD values of 0.02% from 1kHz to 6kHz for the 8- and 4-ohm data. From 20Hz to 1kHz, there is a roughly 5dB increase in THD every time the load is halved. However, even into a 2-ohm load, which the SU-GX70 is not designed to drive, THD ratios range from 0.3% at 20Hz, down to 0.03% from 1kHz to 6kHz.

THD ratio (unweighted) vs. frequency into 8 ohms and real speakers (left channel only)

The chart above shows THD ratios measured at the output of the SU-GX70 as a function of frequency into an 8-ohm load and two different speakers for a constant output voltage of 2.83Vrms (1W into 8 ohms) for the analog line-level input. The 8-ohm load is the blue trace, the purple plot is a two-way speaker (Focal Chora 806, measurements can be found here), and the pink plot is a three-way speaker (Paradigm Founder Series 100F, measurements can be found here). In general, the measured THD ratios for the real speakers were close to the 8-ohm resistive load, hovering between 0.01 and 0.02% from 100Hz to 6kHz. The two-way Focal yielded the highest THD values (0.2% at 20Hz) at very low frequencies.

IMD ratio (CCIF) vs. frequency into 8 ohms and real speakers (left channel only)

The chart above shows intermodulation distortion (IMD) ratios measured at the output of the SU-GX70 as a function of frequency into an 8-ohm load and two different speakers for a constant output voltage of 2.83Vrms (1W into 8 ohms) for the analog line-level input. Here the CCIF IMD method was used, where the primary frequency is swept from 20kHz (F1) down to 2.5kHz, and the secondary frequency (F2) is always 1kHz lower than the primary, with a 1:1 ratio. The CCIF IMD analysis results are the sum of the second (F1-F2 or 1kHz) and third modulation products (F1+1kHz, F2-1kHz). The 8-ohm load is the blue trace, the purple plot is a two-way speaker (Focal Chora 806, measurements can be found here), and the pink plot is a three-way speaker (Paradigm Founder Series 100F, measurements can be found here). All IMD results are similar, hovering from 0.03 to 0.015% across the measured frequency range.

IMD ratio (SMPTE) vs. frequency into 8 ohms and real speakers (left channel only)

The chart above shows IMD ratios measured at the output of the SU-GX70 as a function of frequency into an 8-ohm load and two different speakers for a constant output voltage of 2.83Vrms (1W into 8 ohms) for the analog line-level input. Here, the SMPTE IMD method was used, where the primary frequency (F1) is swept from 250Hz down to 40Hz, and the secondary frequency (F2) is held at 7kHz with a 4:1 ratio. The SMPTE IMD analysis results consider the second (F2 ± F1) through the fifth (F2 ± 4xF1) modulation products. The 8-ohm load is the blue trace, the purple plot is a two-way speaker (Focal Chora 806, measurements can be found here), and the pink plot is a three-way speaker (Paradigm Founder Series 100F, measurements can be found here). Between 40Hz and 60Hz, all result are essentially identical, around -81dB. Above 60Hz, the highest IMD ratios are associate with the Paradigm speakers, rising up to -74dB from 100Hz to 250Hz. All IMD results are essentially identical, from 0.05% from 40Hz to 400Hz, then 0.025% from 500Hz to 1kHz.

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 analog line-level input. We see that the signal’s second (2kHz) and third (3kHz) harmonics are at a relatively high -80dBrA, or 0.01%, while subsequent signal harmonics are at and below -90dBrA, or 0.003%. Since the SU-GX70 uses a switching power supply, there are no obvious peaks at 60Hz or subsequent harmonics. There are, however, several significant noise peaks (as high as -65dB, or 0.06%) that are likely a result of IMD products between the signal, it’s harmonics, and the high-frequency oscillator used in the class-D amplifier section. Of note is that the analyzer would ignore these peaks, which are actually larger in magnitude than the signal harmonics, when calculating THD. There is also a rise in the noise above 20kHz, characteristic of digital amplifiers. This is far from what is considered a clean FFT.

FFT spectrum – 1kHz (line-level input, Pure Amplification off)

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 analog line-level input, but with Pure Amplification turned off. The FFT is similar to the FFT above, where Pure Amplification was turned on, except for low-level peaks (-120dBrA, or 0.0001%) that can be seen here at low frequencies that are not present in the first FFT.

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. Signal harmonics are different to the analog input FFT above. The second (2kHz) harmonic is low at -115dBRa, or 0.0002%, while the third (3kHz) harmonic is much higher, at -75dBrA, or 0.02%. Subsequent signal harmonics are at and below -90dBrA, or 0.003%. The same IMD peaks can also be seen here, as high as -65dB, or 0.06%, flanking the main 1kHz signal peak.

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

Shown above is the fast Fourier transform (FFT) for a 1kHz input sinewave stimulus, measured at the output across an 8-ohm load at 10W for the coaxial digital input, sampled at 24/96. The FFT is very similar to the 16/44.1 input FFT above, but for a more predominant second (2kHz) signal harmonic at -95dBrA, or 0.002%.  

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 see the 1kHz primary signal peak, at the correct amplitude, and no other peaks above the noise floor at -130dBrA.

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. We see the 1kHz primary signal peak, at the correct amplitude, and no other peaks above the noise floor at -135dBrA.

FFT spectrum – 1kHz (MM 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 MM phono input. We see the third (3kHz) signal harmonic dominating at around -75dBrA, or 0.02%. Other signal harmonics can be seen at -95dBrA, or 0.002%, and below. The most significant power-supply-related noise peaks can be seen at 60Hz at -85dBrA, or 0.006%. Higher-order power-supply-related peaks can also be seen at lower amplitudes. The same IMD peaks can also be seen here, as high as -65dB, or 0.06%, flanking the main 1kHz signal peak.

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 analog 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) peak is the third (150Hz) signal harmonic at a high -55dBrA, or 0.02%. Several other signal-related and IMD peaks can be seen throughout at -70dBrA, or 0.03%, and below.

FFT spectrum – 50Hz (MM phono input)

Shown above is the FFT for a 50Hz input sinewave stimulus measured at the output across an 8-ohm load at 10W for the MM phono 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 60Hz power supply fundamental can be seen at -90dBrA, or 0.003%. The most predominant (non-signal) peak is the third (150Hz) signal harmonic at a high -55dBrA, or 0.02%. Several other signal-related and IMD peaks can be seen throughout at -70dBrA, or 0.03%, and below.

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

Shown above is an FFT of the intermodulation distortion (IMD) products for an 18kHz + 19kHz summed sinewave stimulus tone measured at the output across an 8-ohm load at 10W for the analog line-level input. The input RMS values are set at -6.02dBrA so that, if summed for a mean frequency of 18.5kHz, would yield 10W (0dBrA) into 8 ohms at the output. We find that the second-order modulation product (i.e., the difference signal of 1kHz) is at nearly -80dBrA, or 0.01%, while the third-order modulation products, at 17kHz and 20kHz, are at roughly the same level.

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

Shown above is the FFT of the speaker-level output of the SU-GX70 with the APx 32-tone signal applied to the input. The combined amplitude of the 32 tones is the 0dBrA reference, and corresponds to 10W into 8 ohms. The intermodulation products—i.e., the “grass” between the test tones—are distortion products from the amplifier and are below the -90dBrA, or 0.003%, level.

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

Shown above is an FFT of the intermodulation distortion (IMD) products for an 18kHz + 19kHz summed sinewave stimulus tone measured at the output across an 8-ohm load at 10W for the digital coaxial input at 16/44.1. We find that the second-order modulation product (i.e., the difference signal of 1kHz) is at -100dBRa, or 0.001%, while the third-order modulation products, at 17kHz and 20kHz, are around -80dBrA, or 0.01%.

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

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 16/44.1, with MQA turned on. We find that the second-order modulation product (i.e., the difference signal of 1kHz) is at -100dBRa, or 0.001%, while the third-order modulation products, at 17kHz and 20kHz, are around -80dBrA, or 0.01%. This is essentially the same result as with the FFT with MQA turned off.

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

Shown above is an FFT of the intermodulation distortion (IMD) products for an 18kHz + 19kHz summed sinewave stimulus tone measured at the output across an 8-ohm load at 10W for the digital coaxial input at 24/96. We find that the second-order modulation product (i.e., the difference signal of 1kHz) is at -100dBRa, or 0.001%, while the third-order modulation products, at 17kHz and 20kHz, are around -80dBrA, or 0.01%. This is essentially the same result as with the 16/44.1 IMD FFT.

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 MM phono input. We find that the second-order modulation product (i.e., the difference signal of 1kHz) is at -95dBRa, or 0.002%, while the third-order modulation products, at 17kHz and 20kHz, are just below -80dBrA, or 0.01%.

Square-wave response (10kHz)

Above is the 10kHz squarewave response using the analog line-level input, at roughly 10W into 8 ohms. Due to limitations inherent to the Audio Precision APx555 B Series analyzer, this graph should not be used to infer or extrapolate the SU-GX70’s slew-rate performance. Rather, it should be seen as a qualitative representation of the SU-GX70’s mid-tier 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. In this case, because of the digital nature of the amplifier, we see a 400kHz switching frequency (see 1MHz FFT below) riding on top of the squarewave.

Square-wave response (10kHz, restricted 500kHz bandwidth)

Above is the same 10kHz squarewave response using the analog line-level input, at roughly 10W into 8 ohms, this time with a 250kHz input bandwidth on the analyzer to filter out the 400kHz switching frequency. We can see significant over/undershoot in the corners of the squarewave, a consequence of the SU-GX70’s mid-tier bandwidth.

FFT spectrum (1MHz bandwidth)

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 analog line-level input, with an extended 1MHz input bandwidth. This enables us to see the high-frequency noise above 20kHz reaching almost -70dBrA at 80kHz. We also see a clear peak at 400kHz, reaching just past -20dBrA, as well as its harmonics (800kHz, 1.2MHz). These peaks, as well as the noise, are a result of the digital amplifier technology used in the SU-GX70. However, they are far above the audioband—and are therefore inaudible—and so high in frequency that any loudspeaker the amplifier is driving should filter it all out anyway.

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

The final graph above is the damping factor as a function of frequency. We can see here the clear trend of a higher (although still poor in absolute terms) damping factor at low frequencies—around 35 from 20Hz to 3kHz, and then a decline down to 18 at 20kHz. This is a limitation of the digital amplifier technology used in the SU-GX70, and the reason Technics has incorporated their clever Load Adaptive Phase Calibration (LAPC) feature to compensate for losses into low impedances at high frequencies.

Diego Estan
Electronics Measurement Specialist

Details

Parent Category: Products

Category: Amplifier Measurements

Created: 15 November 2023

Link: reviewed by Philip Beaudette on SoundStage! Hi-Fi on November 15, 2023

General Information

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

The Rotel Michi X5 Series 2 was conditioned for one hour at 1/8th full rated power (~43W into 8 ohms) before any measurements were taken. All measurements were taken with both channels driven, using a 120V/20A dedicated circuit, unless otherwise stated.

The X5 Series 2 offers a multitude of inputs, both digital and analog, line-level analog pre-amp outputs, subwoofer line-level outputs and two pairs of speaker-level outputs (for biwiring). There is also a ¼″ TRS headphone output on the front panel. For the purposes of these measurements, unless otherwise stated, the following inputs were evaluated: digital coaxial 1 (RCA), analog balanced (XLR), as well as RCA phono, configured both for moving-magnet (MM) and moving-coil (MC) inputs. Comparisons were made between unbalanced (RCA) and balanced (XLR) line-level inputs, and no differences were seen in terms of THD+N (FFTs for both can be seen in this report); however, the balanced input offers 3.84dB less gain than the unbalanced inputs. Bluetooth is also offered; however, our APx555 does not currently have a Bluetooth board.

Most measurements were made with a 2Vrms line-level, 0dBFS digital input, 5mVrms MM-level, and 0.5mVrms MC-level analog input. The signal-to-noise ratio (SNR) measurements were made with the same input signal values but with the volume set to achieve the rated output power of 350W (8 ohms). For comparison, on the line-level input, a SNR measurement was also made with the volume at maximum, but with a lower input voltage to achieve the same 350W output.

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

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

Published specifications vs. our primary measurements

The table below summarizes the measurements published by Rotel for the Michi X5 Series 2 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 8 ohms and a measurement input bandwidth of 10Hz to 22.4kHz, and the worst-case measured result between the left and right channels.

Our primary measurements revealed the following using the balanced line-level analog input and digital coaxial input (unless specified, assume a 1kHz sinewave at 2Vrms or 0dBFS, 10W output, 8-ohm loading, 10Hz to 22.4kHz bandwidth):

For the continuous dynamic power test, the X5 Series 2 was able to sustain 700W into 4 ohms (3.6% THD) using an 80Hz tone for 500ms, alternating with a signal at -10dB of the peak (70W) 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 sides of the X5 Series 2 were warm to the touch, but not enough to cause discomfort.

Our primary measurements revealed the following using the phono-level input, MM configuration (unless specified, assume a 1kHz sinewave at 5mVrms, 10W output, 8-ohm loading, 10Hz to 22.4kHz bandwidth):

Our primary measurements revealed the following using the phono-level input, MC configuration (unless specified, assume a 1kHz sinewave at 0.5mVrms, 10W output, 8-ohm loading, 10Hz to 22.4kHz bandwidth):

Our primary measurements revealed the following using the balanced line-level inputs at the headphone output (unless specified, assume a 1kHz sinewave 2Vrms input, 2Vrms output, 300 ohms loading, 10Hz to 22.4kHz bandwidth):

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

In our measured frequency response (relative to 1kHz) plot above, the X5 Series 2 is nearly flat within the audioband (-0.2dB at 20Hz, -0.05dB at 20kHz). At the extremes the X5 Series 2 is -0.7dB at 10Hz, and -0.5dB at 100kHz. These data only half corroborate Rotel’s claim of 10Hz to 100kHz (0/-0.6dB). The X5 Series 2 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 ideally matched.

