Sidebar 3: Measurements
I tested the AVM Ovation A6.2 ME with my Audio Precision SYS2722 system (see the January 2008 "As We See It"). I preconditioned the amplifier by following the CEA's recommendation of operating it at one-eighth the specified power into 8 ohms for 30 minutes. The manual states that the A 6.2 ME can get warm. At the end of the preconditioning, its heatsinks were very hot, at 132.3°F (55.8°C), and the top panel's temperature was 110.4°F (43.6°C). Operating the amplifier at the older IHF recommendation for preconditioning—both channels driven at one-third power for 60 minutes—the temperatures increased slightly, with the heatsink measuring 140°F (60.5°C) and the top panel measuring 113.9°F (45.5°C). Be sure to give this amplifier plenty of ventilation.
I looked first at the AVM's behavior via its line inputs, mostly with the volume control set to its maximum of "99.5," then at lower settings. With the optional attenuation bypassed with the menu, the maximum gain at the loudspeaker outputs was a high 47.8dB for both the balanced and single-ended inputs. With the attenuation switched into the circuit, set to 6dB, the gain was reduced by exactly 6dB. (I didn't investigate other attenuation values.) The maximum gain at the headphone outputs was 21.75dB without attenuation, 15.75dB with 6dB attenuation. At the preamplifier output, the maximum gain was 12.2dB with and without attenuation. The Ovation A 6.2 preserved absolute polarity at its loudspeaker, preamplifier, and headphone outputs (ie, was noninverting) for both balanced and single-ended line inputs.
The Ovation A 6.2 ME's single-ended input impedance was 10k ohms at low and midrange frequencies, dropping slightly to 9.4k ohms at 20kHz. The balanced input impedance was lower, at 6.8k ohms across the audioband. The single-ended preamplifier output impedance was a low 46.8 ohms from 20Hz to 20kHz. The balanced impedance was exactly twice that value, as expected. The headphone output impedance was a very low 0.5 ohms at 20Hz and 1kHz but rose to a still-low 7.6 ohms at 20kHz. The AVM should not have problems driving low-impedance headphones (though see later).
The amplifier's output impedance was a very low 0.07 ohms at 20Hz and 1kHz, rising slightly to 0.09 ohms at 20kHz. (The measured values include the series impedance of 6' of spaced-pair speaker 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 minimal (fig.1, gray trace). The small-signal bandwidth into resistive loads with the volume control set to its maximum was flat to 20kHz, not reaching –3dB into 8 ohms until 120kHz (blue and red traces), although a slight, 0.15dB channel imbalance can be seen in this graph. This wide small-signal bandwidth was preserved at lower settings of the volume control. The amplifier's reproduction of a 10kHz squarewave (fig.2) featured very short risetimes. A slight amount of overshoot can be seen, but there was no ringing. A 1kHz squarewave was perfectly square (fig.3).
To reduce the influence of the random noise floor on the distortion measurements, I performed all of these with the attenuation active and the volume control set to –20dB. AVM specifies the Ovation A 6.2 ME's maximum power as 300Wpc into 4 ohms (21.75dBW). With both channels driven and using our definition of clipping, which is when the output's percentage of THD+noise reaches 1%, the amplifier clipped at 190Wpc into 8 ohms (fig.6, 22.8dBW), 315Wpc into 4 ohms (fig.7, 22.0dBW), and 505W (21.0dBW) with one channel driven into 2 ohms (fig.8). The headphone output clipped asymmetrically at just over 1V into 300 ohms under all circumstances measured: with attenuation active or bypassed, with both balanced and unbalanced input signals, and regardless of the volume control setting. This will not be an issue with high-sensitivity headphones, but it correlates with Herb's observation that the AVM couldn't drive his low-sensitivity HiFiMan Susvaras to appropriate levels without clipping.
The distortion was predominantly the third harmonic (fig.10), with the higher odd-order harmonics all lying below –100dB and decreasing in a linear manner with frequency (fig.11). (Other than the third, which remained at –80dB/0.01%, these harmonics all lay below –110dB at the same drive voltage into 8 ohms, not shown.) The reduction in high-frequency linearity seen in fig.7 led to the production of high-order intermodulation products with an equal high-power mix of 19kHz and 20Hz tones (fig.12). The difference product at 1kHz lay at a low –86dB (0.005%), however.
Fig.1 AVM Ovation A 6.2 ME, 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) (0.5dB/vertical div.).
