Luxman SQ-38u integrated amplifier Measurements
Sidebar 3: Measurements
I measured the Luxman SQ-38u with Stereophile's loan sample of the top-of-the-line Audio Precision SYS2722 system (see the January 2008 "As We See It" and www.ap.com). The maximum voltage gain for the line-level inputs measured 39.1dB into 8 ohms, measured at the speaker terminals, and 14.5dB at the Pre Out jacks. This suggests that the power-amplifier section offers a low 24.5dB of gain, which was confirmed by measuring the gain via the Main In jacks. Measured at the Tape Out jacks, the phono stage offered 37.6dB of gain in MM mode, 60.3dB in MC High, and 57.2dB in MC Low. This suggests that the mode names refer to the amount of gain, not the compatibility of each input; ie, the MC High mode will be preferable for low-output moving-coil cartridges, and vice versa. All inputs preserved absolute polarity (ie, were non-inverting), assessed at both the speaker terminals and the Preamp out jacks.
The line-level input impedance was a usefully high 45k ohms at low and middle frequencies, dropping slightly to 37.5k ohms at 20kHz. Assessed at the Main In jacks, the lower-frequency input impedance was higher, at 75k ohms, dropping to 63k ohms at 20kHz. The MM input impedance measured 45k ohms or higher at middle and low frequencies, and 38k ohms at 20kHz. The transformer-coupled MC modes offered input impedances of 160 ohms (High) and 390 ohms (Low).
The Pre Out jacks, which follow the tone-control circuit, featured a fairly high output impedance of 1.1k ohms at high frequencies, 2k ohms at 1kHz, and almost 7k ohms at 20Hz, suggesting that a partnering power amplifier needs to have a high input impedance. The output impedance from the speaker terminals was a moderately high 0.7 ohm over most of the audioband, rising to 0.9 ohm at 20kHz. As a result of the interaction between this impedance and the impedance of our standard simulated loudspeaker, the measured response with this load varies by ±0.5dB (fig.1, gray trace), which will be audible. This graph was taken with the tone controls centered; there is a slight rolloff in the low bass (reaching 3dB at 10Hz), and while the treble is flat to 20kHz into higher impedances, it starts to roll off into 2 ohms (green trace), reaching 0.5dB at 20kHz. This graph was taken with the volume control set at its maximum; the very slight mismatch in gain remained the same at lower settings.
The slight knee in the traces in fig.1, especially into the higher impedances, is due to a well-suppressed ultrasonic resonance at 80kHz. This gives rise to a small amount of overshoot and ringing with a 10kHz squarewave (fig.2), though the risetimes in this graph are very low. The Luxman's response via its phono input (fig.3) was less flat than I'm used to seeing. Even though the Low Cut switch was set to Off, the low-frequency output rolls off sharply below 100Hz, reaching 6dB at 20Hz. Taking the level at 1kHz as a reference, both the lower midrange and the midhigh-treble regions are shelved up. Alternatively, this response could be interpreted as the upper midrange being suppressed by an audible 1dB. How this response is perceived will depend very much on the music being played. The green and gray traces in fig.4 show the line-level response with the tone controls set to their center positions but with the Low Cut filter in circuit. This rolls off the low frequencies, to reach 6dB at 20Hz. The other traces in this graph show the SQ-38u's response with both tone controls set first to their maximum positions, then to the minimum positions, with Low Cut off. The amounts of boost and cut are sensibly arranged and not excessive.
Fig.1 Luxman SQ-38u, frequency response at 2.83V into: simulated loudspeaker load (gray), 8 ohms (left channel blue, right red), 4 ohms (left cyan, right magenta), 2 ohms (green). (0.25dB/vertical div.)
Fig.2 Luxman SQ-38u, small-signal 10kHz squarewave into 8 ohms.
Fig.3 Luxman SQ-38u, phono input RIAA error (left channel blue, right red; 0.25dB/vertical div.).
Fig.4 Luxman SQ-38u, Effect of Low Cut switch and tone controls set to their maximum and minimum positions (left channel blue, right red; 2dB/vertical div.).
Channel separation at 1kHz was modest, at 60dB in both directions, rising to 48dB at 20kHz, presumably because at least one of the small-signal tubes appears from the circuit diagram to be shared between the channels. (That's why there is an odd number of tubes.) Below 1kHz, however, the measured crosstalk was affected by residual hum in the amplifier's output. Fig.5, for example, shows a spectral analysis of the SQ-38u's low-frequency noise floor while it reproduced a 1kHz tone at 10W into 8 ohms. The 68dB spike at 60Hz in both channels is most likely due to magnetic coupling from the power transformer to the output transformers, as are the spuriae at odd harmonics of 60Hz. But there is also a spectral spike visible at 120Hz, which is the ripple frequency of the power-supply voltage rails. Though this lies at a low 90dB (0.003%), a 120Hz sideband at 60dB (0.1%) is associated with the 1kHz tone in this graph. These low-level hum products restricted the unweighted wideband signal/noise ratio (taken with the line-level input shorted but the volume control at its maximum) to 71.6dB left and 73.6dB right (both figures ref. 2.83V into 8 ohms). Switching an A-weighting filter in circuit improved these ratios to 82.6 and 84.3dB, respectively.
