Sidebar 3: Measurements
I tested the Cambridge Audio EVO 150 with my Audio Precision SYS2722 system (see the January 2008 "As We See It") then repeated some tests with the magazine's more recent APx555 system. As the amplifier has an output stage operating in class-D, I inserted an Audio Precision auxiliary AUX-0025 passive low-pass filter between the test load and the Audio Precision analyzers. This filter eliminates RF noise that could drive their input circuitry into slew-rate limiting. I used it for all the loudspeaker output tests other than frequency response. After two hours of operation, the temperature of the amplifier's top panel had stabilized at a slightly warm 94.1°F (34.5°C).
I looked first at the Cambridge's performance via its balanced and single-ended line inputs. The maximum gain at the loudspeaker outputs was 33.9dB for both types of inputs. At the preamplifier output it was 8.2dB, and at the headphone output it was 7.8dB. The EVO 150 preserved absolute polarity at all outputs. The volume control operated in accurate 0.5dB steps up to "90" and accurate 1dB steps from "90" to the maximum of "100." The unbalanced input impedance was 37k ohms at 20Hz and 1kHz, with an inconsequential drop to 30k ohms at 20kHz. The balanced input impedance was 76k ohms at low and middle frequencies, dropping to 70k ohms at the top of the audioband.
The Cambridge amplifier's output impedance at the headphone output was 1 ohm. At the preamplifier output, it ranged from 53 ohms at 20Hz to 47 ohms at 20kHz. The output impedance at the loudspeaker terminals was 0.09 ohm at 20Hz and 1kHz, rising very slightly to 0.1 ohm at 20kHz. (These 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 was therefore a negligible ±0.1dB (fig.1, gray trace). The response into an 8 ohm resistive load (fig.1, blue and red traces) was down by 3dB at 55kHz, which correlates with the slightly lengthened risetimes with the Cambridge's reproduction of a 10kHz squarewave into that load (fig.2). Fig.1 was taken with the volume control set to its maximum; neither the frequency response nor the superb channel matching changed at lower settings of the volume control.
The distortion was predominantly the third harmonic (fig.8), though the asymmetrical shape of the distortion waveform (bottom trace) suggests that some subjectively benign second harmonic is also present. This was confirmed by the spectrum of the EVO 150's output when it drove a 50Hz tone at 50Wpc into 8 ohms (fig.9). Some higher-order harmonics are present in both channels, but these are very low in level. When the Cambridge amplifier 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 Cambridge EVO 150's D/A performance primarily using its TosLink S/PDIF input, repeating some of the tests with the USB input and with the amplifier's Ethernet port connected to my network and sent audio data by Roon. Again, because of what appeared to be a higher-than-expected noisefloor at the headphone output, I examined the digital inputs' performance at the preamplifier output, setting the volume control to "80" to avoid damaging the power amplifier stage.
The Cambridge EVO 150's optical input locked to data sampled up to 192kHz. The USB input can be operated in either USB1.0 or USB2.0 modes. In the EVO 150's USB1.0 mode, Apple's AudioMIDI utility revealed that it accepted 16- and 24-bit integer data sampled at rates up to 96kHz. In its USB2.0 mode, the EVO 150 accepted 16- and 24-bit data sampled at rates up to 705.6kHz. Apple's USB Prober utility identified the Cambridge as "CA Evo 150 2.0" from "CA." The USB port operated in the optimal isochronous asynchronous mode.
The Cambridge's digital inputs all preserved absolute polarity. With the volume control set to its maximum, a 1kHz digital signal at –12dBFS resulted in a level at the preamplifier output of 1.28V, at the headphone output of 4.815V, and at the loudspeaker outputs of 24.56V. The latter is 3dB below the Cambridge's clipping voltage into 8 ohms. It appears, therefore, that the digital inputs have around 9dB of excess gain.
The impulse response with 44.1kHz data (fig.13) indicates that the reconstruction filter is a conventional linear-phase type, with time-symmetrical ringing on either side of the single sample at 0dBFS. With 44.1kHz-sampled white noise (fig.14, red and magenta traces), the 866's response rolled off sharply above 20kHz, reaching full stop-band suppression at 24kHz, just above half the sample rate (vertical green line). The aliased image at 25kHz of a full-scale tone at 19.1kHz (blue and cyan traces) is almost completely suppressed, and the distortion harmonics of the 19.1kHz tone are very low in level.