Fig.2 AVM Ovation A 6.2 ME, small-signal 10kHz squarewave into 8 ohms.
Fig.3 AVM Ovation A 6.2 ME, small-signal 1kHz squarewave into 8 ohms.
The Treble and Bass controls each offer seven steps of boost or cut. Their effect set to "+7" and "–7," with the volume control set to "79.5," is shown as the blue and red traces in fig.4. Each step of the Treble control boosts or cuts the output above 10kHz by 2dB. The Bass control operates in smaller steps, but instead of a conventional Baxandall-type control, it boosts or cuts the output between 50Hz and 500Hz and rolls off the output below 50Hz. The Contour control provides a traditional "Loudness" function for low-level listening and has 10 settings. The green and gray traces in fig.4 were taken with it set to the maximum. The treble is boosted by 5.5dB and the 50–500Hz region by the same amount as when the Bass control was set to "7." With the Contour set to "3," the treble is very slightly attenuated, and the bass boost peaks just above 4dB.
Fig.4 AVM Ovation A 6.2 ME, response with Treble and Bass controls set to "0"and "±7" and Contour set to "Off" (left channel blue, right red) and with Contour set to "10" (left green, right gray) and "3" (left cyan, right magenta) (5dB/vertical div.)
Channel separation (not shown) was excellent, at 100dB in both directions below 1kHz and still 80dB at the top of the audioband. With the unbalanced input shorted to ground, the volume control set to its maximum, and attenuation bypassed, the wideband, unweighted S/N ratio was 59.1dB (average of both channels), ref. 2.83V output into 8 ohms. Restricting the measurement bandwidth to the audioband increased the ratio to 68dB, while switching an A-weighting filter into circuit further improved the ratio to 70.3dB. Activating the attenuation increased all these ratios by around 3dB. The level of the Ovation A 6.2's noise floor depended on the volume control setting. Even with the control set to its maximum, no supply-related spuriae were present in the amplifier's low-frequency output spectrum (fig.5, cyan and magenta traces). When the volume control was set to –20dB (blue and red traces), the random noise was lowered by 20dB, which shows that the source of the noise, which in any case is probably inconsequential, is before the volume control.
Fig.5 AVM Ovation A 6.2 ME, spectrum of 1kHz sinewave, DC–1kHz, at 1W into 8 ohms with attenuation active and volume control set to –20dB (left channel blue, right red) and its maximum (left cyan, right magenta, linear frequency scale).
Fig.6 AVM Ovation A 6.2 ME, distortion (%) vs 1kHz continuous output power into 8 ohms.
Fig.7 AVM Ovation A 6.2 ME, distortion (%) vs 1kHz continuous output power into 4 ohms.
Fig.8 AVM Ovation A 6.2 ME, distortion (%) vs 1kHz continuous output power into 2 ohms.
As the distortion at low powers was very low, I measured how the Ovation A 6.2's distortion changed with frequency at 20V, which is equivalent to 50W into 8 ohms, 100W into 4 ohms, and 200W into 2 ohms. The THD+N percentage was very low into all three impedances at low frequencies (fig.9), but the increase in the treble implies a relatively restricted open-loop bandwidth. (As the signal frequency increases, correspondingly less negative feedback is available to reduce distortion.)
Fig.9 AVM Ovation A 6.2 ME, THD+N (%) vs frequency at 20V into: 8 ohms (left blue, right red), 4 ohms (left cyan, right magenta), and 2 ohms (left green, right gray).
Fig.10 AVM Ovation A 6.2 ME, 1kHz waveform at 20W into 8 ohms, 0.024% THD+N (top); distortion and noise waveform with fundamental notched out (bottom, not to scale).
Fig.11 AVM Ovation A 6.2 ME, spectrum of 50Hz sinewave, DC–1kHz, at 100Wpc into 4 ohms (left channel blue, right red, linear frequency scale).
Fig.12 AVM Ovation A 6.2 ME, HF intermodulation spectrum, DC–30kHz, 19+20kHz at 100Wpc peak into 4 ohms (linear frequency scale).
Other than the random noise that comes before the volume control and the reduction in linearity in the treble octaves, the AVM Ovation A 6.2 ME did well on the test bench, slightly exceeding its specified power. I was puzzled by the headphone jack's limited output voltage, but that might well be a sample-specific problem.—John Atkinson
