Electrocompaniet AW400 monoblock power amplifier Measurements

Sidebar 3: Measurements

To perform the measurements on the Electrocompaniet AW400, I mostly used Stereophile's loan sample of the top-of-the-line Audio Precision SYS2722 system (see the January 2008 "As We See It" and www.ap.com); for some tests, I also used my vintage Audio Precision System One Dual Domain.

Electrocompaniet refers to the AW400 as a "Class A" amplifier, but the heat signature and the idling power consumption suggests that it uses a conventional class-AB output stage. I preconditioned the Electrocompaniet by running it at one-third power into 8 ohms for 60 minutes, a level that maximally stresses a class-B or –AB amplifier's ability to shed waste heat. With the amplifier cold, the THD+noise level was 0.008%; at the end of the preconditioning period, the THD+N had dropped to 0.0035% and the AW400's chassis was warm. The only hot spots were the grilles above the internal heatsinks, but these weren't so hot that I couldn't keep my hand on them.

The voltage gain with the input trim control at its maximum was a high 34.2dB and the amplifier preserved absolute polarity; ie, was non-inverting. (The input XLR jack is wired with pin 2 hot.) The input impedance was very high at all frequencies—my estimate was 450k ohms at low and midrange frequencies compared with the specified 330k ohms, but there is always a large error when measuring such high values of input impedance. It did drop to 100k ohms at 20kHz, though this will be inconsequential.

The output impedance is specified as being below 0.01 ohm, which is very low. However, including 6' of speaker cable, I measured 0.135 ohm at low and midrange frequencies, rising very slightly to 0.17 ohm at the top of the audioband. This resulted in a mild ±0.1dB variation in the AW400's frequency response when it drove our standard simulated loudspeaker (fig.1, gray trace). The high-frequency rolloff was –3dB at 115kHz into 16 ohms, but was negligible at 20kHz (fig.1, magenta). It did increase slightly with decreasing load impedance, reaching a still-modest –0.4dB at 20kHz into 2 ohms (fig.1, red). As a result of this wide bandwidth, the Electrocompaniet's reproduction of a 10kHz squarewave was nicely square (fig.2).

Fig.1 Electrocompaniet AW400, frequency response at 2.83V into: simulated loudspeaker load (gray), 16 ohms (magenta), 8 ohms (blue), 4 ohms (cyan), 2 ohms (red). (0.25dB/vertical div.)

Fig.2 Electrocompaniet AW400, small-signal 10kHz squarewave into 8 ohms.

Even with its high gain, the AW400 was a quiet amplifier. The unweighted audioband signal/noise ratio (ref. 1W into 8 ohms with the input shorted) was 86.9dB, equivalent to a voltage of just 128µV, compared with the specified 130µV (measured over a bandwidth of 400Hz–30kHz). Plotting the THD+N percentage against output power (fig.3) indicates that the distortion lies below this noise at levels up to a few tens of watts into loads of 8 ohms and above. The power at clipping with continuous drive was 425W into 8 ohms (26.3dBW) and 655W into 4 ohms (25.1dBW). The amplifier's protection circuit cut in at 525W into 2 ohms (21.2dBW), which is why the top trace in fig.3 ends at that point. The amplifier more than met its specified power into 8 ohms but fell a bit short into 4 ohms. This is because I don't hold the wall AC voltage constant during this test. It was 124.5V with the amplifier idling, but had dropped to 122.2V with the amplifier clipping into 4 ohms.

Fig.3 Electrocompaniet AW400, distortion (%) vs 1kHz continuous output power into (from bottom to top): 16, 8, 4, 2 ohms.

Fig.4 plots the THD+N percentage against frequency at a level (12.7V, equivalent to 20W into 8 ohms) where the distortion is beginning to rise above the noise floor. All the traces start to rise in the top two audio octaves, due to the amplifier's inevitable reduction in open-loop bandwidth in this region, but the rise is mild and the distortion remains low in level. While the AW400 offers low distortion, what distortion there is contains some higher harmonics (fig.5). Spectral analysis of the waveform in fig.5 indicates that only the second and third harmonics reach –90dB, but the fourth, fifth, seventh, and ninth lie at or above –100dB, which will most likely be inaudible. At higher levels (fig.6), the third harmonic rises to a still-low –84dB (0.006%) and the higher-order harmonics actually decrease in level. While this graph does indicate that some power-supply harmonics are present, the highest-level component, at the full-wave-rectified frequency of 120Hz, lies at –97dB, or low enough not to matter. I did experiment with grounding between the amplifier and the Audio Precision test system to reduce the level of the power-supply spuriae; this was the best I could achieve.

Fig.4 Electrocompaniet AW400, THD+N (%) vs frequency at 12.7V into: 16 ohms (magenta), 8 ohms (blue), 4 ohms (cyan), 2 ohms (red).

Fig.5 Electrocompaniet AW400, 1kHz waveform at 52W into 4 ohms (top), 0.008% THD+N; distortion and noise waveform with fundamental notched out (bottom, not to scale).

Fig.6 Electrocompaniet AW400, spectrum of 50Hz sinewave, DC–10kHz, at 201W into 8 ohms (linear frequency scale).

Finally, even at a level just below visible waveform clipping on the oscilloscope screen, the Electrocompaniet AW400 performed relatively well on the demanding high-frequency intermodulation test (fig.7). The difference product at 1kHz lay at –94dB, and the higher-order products at 18 and 21kHz only just rose above –80dB.

Fig.7 Electrocompaniet AW400, HF intermodulation spectrum, DC–24kHz, 19+20kHz at 401W peak into 4 ohms (linear frequency scale).

The Electrocompaniet AW400 offers high power coupled with low levels of noise and distortion. I am not surprised Art Dudley was impressed by it.—John Atkinson

Electrocompaniet AS
US distributor: Electrocompaniet North America
97 Linden Street
Oakland, CA 94607
(510) 291-1222
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