Sidebar 3: Measurements
I performed the measurements of the Rogue Audio Pharaoh II with my Audio Precision SYS2722 system. As the Rogue's output stages operate in class-D, all the measurements other than frequency and RIAA responses were taken with Audio Precision's auxiliary AUX-0025 passive low-pass filter, which reduces the level of noise above 80kHz and eliminates noise above 200kHz. Without the filter, 376mV of ultrasonic noise was present at the loudspeaker terminals, with a center frequency of 428kHz.
Looking first at the line-level inputs, the Rogue Pharaoh II preserved absolute polarity, ie, was noninverting from its loudspeaker output with both balanced and single-ended inputs but inverted polarity from the headphone output. With single-ended input signals, the maximum voltage gain at 1kHz was 37dB from the loudspeaker output into 8 ohms and 21.4dB from the headphone output. The gains with the balanced input were 6dB lower rather than the usual 6dB higher. The balanced input impedance was a very high 380k ohms at 20Hz and 1kHz and still very high at 310k ohms at 20kHz. The single-ended input impedance was still usefully high, at 28.5k ohms at 20Hz and 1kHz, 23.5k ohms at 20kHz.
The headphone output's source impedance was a low 1 ohm across the audioband. The loudspeaker output impedance was very low, at 0.05 ohms at low and middle frequencies, increasing very slightly to 0.075 ohms at the top of the audioband. The variation in the Rogue amplifier's small-signal frequency response with our standard simulated loudspeaker was therefore less than ±0.1dB (fig.1, gray trace). Into resistive loads (fig.1, blue, red, cyan, magenta, and green traces), the Pharaoh II's frequency response was down by 1dB at 25kHz. Fig.1 was taken with the volume control set to its maximum and the balance control centered. The right channel was 0.2dB higher in level than the left; this slight imbalance was the same at lower settings of the volume control. The Pharaoh II's reproduction of a 10kHz squarewave (fig.2) showed only the slightest hint of overshoot, with no ringing.
Fig.1 Rogue Audio Pharaoh II, frequency response at 2.83V into: simulated loudspeaker load (gray), 8 ohms (left channel blue, right red), 4 ohms (left cyan, right magenta), and 2 ohms (green) (1dB/vertical div.).
Fig.2 Rogue Audio Pharaoh II, small-signal 10kHz squarewave into 8 ohms.
The Pharaoh II's channel separation (not shown) was >70dB in both directions below 1kHz but decreased to 36dB, R–L, and 40dB, L–R, at 20kHz. Taken with the AUX-0025 filter installed, the unbalanced inputs shorted to ground, and the volume control set to its maximum, the unweighted, wideband signal/noise ratio (ref. 1W into 8 ohms) was 64.2dB (average of the two channels). This ratio only improved by 1dB when the measurement bandwidth was restricted to 22Hz–22kHz and by another 2dB when A-weighted.
Spectral analysis of the low-frequency noisefloor while the Rogue amplifier drove a 1kHz tone at 1Wpc into 8 ohms, with the volume control set to the maximum (fig.3, blue and red traces), revealed a relatively high level of AC-related spuriae at 60Hz and its odd-order harmonics, which will be due to magnetic interference from the large toroidal power transformer, perhaps entering via the steel pins of the 12AU7 input tubes. Setting the volume control to –12dB (fig.3, green and gray traces) reduced the levels of these spuriae by approximately the same 12dB, which means that the noise pickup occurs before the volume control. Repeating the spectral analysis with the balanced input (not shown) reduced the levels of these supply-related spuriae by the same 6dB as the reduction in gain with this input.
Fig.3 Rogue Audio Pharaoh II, spectrum of 1kHz sinewave, DC–1kHz, at 1Wpc into 8 ohms with volume control set to its maximum (left channel blue, right red) and to –12dB (left green, right gray) (linear frequency scale).
Rogue specifies the Pharaoh II's maximum power as 250Wpc into 8 ohms (24dBW) and 400Wpc into 4 ohms (23dBW), both powers equivalent to 24dBW. With our usual definition of clipping—when the THD+noise reaches 1%—I measured clipping powers of 240Wpc into 8 ohms (23.8dBW, fig.4), and 380Wpc into 4 ohms (22.8dBW, fig.5) with both channels driven. (I don't hold the AC wall voltage constant in these tests; it had dropped from 119.1V with the Pharaoh II idling to 117.4V with the amplifier clipping into 8 ohms and to 116.8V with it clipping into 4 ohms.) The shape of the traces in these graphs suggests that the measured THD+N is dominated by noise below 2W and that the amplifier uses a limited amount of loop negative feedback, presumably, given the high linearity of the NCore output stage, in the tubed stage.
