Tube Preamp Reviews

I measured the Rogue RP-7 using my Audio Precision SYS2722 system (see the January 2008 "As We See It"). The RP-7's volume control operates in 0.5dB steps, with the maximum level indicated as "192" on the front-panel display. All of its outputs—balanced, single-ended, and headphone—preserved absolute polarity (ie, were non-inverting). The balanced input impedance measured a very high 174k ohms at 20Hz and 1kHz, dropping very slightly to 162k ohms at 20kHz; the unbalanced input impedance was lower, at 24.5k ohms at 20Hz and 1kHz, and 21k ohms at 20kHz. The RP-7's maximum voltage gain into 100k ohms was 15.33dB for both balanced input to balanced output and unbalanced input to unbalanced output. Measured at the headphone output, the maximum gain was a very high 30.2dB into 100k ohms, which is probably why HR noted some noise issues with headphones.

The Rogue's output impedance is specified as 50 ohms. However, I found that this varied with both output and frequency. Measured at the headphone output, the impedance was 1.6 ohms at 20Hz, 0.8 ohm at 1kHz, and 1.2 ohms at 20kHz. The unbalanced output measured a very low 0.8 ohm at 20kHz and 3.5 ohms at 1kHz, but rose to 2.44k ohms at 20Hz, this presumably due to a DC-blocking capacitor in series with the output. The respective balanced line-output impedances were 1.4 ohms, 12.4 ohms, and 5.64k ohms. As a result of the increased output impedance at low frequencies, the RP-7's response drastically rolled off below 400Hz into 600 ohms, reaching –3dB at 200Hz (fig.1, cyan and magenta traces). The RP-7 should not be used with power amplifiers having an input impedance much below 30k ohms. Into 100k ohms, however, the RP-7 offers a flat, wide-bandwidth response with an upper –3dB frequency of 170kHz (fig.1, blue and red traces). Fig.1 was taken with balanced operation and the volume control set to "192." Commendably, the response didn't change with the setting of the volume control or with unbalanced operation. The frequency response at the headphone output was also flat and extended, though the –3dB point dropped to a still-high 120kHz.

Channel separation was similar for balanced and unbalanced operation, and somewhat asymmetrical, at 80dB L–R at 2kHz and 97dB R–L, these respectively decreasing to 60 and 78dB at the top of the audioband. The unweighted, wideband signal/noise ratio, measured at the line outputs with the inputs shorted to ground but the volume control set to its maximum, was only moderate, at 68.3dB ref. 1V output. This improved to 76.1dB, when the measurement bandwidth was restricted to 22Hz–22kHz, and to 80dB with an A-weighting filter switched into circuit. Fig.2 shows spectral analyses of the Rogue's low-frequency noise floor while its balanced output drove a 1kHz tone at 1V into 100k ohms. The random noise-floor components lie at –100dB, with the primary noise contribution from power-supply–related spuriae.

Rogue specifies the RP-7's maximum output voltage as 18V balanced and 9V unbalanced. Figs. 3 and 4 respectively show how the percentages of THD+noise in the balanced and unbalanced outputs vary with voltage. With clipping defined as when the THD+N reaches 1%, the RP-7 clips at 24V balanced and 12V unbalanced. These very high clipping voltages were maintained into the punishing 600 ohm load. The fact that the traces in these two graphs slope downward from left to right below 1–2V means that the distortion actually lies below the noise floor at these lower output levels. When I measured the percentage of THD+N against frequency at 1V into 100k ohms, it didn't change with frequency (not shown).

As with Rogue's RH-5 preamplifier and headphone amplifier, which Herb Reichert reviewed in "November 2017, the distortion is predominantly the subjectively benign second harmonic at a low –76dB (0.015%, fig.5). This graph was taken from the single-ended output; with the balanced output, the second harmonic lay at –80dB (0.01%). Fig.5 was taken with a low-frequency fundamental; when I repeated the test with a 1kHz signal, while the second harmonic was still the highest in level, there were now some higher-frequency enharmonic tones present (fig.6). Fig.6 was taken with the volume control set to "192." Reducing the setting to –20dB ("152") and increasing the input level by 20dB dropped the levels of these spurious tones by the same 20dB (fig.7), which suggests that the circuitry is picking up these tones ahead of the volume control. It turned out that this problem was fixed with v2.2 of the RP-7's firmware. Repeating the spectral analysis with the new firmware eliminated the spurious tones (fig.8).

Keeping the volume control at "152," I looked at the spectrum of the RP-7's single-ended output while it reproduced an equal mix of 19 and 2kHz tones at 1V into 100k ohms, again with the new firmware (fig.9). The high-order intermodulation products are very low in level and the second-order difference product at 1kHz lies at –70dB (0.03%). The picture was similar from the balanced output, though the difference product dropped to –66dB (0.05%).

As long as it's driving a power amplifier with a high input impedance, Rogue's RP-7 offers generally respectable measured performance. Although HR auditioned the RP-7 with the earlier v2.1 firmware, he used sufficiently low volume control settings that the spurious high-frequency tones would not have had audible consequences.—John Atkinson