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

Above is the phase-response plot from 20Hz to 20kHz for the balanced line-level input, measured across the speakers outputs at 10W into 8 ohms. The X5 Series 2 does not invert polarity and exhibits, at worst, +20 degrees (at 20Hz) of phase shift within the audioband.

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

Above are two frequency response plots (relative to 1kHz) for the balanced line-level input, measured at 10W (8-ohms) at the speaker outputs, with the treble/balance controls set at both minimum and maximum. They show that the X5 Series 2 will provide a maximum gain/cut of approximately 12dB at 20Hz, and a maximum gain/cut of approximately 9dB at 20kHz.

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

The chart above shows the X5 Series 2’s frequency response (relative to 1kHz) as a function of input type. The green trace is the same (but limited to 80kHz) analog input data from the previous graph. The blue trace is for a 16-bit/44.1kHz dithered digital input signal from 5Hz to 22kHz using the coaxial input, the purple trace is for a 24/96 dithered digital input signal from 5Hz to 48kHz, and finally pink is 24/192 from 5Hz to 96kHz. The behavior at low frequencies is the same for all the digital sample rates, as well as the analog input: -0.25dB at 20Hz. The behavior at high frequencies for all three digital sample rates is as expected, offering filtering around 22, 48, and 96kHz (half the respective sample rate). The 44.1kHz sampled input signal exhibits typical “brick-wall”-type behavior, with a -3dB point at 21kHz. The -3dB point for the 96kHz sampled data is at 46kHz, and 68kHz for the 192kHz sampled data.

Frequency response (8-ohm loading, MM input)

What is shown above is the moving-magnet (MM) phono stage’s deviation from the RIAA curve, where the input signal sweep is EQ’d with an inverted RIAA curve supplied by Audio Precision. No deviation would yield a flat line at 0dB. So the chart above shows the frequency response (relative to 1 kHz), which displays very small maximum deviations of about -0.25/-0.04dB (20Hz/20kHz) and +0.1dB (100Hz) from 20Hz to 20kHz.

Frequency response (8-ohm loading, MC input)

The chart above shows the frequency response for the phono input (MC configuration). We see essentially the same result as with the MM configuration, with the exception of the left channel deviating by +0.08dB at 4kHz.

Phase response (MM and MC phono inputs)

Above is the phase response plot from 20Hz to 20kHz for the MM phono input (MM and MC configurations behaved identically), measured across the speakers 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 20Hz and +40 degrees at 20kHz.

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

The chart above shows the results of a linearity test for the coaxial digital input (the optical input performed identically) for both 16/44.1 (blue/red) and 24/96 (purple/green) input data, measured at the line-level pre-outs of the X5 Series 2. For this test, the digital input was swept with a dithered 1kHz input signal from -120dBFS to 0dBFS, and the output was analyzed by the APx555. The ideal response would be a straight flat line at 0dB. Both digital input types performed similarly, approaching the ideal 0dB relative level at -100dBFS, then yielding perfect results to 0dBFS. At or near -120dBFS, both sample rates overshot by 3 to 5dB.

Impulse response (24/48 data)

The graph above shows the impulse response for a looped 24/44.1 test file that moves from digital silence to full 0dBFS (all “1”s) for one sample period then back to digital silence, measured at the line level pre-outs of the X5 Series 2. We can see that the X5 Series 2 utilizes a reconstruction filter that favors no pre-ringing and significant post-ringing. Since the initial pulse/peak shows a negative voltage, it's likely that the digital input inverts polarity.

J-Test (coaxial input)

The chart above shows the results of the J-Test test for the coaxial digital input measured at the line-level output of the X5 Series 2. 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 (24bit). 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 X5 Series 2 input shows obvious peaks in the audioband from -95dBrA to -140dBrA. This is a poor J-Test result and an indication that the X5 Series 2’s DAC may be susceptible to jitter.

J-Test (optical input)

The chart above shows the results of the J-Test test for the optical digital input measured at the line-level output of the X5 Series 2. The results here are similar but slightly better than the coaxial input, with the highest peaks just below -100dBrA.

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

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

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 X5 s2, with an additional 100ns of 2kHz sinewave jitter injected by the APx555. The results are very clear, as we see the sidebands at 10kHz and 14kHz (12kHz main signal +/-2kHz jitter signal) manifest at near -50dBrA. This is further indication that the DAC in the X5 Series 2 has poor jitter immunity. For this test, the optical input yielded effectively the same results.

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

The chart above shows a fast Fourier transform (FFT) of the X5 Series 2’s line-level output with white noise at -4dBFS (blue/red) and a 19.1 kHz sinewave at 0dBFS fed to the coaxial digital input, sampled at 16/44.1. The steep roll-off around 20kHz in the white-noise spectrum shows that the X5 Series 2 uses a brick-wall-type reconstruction filter. There are no obvious aliased images within the audioband, with the exception of a small peak at -115dBrA at 15kHz. The primary aliasing signal at 25kHz is highly suppressed at -100dBrA, while the second and third distortion harmonics (38.2, 57.3kHz) of the 19.1kHz tone are at -85 and -75dBrA.

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

The chart above shows RMS level (relative to 0dBrA, which is 1W into 8 ohms or 2.83Vrms) as a function of frequency, for the balanced analog line-level input swept from 5Hz to 100kHz. The blue plot is into an 8-ohm load, the purple is into a 4-ohm load, the pink plot is an actual speaker (Focal Chora 806, measurements can be found here), and the cyan plot is no load connected. The chart below . . .

. . . is the same but zoomed in to highlight differences. Zoomed in, we can see a maximum deviation within the audioband of only about 0.15dB (at 20kHz) from 4 ohms to no load, and much less (0.05dB) within the flatter portion of the curve, which is an indication of a high damping factor, or low output impedance. The maximum variation in RMS level when a real speaker was about the same, deviating by about 0.04dB within the flat portion of the curve (100Hz to 5kHz), with the lowest RMS level, which would correspond to the lowest impedance point for the load, exhibited around 200Hz, and the highest RMS level, which would correspond to the highest impedance point for the load, at around 3-5kHz. The more significant deviations in RMS level between loads at 10kHz and 20kHz is an indication of a dip in damping factor in this frequency range. This can be seen in our damping factor graph (see last graph in the report).

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

The graph 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 the left and right channels at 1W output into 8 ohms, purple/green at 10W, and pink/orange at the full rated power of 350W. The power was varied using the volume control. The 10W and 1W data exhibited effectively the same THD values, and remained commendably flat with the entire audioband, between 0.006% and 0.01%. At the full rated power of 350W, THD values were remarkably close to the 1/10W data, ranging from 0.006% to 0.01% up to 10kHz, then up to 0.2% at 20kHz. 

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

The graph above shows THD ratio as a function of frequency plot for the phono input measured across an 8 ohms load at 10W. The MM configuration is shown in blue/red (left/right channels) and MC in purple/green (left/right channel). The input sweep was EQ’d with an inverted RIAA curve. The THD values for the MM configuration vary from around 0.02% (20Hz) down to just above and below 0.005% (1kHz to 10kHz). The MC THD values were higher, ranging from 0.3/0.2% (20Hz, left/right channel), down to 0.005% (2kHz, left channel). Between 1kHz and 3kHz, the left channel outperformed the right channel for the MC configuration by as much as 10dB.

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

The chart above shows THD ratios measured at the output of the X5 Series 2 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 about 5-6dB, and both data sets show fairly constant THD values across measured output power levels until the “knees” at just past 300W (8 ohms) and 500W (4 ohms). THD levels for the 8-ohm data are around 0.005-0.007%, and 0.01-0.015% for the 4-ohm data. The 1% THD mark for the 8-ohm data is at 400W, and 659W for the 4-ohm data.

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

The chart above shows THD+N ratios measured at the output of the X5 Series 2 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 0.007%. The 4-ohm results were similar, but 2-4 dB worse.

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

The chart above shows THD ratios measured at the output of the X5 Series 2 as a function of load (8/4/2 ohms) for a constant input voltage that yields 40W at the output into 8 ohms (and roughly 80W into 4 ohms, and 160W into 2 ohms) for the balanced line-level input. The 8-ohm load is the blue trace, the 4-ohm load the purple trace, and the 2-ohm load the pink trace. We find increasing levels of THD from 8 to 4 to 2 ohms, with about a 5dB increase between each halving of the load, but nearly 20dB difference at 20kHz between the 8- and 2-ohm data. Overall, even with a 2-ohm load at roughly 160W, THD values ranged from as low as 0.02% through most of the audioband to 0.07% at 20kHz.

THD ratio (unweighted) vs. frequency into 8 ohms and real speakers (left channel only)

The chart above shows THD ratios measured at the output of the X5 Series 2 as a function of frequency into an 8-ohm load and two different speakers for a constant output voltage of 2.83Vrms (1W into 8 ohms) for the analog line-level input. The 8-ohm load is the blue trace, the purple plot is a two-way speaker (Focal Chora 806, measurements can be found here), and the pink plot is a three-way speaker (Paradigm Founder Series 100F, measurements can be found here). Generally, THD ratios into the real speakers were similar to the resistive dummy load, which hovered between 0.01 and 0.005%. THD ratios were higher (5-10dB) at 20Hz for the two-way speaker and at 20kHz for the three-way speaker, but also lower (5-10dB) than the resistive load between 800Hz and 6kHz. This is a strong result, and shows that the X5 Series 2 will yield consistently low THD results into real-world speaker loads.

IMD ratio (CCIF) vs. frequency into 8 ohms and real speakers (left channel only)

The chart above shows intermodulation distortion (IMD) ratios measured at the output of the X5 Series 2 as a function of frequency into an 8-ohm load and two different speakers for a constant output voltage of 2.83Vrms (1W into 8 ohms) for the analog line-level input. Here the CCIF IMD method was used, where the primary frequency is swept from 20kHz (F1) down to 2.5kHz, and the secondary frequency (F2) is always 1kHz lower than the primary, with a 1:1 ratio. The CCIF IMD analysis results are the sum of the second (F1-F2 or 1kHz) and third modulation products (F1+1kHz, F2-1kHz). The 8-ohm load is the blue trace, the purple plot is a two-way speaker (Focal Chora 806, measurements can be found here), and the pink plot is a three-way speaker (Paradigm Founder Series 100F, measurements can be found here). Generally, THD ratios into the real spakers were similar to the resistive dummy load, which hovered around 0.005%. At lower frequencies, both speakers yielded lower IMD results (0.002-0.003%), and at higher frequencies, the 3-way speaker yielded higher IMD results than the resistive load, 0.01% at 20kHz.

IMD ratio (SMPTE) vs. frequency into 8 ohms and real speakers (left channel only)

The chart above shows IMD ratios measured at the output of the X5 Series 2 as a function of frequency into an 8-ohm load and two different speakers for a constant output voltage of 2.83Vrms (1W into 8 ohms) for the analog line-level input. Here, the SMPTE IMD method was used, where the primary frequency (F1) is swept from 250Hz down to 40Hz, and the secondary frequency (F2) is held at 7kHz with a 4:1 ratio. The SMPTE IMD analysis results consider the second (F2 ± F1) through the fifth (F2 ± 4xF1) modulation products. The 8-ohm load is the blue trace, the purple plot is a two-way speaker (Focal Chora 806, measurements can be found here), and the pink plot is a three-way speaker (Paradigm Founder Series 100F, measurements can be found here). All three results are similar enough to be judged identical, hovering around 0.02%.

FFT spectrum – 1kHz (XLR 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 at -85dBrA or 0.005%, and around -100dBrA, or 0.001%, at the fourth (4kHz). The sixth (6kHz) and eighth (8kHz) harmonics follow at -110 and -120dBrA, or 0.0003 and 0.0001%. Below 1kHz, we see peaks from power-supply noise artifacts at 60Hz (around -100dBrA or 0.001%), and then the odd harmonics (180Hz, 300Hz, 420Hz) dominating at between -90dBrA, or 0.003%, and -100dBrA, or 0.001%.

FFT spectrum – 1kHz (RCA 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 unbalanced line-level input. We see effectively the same results as with the XLR balanced input FFT above.

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. We see effectively the same results as with the analog balanced input FFT above.

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 and noise harmonic profile within the audioband as with the 16/44.1 sampled input.

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

Shown above is the FFT for a 1kHz -90dBFS dithered 16/44.1 input sinewave stimulus at the coaxial digital input, measured at the output across an 8-ohm load. We see the 1kHz primary signal peak, at the correct amplitude, along with the 60Hz power-supply peak (-110dBrA) with a multitude of subsequent harmonics at and below -95dBrA.

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. We see the 1kHz primary signal peak, at the correct amplitude, along with the 60Hz power-supply peak (-110dBrA) with a multitude of subsequent harmonics at and below -95dBrA.

FFT spectrum – 1kHz (MM 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, configured for MM. We see the signal harmonic profile is similar to the line-level balanced input, with the second harmonic dominating at -85dBrA, or 0.005%. The noise-related peaks are at and below the -80dBrA level, or 0.01%.

FFT spectrum – 1kHz (MC 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, configured for MC. The main signal harmonic is again the second harmonic (2 kHz) at around -80dBrA or 0.01%. What dominates the FFT are the noise peaks, which is due to the very high gain required for an MC cartridge, and are as high as almost -55dBrA, or around 0.2% at 60, 180, and 300Hz.

FFT spectrum – 50Hz (line-level input)

Shown above is the FFT for a 50Hz input sinewave stimulus measured at the output across an 8-ohm load at 10W for the balanced line-level input. The X axis is zoomed in from 40 Hz to 1kHz, so that peaks from noise artifacts can be directly compared against peaks from the harmonics of the signal. The most predominant (non-signal) peaks are that of the signal’s second (100Hz) harmonic at -85dBrA, or 0.005%, and the fifth power-supply noise harmonic (300Hz) at -90dBrA, or 0.003%.

