LABORATORY TEST REPORT
Although Audio Analogue advertises the AAcento as being capable of ‘100-watts per channel’ it adds some ‘fine print’ in its manual that would indicate that achieving this output power is a ‘best case’ scenario. For example, it says it acheives this output only when one channel is being driven, plus adds that at this level, THD+N will be 1%. Even more fine print adds that the amplifier cannot sustain this output continuously, which it’s supposed to be able to do according to the consumer laws in many countries, including Australia. To be specific, the manual states: ‘ The AAcento is not designed to sustain continuous output power. High power sine wave tests over a load of 8 or less must be evaluated for short time or the over-temperature (sic) will shut down the amplifier.’ This meant that Newport Test Labs had to make very quick measurements before the amplifier’s thermal protection kicked in. So the power output measurements shown in the tabulated chart are not true ‘continuous’ measurements as are recorded by other amplifiers, which can sustain their rated output almost indefinitely. The figures do, however, give a fairly true idea of the amplifier’s power output capabilities when it’s playing music.
The measurements made by Newport Test Labs put the Audio Analogue AAcento’s power output at 90-watts per channel, both channels driven into 8 at 1kHz from 20Hz to 20kHz. As you can see from the table, the 90-watt limitation comes about because this is the amplifier’s maximum output before 0.1% waveform distortion from 1kHz up to 20kHz. At lower frequencies it can deliver 95-watts per channel when both channels are driven. As you can also see, the AAcento delivered 100-watts per channel at 20kHz when only one channel was driven and a touch more than this at lower frequencies… though the differences are insignificantly small, as you can see from the differences in the dBw column—less than 0.1dB at 1kHz and 0.2dB at 20Hz.
When driving 4 and 2 loads, the AAcento didn’t manage to meet its rated output power at any test frequency, even when only a single channel was driven. This is no doubt because Audio Analogue measures output power at the point where the test waveform has reached 1.0% distortion, whereas Newport Test Labs measured at the point where distortion in the test waveform reached 0.1% THD—which is an order of magnitude less.
As you can see from the tabulated chart, in Newport Test Labs’ more stringent power output tests the AAcento delivered 176-watts per channel with one channel driven at low frequencies, and 169-watts per channel at 20kHz. When both channels were driven into 4 loads, maximum output dropped to around 150-watts per channel across the audio band. Into 2 loads the amplifier delivered 220-watts both channels driven at 1kHz, but at the frequency extremes (20Hz and 20kHz) power output dropped to around 180-watts per channel.
Distortion was quite high irrespective of output level, which is of course what I’d expect from an amplifier that doesn’t use global negative feedback.
Looking at Graph 1, you can see the levels of the odd-order components are higher than the even-order, though at these low-order levels, odd-order harmonics are not the problem they’d be if they were at higher orders. The second harmonic is at –75dB (0.01778%), the third at –56dB (0.15848%), the fourth at –87dB (0.00446%), the fifth at –67dB (0.04466%), the sixth at –104dB (0.00063%), the seventh at –79dB (0.01122%), the eighth at –113dB (0.00022%), the ninth at –90dB (0.00316%) and a tenth at –122dB (0.00007%). Except for one higher-order harmonic at 100dB (0.001%) down and one at –111dB (0.00028%) the others are all more than 123dB down (0.00007%).
Graph 2 shows harmonic distortion when the amplifier is driving a 4 load at a level of one watt. You can see the levels of the distortion components are roughly similar to those when the amplifier is driving an 8 load, except for the higher-order components, which increase in level (but are still all more than 110dB down (0.00031%). Note the admirably low noise floor at higher frequencies on both graphs, and that even the low-frequency noise (extreme left of graph) is mostly more than 100dB down.
