Sidebar 3: Measurements
I measured the Esoteric Grandioso C1X's performance with my Audio Precision SYS2722 system (see the January 2008 "As We See It") and checked some results with the magazine's more-recent APx555 system. The C1X's massive power supply chassis sat on the floor, as far away from the preamplifier chassis as I could arrange with the long umbilical cables. I took a complete set of tests using the balanced inputs, repeating some tests using the single-ended inputs. As set up with the Menu by JVS, the display showed the volume in accurate 0.1dB steps from –99.9dB to +12.0dB. The maximum gain was exactly 12dB for the balanced and unbalanced inputs to the balanced outputs. The preamplifier preserved absolute polarity (ie, was noninverting) with both balanced and unbalanced inputs. (Its XLR jacks are wired with pin 2 hot, the AES convention.)
At 63k ohms at 20Hz and 1kHz, the C1X's unbalanced input impedance was close to the specified 68k ohms, but it was lower at 20kHz, at 19k ohms. The balanced input impedance was higher than the specified 50k ohms, at 90k ohms per phase at 20Hz and 1kHz, 40k ohms per phase at 20kHz. Neither input will present any drive issues with source components. The Esoteric's balanced output impedance was a usefully low 60 ohms at 20Hz and an even-lower 35 ohms at 1kHz and 20kHz. It does not have unbalanced outputs.
The Esoteric preamplifier's frequency response into high impedances was flat from 10Hz to 20kHz (fig.1, blue and red traces) with the ultrasonic rolloff reaching –1dB at 80kHz. The C1X's response into the demanding 600 ohm load (fig.1, cyan, magenta traces) was still flat to 20kHz, but at the other end of the audioband it rolled off by 1.5dB at 11Hz owing to the increased output impedance in this region. Fig.1 was taken with the C1X's volume control at its maximum setting of "12.0dB." Both the frequency response and the excellent channel matching were preserved at lower settings of the control.
Figs.4 and 5 respectively plot the percentage of THD+noise in the Esoteric's balanced output against the output voltage into 100k ohms and 600 ohms with the volume control set to unity gain. We specify a preamplifier's clipping voltage as being when the THD+N reaches 1%. The C1X's output level at this THD percentage into both loads was 12.65V, confirming the statement in the manual that the "maximum permissible input voltage" is 12V.
Fig.7 shows the spectrum of the C1X's output at 2V, a typical maximum level that the preamplifier will be sending to a power amplifier. Though the second and third harmonics can be seen, both lie at –120dB (0.0001%), which is not much higher than the levels of these harmonics in the SYS 2722's signal generator. Repeating the analysis with the APx555's higher-precision signal generator gave the same result. Both harmonics rose by 12dB into the current-hungry 600 ohms, but this load is very much a worst case. Tested for intermodulation distortion with an equal mix of 19 and 20kHz tones at the same peak voltage level, the second-order difference product at 1kHz lay below –130dB in the left channel and even lower in the right (fig.8). The higher-order products were all below –120dB (0.0001%).
Esoteric's Grandioso C1X offered superb measured behavior. Its extraordinarily low levels of distortion and noise rival the performance of contenders from Benchmark, MBL, and Pass Labs.—John Atkinson
Fig.1 Esoteric C1X, frequency response with volume control set to "12.0dB" at 1V into: 100k ohms (left channel blue, right red), 600 ohms (left cyan, right magenta) (0.5dB/vertical div.).
The C1X's channel separation was superbly high, at 130dB in both directions below 1kHz, decreasing inconsequentially to 115dB at the top of the audioband (fig.2). The C1X offered extremely low noise, with the power-supply–related spuriae in its output lying close to –130dB ref. 2V (fig.3). The wideband, unweighted signal/noise ratio, measured with the unbalanced input shorted to ground but the volume control set to its maximum, was a high 85dB ref. 1V output (average of both channels). Restricting the measurement bandwidth to the audioband increased the S/N to a superb 98.8dB, while switching an A-weighting filter into circuit further improved this ratio, to 101dB. Muting the Esoteric with the remote control increased the A-weighted S/N ratio by just 2dB; even with the volume control set to its maximum, this is a very quiet preamplifier!
Fig.2 Esoteric C1X, channel separation (dB/vertical div.)
Fig.3 Esoteric C1X, spectrum of 1kHz sinewave, DC–1kHz, at 2V into 100k ohms (left channel blue, right red; linear frequency scale).
Fig.4 Esoteric C1X, THD+N (%) vs 1kHz output voltage into 100k ohms.
Fig.5 Esoteric C1X, THD+N (%) vs 1kHz output voltage into 600 ohms.
Figs.4 and 5 show that the measured THD+N increases with decreasing signal levels below 5V into 100k ohms and below 2V into 600 ohms. This is because the actual distortion lies beneath the noise floor at these levels. To be sure, therefore, that the reading was not dominated by noise, I measured how the C1X's distortion changed with frequency at 5V. The THD+N percentage was extremely low throughout the audioband into 100k ohms (fig.6, blue and red traces), though with a slight increase in the top audio octaves. As predicted by fig.5, the THD+N was higher into 600 ohms (cyan and magenta traces) but still very low in level in absolute terms.
Fig.6 Esoteric C1X, THD+N (%) vs frequency at 5V into: 100k ohms (left channel blue, right red), 600 ohms (left cyan, right magenta).
Fig.7 Esoteric C1X, spectrum of 1kHz sinewave, DC–1kHz, at 2V into 100k ohms (left channel blue, right red; linear frequency scale).
Fig.8 Esoteric C1X, HF intermodulation spectrum, DC–30kHz, 19+20kHz at 2V into 100k ohms (left channel blue, right red; linear frequency scale).