FFT spectrum – 50Hz (MM phono input)

Shown above is the FFT for a 50Hz input sinewave stimulus measured at the output across an 8-ohm load at 10W for the phono input configured for MM. The most predominant (non-signal) peaks are that of the signal’s second (100Hz) harmonic at -85dBrA, or 0.005%, and the primary (60Hz), third (180Hz), and fifth (300Hz) power-supply noise harmonics at nearly the same level.

FFT spectrum – 50Hz (MC phono input)

Shown above is the FFT for a 50Hz input sinewave stimulus measured at the output across an 8-ohm load at 10W for the phono input configured for MC. The most predominant (non-signal) peaks are the primary (60Hz), third (180Hz), and fifth (300Hz) power-supply noise harmonics at around -60dBrA, or 0.1%. The second (2kHz) signal harmonic is just below -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 -90dBRA, or 0.003%, while the third-order modulation products, at 17kHz and 20kHz, are lower, at -100dBrA, or 0.001%.

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

Shown above is an FFT of the intermodulation distortion (IMD) products for an 18kHz + 19kHz summed sinewave stimulus tone measured at the output across an 8-ohm load at 10W for the digital coaxial input at 16/44.1. We find that the second-order modulation product (i.e., the difference signal of 1kHz) is at -90dBRA, or about 0.003%, while the third-order modulation products, at 17kHz and 20kHz, are at -100dBrA, or 0.001%. We also see the main aliased peaks at 25.1kHz and 26.1kHz around -105dBrA.

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

Shown above is an FFT of the intermodulation distortion (IMD) products for an 18kHz + 19kHz summed sinewave stimulus tone measured at the output across an 8-ohm load at 10W for the digital coaxial input at 24/96. We find that the second-order modulation product (i.e., the difference signal of 1kHz) is at -90dBRA, or about 0.003%, while the third-order modulation products, at 17kHz and 20kHz, are at -100dBrA, or 0.001%.

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. Here we find close to the same result as with the balanced line-level analog input. The second order 1kHz peak is at -90dBrA, or 0.003%, while the third-order peaks are at -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 sinewave stimulus tone measured at the output across an 8-ohm load at 10W for the phono input configured for MC. The second order 1kHz peak is at -80dBrA, or 0.01%, while the third-order peaks are at -100dBrA, or 0.001%. 

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

Shown above is the FFT of the speaker-level output of the X5 Series 2 with the APx 32-tone signal applied to the input. The combined amplitude of the 32 tones is the 0dBrA reference, and corresponds to 10W into 8 ohms. The intermodulation products—i.e., the “grass” between the test tones—are distortion products from the amplifier and below the -110dBrA, or 0.0003%, level. The peaks at lower frequencies that reach the -90dBrA level are not IMD products but power-supply-related noise peaks.

Square-wave response (10kHz)

Above is the 10kHz squarewave response using the analog line-level input, at roughly 10W into 8 ohms. Due to limitations inherent to the Audio Precision APx555 B Series analyzer, this graph should not be used to infer or extrapolate the X5 Series 2’s slew-rate performance. Rather, it should be seen as a qualitative representation its extended bandwidth. An ideal squarewave can be represented as the sum of a sinewave and an infinite series of its odd-order harmonics (e.g., 10kHz + 30kHz + 50kHz + 70kHz . . .). A limited bandwidth will show only the sum of the lower-order harmonics, which may result in noticeable undershoot and/or overshoot, and softening of the edges. The X5 Series 2’s reproduction of the 10kHz square wave is very clean, with only very mild softening in the edges.

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

The final graph above is the damping factor as a function of frequency. Both channels show a relatively steady decline in damping factors from low to high frequencies, and track very closely to one another. From 20Hz to 2kHz, damping factors ranged from 600 to just shy of 500, then a decline to 125 at 20kHz. These are strong damping factor results.

Diego Estan
Electronics Measurement Specialist

Details

Parent Category: Products

Category: Amplifier Measurements

Created: 01 November 2023

Link: reviewed by Dennis Burger on SoundStage! Access on November 1, 2023

General Information

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

The Ampster BT II was conditioned for one hour at 1/8th full rated power (~3W 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 Ampster BT II offers one analog input (RCA), one digital optical (S/PDIF), and one Bluetooth input, plus a subwoofer output (RCA) and a pair of speaker-level outputs. For the purposes of these measurements, the following inputs were evaluated: optical (S/PDIF) and the analog line-level unbalanced (RCA) input.

Based on the different results at various volume levels of the left/right channel matching (see table below), the Ampster BT II volume control is likely operating in the analog domain but is digitally controlled. The volume control offers a total range from -62dB to +28dB with step sizes ranging from 1 to 4dB.

Most measurements were made with a 1.1Vrms line-level analog input, or a 0dBFS digital input. We avoided our typical standard 2Vrms analog input level because this caused severe distortion at the input. We found that a 1.5Vrms analog input yielded 1% THD at the output of the Ampster BT II, while maintaining a modest 1W into 8 ohms. This could be considered a significant design flaw, as most modern DAC outputs are between 2 and 2.2Vrms, with some even exceeding 4Vrms for a 0dBFS digital input. This means that a typical DAC connected to the Ampster BT II’s analog input, while decoding a digital track that is recorded with little digital headroom (peaks at or approaching 0dBFS, which are common for modern music), would cause the Ampster BT II to clip regardless of volume position and load.

The signal-to-noise ratio (SNR) measurements were made with the same input signal values but with the volume set to achieve the rated output power of 25W (8 ohms). For comparison, on the line-level input, a SNR measurement was also made with the volume at maximum, but with a lower input voltage to achieve the same 25W output.

Because the Ampster BT II uses digital amplifier technology that exhibits considerable noise above 20kHz (see FFTs below), our typical input bandwidth filter setting of 10Hz–90kHz was necessarily changed to 10Hz–22.4kHz for all measurements, except for frequency response and for FFTs. In addition, THD versus frequency sweeps were limited to 6kHz to adequately capture the second and third signal harmonics with the restricted-bandwidth setting.

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

Published specifications vs. our primary measurements

The table below summarizes the measurements published by Tangent for the Ampster BT II compared directly against our own. The published specifications are sourced from Tangent’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 22.4kHz, and the worst-case measured result between the left and right channels.

Our primary measurements revealed the following using the analog/optical input (unless specified, assume a 1kHz sinewave at 1.1Vrms or 0dBFS, 10W output, 8-ohm loading, 10Hz to 22.4kHz bandwidth):

For the continuous dynamic power test, the Ampster BT II was able to sustain about 52W (5% THD) into 4 ohms using an 80Hz tone for 500ms, alternating with a signal at -10dB of the peak (5.2W) 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 sides of the BT II were only slightly warm to the touch.

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

In our measured frequency-response (relative to 1kHz) chart above, the Ampster BT II is nearly flat at the low end of the audioband (-0.1dB at 20Hz), but deviates from flat at high frequencies (+2.5dB at 20kHz). The -3dB point is just past 60kHz. 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.

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

Above is the phase response plot from 20Hz to 20kHz for the analog input. The Ampster BT II does not invert polarity and exhibits a about +10 degrees of phase shift at 20Hz, and less than -5 degrees at 20kHz.

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

The chart above shows the Ampster BT II’s frequency response (left channel only) as a function of input type. The green trace is the same analog input data from the previous graph. The blue trace is for a 16bit/44.1kHz dithered digital input signal from 5Hz to 22kHz, the purple trace is for a 24/96 dithered digital input signal from 5Hz to 48kHz, and finally pink is 24/192 from 5Hz to 96kHz. The behavior at low frequencies is the same across digital input types (and the analog input)— -1.5dB at 5Hz. The behavior nearing 20kHz for the 16/44.1 digital input is a brick-wall-type attenuation, with a -3dB point at 21kHz, but +0.45dB at 17-18kHz. The -3dB point for the 24/96 data is at 47kHz, and 56kHz for the 24/192 data.

Frequency response (bass and treble controls, line-level input)

Above are two frequency-response plots (relative to 1kHz) for the analog input, measured at 10W (8-ohm loading) at the speaker outputs, with the treble and bass controls set at both minimum and maximum. They show that the Ampster BT II will provide a maximum gain/cut of approximately 5dB centered around 150Hz and 9-10kHz. Due to the Ampster BT II’s inherent rise in frequency response at high frequencies, with the treble set to maximum, we measured +8dB at 20kHz.

Frequency response (subwoofer output)

Above is the frequency response (relative to 20Hz) plot for the analog input, measured at the line-level subwoofer output. The -3dB point is near 600Hz.

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

The chart above shows the results of a linearity test for the optical digital input for both 16/44.1 (blue/red) and 24/96 (purple/green) input data, measured at the speaker outputs of the Ampster BT II. For this test, 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. The 16/44.1 data overshot the ideal output signal amplitude by about 10dB at -120dBFS, but yielded perfectly flat results from -90dBFS to 0dBFS. The 24/96 data undershot by 10dB at -110dBFS (left) and overshot by 10dB at -120dBFS, but yielded perfectly flat results from -80dBFS to 0dBFS. Interestingly, the 16/44.1 data outperformed the 24/96 data, because that usually does not happen.

Impulse response (24/44.1 data)

The graph above shows the impulse response for a looped 24/44.1 test file that moves from digital silence to full 0dBFS (all “1”s) for one sample period then back to digital silence, measured at the speaker outputs (10W 8-ohm). We can see that the Ampster BT II utilizes a reconstruction filter with minimal pre-ringing and significant post-ringing.

J-Test (optical)

The chart above shows the results of the J-Test test for the optical digital input measured at the speaker outputs (10W into 8-ohms) of the Ampster BT II. J-Test was developed by Julian Dunn in the 1990s. It is a test signal—specifically, a -3dBFS undithered 12kHz squarewave sampled (in this case) at 48kHz (24 bits). Since even the first odd harmonic (i.e., 36kHz) of the 12kHz squarewave is removed by the bandwidth limitation of the sampling rate, we are left with a 12kHz sinewave. 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 -144dBFS 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.

We see several and clear peaks in the audioband at -105 to 130dBFS. This is an average-to-mediocre J-Test result, and an indication that the Ampster BT II DAC may have poor jitter immunity.

J-Test with 10ns of injected jitter (optical)

The chart above shows the results of the J-Test test for the optical digital input measured at the speaker outputs (10W into 8 ohms) of the Ampster BT II, 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 indicates that the DAC in the Ampster BT II has poor jitter immunity.

J-Test with 100ns of injected jitter (optical)

The chart above shows the results of the J-Test test for the optical digital input measured at the speaker outputs (10W into 8-ohms) of the Ampster BT II, 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. This is yet another indication that the DAC in the Ampster BT II has poor jitter immunity.

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

The chart above shows a fast Fourier transform (FFT) of the Ampster BT II’s speaker outputs (10W into 8-ohms) with white noise at -4 dBFS (blue/red) and a 19.1 kHz sinewave at -1dBFS, both fed to the optical digital input, sampled at 16/44.1. The sharp roll-off above 20kHz in the white-noise spectrum shows the implementation of a brick-wall-type reconstruction filter. There are several low-level aliased image peaks in the audioband at -90dBrA and below. The main 25kHz alias peak is at -45dBrA.

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

. . . is the same but zoomed in to highlight differences. Zoomed in, we can see a maximum deviation within the audioband of nearly 0.4dB from 4 ohms to no load, which is an indication of an average damping factor, or average output impedance. The maximum variation in RMS level when a real speaker was used is less at about 0.2dB within the flat portion of the response.

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 6kHz) for a sinewave stimulus at the analog input. The blue and red plots are for the left and right channels at 1W output into 8 ohms, purple/green at 10W, and pink/orange at 25W (rated power). The power was varied using the volume control. Between 20Hz and 200Hz, all three THD plots are relatively flat and similar, hovering around 0.04% to 0.1%. From 200Hz to 6kHz, the 1W and 10W data ranged from 0.04% to 0.2% (left, 10W at 6kHz), while the 25W THD data yielded higher results, from 0.1% to 0.5% at 6kHz.

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 Ampster BT II as a function of output power for the analog line level-input, for an 8-ohm load (blue/red for left/right channels) and a 4-ohm load (purple/green for left/right channels). Although there are many fluctuations below the “knees” (roughly 25W into 8 ohms and about 45W into 4 ohms), both the 4-ohm and 8-ohm data are relatively close, from 0.2% down to 0.02%. It’s the right channel that generally outperformed the left channel, by as much as 10dB. The exception is the right channel into 4 ohms from 4W to 20W, where THD ratios reached 0.3%, 10-15dB higher than the other data at these power levels. The 1% THD levels were reached at 28W and 49-50W into 8 and 4 ohms.

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 Ampster BT II as a function of output power for the analog line-level input, for an 8-ohm load (blue/red for left/right) and a 4-ohm load (purple/green for left/right). The plots are virtually identical to the THD vs. output power plot above, which means, even at low power levels, it’s the THD ratios that dominate, and the Ampster BT II is a relatively quiet amplifier.

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 Ampster BT II as a function of load (8/4/2 ohms) for a constant input voltage that yields 10W at the output into 8 ohms (and roughly 20W into 4 ohms, and 40W into 2 ohms) for the analog line-level input. The 8-ohm load is the blue trace, the 4-ohm load the purple trace, and the 2-ohm load the pink trace. We find identical THD ratios between the 8- and 4-ohm loads, hovering around 0.05%, and up to 0.15% at 6kHz. Since the Ampster BT II is not designed to drive 2-ohm loads, predictably, THD ratios were higher into 2 ohms, at 0.1% to 0.2%. Nevertheless, the Ampster BT II was stable into 2 ohms, and did not shut down due to a protection circuit.

THD ratio (unweighted) vs. frequency into 8 ohms and real speakers (left channel only)

The chart above shows THD ratios measured at the output of the Ampster BT II as a function of frequency into an 8-ohm load and two different speakers for a constant output voltage of 2.83Vrms (1W into 8 ohms) for the analog line-level input. The 8-ohm load is the blue trace, the purple plot is a two-way speaker (Focal Chora 806, measurements can be found here), and the pink plot is a three-way speaker (Paradigm Founder Series 100F, measurements can be found here). All three results are similar enough to be judged identical, yielding THD ratios around 0.05% from 20Hz to 6kHz. The exception is the two-way speaker at 20kHz, which typically yields higher THD results in most amps, here at 0.25%. While a strong result in this test is one where the real speaker THD ratios are very close to the THD ratios into a dummy resistive load, as seen here, in this case, since THD results are already high into a dummy load, this result should be taken with a grain of salt.