Distortion levels at 100-watts into 8 (Graph 3)and 170-watts into 4 (Graph 4) are not only high, but also almost identical. The only levels that are different are those of the sixth, eighth and tenth harmonics (of the 1kHz test signal). Roughly speaking, the first two harmonics are at –60dB (0.1%), the fifth is at –70dB (0.03162%) and the fourth, seventh and ninth are around –80dB (0.01%). Note again the noise floor at higher frequencies, which has dropped below –140dB, and is more than 100dB down at low frequencies.
The AAcento’s intermodulation distortion graph (Graph 5) looks more like one from a valve amplifier than from a solid-state amplifier, not least because of the level of the signal regenerated at 1kHz, which is only 75dB down (0.01778%), which has in turn increased the levels of the sidebands either side of the two test signals at 19kHz and 20kHz, the highest of which are 60dB down (0.1%).
The frequency response of the Audio Analogue AAcento was extremely flat and extended. Overall, it extends from less than 1Hz to 91kHz –1dB, and from less than 1Hz to 151kHz –3dB. These are superbly wideband responses. The response within the audio band is shown in Graph 6 for both the response into a standard non-inductive 8 laboratory test load and into a load that simulates that of a two-way bass reflex speaker. The flattest response is the one into the laboratory load, and it’s 5Hz to 20kHz ±0.05dB. The response into the simulated speaker load doesn’t quite reach the same level of perfection, but it’s still 5Hz to 20kHz ±0.2dB, which is outstandingly good.
Channel separation was far more than required, but Newport Test Labs’ tested results of 77dB at 20Hz, 94dB at 1kHz and 67dB at 20kHz were rather less than I expect to see in such a high-quality amplifier.
Interchannel phase, on the other hand, was a little better than I am used to seeing, being perfect at 1kHz and only 0.01° at 20Hz and 0.15° at 20kHz. All these differences would, of course, be imperceptible to the human ear.
Signal-to-noise ratios, as measured, were excellent, with the Audio Analogue AAcento returning figures of 85dB A-weighted referred to an output of one watt, and 102dB A-weighted referred to rated output (exceeding Audio Analogue’s specification).
Newport Test Labs measured the AAcento’s output impedance as being 0.44 , compared to Audio Analogue’s specification of 0.4 , so slightly higher (or perhaps Audio Analogue didn’t bother with the second digit), so the damping factor would be a low and likely just-audible 18. When I say ‘just audible’ this damping factor is about the same as I’d expect from a valve amplifier, which means there will be some interaction between the back-emf from large-coned loudspeakers such that the amplifier won’t be able to deliver quite the same level of control over them as an amplifier with a damping factor of 20 or more. (Once damping factor exceeds 20, higher numbers make no audible difference.) But it’s close, very close. If the amplifier’s output impedance had been exactly 0.4 , its damping factor would have been 20, and variations in production could easily account for a 0.04 difference.
Mains power consumption varied from 46-watts when the AAcento was idling, to 72-watts when it was operating at typical listening levels, to 353-watts when it was delivering its maximum power output into 8 . In stand-by mode, power consumption is less than the one watt specified by Audio Analogue (0.77-watts) but still higher than the Australian standard for stand-by power consumption, which is 0.5-watts.
Square wave performance was outstandingly good, as you’d expect given the linearity and extension of the AAcento’s frequency response. You can see that the 100Hz and 1kHz square waves look almost exactly as if they’d come direct from the square wave generator. The 10kHz square wave has only the tiniest amount of rounding on the leading edge— definitely one of the best results I have ever seen for this test. The square wave that shows the AAcento’s performance into a highly reactive load is also outstandingly good, with just a single one-eighth-height overshoot that’s essentially damped within one cycle. Amplifiers that perform this way in this test are generally held to ‘sound better’ in listening tests.
Other than distortion, which is inevitably higher than usual if a designer decides to eschew global negative feedback (though to put it into perspective, the AAcento’s distortion is no more than I’d expect from a high-quality valve amplifier), the Audio Analogue AAcento returned excellent measured performance on Newport Test Labs’ test bench in all other areas of technical performance. Steve Holding