Rogue Audio 66LSR preamplifier Measurements part 2

The Rogue preamplifier gave the lowest THD+noise readings when its input and output floated above ground, which suggests something peculiar in its internal grounding. Much to my surprise, the 66LSR gave lower distortion into 10k ohms than into 100k ohms. This can be seen in fig.4, which plots the THD+N percentage against the output voltage into the two loads. The lower trace was taken into 10k ohms, and reveals a sensible gain architecture: the preamp offers the lowest distortion, 0.07%, at around 2V output, which is near the level at which the partnering amplifier will clip. Below that level, the measured THD+N is dominated by noise. True distortion increases above that level, but this will be academic. True clipping occurs around 13V output, considerably lower than specified. Into the higher load, the distortion starts to dominate above 1V output, and rises consistently with output voltage.

Fig.4 Rogue 66LSR, distortion (%) vs output voltage at 1kHz into 100k ohms (top) and 10k ohms (bottom).

The distortion level was constant with frequency and consisted primarily of the sonically benign second harmonic (fig.5). This graph was taken at 1V into 10k ohms. At the same level into 100k ohms, the second harmonic rose to 0.1% (-60dB)—still sonically benign, but curious, to say the least. The intermodulation spectrum (fig.6) was dominated by the 1kHz difference product, though "dominated" is hardly the word for something at -63dB (0.07%).

Fig.5 Rogue 66LSR, spectrum of 50Hz sinewave, DC-1kHz, at 1V into 10k ohms (linear frequency scale).

Fig.6 Rogue 66LSR, HF intermodulation spectrum, DC-22kHz, 19+20kHz at 2V into 10k ohms (linear frequency scale).

Other than its poor channel balance, relatively high noise floor, grounding anomalies, and the peculiar fact that it was happier driving 10k ohms than 100k ohms, this little preamp offers acceptable measured performance.—John Atkinson

Rogue Audio
2827 Avery Road
Slatington, PA 18080
(570) 992-9901