IMD ratio (CCIF) vs. frequency into 8 ohms and real speakers (left channel only)

The chart above shows intermodulation distortion (IMD) ratios measured at the output of the Ampster BT II as a function of frequency into an 8-ohm load and two different speakers for a constant output voltage of 2.83Vrms (1W into 8 ohms) for the analog line-level input. Here the CCIF IMD method was used, where the primary frequency is swept from 20kHz (F1) down to 2.5kHz, and the secondary frequency (F2) is always 1kHz lower than the primary, with a 1:1 ratio. The CCIF IMD analysis results are the sum of the second (F1-F2 or 1kHz) and third modulation products (F1+1kHz, F2-1kHz). The 8-ohm load is the blue trace, the purple plot is a two-way speaker (Focal Chora 806, measurements can be found here), and the pink plot is a three-way speaker (Paradigm Founder Series 100F, measurements can be found here). All three results are similar enough to be judged identical, yielding relatively high IMD ratios from around 0.05% up to 0.1%.

IMD ratio (SMPTE) vs. frequency into 8 ohms and real speakers (left channel only)

The chart above shows IMD ratios measured at the output of the Ampster BT II as a function of frequency into an 8-ohm load and two different speakers for a constant output voltage of 2.83Vrms (1W into 8 ohms) for the analog line-level input. Here, the SMPTE IMD method was used, where the primary frequency (F1) is swept from 250Hz down to 40Hz, and the secondary frequency (F2) is held at 7kHz with a 4:1 ratio. The SMPTE IMD analysis results consider the second (F2 ± F1) through the fifth (F2 ± 4xF1) modulation products. The 8-ohm load is the blue trace, the purple plot is a two-way speaker (Focal Chora 806, measurements can be found here), and the pink plot is a three-way speaker (Paradigm Founder Series 100F, measurements can be found here). All three results are similar enough to be judged identical, yielding relatively high IMD ratios around 0.2% across the frequency sweep.

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 analog line-level input. We see that the signal’s second (2kHz) and third (3kHz) harmonics dominate at -70dBrA, or 0.03%; the remaining signal harmonics are at or below -80dBrA, or 0.01%. Below 1kHz, we see power-supply noise-related peaks at the fundamental (60Hz), and second (120Hz), third (180Hz) and fourth (240Hz) harmonics, at a low -105 dBrA, or 0.0006%, and below. Other lower-level noise-related peaks can also be seen. There is a significant rise in the noise floor just above 20kHz, typical for many class-D amps. It’s clear from the FFT above that THD related peaks dominate with the Ampster BT II, while noise levels are relatively low.

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 optical digital input, sampled at 16/44.1. The main differences compared to the analog input FFT above are the fourth (4kHz), fifth (5kHz), and sixth (6kHz) signal harmonic peaks that are higher here, at or near -70dBrA, or 0.03%.

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 optical digital input, sampled at 24/96. We see essentially the same signal harmonic profile as with the 16/44.1 FFT above, but with a slightly lower noise floor due to increased bit-depth.

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 optical digital input, measured at the output across an 8-ohm load. We see the 1kHz primary signal peak, at just slightly below the correct amplitude, and no visible signal harmonic peaks seen above the -145dBrA noise floor. The second (120Hz) and fourth (240Hz) power-supply-related harmonic peaks are slightly below -130dBrA, or 0.00003%.

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 optical digital input, measured at the output across an 8-ohm load. We see the 1kHz primary signal peak, at just below the correct amplitude, and the signal’s third (3kHz, left channel) harmonic can be seen at a very low -140dBrA, or 0.00001%. Power-supply-related harmonic peaks are similar to what is seen in the 16/44.1 FFT above.

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 analog input. The X axis is zoomed in from 40Hz to 1kHz, so that peaks from noise artifacts can be directly compared against peaks from the harmonics of the signal. The most predominant (non-signal) peak is the signal second (100Hz) and third (150Hz) harmonics at -70dBrA, or 0.03%, with other signal harmonics can be seen below -80dBrA, or 0.01%. Very small power-supply-related peaks can be seen, for example, at 60Hz at -105dBrA, or 0.0005%, and 120Hz at -110dBrA, or 0.0003%.

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 analog input. The input RMS values are set at -6.02dBrA so that, if summed for a mean frequency of 18.5kHz, would yield 10W (0dBrA) into 8 ohms at the output. We find that the second-order modulation product (i.e., the difference signal of 1kHz) is at -80/-90dBrA (left/right), or 0.01/0.003%, while the third-order modulation products, at 17kHz and 20kHz, are higher, at around -60dBrA, or 0.1%.

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

Shown above is an FFT of the intermodulation distortion (IMD) products for an 18kHz + 19kHz summed sinewave stimulus tone measured at the output across an 8-ohm load at 10W for the digital optical input at 16/44.1. We find that the second-order modulation product (i.e., the difference signal of 1kHz) is at 70dBrA, or about 0.03%, while the third-order modulation products, at 17kHz and 20kHz, are at -55dBrA, or 0.2%.

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

Shown above is an FFT of the intermodulation distortion (IMD) products for an 18kHz + 19kHz summed sinewave stimulus tone measured at the output across an 8-ohm load at 10W for the digital optical input at 24/96. We find that the second-order modulation product (i.e., the difference signal of 1kHz) is at 70dBrA, or about 0.03%, while the third-order modulation products, at 17kHz and 20kHz, are at -55dBrA, or 0.2%.

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

Shown above is the FFT of the speaker-level output of the Ampster BT II with the APx 32-tone signal applied to the input. The combined amplitude of the 32 tones is the 0dBrA reference, and corresponds to 10W into 8 ohms. The intermodulation products—i.e., the “grass” between the test tones—are distortion products from the amplifier and are below the -100dBrA, or 0.001%, level, below 6kHz.

Square-wave response (10kHz)

Above is the 10kHz squarewave response using the analog input, at roughly 10W into 8 ohms. Due to limitations inherent to the Audio Precision APx555 B Series analyzer, this graph should not be used to infer or extrapolate the Ampster BT II’s slew-rate performance. Rather, it should be seen as a qualitative representation of its average 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. Here we find considerable overshoot in the corner, which may be due to the Ampster BT II’s non-linear frequency response above 10kHz. In addition, we can see the 400kHz switching oscillator frequency used in the digital amplifier section clearly visible modulating the waveform.

Square-wave response (1kHz–250kHz bandwidth)

Above is the 1kHz squarewave response using the analog input, at roughly 10W into 8 ohms, with a 250kHz restricted bandwidth to remove the modulation from the 400kHz oscillator. Here we seen a relatively clean squarewave response, with the exception of the overshoot in the corner, which again may be due to the Ampster BT II’s non-linear frequency response above 10kHz.

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

The Ampster BT II’s class-D amplifier relies on a switching oscillator to convert the input signal to a pulse-width modulated (PWM) squarewave (on/off) signal before sending the signal through a low-pass filter to generate an output signal. The Ampster BT II oscillator switches at a rate of about 400kHz, and this graph plots a wide bandwidth FFT spectrum of the amplifier’s output at 10W into 8 ohms as it’s fed a 1kHz sinewave. We can see that the 400kHz peak is quite evident, and at -35dBrA. There is also a peak at 800kHz (the second harmonic of the 400kHz peak), at -60dBrA, and at 1.2MHz (the third harmonic) at -75dBrA. Those peaks are direct results of the switching oscillators in the Ampster BT II amp modules. The noise around those very-high-frequency signals are in the signal, but all that noise is far above the audioband—and is therefore inaudible—and so high in frequency that any loudspeaker the amplifier is driving should filter it all out anyway.

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

The final plot above is the damping factor as a function of frequency. Both channels track very closely, and range from about 44 from 20Hz to 2kHz, then a dip to 6.5 at 20kHz. This dip in damping factor at high frequencies is typical of inexpensive class-D amp modules.

Diego Estan
Electronics Measurement Specialist

Details

Parent Category: Products

Category: Amplifier Measurements

Created: 15 October 2023

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

General Information

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

The Musical Fidelity Nu-Vista 800.2 was conditioned for one hour at 1/8th full rated power (~37W 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 800.2 offers four unbalanced (RCA) and one balanced (XLR) set of line-level analog inputs, two pairs of line-level outputs (fixed and variable over RCA), and two pairs of speaker-level outputs. For the purposes of these measurements, the following inputs were evaluated: analog line-level balanced inputs (a 1kHz FFT using the unbalanced inputs is also provided).

Most measurements were made with a 2Vrms line-level analog input. The signal-to-noise ratio (SNR) measurements were amade with the same input signal values but with the volume set to achieve the measured output power at 1% THD, which was 276W into 8 ohms (the 800.2 did not make its rated output of 330W into 8 ohms). For comparison, on the line-level input, a SNR measurement was also made with the volume at maximum, but with a lower input voltage to achieve the same 276W output.

Based on the accuracy and non-repeatable results at various volume levels of the left/right channel matching (see table below), the 800.2 volume control is likely digitally controlled but operating in the analog domain. The volume control offers a total range from -69.5dB to +30.9dB between the line-level balanced analog inputs and the speaker outputs. Volume step sizes were 0.5dB steps throughout the range.

The analyzer’s input bandwidth filter was set to 10Hz–22.4kHz for all measurements, except for frequency response (DC to 1 MHz), FFTs (10Hz to 90kHz) and THD vs Frequency (10Hz to 90kHz). The latter to capture the second and third harmonic of the 20kHz output signal. Since the 800.2 is a conventional class-AB amp, there was no issue with excessive noise above 20kHz.

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 800.2 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 was extended from DC to 1MHz, assume, unless otherwise stated, 10W into 8 ohms and a measurement input bandwidth of 10Hz to 22.4kHz, and the worst-case measured result between the left and right channels.

Our primary measurements revealed the following using the line-level analog input (unless specified, assume a 1kHz sinewave at 2Vrms, 10W output, 8-ohm loading, 10Hz to 22.4kHz bandwidth):

For the continuous dynamic power test, the 800.2 was able to sustain 508W into 4 ohms (~4% THD) using an 80Hz tone for 500ms, alternating with a signal at -10dB of the peak (50.8W) 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 800.2 was warm but not hot to the touch.

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

In our measured frequency-response chart above, the blue and red plots show the speaker-level outputs (relative to 1kHz, at 10W into 8 ohms). The 800.2’s speaker outputs are near flat within the audioband   (about -0.1dB at 20Hz and 20kHz), and exhibit an average extended bandwidth (-2dB at 75kHz). The 800.2 appears to be AC-coupled, due the attenuation in response below 20Hz. 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.

Phase response (line-level input)

Above is the phase response plot from 20Hz to 20kHz for the line-level input, measured across the speaker outputs at 10W into 8 ohms. The 800.2 does not invert polarity. Here we find phase shifts of +20 degrees at 20Hz and -20 degrees of at 20kHz.

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. Here we can see only minor deviations of about 0.04dB from 4 ohms to no load through most of the audioband, reaching as high as about 0.1dB at 20kHz. This is an indication of a high damping factor, or low output impedance. The variation in RMS level when a real speaker was used is about the same at 0.04dB through most of the audioband.

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 analog 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 245W. The power was varied using the volume control. At 1W and 10W, THD ratios were very similar and ranged from 0.001–0.002% from 20Hz to 2kHz, then up to 0.01% at 20kHz. The 245W THD data were consistent for the right channel at 0.005% from 20Hz to 8kHz, then up to 0.01% at 20kHz. The left channel THD ratios at 245W varied from 0.02% at 20Hz, down to 0.002% from 100Hz to 2kHz, then up to 0.01% at 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 800.2 as a function of output power for the analog 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 3 to 5dB. The 8-ohm data ranged from 0.01% at 50mW, down to 0.001% at 2-3W, the up to 0.005% at the “knee” at roughly 250W. The “knee” for the 4-ohm data can be seen at roughly 400W. The 1% THD marks are at 276W (shy of the rated 330W) into 8 ohms, and 472W into 4 ohms.

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 800.2 as a function of output power for the analog 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 both loads were similar, ranging from about 0.05% at 50mW, down to 0.003% (8-ohm). The 4-ohm THD+N ratios were 2–3dB worse than the 8-ohm ratios through most of the sweep.

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 800.2 as a function of frequency into three different loads (8/4/2 ohms) for a constant input voltage that yields 40W at the output into 8 ohms (and roughly 80W into 4 ohms, and 160W into 2 ohms) for the analog line-level input. The 8-ohm load is the blue trace, the 4-ohm load the purple trace, and the 2-ohm load the pink trace. We find a 5 to 10dB increase in THD every time the load is halved. Nonetheless, even into 2 ohms, these data show that the 800.2 is not only stable into 2-ohm loads, but will yield acceptably low THD values, ranging from 0.005 to 0.05%.

THD ratio (unweighted) vs. frequency into 8 ohms and real speakers (left channel only)

The chart above shows THD ratios measured at the output of the 800.2 as a function of frequency into an 8-ohm load and two different speakers for a constant output voltage of 2.83Vrms (1W into 8 ohms) for the analog line-level input. The 8-ohm load is the blue trace, the purple plot is a two-way speaker (Focal Chora 806, measurements can be found here), and the pink plot is a three-way speaker (Paradigm Founder Series 100F, measurements can be found here). At lower frequencies, the two-way speaker yielded the highest THD ratios, as high as 0.03% at 20Hz. From 300Hz to 20kHz however, all THD values were very similar, ranging  from around 0.001% up to 0.01%. This shows that the 800.2 will yield consistent and stable THD results into different loads at low power levels.

