Integrated Amp Reviews

I tested the Boulder 866 with my Audio Precision SYS2722 system (see the January 2008 "As We See It"), then repeated some tests with the magazine's more recently acquired APx555 system. But first, I preconditioned the amplifier by following the CEA's recommendation of operating it at one-eighth the specified power into 8 ohms for 30 minutes. At the end of that time, the top panel was very hot, at 115.5°F (44.2°C), and the heatsinks were too hot to touch, at 133.3°F (56.3°C). The Boulder amplifier's output stages appear to be biased heavily into partial class-A operation. Owners must make sure the amplifier has sufficient ventilation. (JVS placed the 866 on the top shelf of his equipment rack.)

I looked first at the Boulder's performance via one of its three balanced line inputs. The specified maximum gain is 40.4dB, though this can be adjusted with the front-panel menu screen. As set up by JVS, the 866's maximum gain at the speaker outputs was 30.75dB. The volume control operated in accurate 1dB steps. The amplifier preserved absolute polarity, and the input XLR jacks were wired with pin 2 positive, the AES standard. The input impedance is specified as 100k ohms; I measured 98k ohms at 20Hz and 1kHz, and an inconsequential drop to 80k ohms at 20kHz.

The Boulder amplifier's output impedance at the loudspeaker terminals was 0.16 ohm at 20Hz and 1kHz, rising slightly to 0.19 ohm at 20kHz. (The figures include the series impedance of 6' of spaced-pair loudspeaker cable.) The modulation of the amplifier's frequency response, due to the Ohm's law interaction between this source impedance and the impedance of my standard simulated loudspeaker load, was a negligible ±0.2dB (fig.1, gray trace). The response into an 8 ohm resistive load (fig.1, blue and red traces) was down by 3dB at 100kHz, which correlates with the Boulder's accurate reproduction of a 10kHz squarewave into that load (fig.2). The level of the right channel in fig.1 was 0.1dB lower than that of the left. Fig.1 was taken with the volume control set to its maximum; neither the frequency response nor the channel matching changed at lower settings of the volume control.

Channel separation was superb, at >110dB in both directions below 1kHz and still 79dB at the top of the audioband. The Boulder's unweighted, wideband signal/noise ratio, taken with the balanced line inputs shorted to ground and the volume control set to its maximum, was very good, at 78.8dB ref. 2.83V into 8 ohms (average of both channels). This ratio improved to 86.4dB when the measurement bandwidth was restricted to the audioband, and to 89dB when A-weighted. The background noise included spuriae at 60Hz and its even- and odd-order harmonics (fig.3), with the latter higher in level than the former. (The odd-order spuriae are probably due to magnetic interference from the massive power transformer.) The cyan and magenta traces in this graph were taken with the volume control set to its maximum, the blue and red traces with it set to "80," which is equivalent to –20dB. The lower setting reduced the levels of both the random noisefloor components and the supply-related spuriae by around 10dB, but even at the maximum setting the noise is low in level.

With both channels driven, the 866 slightly exceeded its specified maximum power into 8 ohms of 200Wpc (23dBW), clipping at 1% THD+noise at 210Wpc (23.2dBW, fig.4). The Boulder didn't meet its specified maximum power of 400Wpc into 4 ohms (23dBW), clipping at 360Wpc (22.55dBW, fig.5). However, according to the manual, Boulder's specified maximum powers were taken with an AC wall voltage of 120V. On the day I measured the amplifier, my wall voltage was 116V—I suspect that everyone on my street had their wall air conditioners operating—dropping to 114.5V with the amplifier clipping into 4 ohms. Boulder doesn't specify a maximum continuous power into 2 ohms, but with one channel driven the amplifier clipped at 650W into 2 ohms (22.1dBW, fig.6). The rear-panel 8A circuit breaker operated just above this power when I performed this test. Muting the 866 and pushing back the breaker button restored the amplifier to normal operation.

The downward slope of the traces below the actual clipping power in figs.4 and 5 is due to the actual distortion lying beneath the noisefloor. (A constant level of noise becomes an increasing percentage of the output signal as the power drops.) I therefore examined how the THD+N percentage changed with frequency at 14.17V, which is equivalent to 25W into 8 ohms, 50W into 4 ohms, and 100W into 2 ohms. The left channel's distortion was very low in the bass and midrange into both 8 and 4 ohms (fig.7, blue and cyan traces, respectively), and though it rose in the top octaves, it was still below 0.02%. The right channel had slightly more distortion into 8 ohms (red trace) and a lot more into 4 ohms (magenta). I have only shown the left channel's distortion into 2 ohms (green trace), as the rear-panel circuit breaker operated while I was measuring how the right channel's THD+N varied with frequency into this load.

