Sidebar 3: Measurements
I performed the measurements of the Audio Note Meishu Phono Tonmeister with my Audio Precision SYS2722 system. The tubes were already installed when I received the review sample. I removed the cover to check for proper installation. All was well. I waited for 30 minutes after powering up the amplifier before starting the testing.
Looking first at its line inputs, the Meishu Tonmeister preserved absolute polarity, ie, was noninverting, from both the 4 ohm and 8 ohm output transformer taps. The maximum voltage gain at 1kHz was a moderately low 29.65dB from the 4 ohm tap into 8 ohms and 31.3dB from the 8 ohm tap into the same load. The input impedance is specified as 100k ohms. I measured a still-high 77k ohms at 20Hz, 74.5k ohms at 1kHz, and 63.7k ohms at 20kHz.
The source impedance from the 8 ohm output tap was a high 3.2 ohms at 20Hz and 1kHz, increasing slightly to 3.45 ohms at 20kHz. The variation in the small-signal frequency response with this output with our standard simulated loudspeaker (fig.1, gray trace) was therefore high, at ±1.8dB. The variation was lower from the 4 ohm tap, at ±1.1dB (fig.2), but with both output taps, there will be audible modifications of loudspeaker responses with almost all loudspeakers. Into resistive loads (figs.1 & 2, blue, red, cyan, magenta, and green traces), the Audio Note amplifier's output started to roll off below 40Hz and above 20kHz, reaching –3dB at 9Hz and 55kHz. Figs.1 & 2 were taken with the volume control set to its maximum; the excellent channel matching was preserved at lower settings of the control. There is the slightest hint of a resonant peak at 60kHz in the Meishu Tonmeister's frequency response, which correlated with some damped ultrasonic ringing in the amplifier's reproduction of a 10kHz squarewave into 8 ohms (fig.3).
Spectral analysis of the low-frequency noisefloor while the Audio Note's 8 ohm taps drove a 1kHz tone at 1Wpc into 8 ohms with the volume control set to the maximum (fig.4) revealed a low level of random noise. However, AC supply–related spuriae at 60Hz and its odd- and even-order harmonics were present. The highest of these, at 120Hz, lay at –76dB (0.015%). Its level didn't change when I experimented with the grounding between the amplifier and the Audio Precision analyzer.
Figs.5–7 indicate that the lowest distortion at low power is obtained from the 4 ohm transformer tap. Fig.8 shows how the percentage of THD+N in both channels varied with frequency with this tap driving 8 and 4 ohms at 1V. The THD+N rose slightly at low frequencies and was significantly higher into 4 ohms (green and gray traces) than it was into 8 ohms (blue and red traces). At the same level from the 8 ohm tap (fig.9), the distortion across the audioband was close to 0.5% into 8 ohms and 1% into 4 ohms.
The distortion waveform (fig.10) was predominantly the subjectively innocuous second harmonic, with higher-order harmonics progressively lower in level (fig.11). However, the levels of the third, fifth, and seventh harmonics were higher at low frequencies (fig.12), which will probably be due to the onset of core saturation in the output transformers.
To examine the behavior of the Audio Note's phono input, I connected a wire from the Audio Precision's ground terminal to the amplifier's chassis ground post on its rear panel to obtain the lowest noise. The phono input preserved absolute polarity and the maximum gain at 1kHz was 67.8dB at the 8 ohm outputs and 66.14dB at the 4 ohm outputs. The gain was fixed at 36.4dB at the single-ended Source output, so, to avoid damaging the amplifier's output stage, I measured the phono input's behavior at the Source output with the volume control set to its minimum.
The input impedance is specified as 47k ohms. I measured 48k ohms at 20Hz, 37.5k ohms at 1kHz, but just 8k ohms at 20kHz. The phono input's RIAA equalization was very accurate, with excellent channel matching (fig.14), though the low frequencies rolled off a little, reaching –3dB at 16Hz. The wideband, unweighted S/N ratio with the inputs shorted to ground was a good 64dB in both channels, ref. 1kHz at 5mV. Restricting the measurement bandwidth to the audioband increased the ratio to 68.9dB, while an A-weighting filter further increased the ratio to 76dB. Negligible power supply–related spuriae were present in the phono stage's noisefloor (fig.15); this is a relatively quiet phono stage.
Fig.1 Audio Note Meishu Tonmeister Phono, line input, 8 ohm output tap, frequency response at 2.83V into: simulated loudspeaker load (gray), 8 ohms (left channel blue, right red), 4 ohms (left cyan, right magenta), and 2 ohms (green) (1dB/vertical div.).
Fig.2 Audio Note Meishu Tonmeister Phono, line input, 4 ohm output tap, frequency response at 2.83V into: simulated loudspeaker load (gray), 8 ohms (left channel blue, right red), 4 ohms (left cyan, right magenta), and 2 ohms (green) (1dB/vertical div.).
Fig.3 Audio Note Meishu Tonmeister Phono, 8 ohm output tap, small-signal 10kHz squarewave into 8 ohms.
The Meishu Tonmeister's channel separation (not shown) was moderate, at 49dB, R–L, and 36dB, L–R, at 1kHz, respectively decreasing to 23dB and 40dB at 20kHz. The unweighted, wideband signal/noise ratio (ref. 1W into 8 ohms) taken from the 8 ohm taps with the inputs shorted to ground and the volume control set to its maximum was 69.7dB (average of the two channels). This ratio improved slightly to 71.5dB, left, and 69.6dB, right, when the measurement bandwidth was restricted to 22Hz–22kHz, and to 86.7dB when A-weighted. With their lower gain, the ratios from the 4 ohm outputs were 1.3dB greater.