IMD ratio (CCIF) vs. frequency into 8 ohms and real speakers (left channel only)

The chart above shows intermodulation distortion (IMD) ratios measured at the output of the 800.2 as a function of frequency into an 8-ohm load and two different speakers for a constant output voltage of 2.83Vrms (1W into 8 ohms) for the analog line-level input. Here the CCIF IMD method was used, where the primary frequency is swept from 20kHz (F1) down to 2.5kHz, and the secondary frequency (F2) is always 1kHz lower than the primary, with a 1:1 ratio. The CCIF IMD analysis results are the sum of the second (F1-F2 or 1kHz) and third modulation products (F1+1kHz, F2-1kHz). The 8-ohm load is the blue trace, the purple plot is a two-way speaker (Focal Chora 806, measurements can be found here), and the pink plot is a three-way speaker (Paradigm Founder Series 100F, measurements can be found here). The results for the dummy load were fairly consistent, hovering around 0.001% throughout the sweep. We find that both speakers yielded IMD ratios that were close to the dummy load, but at times 5dB higher than the dummy load.

IMD ratio (SMPTE) vs. frequency into 8 ohms and real speakers (left channel only)

The chart above shows IMD ratios measured at the output of the 800.2 as a function of frequency into an 8-ohm load and two different speakers for a constant output voltage of 2.83Vrms (1W into 8 ohms) for the analog line-level input. Here, the SMPTE IMD method was used, where the primary frequency (F1) is swept from 250Hz down to 40Hz, and the secondary frequency (F2) is held at 7kHz with a 4:1 ratio. The SMPTE IMD analysis results consider the second (F2 ± F1) through the fifth (F2 ± 4xF1) modulation products. The 8-ohm load is the blue trace, the purple plot is a two-way speaker (Focal Chora 806, measurements can be found here), and the pink plot is a three-way speaker (Paradigm Founder Series 100F, measurements can be found here). All three results are similar enough to be judged identocal, hovering around the 0.006% level.

FFT spectrum – 1kHz (balanced 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 analog line-level input. We see that the signal’s second harmonic (2kHz) dominates at around -100dBrA, or 0.001%. The other signal harmonics are below -110dBrA, or 0.0003%. There are clearly visible power-supply-related noise peaks at even and odd harmonics (60Hz, 120Hz, 180Hz, 240Hz, etc.) on the left side of the main 1kHz peak, with the 60Hz (right) and 120Hz (left) peaks dominating at -105dBrA, or 0.0006%.

FFT spectrum – 1kHz (unbalanced 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 unbalanced analog line-level input. The FFT is virtually identical to the FFT shown above using the balanced input.

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 analog 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. We see that the signal’s second harmonic (100Hz) dominates at around -100dBrA, or 0.001%. The subsequent signal harmonic peaks are below -110dBrA, or 0.0003%. There are clearly visible power-supply related noise peaks at even and odd harmonics (60Hz, 120Hz, 180Hz, 240Hz, etc.) on the left side of the main 1kHz peak, with the 60Hz (right) and 120Hz (left) peaks dominating at -105dBrA, or 0.0006%.

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 analog line-level input. The input RMS values are set at -6.02dBrA so that, if summed for a mean frequency of 18.5kHz, would yield 10W (0dBrA) into 8 ohms at the output. We find that the second-order modulation product (i.e., the difference signal of 1kHz) is just below -110dBrA, or 0.0003%. The third-order modulation products, at 17kHz and 20kHz, are higher at -105dBrA, or 0.0006%. This is a very clean IMD result.

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

Shown above is the FFT of the speaker-level output of the 800.2 with the APx 32-tone signal applied to the input. The combined amplitude of the 32 tones is the 0dBrA reference, and corresponds to 10W into 8 ohms. The intermodulation products—i.e., the “grass” between the test tones—are distortion products from the amplifier and below the -120dBrA, or 0.0001%, level. This is another clean IMD result. The peaks that reach the -105dBrA level at lower frequencies are not IMD products but power-supply-related noise peaks.

Square-wave response (10kHz)

Above is the 10kHz squarewave response using the analog line-level input, at roughly 10W into 8 ohms. Due to limitations inherent to the Audio Precision APx555 B Series analyzer, this graph should not be used to infer or extrapolate the 800.2’s slew-rate performance. Rather, it should be seen as a qualitative representation of the 800.2’s above-average 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. Here we can see a relatively clean squarewave reproduction, with some mild softening of the corners, and no overshoot.

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

The final graph above is the damping factor as a function of frequency. Both channels track closely, with a higher damping factor (around 350 to 370) between 20Hz and 2kHz. Above 2kHz, we see a decline in the damping factor, as low as 150 at 20kHz.

Diego Estan
Electronics Measurement Specialist

Details

Parent Category: Products

Category: Amplifier Measurements

Created: 01 October 2023

Link: reviewed by Jason Thorpe on SoundStage! Ultra on October 1, 2023

General Information

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

The Fezz Audio Lybra 300B was conditioned for one hour at 1/8th full rated power (~2W 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 Lybra offers three unbalanced (RCA) inputs and two pairs of speaker level outputs (one for 8 ohms, and one for 4 ohms). For the purposes of these measurements, the following input was evaluated: analog line-level input. Unless otherwise stated, when a measurement was made into an 8-ohm load the, 8-ohm speaker output was used, and for a 4-ohm (or 2-ohm) load, the 4-ohm speaker output was used. By default (if no load is mentioned), an 8-ohm load was used with the 8-ohm speaker outputs.

Most measurements were made with a 2Vrms line-level analog input. The signal-to-noise ratio (SNR) measurements were made with the same input signal values but with the volume set to achieve the rated output power of 15W (8 ohms). For comparison, on the line-level input, a SNR measurement was also made with the volume at maximum, but with a lower input voltage to achieve the same 15W output.

Based on the accuracy and non-repeatable results at various volume levels of the left/right channel matching (see table below), the Lybra volume control is a potentiometer operating in the analog domain. The volume control offers a total range from -75dB to +27.9dB for the 8-ohm output and 24.6dB for the 4-ohm output.

The analyzer’s input bandwidth filter was set to 10Hz–22.4kHz for all measurements, except for frequency response (DC to 1MHz), FFTs (10Hz to 90kHz), and THD vs. frequency (10Hz to 90kHz). The latter to capture the second and third harmonic of the 20kHz output signal. Since the Lybra is not a class-D amp, there was no issue with excessive noise above 20kHz.

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 Fezz Audio for the Lybra 300B compared directly against our own. The published specifications are sourced from Fezz Audio’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 extended from DC to 1MHz, assume, unless otherwise stated, 10W into 8 ohms and a measurement input bandwidth of 10Hz to 22.4kHz, and the worst-case measured result between the left and right channels.

Our primary measurements revealed the following using the line-level analog input (unless specified, assume a 1kHz sinewave at 2Vrms, 10W output, 8-ohm loading, 10Hz to 22.4kHz bandwidth):

For the continuous dynamic power test, the Lybra 300B was able to sustain 15W into 4 ohms (~10% THD) using an 80Hz tone for 500ms, alternating with a signal at -10dB of the peak (50.8W) 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 Lybra was very warm; however, this is the normal operating condition of the amplifier.

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

In our measured frequency response (relative to 1kHz) chart above, the blue and red plots show the speaker-level outputs (relative to 1kHz, at 10W into 8 ohms). The Lybra’s speaker outputs are near flat within the audioband (about -0.5dB at 20Hz and -0.2dB at 20kHz), and exhibit an average bandwidth (-3dB at 61kHz). The small ~0.2dB blip at around 650Hz is real, was repeatable, and was also observed with constant signals in the analyzer’s bench mode. With 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.

Phase response (line-level input)

Above is the phase response plot from 20Hz to 20kHz for the line-level input, measured across the speaker outputs at 10W into 8 ohms. The Lybra does not invert polarity. Here we find +70 degrees at 20Hz, and -20 degrees at 20kHz.

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 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. All sweeps were performed using the Lybra’s 8-ohm speaker outputs, to show the effects of load on frequency response. Here we can see significant deviations of about 4dB from 4 ohms to no load through the flat part of the audioband (100Hz to 10kHz), reaching as high as about 6.5dB at 20Hz. This is an indication of a very low damping factor, or high output impedance. The variation in RMS level when a real speaker was used is about 3.5dB through most of the audioband.

To expand on the Lybra’s frequency response when using a real speaker, the chart below . . .

. . . shows the frequency response (relative to 1kHz) using a continuous sweep for the Focal Chora 806. Again we see deviations of up to 3.5dB within the audioband. It’s important to mention that deviations of this magnitude would be clearly audible, giving the Lybra amplifier a “sound” that would change based on the characteristic impedance curve of the speaker it’s connected to.

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 analog line-level input. The blue and red plots are for left and right channels at 0.1W output into 8 ohms, purple/green at 1W, and pink/orange at 10W. The power was varied using the volume control. At 0.1W, THD ratios hovered between from 0.2 an 0.1% from 20Hz to 20kHz. At 1W, THD ratios hovered between from 0.7 to 0.3% from 20Hz to 20kHz. At 10W, THD ratios hovered between from 5 and 2% from 20Hz to 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 Lybra as a function of output power for the analog line-level input for an 8-ohm load (blue/red for left/right channels) and a 4-ohm load (purple/green for left/right channels). The 8-ohm and 4-ohm data tracked closely. THD ratios started at 0.03% at 10mW, with a steady rise to 2-3% at just over 10W, then a shaper rise to 10% THD at 15W.

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 Lybra as a function of output power for the analog line-level-input for an 8-ohm load (blue/red for left/right channels) and a 4-ohm load (purple/green for left/right channels). The 8-ohm and 4-ohm data tracked closely. THD+N ratios started at 0.1 to 0.2% from 10mW to 200mW, followed by a steady rise to 2 to 3% at just over 10W, then a shaper rise to 10% THD at 15W.

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 Lybra as a function of frequency into three different loads (8/4/2 ohms) for a constant input voltage of 2.83Vrms that yields 1W at the output into 8 ohms using the 8-ohm speaker output, and roughly 2/4W into 4/2 ohms using the 4-ohm speaker outputs, for the analog line-level input. The 8-ohm load is the blue trace, the 4-ohm load the purple trace, and the 2-ohm load the pink trace. We find very similar THD ratios for the 8- and 4-ohm data (the 8-ohm data is about 2-3dB lower than the 4-ohm data), ranging from around 1% down to 0.3% from 20Hz to 20kHz. For the 2-ohm load, THD ratios were higher, hovering around the 2% mark through most of the audioband.

THD ratio (unweighted) vs. frequency into 8 ohms and real speakers (left channel only)

The chart above shows THD ratios measured at the output of the Lybra as a function of frequency into an 8-ohm load and two different speakers for a constant output voltage of 2.83Vrms (1W into 8 ohms) for the analog line-level input. The 8-ohm load is the blue trace, the purple plot is a two-way speaker (Focal Chora 806, measurements can be found here), and the pink plot is a three-way speaker (Paradigm Founder Series 100F, measurements can be found here). Both speaker THD plots showed signficant variations in THD ratios, both above and below the 0.6–0.4% line for the 8-ohm dummy load. The two-way speaker ranged from 5% THD at 20Hz to a low of 0.15% at 2–3kHz. The three-way speaker ranged from 2% at 100Hz to a low of 0.06% at 3kHz. This shows that THD ratios can vary signficanlty for the Lybra, depending on the speaker’s impedance at a given frequency.

IMD ratio (CCIF) vs. frequency into 8 ohms and real speakers (left channel only)

The chart above shows intermodulation distortion (IMD) ratios measured at the output of the Lybra as a function of frequency into an 8-ohm load and two different speakers for a constant output voltage of 2.83Vrms (1W into 8 ohms) for the analog line-level input. Here the CCIF IMD method was used, where the primary frequency is swept from 20kHz (F1) down to 2.5kHz, and the secondary frequency (F2) is always 1kHz lower than the primary, with a 1:1 ratio. The CCIF IMD analysis results are the sum of the second (F1-F2 or 1kHz) and third modulation products (F1+1kHz, F2-1kHz). The 8-ohm load is the blue trace, the purple plot is a two-way speaker (Focal Chora 806, measurements can be found here), and the pink plot is a three-way speaker (Paradigm Founder Series 100F, measurements can be found here). The results for the dummy load were fairly consistent, hovering around 0.4% throughout the sweep. We find that both speakers yielded IMD ratios that were lower (by 10dB) compared to the dummy load between 2.5kHz and 5kHz. From the 6kHz to 20kHz, IMD ratios were almost 10dB higher for the three-way speaker compared to the dummy load, while the two-way speaker yielded IMD ratios lower than the dummy load throughout the sweep.

IMD ratio (SMPTE) vs. frequency into 8 ohms and real speakers (left channel only)

The chart above shows IMD ratios measured at the output of the Lybra as a function of frequency into an 8-ohm load and two different speakers for a constant output voltage of 2.83Vrms (1W into 8 ohms) for the analog line-level input. Here, the SMPTE IMD method was used, where the primary frequency (F1) is swept from 250Hz down to 40Hz, and the secondary frequency (F2) is held at 7kHz with a 4:1 ratio. The SMPTE IMD analysis results consider the second (F2 ± F1) through the fifth (F2 ± 4xF1) modulation products. The 8-ohm load is the blue trace, the purple plot is a two-way speaker (Focal Chora 806, measurements can be found here), and the pink plot is a three-way speaker (Paradigm Founder Series 100F, measurements can be found here). All three results are similar but still varied by as much as +5dB and -10dB relative to the constant 1.5% measured across the resistive dummy load. The lowest IMD level for the 2-way speaker was found at 80Hz at 0.3%, while the two-way speaker yielded 0.7% around 60Hz.

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 analog line-level input. We see that the signal’s second (2kHz) and third (3kHz) harmonic dominate at around -35dBrA, or 2%. The other signal harmonics are below -50dBrA, or 0.3%. There are clearly visible power-supply-related noise peaks at even and odd harmonics (60Hz, 120Hz, 180Hz, 240Hz, etc.) on the left side of the main 1kHz peak, with the 60Hz peak dominating at -90dBrA, or 0.003%.