Fig.4 Rogue Audio Pharaoh II, distortion (%) vs 1kHz continuous output power into 8 ohms.
Fig.5 Rogue Audio Pharaoh II, distortion (%) vs 1kHz continuous output power into 4 ohms.
Fig.6 shows how the percentage of THD+N in both channels varied with frequency into 8 and 4 ohms at 20V, equivalent to 50W into 8 ohms (blue and red traces) and 100W into 4 ohms (green and gray traces). The THD+N was close to 0.2% (–34dB) across the audioband into both load impedances. This level of distortion might be audible if it was comprised of high-order harmonics like the 5th and 7th. Fortunately, the distortion waveform was predominantly the subjectively innocuous second harmonic (figs.7 and 8). Intermodulation distortion, even at high powers, was relatively low, with the 1kHz difference product produced by the Pharaoh II driving an equal mix of 19 and 20kHz tones at 100Wpc peak into 4 ohms lying at –60dB (0.1%, fig.9). The higher-order products at 18 and 21kHz were much lower in level.
Fig.6 Rogue Audio Pharaoh II, THD+N (%) vs frequency at 20V into 8 ohms (left channel blue, right red), 4 ohms (left green, right gray).
Fig.7 Rogue Audio Pharaoh II, left channel, 1kHz waveform at 50W into 8 ohms, 0.19% THD+N (top); distortion and noise waveform with fundamental notched out (bottom, not to scale).
Fig.8 Rogue Audio Pharaoh II, spectrum of 50Hz sinewave, DC–1kHz, at 50Wpc into 8 ohms (left channel blue, right red; linear frequency scale).
Fig.9 Rogue Audio Pharaoh II, HF intermodulation spectrum, DC–30kHz, 19+20kHz at 100Wpc peak into 4 ohms (left channel blue, right red; linear frequency scale).
To examine the behavior of the Rogue's phono input, I connected a wire from the Audio Precision's ground terminal to the grounding lug on the Rogue's rear panel to obtain the lowest noise. The phono input inverted absolute polarity at the headphone output in both MM and MC modes but preserved polarity at the loudspeaker outputs. The maximum gain at 1kHz at the speaker outputs was 76.8dB (MM) and 97.5dB (MC). At the headphone output, the maximum gain was 61.2dB (MM) and 81.9dB (MC). I performed all the subsequent testing using the headphone output, and, other than S/N ratio, with the volume control set to –20dB in order to avoid clipping.
With the input impedance set to 47k ohms, I measured 47k ohms from 20Hz to 20kHz. With the impedance set to 1000 ohms, I measured 986 ohms across the audioband; set to 300 ohms, the impedance was 302 ohms; and set to 100 ohms, it was 101 ohms. The phono input's audioband RIAA error was very low in the right channel (fig.10, red trace), though a little higher above 2kHz in the left channel (blue trace). The low frequencies rolled off below 80Hz, reaching –3dB at 20Hz.
Fig.10 Rogue Audio Pharaoh II, phono input, response with RIAA correction (left channel blue, right red) (1dB/vertical div.).
With the MM phono input, the wideband, unweighted S/N ratio with the inputs shorted to ground and the volume control set to the maximum was a very good 74.1dB in both channels, ref. 1kHz at 5mV. Restricting the measurement bandwidth to the audioband increased the ratio to 75dB, while an A-weighting filter further increased the ratio to 80dB. The S/N ratios in MC mode were 20dB lower, but this is still a relatively quiet phono stage.
Measured at the headphone output, the phono input's overload margins were relatively high at all audio frequencies: >16dB in both MM and MC modes, ref. 1kHz at 5mV for MM and ref. 1kHz at 500ÊV for MC. The phono distortion signature was pure second harmonic, lying at –60dB (0.1%) with the nominal MM input level of 1kHz at 5mV (fig.11). The level of the 1kHz difference product with an equal mix of 19 and 20kHz tones (fig.12) was similar to that with the line inputs, though high-order intermodulation products were much lower in level.
Fig.11 Rogue Audio Pharaoh II, MM phono input, spectrum of 1kHz sinewave, DC–10kHz, for 5mV input, measured at headphone output (left channel blue, right red, linear frequency scale).
Fig.12 Rogue Audio Pharaoh II, MM phono input, HF intermodulation spectrum, DC–30kHz, 19+20kHz for 100mV input, measured at headphone output (left channel blue, right red; linear frequency scale).
The Rogue Audio Pharaoh II's measured performance indicates that it resembles an intriguing marriage between a low-distortion, low-noise, high-power class-D output stage and a higher-distortion, higher-noise, tubed input stage. At the time of writing I had not seen HR's review; I'll be interested to read his take on the Pharaoh II's sound character.—John Atkinson
