Tube Preamp Reviews

I measured the Rogue RH-5 using my Audio Precision SYS2722 system (see the January 2008 "As We See It"). The procedure was a little more complicated than I had anticipated, due to the fact that the RH-5 has three gain options (indicated on the display, in order of increasing gain, as "1," "2," and "3"), balanced and unbalanced line inputs and outputs, balanced and unbalanced headphone outputs, and a phono stage. I tried, therefore, to characterize the amplifier's behavior in all operational modes.

The RH-5's line inputs could handle high levels without overloading and the volume control operates in 0.5dB steps, and all of its outputs preserved absolute polarity (ie, were non-inverting). The balanced input impedance measured a very high 160k ohms from 20Hz to 20kHz; the unbalanced input impedance was also high, at 106k ohms at 20Hz and 1kHz, dropping inconsequentially to 82k ohms at 20kHz. Via its line inputs, the RH-5's voltage gain into 100k ohms, measured at the line outputs and with the volume control set to its maximum of "185," was 1.1, 4.9, and 9.2dB for gain options 1, 2, and 3 for both balanced input to balanced output and unbalanced input to unbalanced output. Measured at the headphone outputs into 100k ohms, the maximum gains were 11, 14.8, and 19.1dB for the three gain settings at both the balanced and unbalanced jacks.

The output impedance varied with both output and frequency. The unbalanced line output measured 0.5 ohm at 20kHz, 9 ohms at 1kHz, and 3.3k ohms at 20Hz, the last presumably due to a DC-blocking capacitor in series with the output. The balanced line-output impedances were twice these values, as expected. The impedances measured at the headphone outputs were much lower. The conventional unbalanced impedance was very low, at around 1 ohms at 20 and 1kHz, rising slightly to 1.6 ohms at the bottom of the audioband. The balanced impedance was again around 1 ohm over most of the audioband, rising to 3.2 ohms at 20Hz.

The top blue and red traces in fig.1 show the RH-5's frequency response measured at the balanced line output with the gain set to "3" and the volume control to "185." The two sets of traces (cyan and blue, green and gray) respectively show the frequency responses with the gain set to "2" and "1"; they're identical, revealing that the response didn't change with the gain setting. It also didn't change with the setting of the volume control, and the headphone output responses were identical to the top blue and red traces in this graph. The blue and red traces that roll off below 1kHz in fig.1 reveal that the line outputs are not appropriate for driving low impedances. When used as a preamplifier, the Rogue should be used with power amplifiers having an input impedance of at least 20k ohms if the tonal balance is not to sound lean.

Channel separation was the same via both line and headphone outputs, at 80dB R–L and 97dB L–R at 1kHz, decreasing to 53 and 70dB 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 76dB ref. 1V output, improving to 83dB when the measurement bandwidth was restricted to 22Hz–22kHz, and to 87dB with an A-weighting filter switched into circuit. Fig.2 shows spectral analyses of the Rogue's low-frequency noise floor while it drove a 1kHz tone into 100k ohms in the three gain conditions. You can see that, compared with "1" (green and gray traces), each increase in gain raises the noise floor by the same amount of gain. The primary noise contribution is from power-supply–related spuriae, though even the highest-level of these, at 120Hz, is still close to 90dB down from the peak signal level.

The Rogue RH-5 can swing a lot of volts before it clips, even from its unbalanced line output (fig.3), where the THD+noise reaches 1% at just above 6V. As expected, the balanced line output offers twice this voltage at clipping (fig.4), and when the RH-5 is used as a preamplifier, both balanced and unbalanced operation offer low levels of harmonic distortion. The headphone outputs also offer very high maximum output voltages, as shown in fig.5 (balanced) and fig.6 (unbalanced), though the minimum distortion levels are a little higher into low impedances. This can be seen in fig.7, which plots the THD+N percentage against frequency from the unbalanced headphone output at 2V into 100k, 300, and 30 ohms. Into the lower impedances, the distortion rises in the treble.

Fortunately, the distortion is predominantly the subjectively benign second harmonic (fig.8), and though this harmonic is higher in level from the unbalanced outputs (fig.9), it is still low in absolute terms. Despite the rise in harmonic distortion at high frequencies into low impedances, intermodulation distortion with an equal mix of 19 and 20kHz tones was relatively low in level (fig.10).

Ass used by Herb, the RH-5's phono input offered an input impedance of 230 ohms, which is appropriate for an MC cartridge, and preserved absolute polarity. The gain, measured at the unbalanced output jacks, depended on the gain setting: it measured 42.8, 46.6, or 57.45dB. The RIAA error is shown in fig.11; the channels don't match as well as I would have liked, the left channel (blue trace) offering more of a slight boost in the midrange than the right (red). Both channels roll off below the midbass region, reaching –3dB at 11Hz. Phono input separation was good, at >75dB in both directions at 1kHz, while the unweighted, wideband S/N ratio, ref. 1kHz at 500µV and measured with the input shorted to ground, was 67.2dB left and 61.3dB right. These ratios improved to 77.3 and 71.3dB when A-weighted, which is low noise for an MC-compatible phono stage.

The phono-stage overload margin was not affected by the RH-5's gain setting and was excellent, at 19dB from 20Hz to 20kHz. Harmonic distortion via the phono input was, again, predominantly second harmonic (fig.12), and intermodulation was low, with the 1kHz difference product resulting from an equal mix of 19 and 20kHz tones having a peak level of 26mV (fig.13).

Overall, its measured performance indicates that Rogue's RH-5 is a well-balanced design.—John Atkinson