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 analog 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. We see that the signal’s second (2kHz) and third (3kHz) harmonic dominate at around -35dBrA, or 2%. The other signal harmonics are below -50dBrA, or 0.3%. There are clearly visible power-supply-related noise peaks at even and odd harmonics (60Hz, 120Hz, 180Hz, 240Hz, etc.) on the left side of the main 1kHz peak, with the 60Hz peak dominating at -90dBrA, or 0.003%.

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 analog line-level input. The input RMS values are set at -6.02dBrA so that, if summed for a mean frequency of 18.5kHz, would yield 10W (0dBrA) into 8 ohms at the output. We find that the second-order modulation product (i.e., the difference signal of 1kHz) is at -45dBrA, or 0.6%. The third-order modulation products, at 17kHz and 20kHz, are at around the same level.

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

Shown above is the FFT of the speaker-level output of the Lybra with the APx 32-tone signal applied to the input. The combined amplitude of the 32 tones is the 0dBrA reference, and corresponds to 10W into 8 ohms. The intermodulation products—i.e., the "grass" between the test tones—are distortion products from the amplifier that are below the -70dBrA, or 0.03%, level.

Square-wave response (10kHz)

Above is the 10kHz squarewave response using the analog line-level input, at roughly 10W into 8 ohms. Due to limitations inherent to the Audio Precision APx555 B Series analyzer, this graph should not be used to infer or extrapolate the Lybra’s slew-rate performance. Rather, it should be seen as a qualitative representation of the Lybra’s average 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. Here we can see an average squarewave reproduction, with some softening and over/under-shoot in the corners.

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

The graph above is the damping factor as a function of frequency with an 8-ohm load connected to the 8-ohm speaker output. Both channels track closely, with a very low damping factor of between 2 (20Hz) and 3 (100Hz to 20kHz). This very high output impedance (2.67 ohms at 1kHz) explains the significant variations measured in frequency response with a real speaker load.

The graph above is the damping factor as a function of frequency with a 4-ohm load connected to the 4-ohm speaker output. It is effectively identical to the 8-ohm damping factor graph. Since damping factor is defined as the ratio of the refrence load impedance over the output impedance, this means that the output impedance on the 4-ohm tap is half that of the output impedance on the 8-ohm tap.

Diego Estan
Electronics Measurement Specialist

Details

Parent Category: Products

Category: Amplifier Measurements

Created: 01 October 2023

Link: reviewed by Roger Kanno on SoundStage! Hi-Fi on October 1, 2023

General Information

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

The NuPrime Evolution Two was conditioned for one hour at 1/8th full rated power (~35W 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 Evolution Two is a monoblock (i.e., single channel) amplifier with one unbalanced (RCA) and one balanced (XLR) input, and one speaker-level output. 400mVrms was required at the input to achieve the reference 10W into 8 ohms. For the purposes of these measurements, unless otherwise specified, the balanced input was used.

Because the Evolution Two uses a digital amplifier technology that exhibits considerable noise above 20kHz (see FFTs below), our typical input bandwidth filter setting of 10Hz–90kHz for frequency sweeps was necessarily changed to 10Hz–22.4kHz, and limited to 6kHz to adequately capture the second and third signal harmonics with the restricted-bandwidth setting.

Published specifications vs. our primary measurements

The table below summarizes the measurements published by NuPrime for the Evolution Two compared directly against our own. The published specifications are sourced from NuPrime’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 extended from DC to 1MHz, assume, unless otherwise stated, 10W into 8 ohms and a measurement input bandwidth of 10Hz to 22.4kHz, and the worst-case measured result between the left and right channesl.

* protection circuit engages after a few seconds at THD = 0.04%

Our primary measurements revealed the following using the Line 2 unbalanced analog input (unless specified, assume a 1kHz sinewave at 400mVrms at the input, 10W output, 8-ohm loading, 10Hz to 22.4kHz bandwidth):

* protection circuit engages after a few seconds at THD = 0.04%

For the continuous dynamic power test, the Evolution Two was able to sustain about 630W into 4 ohms (~1% THD) using an 80Hz tone for 500ms, alternating with a signal at -10dB of the peak (63W) for five seconds, for five continuous minutes. However, during the test, the initiation of a protection circuit did occur several times. The protection circuit engages and disengages very quickly, allowing the Evolution Two to run through the test mostly uninterrupted. Therefore, we are calling a conditional pass on this test. This test is meant to simulate sporadic dynamic bass peaks in music and movies. During the test, the Evolution Two was cool to the touch. Note that the Evolution Two is not able to sustain 630W into 4 ohms continuously for more than about one second before a protection circuit is triggered.

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

In our frequency-response plot above, measured across the speaker outputs at 10W into 8 ohms, the Evolution Two exhibits a clear rise at high frequencies. We have confirmed with NuPrime that this is intentional and not due to a defective unit. At low frequencies, the Evolution Two is essentially flat down to 5Hz. The rise at high frequencies was measured at about 0.6dB at 10kHz and 2dB at 20kHz. The rise peaks around 80–90kHz, at +6.5dB. Whether or not this deviation from a flat response would be audible would depend on the speakers used, musical content, and most importantly, the age of the listener. The -2dB point is at 200kHz, which is the maximum allowable frequency using the AP analyzer.

Phase response (8-ohm loading)

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 Evolution Two does not invert polarity and exhibits, at worst, about +5 degrees of phase shift within the audioband between 10kHz and 20kHz.

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 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. Here we find the maximum deviation between an 8-ohm load and no load to be around 0.08dB. This is an indication of a very high damping factor, or low output impedance. With a real speaker, the deviations are much lower, at about 0.02dB between 20Hz and 1kHz.

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 analog line-level input. The blue plot is at 1W output into 8 ohms, purple at 10W, and pink at 265W. The 1W data yielded fairly constant THD figures, at 0.003% from 20Hz to 3kHz, then a rise to 0.01% at 6kHz. At 10W, THD data ranged from 0.001% from 20Hz to 100Hz, then a steady rise to 0.03% at 6kHz. At 265W, THD data ranged from 0.002% from 20Hz to 100Hz, then a steady rise to 0.1% at 6kHz.

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 Evolution Two as a function of output power for the analog line-level input for an 8-ohm load (blue) and a 4-ohm load (purple). The 8-ohm data ranged from about 0.1% to 0.05% from 50 to 400mW, then a dip down to 0.003% to 0.005% from 500mW to 20W, a rise to 0.03% at the “knee” at just shy of 300W, then reaching the 1% THD mark at roughly 350W. Please note that each measurement in this sweep is only two seconds long, and by contrast, we were not able to sustain more than 294W into 8 ohms continuously. The 4-ohm data ranged from about 0.1% from 50 to 100mW, then a dip down to 0.003% to 0.01% from 150mW to 100W, a rise to 0.03% at the “knee” at 300W, then the Evolution Two protection circuit was triggered, precluding the collection of reliable data points above 300W.

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 Evolution Two as a function of output power for the analog line-level input, for an 8-ohm load (blue) and a 4-ohm load (purple). The 8-ohm data ranged from about 0.1% to 0.05% from 50 to 400mW, then a dip down to 0.005% from 500mW to 20W, a rise to 0.05% at the “knee” at just shy of 300W, then reaching the 1% THD mark at roughly 350W. Please note that each measurement in this sweep is only two seconds long, and by contrast, we were not able to sustain more than 294W into 8 ohms continuously. The 4-ohm data ranged from about 2% from 50 to 100mW, then a dip down to 0.01% from 150mW to 100W, a rise to 0.03% at the “knee” at 300W, then the Evolution Two protection circuit was triggered, precluding the collection of reliable data points above 300W.

THD ratio (unweighted) vs. frequency vs. load

The chart above shows THD ratios measured at the output of the Evolution Two as a function of frequency into three different loads (8/4/2 ohms) for a constant input voltage that yields roughly 30W at the output into 8 ohms (blue), 60W into 4 ohms (purple), and 120W into 2 ohms (pink). The 8-ohm data ranged from 0.001% from 20Hz to 100Hz, then a steady rise to 0.03% at 6kHz. The 4-ohm THD data were only about 5dB higher compared to the 8-ohm data, but converging at the same 0.03% at 6kHz. The 2-ohm data were considerably higher from 20Hz to 500Hz, yielding a fairly constant 0.005% to 0.008%, then a rise to 0.06% at 6kHz. The maximum achieved continuous power into 2 ohms was about 190W, where the protection circuit engaged after about four seconds. Nevertheless, the Evolution Two proved to be stable into 2 ohms with continuous power in the 60W to 100W range.

THD ratio (unweighted) vs. frequency into 8 ohms and real speakers

The chart above shows THD ratios measured at the output of the Evolution Two as a function of frequency into an 8-ohm load and two different speakers for a constant output voltage of 2.83Vrms (1W into 8 ohms) for the analog line-level input. The 8-ohm load is the blue trace, the purple plot is a two-way speaker (Focal Chora 806, measurements can be found here), and the pink plot is a three-way speaker (Paradigm Founder Series 100F, measurements can be found here). At very low freqencies, the two-way speaker yielded the highest THD ratios (0.02%), which is typical with many amps. From 30Hz to 500Hz, all three THD plots are very close to one another, around the 0.003% mark. From 500Hz to 6kHz, the Evolution Two yielded lower THD ratios with real speakers compared to the dummy resistive load, ranging from 0.001 to 0.002%.

IMD ratio (CCIF) vs. frequency into 8 ohms and real speakers

The chart above shows intermodulation distortion (IMD) ratios measured at the output of the Evolution Two as a function of frequency into an 8-ohm load and two different speakers for a constant output voltage of 2.83Vrms (1W into 8 ohms) for the analog line-level input. Here the CCIF IMD method was used, where the primary frequency is swept from 20kHz (F1) down to 2.5kHz, and the secondary frequency (F2) is always 1kHz lower than the primary, with a 1:1 ratio. The CCIF IMD analysis results are the sum of the second (F1-F2 or 1kHz) and third modulation products (F1+1kHz, F2-1kHz). The 8-ohm load is the blue trace, the purple plot is a two-way speaker (Focal Chora 806, measurements can be found here), and the pink plot is a three-way speaker (Paradigm Founder Series 100F, measurements can be found here). Here the three-way speaker yielded the highest IMD results, from 0.001% to 0.02%. The dummy load was in between, while the two-way speaker yielded the lowest IMD results, between 0.0005% and 0.003%.

IMD ratio (SMPTE) vs. frequency into 8 ohms and real speakers

The chart above shows IMD ratios measured at the output of the Evolution Two as a function of frequency into an 8-ohm load and two different speakers for a constant output voltage of 2.83Vrms (1W into 8 ohms) for the analog line-level input. Here, the SMPTE IMD method was used, where the primary frequency (F1) is swept from 250Hz down to 40Hz, and the secondary frequency (F2) is held at 7kHz with a 4:1 ratio. The SMPTE IMD analysis results consider the second (F2 ± F1) through the fifth (F2 ± 4xF1) modulation products. The 8-ohm load is the blue trace, the purple plot is a two-way speaker (Focal Chora 806, measurements can be found here), and the pink plot is a three-way speaker (Paradigm Founder Series 100F, measurements can be found here). The resistive dummy load was fairly constant at 0.008%, while the speaker IMD results fluctuated above and below this value by as much as 10dB.

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 analog line-level input. We see that the signal’s third (3kHz) harmonic dominates at -90dBrA, or 0.003%. The second, fourth, and fifth harmonics are a little lower, around -100dBrA, or 0.001%. Power-supply-related noise peaks can be seen but at low levels: -120dBrA, or 0.0001%, and below.

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 analog 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 dominant (non-signal) peaks are the second (100Hz) and third (150Hz) signal harmonics at roughly -100dBrA, or 0.001%. Power-supply-related noise peaks can be seen but at low levels: -120dBrA, or 0.0001%, and below.

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

Shown above is an FFT of the intermodulation distortion (IMD) products for an 18kHz + 19kHz summed sinewave stimulus tone measured at the output across an 8-ohm load at 10W for the analog line-level input. The input RMS values are set at -6.02dBrA so that, if summed for a mean frequency of 18.5kHz, would yield 10W (0dBrA) into 8 ohms at the output. We find that the second-order modulation product (i.e., the difference signal of 1kHz) is at -100dBrA, or 0.001%, and the third-order modulation products, at 17kHz and 20kHz, are higher at below -75dBrA, or 0.02%.

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

Shown above is the FFT of the speaker-level output of the Evolution Two with the APx 32-tone signal applied to the input. The combined amplitude of the 32 tones is the 0dBrA reference, and corresponds to 10W into 8 ohms. The intermodulation products—i.e., the "grass" between the test tones—are  distortion products from the amplifier and are below the -100dBrA, or 0.001%, level. Higher-amplitude distortion products are seen at higher frequencies.  

Square-wave response (10kHz)

Above is the 10kHz squarewave 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 Evolution Two’s slew-rate performance. Rather, it should be seen as a qualitative representation of the Evolution Two’s mid-tier 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. In the case of the Evolution Two, however, what dominates the plateaus of the squarewave in the top graph is a 600kHz sinewave, the frequency at which the switching oscillator in the class-D amp is operating (see FFT below).

Square-wave response (10Hz–250kHz bandwidth)

Above is the 10kHz squarewave response using the analog input, at roughly 10W into 8 ohms, with a 250kHz restricted bandwidth to remove the modulation from the 600kHz oscillator. Here we find a squarewave with significant over-shoot in the corners, likely due to the Evolution Two’s rise in frequency response at high frequencies.

