Australian HIFI - - LAB 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

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