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

Volume position Channel deviation
-99dB 0.02dB
-70dB 0.026dB
-60dB 0.026dB
-40dB 0.022dB
-30dB 0.024dB
-20dB 0.022dB
-10dB 0.024dB
0dB 0.025dB

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.

Parameter Manufacturer SoundStage! Lab
Amplifier rated output power into 8 ohms (1% THD) 40W 50W
Amplifier rated output power into 4 ohms (1% THD) 80W 94W
Frequency response (analog line-level in, speaker out 4-ohm) 20Hz-40kHz (-3dB) 20Hz-46kHz (-3dB)
Frequency response (digital in, speaker out 4-ohm) 20Hz-40kHz (-3dB) 20Hz-46kHz (-3dB)
Frequency response (phono MM, speaker out 4-ohm) RIAA 20Hz-20kHz (±1dB) RIAA 20Hz-20kHz (±0.5dB)
Input sensitivity (analog line-level in) 200mVrms 187mVrms
Input impedance (analog line-level in) 23k ohms 29.6k ohms
Input sensitivity (phono MM) 2mVrms 1.81mVrms
Input impedance (phono MM) 47k ohms 53.9k ohms

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):

Parameter Left channel Right channel
Maximum output power into 8 ohms (1% THD+N, unweighted) 50W 50W
Maximum output power into 4 ohms (1% THD+N, unweighted) 94W 94W
Maximum burst output power (IHF, 8 ohms) 50W 50W
Maximum burst output power (IHF, 4 ohms) 94W 94W
Continuous dynamic power test (5 minutes, both channels driven) passed passed
Crosstalk, one channel driven (10kHz) -83.5dB -83.2dB
Damping factor 38 38
Clipping no-load output voltage 20.8Vrms 20.8Vrms
DC offset N/A N/A
Gain (pre-out) 21.4dB 21.5dB
Gain (maximum volume) 39.7dB 39.6dB
IMD ratio (CCIF, 18kHz + 19kHz stimulus tones, 1:1) <-68dB <-68dB
IMD ratio (SMPTE, 60Hz + 7kHz stimulus tones, 4:1 ) <-55dB <-55dB
Input impedance (line input, RCA) 29.6k ohms 29.6k ohms
Input sensitivity (40W, maximum volume) 187mVrms 187mVrms
Noise level (with signal, A-weighted) <654uVrms <654uVrms
Noise level (with signal, 20Hz to 20kHz) <745uVrms <745uVrms
Noise level (no signal, A-weighted, volume min) <58uVrms <51uVrms
Noise level (no signal, 20Hz to 20kHz, volume min) <73uVrms <65uVrms
Output impedance (pre-out) 1.39k ohms 1.39k ohms
Signal-to-noise ratio (40W, A-weighted, 2Vrms in) 100.5dB 100.6dB
Signal-to-noise ratio (40W, 20Hz to 20kHz, 2Vrms in) 95.8dB 93.7dB
Signal-to-noise ratio (40W, A-weighted, max volume) 80.4dB 80.5dB
Dynamic range (full power, A-weighted, digital 24/96) 110.4dB 111.6dB
Dynamic range (full power, A-weighted, digital 16/44.1) 95.6dB 95.6dB
THD ratio (unweighted) <0.020% <0.019%
THD ratio (unweighted, digital 24/96) <0.017% <0.018%
THD ratio (unweighted, digital 16/44.1) <0.017% <0.018%
THD+N ratio (A-weighted) <0.024% <0.023%
THD+N ratio (A-weighted, digital 24/96) <0.020% <0.021%
THD+N ratio (A-weighted, digital 16/44.1) <0.020% <0.021%
THD+N ratio (unweighted) <0.022% <0.021%
Minimum observed line AC voltage 125VAC 125VAC

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):