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

The Evolution Two’s amplifier relies on a switching oscillator to convert the input signal to a pulse-width-modulated (PWM) squarewave (on/off) signal before sending the signal through a low-pass filter to generate an output signal. The Evolution TWo oscillator switches at a rate of about 600kHz, and this graph plots a wide bandwidth FFT spectrum of the amplifier’s output at 10W into 8 ohms as it’s fed a 1kHz sinewave. We can see that the 600kHz peak is quite evident, and at -30dBrA. There is also a peak at 1.2MHz (the second harmonic of the 600kHz peak), at -75dBrA. Those two peaks—the fundamental and its second harmonics—are direct results of the switching oscillators in the Evolution Two amp module. The noise around those very-high-frequency signals are in the signal, but all that noise is far above the audioband—and therefore inaudible—and so high in frequency that any loudspeaker the amplifier is driving should filter it all out anyway.

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

The final graph above is the damping factor as a function of frequency. We find very high damping factor values, from 500 up to over 1000 at higher frequencies. We were unable to reliably measure damping factor above around 6–7kHz, where we actually measured negative output impedances. This is impossible, and we can only speculate as to what the amplifier is doing. Based on the measurements, it seems as though the Evolution Two increases gain slightly into small loads at high frequencies.

Diego Estan
Electronics Measurement Specialist

Details

Parent Category: Products

Category: Amplifier Measurements

Created: 01 September 2023

Link: reviewed by Matt Bonaccio on SoundStage! Hi-Fi on September 1, 2023

General Information

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

The Ars-Sonum Armonía was conditioned for one hour at 1/8th full rated power (~3.5W 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 Armonía has two sets of unbalanced inputs (RCA) and a pair of speaker-level outputs. A level of 360mVrms was required at the input to achieve the reference 10W into 8 ohms for the Line 2 input, which we measured yielding a typical gain of about 28dB. The Line 1 input offers more gain—we measured about 34dB. For the purposes of these measurements, unless otherwise specified, Line 2 was used.

Published specifications vs. our primary measurements

The table below summarizes the measurements published by Ars-Sonum for the Armonía compared directly against our own. The published specifications are sourced from Ars-Sonum’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 extended from DC to 1MHz, assume, unless otherwise stated, 10W into 8 ohms and a measurement input bandwidth of 10Hz to 22.4kHz, and the worst-case measured result between the left and right channels.

Our primary measurements revealed the following using the Line 2 unbalanced analog input (unless specified, assume a 1kHz sinewave at 360mVrms at the input, 10W output, 8-ohm loading, 10Hz to 22.4kHz bandwidth):

For the continuous dynamic power test, the Armonía was able to sustain 20W into 4 ohms (~5% THD) using an 80Hz tone for 500ms, alternating with a signal at -10dB of the peak (2W) 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 Armonía was very hot to the touch, which is the same as its heat output when powered on with no signal. 

Frequency response (8-ohm loading)

In our frequency-response plots above, measured across the speaker outputs at 10W into 8 ohms, the Armonía is near flat within the audioband (-0.5/-0.4dB, 20Hz/20kHz). The -3dB points are at about 5.5Hz and 56kHz. There is also a rise in the response at around 8Hz—nearly 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.

Phase response (8-ohm loading)

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

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 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 cyan plot is an actual speaker (Focal Chora 806, measurements can be found here). As per the manufacturer, we avoided applying a signal to the Armonía with no load connected. The chart below . . .

. . . is the same but zoomed in to highlight differences. Here we find the maximum deviation between an 8-ohm and 4-ohm load to be around 1.4dB. This is an indication of a very low damping factor, or high output impedance. With a real speaker, the deviations are worse—as much as 3dB (which would be clearly audible) between 200Hz and 1.5kHz.

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 analog 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 24W. The 1W data yielded the lowest THD figures, from 0.4% (left) at 20Hz, down to 0.02% from 200Hz to 3kHz, then up to 0.1% at 20kHz. The right channel had a flatter response, but consistently yielded higher THD figures (5dB) from 200Hz to 3kHz at all power levels. At 10W, THD data for the left channel ranged from 0.5%, down to 0.05%, then up to 0.3%. At 24W, THD data were flatter, around 1 to 3%.

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 Armonía as a function of output power for the analog line level-input, for an 8-ohm load (blue/red for left/right channels) and a 4-ohm load (purple/green for left/right channels). The 8-ohm data ranged from about 0.02% from 10 to 300mW, then a steady climb to 0.3/0.5% (left/right at 20W), reaching the 1% THD mark around 24W. The 4-ohm data ranged from about 0.02% from 10 to 200mW, then a steady climb to 0.5/1% (left/right at 10W), reaching the 1% THD mark jut past 11W, and the 5% THD mark just shy of 20W. Again here, the left channel yielded lower THD ratios (5-10dB) compared to the right channel through the most-likely-used power band of the amplifier (1W to 10W).

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 Armonía as a function of output power for the analog line-level input, for an 8-ohm load (blue/red for left/right channels) and a 4-ohm load (purple/green for left/right channels). THD+N values for the 8-ohm data ranged from 0.4% at 10mW down to just 0.04% (left channel) at 2-5W. The 4-ohm data yielded similar THD+N values up to about 300mW, then higher values (10dB) then the 8-ohm data due to the higher THD.

THD ratio (unweighted) vs. frequency at 8 and 4 ohms

The chart above shows THD ratios measured at the output of the Armonía as a function of frequency into two different loads (8/4 ohms) for a constant input voltage that yields 10W at the output into 8 ohms.  As per the manufactuerer, we avoided applying a signal to the Armonía with a 2-ohm load connected, which we would normally do. The 8-ohm load data are the blue/red traces and the 4-ohm load are the purple/green traces. The 8-ohm THD data for the left channel ranged from 0.5%, down to 0.05%, then up to 0.3%, with the right channel yielding higher THD ratios (5-10dB) from 200Hz to 3kHz. The 4-ohm THD data were much higher, between 1 and 2% from 20Hz to 20kHz.

THD ratio (unweighted) vs. frequency into 8 ohms and real speakers (left channel only)

The chart above shows THD ratios measured at the output of the Armonía as a function of frequency into an 8-ohm load and two different speakers for a constant output voltage of 2.83Vrms (1W into 8 ohms) for the analog line-level input. The 8-ohm load is the blue trace, the purple plot is a two-way speaker (Focal Chora 806, measurements can be found here), and the pink plot is a three-way speaker (Paradigm Founder Series 100F, measurements can be found here). There are significant varations in THD ratios between the resisitve dummy load and real speakers. The two-way speaker yielded the largest swings, from 4% at 20Hz, down to 0.007% at 2kHz. This is likely due to the Armonía’s low damping factor, and just like with frequency repsonse, shows that THD results will vary greatly depending on the speaker connected to it.

IMD ratio (CCIF) vs. frequency into 8 ohms and real speakers (left channel only)

The chart above shows intermodulation distortion (IMD) ratios measured at the output of the Armonía as a function of frequency into an 8-ohm load and two different speakers for a constant output voltage of 2.83Vrms (1W into 8 ohms) for the analog line-level input. Here the CCIF IMD method was used, where the primary frequency is swept from 20kHz (F1) down to 2.5kHz, and the secondary frequency (F2) is always 1kHz lower than the primary, with a 1:1 ratio. The CCIF IMD analysis results are the sum of the second (F1-F2 or 1kHz) and third modulation products (F1+1kHz, F2-1kHz). The 8-ohm load is the blue trace, the purple plot is a two-way speaker (Focal Chora 806, measurements can be found here), and the pink plot is a three-way speaker (Paradigm Founder Series 100F, measurements can be found here). Here the three-way speaker yielded the highest IMD results, from 0.05% to 0.1%. The dummy load and two-way speaker were as low as 0.01-0.02%.

IMD ratio (SMPTE) vs. frequency into 8 ohms and real speakers (left channel only)

The chart above shows IMD ratios measured at the output of the Armonía as a function of frequency into an 8-ohm load and two different speakers for a constant output voltage of 2.83Vrms (1W into 8 ohms) for the analog line-level input. Here, the SMPTE IMD method was used, where the primary frequency (F1) is swept from 250Hz down to 40Hz, and the secondary frequency (F2) is held at 7kHz with a 4:1 ratio. The SMPTE IMD analysis results consider the second (F2 ± F1) through the fifth (F2 ± 4xF1) modulation products. The 8-ohm load is the blue trace, the purple plot is a two-way speaker (Focal Chora 806, measurements can be found here), and the pink plot is a three-way speaker (Paradigm Founder Series 100F, measurements can be found here). The resistive dummy load was fairly constant at 0.06-0.08%, with the speaker IMD results fluctuating above and below these values by as much as 10dB.

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 analog line-level input. We see that the signal’s second (2kHz) and fifth (5kHz) harmonic dominate, as high as -60dBrA (right channel), or 0.1%. Overall, the right channel yielded higher signal harmonic related peaks, but not always. At 3kHz, the right channel dominates at -70dBrA, or 0.03%, while at 4kHz, the left channel dominates at -80dBRa, or 0.01%. Power-supply-related noise peaks are pervasive, with the fundamental (60Hz) and second (120Hz) harmonics dominating between -80 and -90dBrA, or 0.01% and 0.003%.

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 analog 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 dominant (non-signal) peaks are the second (100Hz) and third (150Hz) signal harmonics at roughly -60dBrA, or 0.1%. Power-supply-related harmonics can be seen here at the fundamental (60Hz) and second harmonic (120Hz) at -80dBrA, or 0.01%, as well as multiples and resulting IMD products at lower levels.

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 analog line-level input. The input RMS values are set at -6.02dBrA so that, if summed for a mean frequency of 18.5kHz, would yield 10W (0dBrA) into 8 ohms at the output. We find that the second-order modulation product (i.e., the difference signal of 1kHz) is at -80/-60dBrA (left/right), or 0.01/0.1%, and the third-order modulation products, at 17kHz and 20kHz, are slightly below -80/-70dBrA (left/right), or 0.01/0.03%.

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

Shown above is the FFT of the speaker-level output of the Armonía with the APx 32-tone signal applied to the input. The combined amplitude of the 32 tones is the 0dBrA reference, and corresponds to 10W into 8 ohms. The intermodulation products—i.e.,  the "grass" between the test tones—are distortion products from the amplifier and are below the -70dBrA, or 0.03%, level.

Square-wave response (10kHz)

Above is the 10kHz squarewave response using the analog line-level input, at roughly 10W into 8 ohms. Due to limitations inherent to the Audio Precision APx555 B Series analyzer, this graph should not be used to infer or extrapolate the Armonía’s slew-rate performance. Rather, it should be seen as a qualitative representation of the Armonía’s mid-tier 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. In the case of the Armonía however, we see a relatively clean squarewave reproduction, although with softened cornered, but no over/undershoot.

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

The final graph above is the damping factor as a function of frequency. We find poor damping factor values, characteristic of tube amplifiers. The right channel was higher, hovering between 6 and 8, while the left channel yielded a fairly constant 3.4 from 30Hz to 20kHz.

Diego Estan
Electronics Measurement Specialist

Details

Parent Category: Products

Category: Amplifier Measurements

Created: 01 August 2023

Link: reviewed by Philip Beaudette on SoundStage! Hi-Fi on August 1, 2023

General Information

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

The Hegel H600 was conditioned for 1 hour at 1/8th full rated power (~37W 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 H600 offers two unbalanced (RCA) and two balanced (XLR) line-level analog inputs, seven digital inputs (one BNC, one RCA, three optical, one ethernet, and on USB), two pairs of line-level outputs (fixed and variable over RCA), and two pairs of speaker-level outputs (left and right). For the purposes of these measurements, the following inputs were evaluated: RCA digital coaxial, and the analog line-level balanced inputs (a 1kHz FFT using the unbalanced inputs is also provided).

Most measurements were made with a 2Vrms line-level analog input or a 0dBFS digital input. The signal-to-noise ratio (SNR) measurements were made with the same input signal values but with the volume set to achieve the rated output power of 300W (8 ohms). For comparison, on the line-level input, a SNR measurement was also made with the volume at maximum, but with a lower input voltage to achieve the same 300W output.

Based on the accuracy and non-repeatable results at various volume levels of the left/right channel matching (see table below), the H600 volume control is likely digitally controlled but operating in the analog domain. The volume control offers a total range from -62.3dB to +32.8dB between the line-level balanced analog inputs and the speaker outputs. Volume step sizes varied, from 5 to 2dB for the first nine steps, then 1dB from levels 10 to 55, then 0.5dB steps from 56 to 100.

The analyzer’s input bandwidth filter was set to 10Hz–22.4kHz for all measurements, except for frequency response (DC to 1 MHz), FFTs (10Hz to 90kHz), and THD vs. frequency (10Hz to 90kHz). The latter to capture the second and third harmonics of the 20kHz output signal. Since the H600 is a conventional class-AB amp, there was no issue with excessive noise above 20kHz.

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 Hegel for the H600 compared directly against our own. The published specifications are sourced from Hegel’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 extended from DC to 1MHz, assume, unless otherwise stated, 10W into 8 ohms and a measurement input bandwidth of 10Hz to 22.4kHz, and the worst-case measured result between the left and right channels.

* Hegel measures damping factor directly at the output stage whereas we measure at the output terminals.

Our primary measurements revealed the following using the line-level analog input and digital coaxial input (unless specified, assume a 1kHz sinewave at 2Vrms or 0dBFS, 10W output, 8-ohm loading, 10Hz to 22.4kHz bandwidth):

For the continuous dynamic power test, the H600 was able to sustain 520W into 4 ohms (~2% THD) using an 80Hz tone for 500ms, alternating with a signal at -10dB of the peak (52W) 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 H600 was hot enough to the touch to cause pain after a few seconds.

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

In our measured frequency-response chart above, the blue and red plots show the speaker-level outputs (relative to 1kHz, at 10W into 8 ohms). The H600’s speaker outputs are near flat within the audioband (-0.15dB at 20Hz, 0dB at 20kHz), and show a very extended bandwidth (0dB at 80kHz). The H600 appears to be AC-coupled, due the attenuation in response below 20Hz. 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.

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

Above is the phase response plot from 20Hz to 20kHz for the line-level input, measured across the speaker outputs at 10W into 8 ohms. The H600 does not invert polarity. Here we find essentially no phase shift at 20kHz, and 10 degrees at 20Hz, owing to the H600’s extended bandwidth and AC-coupled design.

