Australian HIFI - - LAB REPORT - Steve Hold­ing

New­port Test Labs’ mea­sure­ments proved be­yond any shadow of a doubt that the Bel Canto ACI 600 is a pow­er­ful am­pli­fier. It de­liv­ered more than 300-watts per chan­nel into 8 , ir­re­spec­tive of whether one or both chan­nels was driven, and more than 600-watts per chan­nel into 4 loads, again ir­re­spec­tive of whether one or both chan­nels was driven. It also was also able to de­liver this above-spec­i­fi­ca­tion power right across the fre­quency band, from 20Hz to 20kHz. The re­sults are tab­u­lated in the test re­sult ta­ble, and shown graph­i­cally as a bar graph. Note that we haven’t shown the usual two bar graphs (one for sin­gle-chan­nel-driven and the other for both-chan­nels-driven) be­cause the two graphs were iden­ti­cal.

Chan­nel sep­a­ra­tion was su­perb, best­ing 100dB at low and midrange fre­quen­cies and still 91dB at 20kHz. Chan­nel bal­ance was also ex­cel­lent, with New­port Test Labs mea­sur­ing 0.116dB at 1kHz. In­ter­chan­nel phase was ex­cel­lent at low and midrange fre­quen­cies, as you can see from the tab­u­lated re­sults, and only 1.94° out at 20kHz.

Distortion was very, very low, mea­sured either into 8 or 4 loads and also either at low out­put lev­els or at rated out­put. Graph 1 shows har­monic distortion lev­els when the am­pli­fier is de­liv­er­ing one watt into an 8 load. You can see a sin­gle third har­monic com­po­nent at –110dB (0.00031%) and then fifth and sixth har­monic com­po­nents at around –115dB (0.00017%), but that’s about it. Equally re­mark­able is the low level of the noise floor—down close to –130dB—and the lack of low-fre­quency noise. Per­for­mance was al­most as good when the Bel Canto ACI 600 was driv­ing a 4 load. This time there’s a se­cond har­monic at –105dB (0.00056%) and the third har­monic is a lit­tle higher in level, at around –108dB (0.00039%). There’s also a fourth har­monic at –122dB (0.00007%), plus the fifth and sixth har­mon­ics are frac­tion­ally higher than they were when the am­pli­fier was driv­ing an 8 load, at around –113dB (0.00022%). Over­all wide­band distortion at one watt was just 0.005%, as you can see in the tab­u­lated fig­ures.

As I noted ear­lier, distortion was also very low at rated out­put, as you can see from the mea­sure­ments made by New­port Test Labs that are shown in Graph 3 and Graph 4. At an out­put of 300-watts into 8 , apart from the se­cond har­monic at –90dB (0.00316%) and the third har­monic at –92dB (0.00251%), all other distortion com­po­nents were more than 100dB down (0.001%). Distortion in­creased slightly when the ACI 600 was de­liv­er­ing 600-watts into 4 (Graph 4) but apart from the se­cond and third har­mon­ics, all other distortion com­po­nents were more than 100dB down as well. In both graphs you can see the noise floor has dropped even lower—down close to –140dB. The ex­tra distortion com­po­nents vis­i­ble in Graph 4 ap­pear to be the re­sult of an in­creased level of 100Hz sig­nal from the power sup­ply. Over­all wide­band distortion at rated out­put was mea­sured at 0.004%.

In­ter­mod­u­la­tion distortion was ex­cep­tion­ally low, as you can see from Graph 6. The 18kHz and 21kHz side­bands along­side the two test sig­nals are at –95dB (0.00177%)

and –97dB (0.00141%), while the 17kHz and 22kHz side­bands are both around –114dB (0.00019%). The un­wanted re­gen­er­ated sig­nal at 1kHz is nearly 120dB down. In­ter­est­ingly, there’s an ob­vi­ous sig­nal at 2kHz, though as it’s 111dB down (0.00028%) it would not be au­di­ble. Fre­quency re­sponse was mea­sured as ex­tend­ing from 7Hz to 21kHz –1dB, and from 4Hz to 33kHz –3dB. This re­sponse is shown in Graph 5 and was mea­sured with a lab­o­ra­tory stan­dard non-in­duc­tive 8 load.

The ef­fect of the Bass Eq con­trol is shown in Graph 7. You can see that it de­liv­ers a 3dB boost or cut from 20Hz up to 100Hz, af­ter which the boost or cut rapidly di­min­ishes, so there’s ba­si­cally no ef­fect on the re­sponse above 300Hz.

The ef­fect of the Tilt con­trols is shown in Graph 8. Al­though at first glance the boost and cut ef­fect seems to be the same as stan­dard bass and tre­ble con­trols, if you look closely you can see quite a few dif­fer­ences.

The per­for­mance of Class-D am­pli­fiers con­tin­ues to im­prove apace and now mostly ri­vals or ex­ceeds that of lin­ear am­pli­fiers

Firstly, max­i­mum boost and cut is re­stricted to around 3dB, whereas tone con­trols usu­ally of­fer around 8–12dB. Se­cond, the fre­quency where there’s no boost or cut at all when either of the con­trols is used is some­what lower than usual, at 780Hz (it’s usu­ally at 1kHz). Fi­nally, the range of fre­quen­cies that is boosted (or cut) is some­what wider than is usual with tone con­trols, both in the bass and tre­ble re­gions.

Sig­nal-to-noise ra­tios were mea­sured at 81dB un­weighted and 91dB A-weighted ref­er­enced to one-watt out­put, and at 94dB un­weighted and 100dB A-weighted ref­er­enced to rated out­put. Al­though these are good re­sults, the rea­son they’re not bet­ter is be­cause of the pres­ence of high-fre­quency switch­ing noise from the Class-D out­put stage. Al­though this noise is so high in fre­quency that it is not au­di­ble, it af­fects the mea­sure­ments.

The switch­ing noise cer­tainly af­fects the square wave mea­sure­ments, so for the square waves shown with this re­port, New­port Test Labs used a low-pass fil­ter to re­move it. The 100Hz wave shows a de­gree of tilt, in­di­cat­ing that the Bel Canto ACI 600’s low-fre­quency re­sponse rolls off at low fre­quen­cies and does not ex­tend to d.c., but there’s no bend­ing, so there’s no phase shift at low fre­quen­cies. The 1kHz square wave shows a slight rise-time lim­i­ta­tion, plus an over­shoot that would sug­gest a lift in the fre­quency re­sponse at some ul­tra­sonic fre­quency. The 10kHz square wave is of course af­fected by the pres­ence of the low-pass fil­ter used by the lab, but you can see the rise-time lim­i­ta­tion, the high-fre­quency re­sponse roll-off, and the ul­tra­sonic lift. I have in­cluded a square wave of a the same 10kHz wave with­out the fil­ter in place, and you can see that the high-fre­quency switch­ing noise ob­scures the in­for­ma­tion the square wave test could oth­er­wise re­veal. The square wave show­ing the Bel Canto ACI 600’s per­for­mance into a highly ca­pac­i­ta­tive load shows that the am­pli­fier it will be un­con­di­tion­ally sta­ble when driv­ing highly reactive loads, such as elec­tro­static loud­speak­ers.

The per­for­mance of Class-D am­pli­fiers con­tin­ues to im­prove apace and now mostly ri­vals or ex­ceeds that of lin­ear am­pli­fiers in al­most all ar­eas. The am­pli­fiers in­side the ACI 600 are ex­cep­tion­ally good ex­am­ples of Class-D en­gi­neer­ing.

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