The line and phono THD+noise vs frequency results are shown in fig.4. Both are good results, and also reveal good in-band noise performance. The input voltages used for both the crosstalk and THD+noise figures were 2.45V (line) and 1mV (phono). These are the values that resulted in the lowest THD figures shown in fig.5—the THD+noise vs output results—and therefore minimized the effect of noise on the readings.

Fig.4 Nagra PL-P, THD+noise vs frequency at 2.45V input (line, bottom), and 1mV input (MC phono, top) (right channel dashed).

Fig.5 Nagra PL-P, distortion (%) vs output voltage into 100k ohms of MC phono (top) and line stage (bottom).

The spectra of the PL-P's line stage output at levels of 5V and 10V at 50Hz are shown in figs.6 and 7, respectively. The latter response, just prior to clipping, is an excellent result, with the third harmonic (the largest artifact) at -72dB (0.025%). At a more realistic 5V output, the largest artifact is the second harmonic, well down at almost -90dB (0.003%).

Fig.6 Nagra PL-P, spectrum of 50Hz sinewave, DC-1kHz, at 5V into 100k ohms (linear frequency scale). Note that the second harmonic is the highest in level at -90dB (0.003%).

Fig.7 Nagra PL-P, spectrum of 50Hz sinewave, DC-1kHz, at 10V into 100k ohms (linear frequency scale). Note that the third harmonic is the highest in level at -72dB (0.025%).

Altogether, this is a good if not exceptional set of measurements for a tube preamplifier, though the distortion performance is excellent.—Thomas J. Norton