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

The chart above shows the H600’s frequency response (relative to 1kHz) as a function of input type measured across the speaker outputs at 10W into 8 ohms. The blue trace is for a 16-bit/44.1kHz dithered digital input signal from 10Hz to 22kHz using the coaxial digital input, and the purple trace is for a 24/96 dithered digital input signal from 10Hz to 48kHz, while the pink trace is for a 24/192 dithered digital input signal from 10Hz to 96kHz. The green trace is the same analog input frequency response seen above. All three digital responses show a rise in output above 20kHz, with the 16/44.1 data peaking at +0.15dB at 19.2kHz, the 24/96 data peaking at +0.72dB at 43.3kHz, and the 24/192 data peaking at +1.8dB at 78.3kHz. All three digital responses exhibit brick-wall-type filtering, with -3dB points at 21.2kHz (16/44.1), 46.8kHz (24/96), and 93.8kHz (24/192).

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

The chart above shows the results of a linearity test for the coaxial digital input for both 16/44.1 (blue/red) and 24/96 (purple/green) input data, measured at the fixed line-level outputs of the H600 for a 2.4Vrms (at 0dBFS) output signal. 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 data were essentially perfect as of -100dBFS down to 0dBFS. At -120dBFS, the 16/44.1 data were about +2dB (left/right) above reference, while the 24/96 data were just below +1dB above reference.

Impulse response (24/44.1 data)

The graph above shows the impulse response for a looped 24/44.1 test file that moves from digital silence to full 0dBFS (all “1”s) for one sample period then back to digital silence, measured at the fixed line-level outputs of the H600. We can see that the H600 DAC utilizes a reconstruction filter with no pre-ringing.

J-Test (coaxial)

The chart above shows the results of the J-Test test for the coaxial digital input measured at the line-level pre-outs of the H600. J-Test was developed by Julian Dunn in the 1990s. It is a test signal—specifically a -3dBFS undithered 12kHz squarewave sampled (in this case) at 48kHz (24 bits). Since even the first odd harmonic (i.e., 36kHz) of the 12kHz squarewave is removed by the bandwidth limitation of the sampling rate, we are left with a 12kHz sinewave (the main peak). In addition, an undithered 250Hz squarewave at -144dBFS is mixed with the signal. This test file causes the 22 least significant bits to constantly toggle, which produces strong jitter spectral components at the 250Hz rate and its odd harmonics. The test file shows how susceptible the DAC and delivery interface are to jitter, which would manifest as peaks above the noise floor at 500Hz intervals (e.g., 250Hz, 750Hz, 1250Hz, etc.). Note that the alternating peaks are in the test file itself, but at levels of -144dBrA and below.  The test file can also be used in conjunction with artificially injected sinewave jitter by the Audio Precision, to show how well the DAC rejects jitter.

The coaxial input exhibits some low-level peaks below 1kHz, between -120 and -140dBrA. This is a relatively strong J-test result, indicating that the H600 DAC should be adequate at rejecting jitter.

J-Test (optical)

The chart above shows the results of the J-Test test for the optical digital input measured at the fixed line-level pre-outs of the H600. The optical input produced essentially the same result as with the coax input.

J-Test with 100ns of injected jitter (coaxial)

Both the coaxial and optical inputs were also tested for jitter immunity by injecting artificial sinewave jitter at 2kHz, which would manifest as sidebands at 10kHz and 14kHz without any jitter rejection. Jitter immunity proved strong. Although sidebands were visible at the 100ns jitter level, they did not exceed -135dBrA in amplitude. The optical input jitter result was very similar to the coaxial input result.

J-Test with 300ns of injected jitter (coaxial)

Both the coaxial and optical inputs were also tested for jitter immunity by injecting artificial sinewave jitter at 2kHz, which would manifest as sidebands at 10kHz and 14kHz without any jitter rejection. Jitter immunity again proved to be solid but not perfect, with visible sidebands at a very low -125dBrA at the 300ns jitter level. The H600 DAC did lose sync with the signal when jitter was increased beyond approximately 400ns. The optical input jitter result was very similar to the coaxial input result.

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

The plot above shows a fast Fourier transform (FFT) of the H600’s line-level pre-outs with white noise at -4 dBFS (blue/red), and a 19.1 kHz sinewave at 0dBFS fed to the coaxial digital input, sampled at 16/44.1. The medium roll-off above 20kHz in the white-noise spectrum shows the implementation of a typical brickwall-type reconstruction filter. There are no clear aliased image peaks in the audioband above the -135dBrA noise floor. Some very low frequency peaks can be seen in the FFT, however; these are at 60/120Hz due to power-supply noise. The main 25kHz alias peak is highly suppressed at -110dBrA. The second, third, and fourth distortion harmonics (38.2, 57.3, 76.4kHz) of the 19.1kHz tone are below -100dBrA.

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. Here we can see only minor deviations of about 0.04dB from 4 ohms to no load through most of the audioband, reaching as high as 0.08dB at 20kHz. This is an indication of a very high damping factor, or low output impedance. The variations in RMS level when a real speaker was used are smaller, deviating by at worst 0.04dB through most of the audioband.

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 analog 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 240W. The power was varied using the volume control. At 1W and 10W, THD ratios ranged from 0.006% at 20Hz, down to 0.003% from 100Hz to 4kHz, then up to 0.01% at 20kHz. The 240W THD values were slightly higher and ranged from 0.004% at 20Hz through to about 1kHz, then up to 0.04% at 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 H600 as a function of output power for the analog line-level input, for an 8-ohm load (blue/red for left/right channels) and a 4-ohm load (purple/green for left/right channels). The 8-ohm and 4-ohm data track closely and are relatively constant below the “knees,” at 0.002 to 0.005%, with the 8-ohm data outperforming the 4-ohm THD data by only 4-5dB. The “knee” for the 8-ohm data is just past 250W, then up to the 1% THD mark at 313W. With a 4-ohm load, the “knee” occurs at about 400W, and the 1% THD value was reached at 519W.

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 H600 as a function of output power for the analog line-level input, for an 8-ohm load (blue/red for left/right channels) and a 4-ohm load (purple/green for left/right channels). Overall, THD+N values for both loads were similar, ranging from about 0.02-0.03%, down to just above 0.003% (8-ohm). The 4-ohm THD+N ratios were a few dB worse than the 8-ohm ratios through most of the sweep.

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 H600 as a function of frequency into three different loads (8/4/2 ohms) for a constant input voltage that yielded 40W at the output into 8 ohms (and roughly 80W into 4 ohms, and 160W into 2 ohms) for the analog line-level input. The 8-ohm load is the blue trace, the 4-ohm load the purple trace, and the 2-ohm load the pink trace. We find a roughly 3-4dB increase between the 8- and 4-ohm data, then another 5-7dB increase from 4 to 2 ohms. We find the 8-ohm trace ranging from 0.003% from 60Hz up to 4kHz, then up to 0.007% at 20kHz. These data show that the H600 is not only stable into 2-ohm loads, but will perform well in terms of THD, comparable to an 8- and 4-ohm load.

THD ratio (unweighted) vs. frequency into 8 ohms and real speakers (left channel only)

The chart above shows THD ratios measured at the output of the H600 as a function of frequency into an 8-ohm load and two different speakers for a constant output voltage of 2.83Vrms (1W into 8 ohms) for the analog line-level input. The 8-ohm load is the blue trace, the purple plot is a two-way speaker (Focal Chora 806, measurements can be found here), and the pink plot is a three-way speaker (Paradigm Founder Series 100F, measurements can be found here). Overall, THD ratios were mostly within 5-10dB of the 8-ohm THD data, sometimes above, sometimes below. The worst-case result was at 20Hz with the two-way speaker, where THD ratios reached 0.03%. By and large, THD ratios ranged from 0.01 to 0.001%. This shows once again, that the H600 will yield consistent and stable THD results into different loads.

IMD ratio (CCIF) vs. frequency into 8 ohms and real speakers (left channel only)

The chart above shows intermodulation distortion (IMD) ratios measured at the output of the H600 as a function of frequency into an 8-ohm load and two different speakers for a constant output voltage of 2.83Vrms (1W into 8 ohms) for the analog line-level input. Here the CCIF IMD method was used, where the primary frequency is swept from 20kHz (F1) down to 2.5kHz, and the secondary frequency (F2) is always 1kHz lower than the primary, with a 1:1 ratio. The CCIF IMD analysis results are the sum of the second (F1-F2 or 1kHz) and third modulation products (F1+1kHz, F2-1kHz). The 8-ohm load is the blue trace, the purple plot is a two-way speaker (Focal Chora 806, measurements can be found here), and the pink plot is a three-way speaker (Paradigm Founder Series 100F, measurements can be found here). The results for the dummy load were fairly consistent, hovering between 0.005 and 0.01% throughout the sweep. We find that the two-way speaker yielded IMD ratios 2-10dB lower than the dummy load, whereas the 3-way speaker yielded IMD ratios roughly 5dB lower than the dummy load at lower frequences, and 5dB higher at higher frequencies.

IMD ratio (SMPTE) vs. frequency into 8 ohms and real speakers (left channel only)

The chart above shows IMD ratios measured at the output of the H600 as a function of frequency into an 8-ohm load and two different speakers for a constant output voltage of 2.83Vrms (1W into 8 ohms) for the analog line-level input. Here, the SMPTE IMD method was used, where the primary frequency (F1) is swept from 250Hz down to 40Hz, and the secondary frequency (F2) is held at 7kHz with a 4:1 ratio. The SMPTE IMD analysis results consider the second (F2 ± F1) through the fifth (F2 ± 4xF1) modulation products. The 8-ohm load is the blue trace, the purple plot is a two-way speaker (Focal Chora 806, measurements can be found here), and the pink plot is a three-way speaker (Paradigm Founder Series 100F, measurements can be found here). All three results are similar, at around the 0.01% level.

FFT spectrum – 1kHz (balanced 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 analog line-level input. We see that the signal’s second (2kHz) and third harmonic (3kHz) dominate at near -90dBrA, or 0.003%. The other signal harmonics are around or below -110dBrA, or 0.0003%. There are clearly visible power-supply-related noise peaks at even and odd harmonics (60Hz, 120Hz, 180Hz, 240Hz, etc.) on the left side of the main 1kHz peak, with the 60/180/300Hz peaks dominating at near -110dBrA or 0.0003%.

FFT spectrum – 1kHz (unbalanced 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 unbalanced analog line-level input. While the signal’s second (2kHz) and third harmonics (3kHz) are nearly identical to the balanced input FFT above, this FFT shows more higher odd-order signal harmonics, all the way out to 100kHz at -120dBrA, or 0.0001%, and below. The power-supply-related noise peaks are very similar to the balanced input FFT above.

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. The signal and noise-related harmonics are very similar to the balanced analog input FFT above, but with a slightly elevated noise floor due to the limitations of the 16-bit word depth.

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. The signal and noise-related harmonics are very similar to the balanced analog input 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 see the 1kHz primary signal peak, at the correct amplitude, and non-existent signal harmonics. Power-supply-related noise peaks can be seen at the fundamental (60Hz) and higher harmonics, at -110dBrA, or 0.0003%, and below.

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. We see the 1kHz primary signal peak, at the correct amplitude, and non-existent signal harmonics. Power-supply-related noise peaks can be seen at the fundamental (60Hz) and higher harmonics, at -110dBrA, or 0.0003%, and below.

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 analog 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. We see that the signal’s third harmonic (150Hz) dominates at around -90dBrA, or 0.003%. The second signal harmonic (100Hz), and subsequent signal harmonic peaks, are at or below -100dBrA, or 0.001%. There are clearly visible power-supply-related noise peaks at even and odd harmonics (60Hz, 120Hz, 180Hz, 240Hz, etc.) on the left side of the main 1kHz peak, with the 200Hz peak dominating at -110dBrA (left), or 0.0003%.

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 analog line-level input. The input RMS values are set at -6.02dBrA so that, if summed for a mean frequency of 18.5kHz, would yield 10W (0dBrA) into 8 ohms at the output. We find that the second-order modulation product (i.e., the difference signal of 1kHz) is at -100dBrA, or 0.001%. The third-order modulation products, at 17kHz and 20kHz, are higher at almost -90dBrA, or 0.003%.

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

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 digital coaxial input at 16/44.1. We find that the second-order modulation product (i.e., the difference signal of 1kHz) is at -100dBrA, or 0.001%. The third-order modulation products, at 17kHz and 20kHz, are higher at almost -90dBrA, or 0.003%.

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

Shown above is an FFT of the intermodulation distortion (IMD) products for an 18kHz + 19kHz summed sinewave stimulus tone measured at the output across an 8-ohm load at 10W for the digital coaxial input at 24/96. We find that the second-order modulation product (i.e., the difference signal of 1kHz) is at -100dBrA, or 0.001%. The third-order modulation products, at 17kHz and 20kHz, are higher at almost -90dBrA, or 0.003%.

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

Shown above is the FFT of the speaker-level outputs of the H600 with the APx 32-tone signal applied to the input. The combined amplitude of the 32 tones is the 0dBrA reference, and corresponds to 10W into 8 ohms. The intermodulation products—i.e., the "grass" between the test tones—are distortion products from the amplifier and reach between the -120 and -110dBrA, or 0.0001-0.0003%, level.

Square-wave response (10kHz)

Above is the 10kHz squarewave response using the analog line-level input, at roughly 10W into 8 ohms. Due to limitations inherent to the Audio Precision APx555 B Series analyzer, this graph should not be used to infer or extrapolate the H600’s slew-rate performance. Rather, it should be seen as a qualitative representation of the H600’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. Here we can see a very clean squarewave reproduction, with sharp corners, and no overshoot.

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

The final graph above is the damping factor as a function of frequency. Both channels track closely, with a higher damping factor (as high as 550) between 20Hz and 2kHz. Above 2kHz, we see a decline in the damping factor, as low as 220 at 20kHz.

Diego Estan
Electronics Measurement Specialist