The digital-input frequency response was flat in the audioband and follows the same basic shape, with then a sharp rolloff just below half of each sample rate (fig.15). The levels of the two channels were perfectly matched. When I increased the bit depth from 16 to 24 with a dithered 1kHz tone at –90dBFS (fig.16), the noisefloor dropped by 21dB, which implies that the EVO 150 offers close to 20 bits' worth of resolution. With undithered data representing a tone at exactly –90.31dBFS (fig.17), the three DC voltage levels described by the data were well resolved. With undithered 24-bit data (fig.18), the result was a relatively clean sinewave.
Fig.1 Cambridge EVO 150, volume control set to maximum, frequency response at 2.83V into: simulated loudspeaker load (gray), 8 ohms (left channel blue, right red) 4 ohms (left cyan, right magenta), (left green) (1dB/vertical div.).
Fig.2 Cambridge EVO 150, small-signal 10kHz squarewave into 8 ohms.
The bass and treble controls offered a maximum boost and cut of ±10dB (fig.3), but, peculiarly, the response with the tone controls active rolled off sharply above 20kHz, which suggests they operate in the digital domain. With the tone controls bypassed, the frequency response via the preamplifier and headphone outputs was flat to 200kHz. The subwoofer output rolled off above 500Hz, reaching –1dB at 1.5kHz and –3dB at 2.2kHz (not shown).
Fig.3 Cambridge EVO 150, frequency response at 2.83V into 8 ohms with treble and bass controls set to their maximum and minimum and switched out of circuit (left channel blue, right red, 2.5dB/vertical div.).
Channel separation was excellent, at >100dB in both directions below 1kHz, and still 80dB at the top of the audioband. Without the auxiliary low-pass filter, 319mV of ultrasonic noise was present at the loudspeaker outputs. With the filter, the Cambridge's unweighted, wideband signal/noise ratio, taken with the unbalanced line inputs shorted to ground but the volume control set to its maximum, was good at 73.2dB ref. 2.83V into 8 ohms (average of both channels). This ratio improved to 86dB when the measurement bandwidth was restricted to the audioband, and to 88.4dB when A-weighted. The background noise included spuriae at 60Hz and its odd-order harmonics (fig.4). The blue and red traces in this graph were taken with the volume control set to "100." Reducing the volume to "80" lowered the levels of the spuriae in the left channel (green trace) but not the right (gray trace). However, even at the maximum setting the noise is relatively low in level.
Fig.4 Cambridge EVO 150, spectrum of 1kHz sinewave, DC–1kHz, at 1W into 8 ohms, volume control set to maximum (left channel blue, right red) and to –15dB (left green, right gray; linear frequency scale).
With both channels driven, the EVO 150 met its specified maximum power into 8 ohms of 150Wpc (21.76dBW) at 1% THD+noise (fig.5). Into 4 ohms, the Cambridge clipped at 280Wpc (21.46dBW, fig.6). I didn't test clipping power into 2 ohms, as the amplifier isn't specified into that load. The distortion is very low at low powers, so I examined how the THD+N varied with frequency at 20V, which is equivalent to 50W into 8 ohms and 100W into 4 ohms. The results are shown in fig.7. Below 2kHz, the distortion is the same into both impedances, at around 0.005%. It then rises in the top two octaves, slightly more into 8 ohms (blue and red traces) than into 4 ohms (green and gray traces).
Fig.5 Cambridge EVO 150, distortion (%) vs 1kHz continuous output power into 8 ohms.
Fig.6 Cambridge EVO 150, distortion (%) vs 1kHz continuous output power into 4 ohms.
Fig.7 Cambridge EVO 150, THD+N (%) vs frequency at 20V into: 8 ohms (left channel blue, right red), 4 ohms (left green, right gray).
Fig.8 Cambridge EVO 150, 1kHz waveform at 50W into 8 ohms, 0.0046% THD+N (top); distortion and noise waveform with fundamental notched out (bottom, not to scale).
Fig.9 Cambridge EVO 150, spectrum of 50Hz sinewave, DC–1kHz, at 50W into 8 ohms (left channel blue, right red; linear frequency scale).
Fig.10 Cambridge EVO 150, HF intermodulation spectrum, DC–30kHz, 19+20kHz at 50W peak into 8 ohms (left channel blue, right red; linear frequency scale).
