Sidebar 3: Measurements
Nostalgia struck when I unpacked the Luxman SQ-N150 and hoisted it onto the test bench. This compact integrated amplifier uses a pair of EL84 tubes for each channel's output—I used to own a sweet-sounding Vox AC30 guitar amplifier back in the 1960s, which also used a quartet of EL84s.
Down to business: I measured the Luxman using my Audio Precision SYS2722 system (see the January 2008 "As We See It"). Looking first at the line inputs, the voltage gain with the volume control set to its maximum was 32.8dB into 8 ohms at the loudspeaker terminals and 27.45dB into 100k ohms at the headphone jack. Both sets of outputs inverted absolute polarity. The line input impedance is specified as 47k ohms; I measured 45k ohms at 20Hz and 1kHz, dropping slightly to 33.7k ohms at 20kHz.
The headphone output impedance was 232 ohms from 20Hz to 20kHz, which is on the high side for low-impedance, high-performance headphones, even with the higher-than-usual gain for this output.
The Luxman amplifier's output transformers have a single tap, which is optimized for a 6 ohm load. The output impedance from this tap was relatively low for a tube design at 0.7 ohm at 20Hz and 1kHz, rising to 0.83 ohm at 20kHz. As a result, the modulation of the Luxman's frequency response with our standard simulated loudspeaker was a modest ±0.5dB (fig.1, gray trace). The response into resistive loads was flat in the midrange but sloped down slightly in the treble, lying at –0.2dB at 20kHz into 8 ohms (blue and red traces). This slight downward tilt continued above the audioband before the amplifier's output dropped sharply at 100kHz, with a hint of a peak at that frequency. The response was identical at lower settings of the volume control, but the excellent channel matching at full volume worsened a little, the right channel being 0.2dB louder than the left at the 12 o'clock setting.
Luxman specifies the SQ-N150 as offering up to 10Wpc into 6 ohms (8.25dBW). I don't have a high-power 6 ohm test load, but with clipping defined as when the proportion of THD+noise in the output reaches 1%, I measured clipping powers with both channels driven of 12.5W into 8 ohms (11.0dBW, fig.5) and 11Wpc into 4 ohms (7.4dBW, fig.6). I examined how the SQ-N150's THD+N percentage varied with frequency at 2.83V (equivalent to 1W into 8 ohms and 2W into 4 ohms). The THD+N was low in the midrange into 8 ohms (fig.7, blue and red traces), but rose both into 4 ohms (cyan, magenta) and at the frequency extremes.
The MM input's overload margins were good at 17.5dB across the audioband and slightly better for the MC input setting. Both harmonic distortion and intermodulation distortion (measured with the MM input at +12dB ref. 1kHz at 5mV) were very low in level.
As with other Luxman amplifiers I have looked at, the SQ-N150 is a tube design you don't have to make apologies for. It is well-engineered and offers excellent measured performance within its limited power envelope.—John Atkinson
Fig.1 Luxman SQ-N150, 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) (1dB/vertical div.).
The SQ-N150's reproduction of a small-signal 10kHz squarewave (fig.2) featured short risetimes, but there was a slight amount of overshoot accompanied by three cycles of damped ringing, this correlating with the slight response peak at 100kHz. Even so, the SQ- N150's squarewave performance illustrates that it has well-designed output transformers, something it shares with the other Luxman amplifiers we have reviewed.
Fig.2 Luxman SQ-N150, small-signal 10kHz squarewave into 8 ohms.
The SQ-N150's tone controls offer maximum cuts and boosts of ±13dB at 30Hz and ±9dB at 10kHz (fig.3); the former is unnecessarily high, in my opinion. Channel separation was good rather than great, measuring 69dB in both directions at 1kHz and below, decreasing to 50dB at 20kHz due to the usual capacitive coupling between channels. The levels of power-supply–related spuriae in the Luxman's output were all at or below –90dB ref. 1W into 8 ohms (fig.4). The unweighted, wideband signal/noise ratio, taken with the inputs shorted to ground but the volume control set to its maximum, was 75.5dB ref. 1W into 8 ohms. The S/N ratio improved to a very good 82.7dB when the measurement bandwidth was restricted to the audioband, and to 87.5dB with an A-weighting filter in circuit. This is a quiet amplifier, though this will be due in part to the low maximum gain.
Fig.3 Luxman SQ-N150, frequency response at 300mV into 8 ohms with Bass and Treble controls set to "0dB," maximum, and minimum (left channel blue, right red) (5dB/vertical div.).
Fig.4 Luxman SQ-N150, spectrum of 1kHz sinewave, DC–1kHz, at 1Wpc into 8 ohms (left channel blue, right red; linear frequency scale).
Fig.5 Luxman SQ-N150, distortion (%) vs 1kHz continuous output power into 8 ohms.
Fig.6 Luxman SQ-N150, distortion (%) vs 1kHz continuous output power into 4 ohms.
Fig.7 Luxman SQ-N150, THD+N (%) vs frequency at 2.83V into: 8 ohms (left channel blue, right red), 4 ohms (left cyan, right magenta).
Spectral analysis of the amplifier's output while it drove 50Hz at 3Wpc into 8 ohms (fig.8) indicates that the third harmonic is the highest in level at –56dB (0.15%), presumably due to the output transformer's core starting to saturate. At higher frequencies (fig.9), the second harmonic is a little higher in level than the third, both lying below –60dB (0.1%), this confirmed by the THD+N waveform (fig.10). Tested with an equal mix of 19kHz and 20kHz tones with the signal peaking at 3Wpc into 8 ohms, the Luxman produced relatively low levels of high-order intermodulation distortion, though the difference product at 1kHz was fairly high in level, at –54dB (0.2%, fig.11). Turning to the SQ-N150's phono input, the MM input's voltage gain at 1kHz with the volume control at its maximum setting measured 68.5dB from the speaker outputs, as specified. The MC input's gain was a very high 85.8dB, again as specified. The phono input inverted absolute polarity set to both MM and MC input signals. The MM input impedance was 45k ohms at 20Hz and 1kHz, and 35k ohms at 20kHz, these impedances appropriate for MM cartridges. Switched to MC, the phono input's impedance was 100 ohms across the band.
Fig.8 Luxman SQ-N150, spectrum of 50Hz sinewave, DC–1kHz, at 3W into 8 ohms (left channel blue, right red; linear frequency scale).
Fig.9 Luxman SQ-N150, spectrum of 1kHz sinewave, DC–10kHz, at 3W into 8 ohms (left channel blue, right red; linear frequency scale).
Fig.10 Luxman SQ-N150, 1kHz waveform at 1W into 8 ohms, 0.044% THD+N (top); distortion and noise waveform with fundamental notched out (bottom, not to scale).
Fig.11 Luxman SQ-N150, HF intermodulation spectrum, DC–30kHz, 19+20kHz at 3W peak into 8 ohms (left channel blue, right red; linear frequency scale).
The phono-input RIAA error was low in the treble but with a slight positive plateau in the midrange (fig.12). With the phono inputs shorted to ground and the volume control set to the maximum, the MM input's wideband, unweighted signal/noise ratio (ref. 1kHz input signal at 5mV and measured at the headphone outputs) measured an excellent 77dB (average of both channels). An A-weighting filter increased the ratios to 89.5dB. The MC input's unweighted, wideband S/N ratios were >20dB worse, however.
Fig.12 Luxman SQ-N150, MM input, response with RIAA correction (left channel blue, right red) (0.5dB/vertical div.).