Fig.4 Audio Note Meishu Tonmeister Phono, 8 ohm output tap, spectrum of 1kHz sinewave, DC–1kHz, at 1Wpc into 8 ohms with volume control set to its maximum (left channel blue, right red) (linear frequency scale).
Audio Note specifies the Meishu Tonmeister's maximum power as 8W into both 4 and 8 ohms (6.02dBW and 9.03dBW, respectively), though no distortion percentage is mentioned. With our usual definition of clipping—when the THD+noise reaches 1%—and with both channels driven, I measured a clipping power of just 1Wpc from the 8 ohm tap into 8 ohms (fig.5) and from the 4 ohm tap into 4 ohms (fig.6). At 3% THD+N, I measured 4.8Wpc with each output tap matched to the load, and at 10%, 12.2Wpc from the 8 ohm tap (10.9dBW) and from the 4 ohm tap (7.9dBW), again with the taps matched to the loads. Less power was available from the 8 ohm tap into 4 ohms, but with the 4 ohm tap driving 8 ohms (fig.7), 5.75Wpc was available at 3% THD+N and 10Wpc at 10% (10dBW). The shape of the traces in these graphs suggests that the amplifier's circuit doesn't use loop negative feedback. It is also fair to note that the waveform wasn't actually clipped at these high levels of distortion; instead, it was asymmetrically rounded off.
Fig.5 Audio Note Meishu Tonmeister Phono, 8 ohm output tap, distortion (%) vs 1kHz continuous output power into 8 ohms.
Fig.6 Audio Note Meishu Tonmeister Phono, 4 ohm output tap, distortion (%) vs 1kHz continuous output power into 4 ohms.
Fig.7 Audio Note Meishu Tonmeister Phono, 4 ohm output tap, distortion (%) vs 1kHz continuous output power into 8 ohms.
Fig.8 Audio Note Meishu Tonmeister Phono, 4 ohm output tap, THD+N (%) vs frequency at 1V into: 8 ohms (left channel blue, right red), 4 ohms (left green, right gray).
Fig.9 Audio Note Meishu Tonmeister Phono, 8 ohm output tap, THD+N (%) vs frequency at 1V into: 8 ohms (left channel blue, right red), 4 ohms (left green, right gray).
Fig.10 Audio Note Meishu Tonmeister Phono, 8 ohm output tap, left channel, 1kHz waveform at 0.5W into 8 ohms, 0.72% THD+N (top); distortion and noise waveform with fundamental notched out (bottom, not to scale).
Fig.11 Audio Note Meishu Tonmeister Phono, 8 ohm output tap, spectrum of 1kHz sinewave, DC–10kHz, at 1V into 8 ohms (left channel blue, right red; linear frequency scale).
Fig.12 Audio Note Meishu Tonmeister Phono, 4 ohm output tap, spectrum of 50Hz sinewave, DC–1kHz, at 1V into 8 ohms (left channel blue, right red; linear frequency scale).
Due to masking, in itself the level of the second harmonic may not result in audible distortion, but this will only be true if it is not accompanied by intermodulation distortion. With the Meishu Tonmeister's 4 ohm taps driving an equal mix of 19 and 20kHz tones at 1Wpc peak into 8 ohms (fig.13), the 1kHz difference product lay just below –50dB (0.3%), with the higher-order products at 18 and 21kHz 10dB lower in level. This is marginal performance, in my opinion.
Fig.13 Audio Note Meishu Tonmeister Phono, 8 ohm output tap, HF intermodulation spectrum, DC–30kHz, 19+20kHz at 1Wpc peak into 4ohms (left channel blue, right red; linear frequency scale).
Fig.14 Audio Note Meishu Tonmeister Phono, phono input, response with RIAA correction, measured at Source output (left channel blue, right red) (1dB/vertical div.).
Fig.15 Audio Note Meishu Tonmeister Phono, MM phono input, spectrum of 1kHz sinewave, DC–1kHz, for 5mV input, measured at Source output (left channel blue, right red, linear frequency scale).
The low-frequency and midrange overload margins, calculated from the difference between the nominal 1kHz input level of 5mV and the input voltage where the THD+N reached 1%, were superbly high, at 37.8dB at 20Hz and 32dB at 1kHz. The margin at 20kHz was lower at 14dB. The phono input's harmonic distortion was respectably low in level, with the second harmonic the highest in level at –60dB (0.1%, fig.16). With the relatively low overload margin at the top of the audioband, I wasn't surprised to find that the second-order difference product with an equal mix of 19 and 20kHz tones peaking at 25mV lay at –40dB (1%, fig.17). High-order intermodulation products were vanishingly low in level, however, until I increased the signal level by 10dB.
Fig.16 Audio Note Meishu Tonmeister Phono, MM phono input, spectrum of 1kHz sinewave, DC–10kHz, for 5mV input, measured at Source output (left channel blue, right red, linear frequency scale).
Fig.17 Audio Note Meishu Tonmeister Phono, MM phono input, HF intermodulation spectrum, DC–30kHz, 19+20kHz for 25mV input (left channel blue, right red; linear frequency scale).
The Audio Note Meishu Tonmeister Phono's measured performance is what I would expect from an amplifier with a single-ended output stage that uses a single 300B tube for each channel. In this respect, its behavior resembles that of the Western Electric Type No.91E integrated amplifier that Ken Micallef reviewed in November 2022.—John Atkinson
