Sidebar 3: Measurements
Before I tested one of the Esoteric M1X amplifiers, serial number 20014, with my Audio Precision SYS2722, I preconditioned the amplifier, following the CEA's recommendation of operating it at one-eighth the specified power into 8 ohms for 30 minutes. At the end of that time, the internal heatsinks were fairly warm, at 105.7°F (40.8°C), though the top panel's temperature was just 82.6°F (28.1°C). As JVS had told me that the M1X's sound quality improved after a lengthy break-in, I then ran the amplifier at one-third the specified power into 8 ohms for another 30 minutes before examining its measured performance. The heatsink and top panel temperatures had increased only slightly after this period, to107.4°F (41.9°C) and 83.4°F (28.5°C), respectively.
The Esoteric's voltage gain is specified as 28.5dB; I measured 28.13dB into 8 ohms for the balanced input, 28.27dB for the unbalanced input. The M1X preserved absolute polarity (ie, was noninverting) with both input types. The specified input impedances are 100k ohms, balanced, and 10k ohms, unbalanced. I measured an even higher 180k ohms at 20Hz and 1kHz for the balanced input, dropping slightly to a still very high 146k ohms at 20kHz. The unbalanced input was 9.8k ohms at low and midrange frequencies, dropping to 9.2k ohms at the top of the audioband.
Fig.1 Esoteric M1X, frequency response at 2.83V into: simulated loudspeaker load (gray), 8 ohms (blue), 4 ohms (magenta), 2 ohms (red) (0.5dB/vertical div.).
Fig.2 Esoteric M1X, small-signal 10kHz squarewave into 8 ohms.
The Esoteric's output impedance was a low 0.24 ohm at 20Hz and 1kHz, increasing slightly to 0.265 ohm at 20kHz. (These figures include the series impedance of 6' of spaced-pair loudspeaker cable.) The modulation of the amplifier's frequency response due to the Ohm's law interaction between this source impedance and the impedance of our standard simulated loudspeaker was very small, at ±0.1dB (fig.1, gray trace). The response into an 8 ohm resistive load (fig.1, blue trace) was down by 3dB just above 100kHz, though the increasing output impedance at very high frequencies means that the ultrasonic rolloff into 4 ohms (magenta) and 2 ohms (red) was greater. The M1X's reproduction of a 10kHz squarewave into 8 ohms (fig.2) was superb, with no overshoot or ringing.
Fig.3 Esoteric M1X, spectrum of 1kHz sinewave, DC–1kHz, at 1W into 8 ohms (linear frequency scale).
Measured with the unbalanced input shorted to ground, the amplifier's unweighted, wideband signal/noise ratio was a very good 77.4dB ref. 1W into 8 ohms. This ratio improved to 85dB when the measurement bandwidth was restricted to 22Hz–22kHz and to 87.7dB when A-weighted. While spuriae at the even-order harmonics of the 60Hz power-supply frequency were present in the Esoteric's noisefloor (fig.3), these are negligible, at –100dB and below ref. 1W into 8 ohms. I repeated this spectral analysis with a wire connecting the grounding terminal on the amplifier's back panel to the Audio Precision's chassis ground, but there was no difference in the levels of the power supply–related spuriae.
Fig.4 Esoteric M1X, distortion (%) vs 1kHz continuous output power into 8 ohms.
Fig.5 Esoteric M1X, distortion (%) vs 1kHz continuous output power into 4 ohms.
The Esoteric M1X exceeded its specified output powers of 300W into 8 ohms and 600W into 4 ohms (both powers equivalent to 24.8dBW). With our definition of clipping, which is when the output's percentage of THD+noise reaches 1%, the M1X clipped with a 1kHz signal at 340W into 8 ohms (25.3dBW, fig.4) and at 620W into 4 ohms (24.9dBW, fig.5). Although the Esoteric amplifier's maximum power into 2 ohms isn't specified, I measured 1050W into this load at 1% THD+N. The wall voltage had dropped from 120.2V to 116V at this power.
Fig.6 Esoteric M1X, THD+N (%) vs frequency at 20V into: 8 ohms (blue), 4 ohms (magenta), 2 ohms (red).
Figs.4 and 5 indicate that distortion was very low below actual waveform clipping. I examined how the percentage of THD+N changed with frequency at 20V, which is equivalent to 50W into 8 ohms, 100W into 4 ohms, and 200W into 2 ohms. The distortion was extremely low in the midrange into 8 ohms (fig.6, blue trace) but rose in the top octaves, and it was higher into 4 ohms (magenta) and 2 ohms (red). However, even at this very high power into 2 ohms, the THD+N remained well below 0.1%.
Fig.7 Esoteric M1X, 1kHz waveform at 50W into 8 ohms, 0.051% THD+N (top); distortion and noise waveform with fundamental notched out (bottom, not to scale).
Fig.8 Esoteric M1X, spectrum of 50Hz sinewave, DC–1kHz, at 50W into 8 ohms (linear frequency scale).
The Esoteric's distortion driving 1kHz at 50W into 8 ohms was predominantly the third harmonic (fig.7), lying at –66dB (0.05%, fig.8). Though higher-order, predominantly odd-order harmonics are present in this graph, these are all low in level. The third harmonic rose almost to –60dB (0.1%) at the same voltage into 4 ohms (not shown).
Fig.9 Esoteric M1X, HF intermodulation spectrum, DC–24kHz, 19+20kHz at 50W peak into 8 ohms (linear frequency scale).
When the M1X drove an equal mix of 19 and 20kHz tones with a peak level of 50W into 8 ohms (fig.9), the second-order difference product at 1kHz lay at a very low –94dB (0.002%). Higher-order intermodulation products lay at the same level or lower. At the same voltage into 4 ohms, the intermodulation products rose by 3dB.
The Esoteric M1X did well on the test bench, offering very high power with low distortion and noise.—John Atkinson