I usually measure an integrated amplifier's phono input with the speaker outputs turned off (if possible) or from the headphone output, which mutes the loudspeaker outputs, to avoid clipping the power amplifier's output with high-level signals. However, because the EVO 150's headphone output had a fairly high noisefloor, I performed the phono input measurements at the preamplifier outputs with the volume control set to "80." The Cambridge's phono input had an input impedance of 44.2k ohms at 20Hz and 1kHz, this appropriate for a MM cartridge, declining to 30k ohms at 20kHz. The voltage gain at 1kHz was 42.8dB at the loudspeaker outputs, 17.1dB at the preamplifier output, and a very high 46.6dB at the headphone output. Every output preserved absolute polarity.
The RIAA correction (fig.11) had slight boosts in the midrange and high treble—alternatively, it had a slight lack of energy in the presence region—and the response started to roll off in the bass, reaching –3dB at 20Hz. Channel separation via this input was good, at 70dB in both directions across the audioband. The phono input's unweighted, wideband S/N ratio, measured with the input shorted to ground, was a very good 77.8dB (average of both channels) referred to an input signal of 1kHz at 5mV. Restricting the measurement bandwidth to 22Hz–22kHz increased the ratio to 85.4dB, while switching an A-weighting filter into circuit increased the ratio to 89.2dB. This is a quiet phono stage!
Fig.11 Cambridge EVO 150, phono input, response with RIAA correction into 100k ohms (left channel blue, right red), with subsonic filter (left cyan, right magenta.) (0.5dB/vertical div.).
The Cambridge phono input overload margin was a very good 22dB at 20Hz and 1kHz, dropping slightly to 18dB at 20kHz. The phono stage's distortion was very low, with the only harmonic visible above the noisefloor the second, at just –100dB (0.001%, fig.12). Intermodulation distortion with an equal mix of 19kHz and 20kHz tones, at a peak input level equivalent to 1kHz at 10mV, was low in level, with the second-order difference product at 1kHz the highest in level, at –72dB (not shown).
Fig.12 Cambridge EVO 150, phono input, spectrum of 1kHz sinewave, DC–10kHz, into 100k ohms for 10mV input (left channel blue, right red; linear frequency scale).
Fig.13 Cambridge EVO 150, digital inputs, impulse response (one sample at 0dBFS, 44.1kHz sampling, 4ms time window).
Fig.14 Cambridge EVO 150, digital inputs, wideband spectrum of white noise at –4dBFS (left channel red, right magenta) and 19.1kHz tone at 0dBFS (left blue, right cyan), with data sampled at 44.1kHz (20dB/vertical div.).
Fig.15 Cambridge EVO 150, digital inputs, frequency response at –12dBFS into 100k ohms with data sampled at: 44.1kHz (left channel green, right gray), 96kHz (left channel cyan, right magenta), 192kHz (left blue, right red) (1dB/vertical div.).
Fig.16 Cambridge EVO 150, digital inputs, spectrum with noise and spuriae of dithered 1kHz tone at –90dBFS with: 16-bit data (left channel cyan, right magenta), 24-bit data (left blue, right red) (20dB/vertical div.).
Fig.17 Cambridge EVO 150, digital inputs, waveform of undithered 1kHz sinewave at –90.31dBFS, 24-bit data (left channel blue, right red).
Fig.18 Cambridge EVO 150, digital inputs, waveform of undithered 1kHz sinewave at –90.31dBFS, 16-bit data (left channel blue, right red).
Intermodulation distortion via the Cambridge amplifier's digital inputs (fig.19) was as low as it had been with analog input signals. I tested the EVO 150 for its rejection of word-clock jitter via its TosLink, USB, and network inputs. All the odd-order harmonics of the 16-bit J-Test signal's LSB-level, low-frequency squarewave were at the correct levels with all inputs (fig.20, sloping green line), and the spectral spike that represents the high-level tone at exactly one-quarter the sample rate was clean.
Fig.19 Cambridge EVO 150, digital inputs, HF intermodulation spectrum, DC–30kHz, 19+20kHz at 0dBFS peak (left channel blue, right red; linear frequency scale).
Fig.20 Cambridge EVO 150, digital inputs, high-resolution jitter spectrum of analog output signal, 11.025kHz at –6dBFS, sampled at 44.1kHz with LSB toggled at 229Hz: 16-bit data (left channel blue, right red). Center frequency of trace, 11.025kHz; frequency range, ±3.5kHz.
With the exception of the higher-than-expected levels of noise in its headphone output, the Cambridge EVO 150's measured performance reveals excellent audio engineering. This is especially notable given the close proximity of the low-level circuits to the class-D amplifier modules, with their high levels of high-frequency switching noise.—John Atkinson