Parameter Left channel Right channel
Crosstalk, one channel driven (10kHz) -75dB -76dB
DC offset N/A N/A
Gain (default phono preamplifier) 40.2dB 40.2dB
IMD ratio (CCIF, 18kHz + 19kHz stimulus tones, 1:1) <-68dB <-69dB
IMD ratio (CCIF, 3kHz + 4kHz stimulus tones, 1:1) <-67dB <-67dB
Input impedance 53.9k ohms 52.4k ohms
Input sensitivity (to 40W with max volume) 1.81mVrms 1.83mVrms
Noise level (with signal, A-weighted) <870uVrms <800uVrms
Noise level (with signal, 20Hz to 20kHz) <1300uVrms <1300uVrms
Noise level (no signal, A-weighted, volume min) <58uVrms <50uVrms
Noise level (no signal, 20Hz to 20kHz, volume min) <73uVrms <65uVrms
Overload margin (relative 5mVrms input, 1kHz) 26.3dB 26.4dB
Signal-to-noise ratio (40W, A-weighted, 5mVrms in) 83.8dB 83.8dB
Signal-to-noise ratio (40W, 20Hz to 20kHz, 5mVrms in) 77.5dB 78.8dB
Signal-to-noise ratio (40W, A-weighted, max volume) 74.7dB 74.8dB
THD (unweighted) <0.018% <0.018%
THD+N (A-weighted) <0.022% <0.022%
THD+N (unweighted) <0.023% <0.023%

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):

Parameter Left and right channels
Maximum gain 16.0dB
Maximum output power into 600 ohms (1% THD) 2.5mW
Maximum output power into 300 ohms (1% THD) 4.1mW
Maximum output power into 32 ohms (1% THD) 6.8mW
Output impedance 60 ohms
Maximum output voltage (1% THD into 100k ohm load) 1.34Vrms
Noise level (with signal, A-weighted) <15uVrms
Noise level (with signal, 20Hz to 20kHz) <28uVrms
Noise level (no signal, A-weighted, volume min) <13uVrms
Noise level (no signal, 20Hz to 20kHz, volume min) <16uVrms
Signal-to-noise ratio (A-weighted, 1% THD, 1.1Vrms out) 96.7dB
Signal-to-noise ratio (20Hz - 20kHz, 1% THD, 1.1Vrms out) 91.7dB
THD ratio (unweighted) <0.02%
THD+N ratio (A-weighted) <0.024%
THD+N ratio (unweighted) <0.021%

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

frequency response

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)

frequency response bass treble

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)

phase response

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)

frequency response vs input type

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)

frequency response vs input type

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)

frequency response phono mm

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)

phase response phono mm

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)

digital linearity

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)

impulse response 2444 1

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)

jtest coax 2448

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)

jtest optical 2448

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)

jtest coax 2448

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)

jtest coax 2448 100ns

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)

wideband fft noise plus 19 1khz 1644 1kHz

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)

wideband fft noise plus 19 1khz 1644 1kHz

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)

rms level vs frequency vs load impedance

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 . . .

rms level vs frequency vs load impedance

. . . 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)

rms level vs frequency vs load impedance

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

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)

thd ratio unweighted vs frequency phono mm mc

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

thd ratio unweighted vs output power at 4 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

thd n ratio unweighted vs output power at 4 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)

thd vs frequency load

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)

thd vs frequency vs speakers

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)

IMD CCIF vs frequency vs speakers

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)

IMD SMPTE vs frequency vs speakers

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)

FFT spectrum 1khz

Shown above is the fast Fourier transform (FFT) for a 1kHz input sinewave stimulus, measured at the output across an 8-ohm load at 10W for the 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)

FFT spectrum 1khz

Shown above is the fast Fourier transform (FFT) for a 1kHz input sinewave stimulus, measured at the output across an 8-ohm load at 10W for the 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)

fft spectrum 1khz 1644 1 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)

fft spectrum 1khz 2496 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)

fft spectrum 1khz 2444 1 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)

fft spectrum 1khz 2496 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)

fft spectrum 1khz phono mm

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)

fft spectrum 50hz

Shown above is the FFT for a 50Hz input sinewave stimulus measured at the output across an 8-ohm load at 10W 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)

fft spectrum 50hz phono mm

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)

intermodulation distortion fft 18khz 19khz summed stimulus

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)

fft spectrum 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)

intermodulation distortion fft 18khz 19khz summed stimulus

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)

intermodulation distortion fft 18khz 19khz summed stimulus

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)

intermodulation distortion fft 18khz 19khz summed stimulus

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)

intermodulation distortion FFT 18kHz 19kHz summed stimulus phono mm

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)

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)

square wave response 10kHz 250kHz BW

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)

fft spectrum 1khz 1MHz BW

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)

damping factor vs frequency

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