NAD PP 2e Phono Preamplifier Measurements

Links: reviewed by Doug Schneider on SoundStage! Hi-Fi on January 1, 2021 and by James Hale on SoundStage! Xperience on January 1, 2021

General information

All measurements taken using an Audio Precision APx555 B Series analyzer.

The NAD PP 2e was conditioned for 30 minutes at 1Vrms at the output before any measurements were taken.

On the PP 2e’s back panel are one switch, to switch between moving-magnet (MC) and moving-coil (MC) operation, and two sets of single-ended RCA inputs, with one set labeled MC and the other set MM. The rear panel also has a pair of single-ended RCA outputs. For the MM input, a 15mVrms 1kHz sinewave was required to achieve the reference output voltage of 1Vrms, while a 1.2mVrms 1kHz sinewave was required at the MC input.

Published specifications vs. our primary measurements

The tables below summarize our primary measurements performed on the PP 2e. Here we can compare directly against NAD’s own published specifications for the PP 2e, which are stated as follows:

• Input impedance: MM, 47k ohms; MC, 100 ohms
• Gain at 1kHz: MM, 35dB; MC, 60dB
• Input sensitivity (ref. 200mV output): MM, 2.5mV; MC, 0.3mV
• Signal-to-noise ratio (A-weighted): MM, 80dB; MC, 78dB
• Input overload (20Hz/1kHz/20kHz): MM, 10/102/950mV; MC, 0.8/9/84 mV
• Rated distortion (THD 20Hz to 20kHz): MM, <0.03%; MC, <0.03%
• RIAA response accuracy: MM, +/-0.3dB; MC, +/-0.3dB
• Output impedance: 100 ohms
• Maximum output level: 5.3V

Our primary measurements below revealed the following using the unbalanced MM input (unless specified, assume a 1kHz sinewave, 1Vrms output into 100k ohms load, 10Hz to 90kHz bandwidth):

Our primary measurements below revealed the following using the unbalanced MC input (unless specified, assume a 1kHz sinewave, 1Vrms output into 100k ohms load, 10Hz to 90kHz bandwidth):

Our measured input impedances for both the MM (47.65/47.81k ohms) and MC inputs (117.6/118.6 ohms) match very closely NAD’s specs of 47k ohms and 100 ohms.

Our measured gain values for both inputs (MM: 37dB, MC: 59dB) also corroborate the published specifications of 35dB and 60dB. Input sensitivity is another way to express gain (in volts per volts). Interestingly, NAD’s input sensitivity specs when converted to dB are 38dB for the MM input and 56.5dB for the MC input, which are slightly different than their specs for gain. Nevertheless, all published gain values are very close to our measured values.

NAD’s signal-to-noise ratio (SNR) specs of 80dB and 78dB (MM and MC, A-weighted) are difficult to compare to our measured values, because we do not know what NAD used as a reference output voltage. Nevertheless, our measured values of 93dB and 84dB (MM and MC, A weighted, with a 1Vrms reference) exceed those published by NAD.

NAD’s maximum output voltage value of 5.3mVrms was corroborated by our measurements, where we saw 1% THD+N at 5.32Vrms at the output (1kHz).

Our input overload values are expressed in dB, as overload margin, with a reference 5mVrms and 0.5mVrms input signal for the MM and MC inputs respectively. We measured the input signals required at 20Hz, 1kHz, and 20kHz to achieve 5.32Vrms at the output for both input types. We expressed those values in dB as a ratio over the reference values (5 and 0.5Vrms), but we also show the actual input voltages in parentheses so as to directly compare against the NAD values. We measured lower input overload values; with some rounding, our values compare to NADs as follows: 8/75/711 vs. 10/102/950mVrms for the MM input, and 0.7/6/57 vs. 0.9/8/84Vrms for the MC input.

NAD’s published rated distortion values are <0.03% for both input types; unfortunately, we only know these were measured with a 20Hz to 20kHz filter. We don’t know if NAD measured THD or THD+N, and what they used as a reference output voltage. Our measured THD+N values of about 0.003% and 0.006% (MM and MC) show better performance than the published values, but these are A-weighted. We also measured THD+N ratios with a 20Hz to 20kHz bandwidth (instead of A-weighting), using the fourth-order low-/high-pass Butterworth filters in the APx555, and measured less than 0.008% for the MM input, and less than 0.03% for the MC input, corroborating NAD’s claim.

In terms of our measured output impedance, there is a significant discrepancy, where we measured about 522 ohms vs. NAD’s published 100 ohms. While 500 ohms is on the high side for a line level device output impedance, it should pose no issues for any typical active preamp or integrated amp input.

