Australian HiFi - March-April 2016_

34 Australian

LAB REPORT Bel Canto REF600M Mono Power Amplifi er

CONTINUED FROM PAGE 32

LABORATORY TEST RESULTS

Newport Test Labs measured the power output

of the Bel Canto REF600M while it was be-

ing driven into 8Ω, 4Ω and 2Ω loads. Into

8Ω loads, the REF600M delivered 338-watts

continuously into 20Hz and 1kHz loads, and

328-watts continuously into 20kHz loads.

Driven into 4Ω loads, the Bel Canto REF600M

proved the veracity of its model number by

delivering 600-watts continuously at 1kHz,

and 645-watts continuously at 20kHz. How-

ever when driven with a 20Hz test frequency,

power output fell by 0.4dB to 557-watts

continuous. When Newport Test Labs loaded

down the Bel Canto REF600M’s output with a

2Ω load it delivered 650-watts at all three test

frequencies, at which point the amplifi er’s

internal protection cut in, presumably as a

result of the current draw, which would have

been just on 18-amps.

The frequency response of the Bel Canto

REF600M is shown in Graph 6. The black

trace shows the response into a standard non-

inductive 8Ω laboratory test load, while the

red trace shows the frequency response into a

load that simulates that of a typical two-way

stand-mount loudspeaker (the same one

developed by Ken Kantor, but with a Zobel

network added across the input terminals).

You can see on the graph that—unusually

for a Class-D amplifi er—the traces are almost

identical, meaning that the sound quality

of the Bel Canto will be the same, no matter

what loudspeakers you connect to it.

The almost-identical responses into the

two loads also suggest a high damping factor,

which proved to be the case. (Newport Test

Labs measured damping factor at 1kHz as

being 133, which is an excellent result, and

more than will ever be required, though it

falls somewhat short of Bel Canto’s specifi ca-

tion of 1,000.) As for the frequency response

itself, you can see that at low frequencies

the trace is 3dB down at 6Hz and 1dB down

at 11Hz, while at high frequencies, it’s 1dB

down at 22kHz and 2dB down at 32kHz (the

graphing limit). It turned out to be 3dB down

at 66kHz. All of which meant the Bel Canto

REF600M easily fulfi lled its manufacturer’s

specifi cation of 20Hz to 50kHz ±3dB, with a

measured result of 6Hz to 66kHz ±3dB. Across

the audio band, the response was even fl atter,

with the graph showing a frequency response

of 20Hz to 20kHz ±0.3dB.

Distortion was very low at all power levels

and into both test loads. At an output of one

watt into 8Ω, overall THD+N was measured

at 0.005%, well below the capacity of the

human ear to determine it. Spectrum analysis

(Graph 1) showed only three harmonically-

related distortion components, a 2ndharmon-

ic at –108dB (0.00039%), a third harmonic at

–88dB (0.00398%) and a fourth harmonic at

–118dB (0.00012%). When the test load im-

pedance was halved, to 4Ω, distortion levels

dropped even lower, with a second harmonic

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at –100dB (0.001%) and a third harmonic

at –107dB (0.00044%). This is exceptionally

good performance.

When the amplifi er was driven at its rated

power, distortion levels increased, but still

remained very low, with the overall THD+N

result being measured by Newport Test Labs at

just 0.014%, a level low enough it would not

be detectable by the human ear. Spectrum

analysis of overall distortion into an 8Ω load

is shown in Graph 3. You can see second- and

third-order components at around –85dB

In sum, I think

this may be the

best-performing

Class-D amplifi er

I have ever seen...