Sidebar 3: Measurements
Before measuring the Aesthetix Mimas with my Audio Precision SYS2722 system (see the January 2008 "As We See It"), I installed its two Electro-Harmonix 6922 tubes and left the amplifier's internal jumpers as set by the factory. Before I test an amplifier, I precondition it with both channels driving a 1kHz tone at one-third power into 8 ohms for an hour. The Mimas turned itself off after 30 minutes, however, its front panel displaying the message "Temp HIGH RITE." The amplifier's top panel was hot, at 126.6°F (52.6°C), the temperature of the side panels was 110.4°F (43.6°C), and that of the internal heatsinks was 163.5°F (73.1°C). The Mimas doesn't have quite enough heatsink capacity for sustained high-power use.
After letting the Mimas cool down, I continued the testing. Looking at the line inputs—our review sample didn't include the optional phono and digital input modules—the maximum voltage gain at 1kHz from the speaker terminals into 8 ohms, with the volume control set to the maximum, measured 48.2dB for both balanced and unbalanced signals. The maximum gains at the preamplifier and headphone jacks were 29.3 and 27.1dB, respectively, both higher than the norm. Both inputs preserved absolute polarity from the speaker and preamplifier outputs (ie, were noninverting), meaning that the XLR input jacks are wired with pin 2 hot. The headphone output inverted polarity, however. The unbalanced input impedance was close to specification at a relatively high 19k ohms from 20Hz to 20kHz; the balanced input impedance was twice that value, as specified and expected.
The headphone output impedance was an appropriately low 6 ohms at all audio frequencies, while the preamplifier output impedance was 100 ohms from the unbalanced jacks, 200 ohms from the balanced XLRs. The output impedance at the speaker terminals was 0.25 ohm at low and middle frequencies, rising to 0.26 ohm at the top of the audioband. As a result, the modulation of the Mimas's frequency response with our standard simulated loudspeaker was ±0.3dB (fig.1, gray trace). This graph was taken with the volume control set to its maximum; commendably, there was no change in response or channel balance at lower volume-control settings. The Aesthetix's frequency response at the speaker terminals is down by 3dB at 120kHz; as a result, a 10kHz squarewave was reproduced with very short risetimes (fig.2), and there was no overshoot or ringing with a resistive load. A 1kHz squarewave was perfectly square (fig.3). The response from the headphone outputs was down by just 0.4dB at 200kHz.
Fig.1 Aesthetix Mimas, 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.).
Fig.2 Aesthetix Mimas, small-signal 10kHz squarewave into 8 ohms.
Fig.3 Aesthetix Mimas, small-signal 10kHz squarewave into 8 ohms.
Channel separation was superb below 2kHz, at >95dB R–L and >100dB L–R, though at 20kHz these ratios respectively decreased to 72 and 78dB. The unweighted, wideband signal/ noise ratio, taken with the unbalanced inputs shorted to ground and the volume control set to its maximum, was a disappointing 46dB (average of both channels), this improving to 57.5dB when the measurement bandwidth was restricted to 22Hz–22kHz, and to 61.2dB with an A-weighting filter in circuit. These ratios improved at lower settings of the volume control and are related, I believe, to the Mimas's higher-than-usual preamplifier gain.
Spectral analysis of the low-frequency noise floor (fig.4) indicated that the AC-supply spuriae were at 60Hz and its odd-order harmonics, which suggests that they were due to magnetic interference from the power transformer perhaps being picked up by the tubes' steel pins. The spectra in fig.4 were taken with the volume control set to "88" (blue and red traces) and to "68" (green, gray). Though the levels of the random noise components remain the same, lowering the volume reduced the levels of the AC spuriae, particularly the 60Hz component in the right channel (red and gray traces).
Fig.4 Aesthetix Mimas, spectrum of 1kHz sinewave, DC–1kHz, at 1W into 8 ohms with volume control at "88" (left channel blue, right red) and "68" (left green, right gray) (linear frequency scale).
Aesthetix specifies the Mimas as offering 150Wpc into 8 ohms (21.76dBW) and says it will typically deliver 185W (22.67dBW). With both channels driven and with clipping defined as when the THD+noise in the output reaches 1%, I measured the clipping power into 8 ohms as 180Wpc (22.55dBW, fig.5). Aesthetix specifies the maximum power into 4 ohms as "nearly double" that into 8 ohms; I measured 275Wpc into 4 ohms (21.4dBW, fig.6).
Fig.5 Aesthetix Mimas, distortion (%) vs 1kHz continuous output power into 8 ohms.
Fig.6 Aesthetix Mimas, distortion (%) vs 1kHz continuous output power into 4 ohms.
I also examined how the THD+N percentage varied with frequency at a fairly high level, 20V, which is equivalent to 50W into 8 ohms and 100W into 4 ohms. The THD+N was low into 8 ohms (fig.7, blue and red traces), but higher into 4 ohms (cyan, magenta). The usual rise in the treble due to the reduction in the circuit's open-loop gain at high frequencies was absent.
Fig.7 Aesthetix Mimas, THD+N (%) vs frequency at 20V into: 8 ohms (left channel blue, right red), 4 ohms (left cyan, right magenta).
The right channel's THD+N waveform at this level (fig.8) indicates that the distortion is predominantly third-harmonic in nature, though spectral analysis (fig.9) reveals that there are also higher harmonics present, and there was a much higher level of second harmonic in the left channel (blue trace) than the right (red). Tested with an equal mix of 19 and 20kHz tones a few dB below clipping into 4 ohms, the Aesthetix produced fairly low levels of high-order intermodulation distortion (fig.10). However, while the difference product at 1kHz lay at a very low –106dB (0.0006%) in the right channel, it was much higher in the left channel (blue trace), at –84dB (0.006%). This is still low in absolute terms, however.
Fig.8 Aesthetix Mimas, right channel, 1kHz waveform at 50W into 8 ohms, 0.069% THD+N (top); distortion and noise waveform with fundamental notched out (bottom, not to scale).
Fig.9 Aesthetix Mimas, spectrum of 50Hz sinewave, DC–1kHz, at 50W into 8 ohms (left channel blue, right red; linear frequency scale).
Fig.10 Aesthetix Mimas, HF intermodulation spectrum, DC–30kHz, 19+20kHz at 100W peak into 4 ohms (left channel blue, right red; linear frequency scale).
Its measured performance indicates that Aesthetix Audio's Mimas integrated amplifier is generally well engineered, though its noise floor at high settings of the volume control is higher than it need be. However, as the high gain will mean keeping the volume control low, this will not be a factor in the amplifier's sound quality.—John Atkinson
