Nagra PL-P preamplifier Measurements
Except as noted, these measurements were taken with the Nagra PL-P's input-level controls at 0dB, and the output level at maximum. Line gain in this configuration measured 6.2dB. The line input impedance measured 72.6k ohms in the left channel, 68.2k ohms in the right. The output impedance measured 37 ohms. Neither input nor output impedance was significantly affected by changes in the output-level setting. The source impedance of the tape output was 5 ohms regardless of the source component's output impedance, indicating buffered tape outputs (not surprising from this manufacturer!).
The line-stage S/N ratio measured 82dB from 22Hz to 22kHz, 58dB, 10Hz-500kHz, and 84.3dB A-weighted (all ref. 1V). The middle figure is a little troubling, but should have no audible consequences—the noise was not hum-related, but appeared instead to be at ultrasonic frequencies. DC offset measured 4.4mV in the left channel, 3.4mV in the right.
Measurements of the phono stage were taken primarily in the transformer-coupled moving-coil mode; unless otherwise noted, the results here reflect MC operation:
MC phono gain is 58.7dB. The phono input impedance, via the transformers with no additional loading applied via the internal jumpers, was 5.1k ohms in the left channel, 4.9k ohms in the right. This tenfold difference from the specified value was confirmed by measuring the input impedance with the 1k ohm jumpers installed. In this condition the input impedance measured 860 ohms—the value you would expect with 5.1k ohms in parallel with 1k ohm, with the effect of the Audio Precision System One's 25 ohm source impedance taken into account.
I checked the phono input impedance in the moving-magnet configuration and obtained a value of 44.8k ohms (left channel)—close to the specified 47k ohms. Unless you're dedicated to an input impedance of 47k ohms for a moving-coil cartridge (far higher than is usually specified by MC manufacturers, but favored by some audiophiles), the minimum available input impedance of around 5k ohms should not be a problem.
Phono signal/noise (ref. 1V) measured 69.2dB from 22Hz to 22kHz, 62dB from 10Hz to 500kHz, and 82.2dB A-weighted. MC phono overload (1% THD+noise) was 7.4mV at 1kHz (23.4dB margin relative to 500;uV), 56.5mV at 20kHz (21.1dB), and 0.4mV at 20Hz (18.1dB)—satisfactory figures without being exceptional.
The PL-P's line-stage frequency response is shown in fig.1. The PL-P's output is 1dB down at 30Hz. Note that when the input-level controls are adjusted for matched output at the maximum setting of the output-level control (upper traces), the output levels deviate by 1dB if the output level is reduced to -20dB—by modern standards, rather poor tracking of the output-level control. Fig.2 shows the frequency response of the phono stage measured at the tape outputs. The bump-up in the upper bass—this can also be seen in the manufacturer's supplied graph—might well produce a rather warm sound. And even without the LF filter, there is a noticeable drop-off in the low-bass response.
Fig.1 Nagra PL-P, line-stage frequency response into 100k ohms at 1V output (bottom) and 100mV (top) (right channel dashed, 0.5dB/vertical div.).
Fig.2 Nagra PL-P, phono stage frequency response into 100k ohms with (bottom) and without (top) LF high-pass filter (right channel dashed, 1dB/vertical div.).
The PL-P's crosstalk performance is shown in fig.3. The difference between the left and right phono channel separation is a minor concern, but should not create any audible problems. The line-stage separation is only fair but it does basically meet the manufacturer's specification. The decrease in separation with frequency is due to capacitive coupling between channels.
Fig.3 Nagra PL-P, crosstalk (from bottom to top at 8kHz): L-R, R-L, MC phono; L-R, R-L, line (10dB/vertical div.).