Fig.5 Luxman SQ-38u, spectrum of 1kHz sinewave, DC1kHz, at 10W into 8 ohms (left channel blue, right red; linear frequency scale).
The Luxman is specified as delivering a maximum power of 30Wpc into the nonstandard 6 ohms, or 25Wpc into 8 or 4 ohms. Fig.6 shows how the THD+noise percentage in the output varies with power into 2, 4, 8, and 16 ohms. The upward slope of the traces below 3W or so means that any distortion is buried beneath the noise floor at these levels. The distortion rises above the noise at an increasing power level as the impedance increases: into 16 ohms, the SQ-38u offers very low distortion, but into 8 ohms, the distortion doesn't drop below 0.1%. With clipping defined as 1% THD+N, the Luxman clips at 27W into 16 ohms (17.3dBW), 38W into 8 ohms (15.8dBW), 41W into 4 ohms (13.1dBW), and 27W into 2 ohms (8.3dBW). The amplifier exceeds its specified power output, but fig.6 shows that THD+N increases significantly well below clipping into the lower impedances.
Fig.6 Luxman SQ-38u, distortion (%) vs 1kHz continuous output power into (from bottom to top at 1W): 16, 8, 4, 2 ohms.
Fig.7 plots the THD+N percentage at 8V, equivalent to a power of 8W into 8 ohms, against frequency into loads varying from 2 to 16 ohms. As expected, the THD+N is high into the lower impedances at this output voltage, but the distortion rises at the frequency extremes even into the higher impedances. Fortunately, the distortion is predominantly the subjectively benign low-order (fig.8).
Fig.7 Luxman SQ-38u, THD+N (%) vs frequency at 8V into: 16 ohms (green), 8 ohms (left channel blue, right red), 4 ohms (left cyan, right magenta), 2 ohms (gray).
Fig.8 Luxman SQ-38u, 1kHz waveform at 5W into 8 ohms (top), 0.077% THD+N; distortion and noise waveform with fundamental notched out (bottom, not to scale).
Fig.9 repeats the spectral analysis shown in fig.5, this time plotted to10kHz to reveal the distortion harmonics. The second harmonic is higher in the left channel (blue trace) than the right (red), but a regular series of higher-order harmonics can be seen, albeit at a lower level. Probably of greater subjective significance are the supply-related sidebands around the fundamental and each of the harmonics. While the SQ-38u doesn't actually clip until 38W into 8 ohms, the power supply is obviously being stressed at the 10W power used to generate this graph.
Fig.9 Luxman SQ-38u, spectrum of 1kHz sinewave, DC10kHz, at 10W into 8 ohms (left channel blue, right red; linear frequency scale).
The phono stage was well behaved in terms of distortion, fig.10 indicating that there were no harmonics other than the benign second and third, at 70dB (0.03%) and 90dB (0.003%), respectively. This graph was taken with the phono input set to MM and a level of 10mV, 6dB greater than the nominal MM level of 5mV. In fact, I could drive the MM input with 187mV at 1kHz before the THD+N reached 1%, implying an overload margin of 31.4dB, which is superb. The margins at 20Hz and 20kHz were 24.3 and 22.5dB, respectively, which are still excellent. I noted similarly superb margins in the MC High and MC Low modes. Only when it came to noise did the phono stage stumble a little, the wideband, unweighted S/N ratio measuring 48.5/44dB, L/R MM; 51.5/48dB, L/R MC High; and 50.5/47.6dB, L/R MC Low. An A-weighting filter increased these figures, which were taken at the Tape Out jacks, to, respectively, 65/61.5dB, 66.5/63.6dB, and 66.8/53.6dB.
Fig.10 Luxman SQ-38u, MM phono input, spectrum of 1kHz sinewave, DC10kHz, at 10mV input (left channel blue, right red; linear frequency scale).
Finally, the Luxman SQ-38u didn't do as well as I had expected on the high-frequency intermodulation test, the 1kHz difference product with the equal mix of 19 and 20kHz tones at a level of 10W into 8 ohms reaching 53dB left and 56dB right (fig.11). Again, the supply-related sidebands can be seen in this graph. The SQ-38u's phono stage, however, did well on this test (fig.12). Though the 1kHz intermodulation product lay at 57dB (0.14%), the higher-order products were suppressed by the RIAA equalization to 100dB (0.001%).
Fig.11 Luxman SQ-38u, HF intermodulation spectrum, DC24kHz, 19+20kHz, at 10W peak into 4 ohms (left channel blue, right red; linear frequency scale).
Fig.12 Luxman SQ-38u, phono input, HF intermodulation spectrum, DC24kHz, 19+20kHz, at 5V peak (left channel blue, right red; linear frequency scale).
The Luxman SQ-38u measures about as well as I expected for a classic circuit using a pair of EL34 tubes per channel, and transformers limited by the amount of iron that can be packed into the amplifier's modest-size chassis.John Atkinson