The distortion was predominantly the subjectively benign third harmonic (fig.8), though it was higher in the right channel (fig.9, red trace) than the left (blue trace). Some higher-order harmonics are present in both channels, but these are very low in level. A low level of second harmonic distortion is present in the right channel, which suggests that in this channel the complementary output devices are not optimally matched. (Even-order harmonics completely cancel with a perfect push-pull output stage.) When the 866 drove an equal mix of 19 and 20kHz tones at 50W peak into 8 ohms, all the intermodulation products lay at or below –100dB (0.001%) in both channels (fig.10).

I examined the Boulder 866's D/A performance primarily using its AES/EBU input, repeating some tests with the TosLink S/PDIF input and with the amplifier connected to my network and sent audio data by Roon. The Boulder 866's optical input locked to data sampled up to 96kHz, the AES/EBU input up to 192kHz.

With the volume control set to its maximum, a 1kHz digital signal at –20dBFS resulted in a level at the loudspeaker outputs of 18.5V, which is 6.9dB below the clipping voltage and equivalent to 42.8W into 8 ohms. It appears, therefore, that the Boulder's digital inputs have around 14dB of excess gain. To avoid damaging the Boulder's power amplifier with high- level digital signals, I performed all the measurements of the digital inputs' performance with the volume control set to –20dB.

The Boulder's digital inputs all pre- served absolute polarity. The impulse response with 44.1kHz data (fig.11) indicates that the reconstruction filter is a conventional linear-phase type, with symmetrical ringing on either side of the single sample at 0dBFS. With white noise sampled at 44.1kHz (fig.12, red and magenta traces), the 866's rsponse rolled off sharply above 20kHz, though it didn't reach full stop-band suppression until 57kHz. The steep rolloff slope became shallower just above half the sample rate (vertical green line). An aliased image at 25kHz of a full-scale tone at 19.1kHz (blue and cyan traces) is barely visible above the noisefloor, though the distortion harmonics of the 19.1kHz tone can be seen, including the second, which was not present with the analog inputs.

The 866's digital-input frequency response was flat in the audioband and follows the same basic shape, with a sharp rolloff just below half of each sample rate (fig.13). The levels of the two channels were perfectly matched. When I increased the bit depth from 6 to 24 with a dithered 1kHz tone at –90dBFS (fig.14), the noisefloor components dropped by around 14dB, which implies that the 866 offers close to 18 bits of resolution. However, the noisefloor contains a large number of low-level, power-supply–related spuriae, particularly in the left channel (red and magenta traces). Pairs of low-frequency sidebands are visible around each of the harmonic frequencies. With undithered data representing a tone at exactly –90.31dBFS (fig.15), the three DC voltage levels described by the data were well resolved. With undithered 24-bit data (fig.16), the result was a relatively clean sinewave.

Intermodulation distortion via the Boulder amplifier's digital inputs (not shown) was a little higher than it had been with analog input signals, the difference product at 1kHz lying at –89dB (0.003%) compared with –101dB (0.001%).

I tested the 866 for its rejection of word-clock jitter via its TosLink, AES/ EBU, and network inputs. Though all the odd-order harmonics of the 16-bit J-Test signal's LSB-level, low-frequency squarewave were at the correct levels (fig.17, sloping green line), the spectral spike that represents the high-level tone at exactly one-quarter the sample rate was surrounded by power supply– related sidebands. All three digital inputs behaved identically in this respect. The same supply-related sidebands were present with 24-bit J-Test data (not shown).

I investigated whether the sidebands were due to jitter or to supply noise on the DAC chip's voltage reference pin by performing narrow- band spectral analyses with 24-bit data representing full-scale tones at 5, 10, 15, and 20kHz. If the sidebands were due to supply noise, the levels of the sidebands would remain the same at all four signal frequencies. If they were due to jitter, their levels would drop as the frequency of the signal reduced. The levels of the sidebands were the same with all the signal frequencies, so they must be due to supply-related spuriae.

The Boulder products that I have measured in the past have all demonstrated excellent audio engineering. With the exception of the higher harmonic distortion in the right channel, which may well have been due to an out-of-specification output device, the 866 performed well on the test bench. It offers relatively high power with very low harmonic and intermodulation distortion. The digital inputs generally performed well, though I was a little bothered by the presence of power supply–related spuriae in figs.13 and 15. I must assume that these are due to the sensitive digital circuitry sharing space in a relatively small enclosure with a massive power transformer.—John Atkinson