Frequency response - MM and MC inputs

In the measured frequency-response plot (above), the PP 2e is within 0.3dB of flat from 20Hz to 20kHz, corroborating the NAD claim. We found the same measured response with both the MM and MC input. An inverse RIAA EQ is applied to the input sweep, so that if a device were to track the RIAA curve perfectly, a flat line would emerge. In the graph above and some of the graphs below, we see two visible traces; the left channel (blue or purple trace) and the right channel (red or green trace). On other graphs, only one trace may be visible, this is because the left and right channels are tracking extremely closely, so as not to show a difference with the chosen axis scales.

Phase response - MM and MC inputs

Above is the phase response of the PP 2e for both the MM and MC inputs (they measured effectively identically), from 20Hz to 20kHz. The PP 2e does not invert polarity. Since phono preamplifiers must implement the RIAA equalization curve, which ranges from +19.9dB (20Hz) to -32.6dB (90kHz), phase shift at the output is inevitable. Here we find a worst case +28 degrees at 20Hz, and just above -60 degrees between 200 and 300Hz, and right around -60 degrees between 5 and 8kHz.

THD ratio (unweighted) vs. frequency - MM input

This is the THD ratio as a function of frequency plot for the MM input, where the input sweep is EQ’d with an inverted RIAA curve. The output voltage is maintained at the refrence 1Vrms. The THD values vary from 0.005% at 20Hz, down to below 0.001% from 300Hz to 500Hz, then back up just above 0.01% at 20kHz. One curiosity is the deviation in THD between left and right channels, where we find up to 5dB difference in favor of the right channel, which is most obvious between 1 and 5kHz.

THD ratio (unweighted) vs. frequency - MC input

Above is the THD ratio as a function of frequency for the MC input. The THD values vary from 0.01% at 20Hz, down to just above 0.001% at around 500Hz, then back up just above 0.01% at 20kHz. Here again, we see a deviation in THD between left and right channels.

THD ratio (unweighted) vs. output voltage at 1kHz (input voltage from 1mVrms to 100mVrms) - MM input

Above is the THD ratio as a function of output voltage plot for the MM input. We can see very low THD ratio values ranging from just above 0.01% down to 0.001% at around 2Vrms at the output, then to a sharp rise in THD at the “knee” at just shy of 5Vrms at the output. The deviation in THD performance (5-10dB) in favor of the right channel can be seen with output voltages between 1 and 5Vrms. The 1% THD ratio value is reached at 5.32Vrms at the output. It’s important to mention that anything above 1-2Vrms is not typically required for most line-level preamps or integrated amps.

THD+N ratio (unweighted) vs. output voltage at 1kHz (input voltage from 1mVrms to 100mVrms) - MM input

Above is the THD+N ratio as a function of output voltage plot for the MM input. We can see THD+N ratio values ranging from just above 0.1% below 100mVrms, down to 0.002% (R) and 0.005% (L) at around 3-4Vrms at the output.

THD ratio (unweighted) vs. output voltage at 1kHz (input voltage from 1mVrms to 100mVrms) - MC input

Above is the THD ratio as a function of output voltage plot for the MC input. THD ratio values range from about 0.015% at 100mVrms to 0.001% at around 2Vrms at the output, then to a sharp rise in THD at the “knee” at just shy of 5Vrms at the output. Beyond this output voltage, the DUT distortion begins to rise exponentially. The 1% THD ratio value is reached at 5.32Vrms at the output.

THD+N ratio (unweighted) vs. output voltage at 1kHz (input voltage from 1mVrms to 100mVrms) - MC input

The chart above is the THD+N ratio as a function of output voltage plot for the MC input. We can see THD+N ratio values ranging from just above 0.2% below 100mVrms, down to between 0.005% and 0.01% at around 3-4Vrms at the output.

FFT spectrum, 1kHz - MM input

Shown above is a fast Fourier transform (FFT) of a 1kHz 15mVrms input sinewave stimulus for the MM input, which results in the reference output voltage of 1Vrms. Here we see that the second signal harmonic at 2kHz is at -100dBrA (L) and -105dBRa (R). At 3kHz, or the third signal harmonic, we see at 10dB difference between the left (-100dBrA) and right (-110dBrA) channels. The worst noise peak is at 120Hz (second harmonic of 60Hz) and can be seen at around -90dBrA (left) and -100dBrA (right).

FFT spectrum, 1kHz - MC input

Above is a fast Fourier transform (FFT) of a 1kHz 1.2mVrms input sinewave stimulus for the MC input. The second (2kHz) and third (3kHz) signal harmonics are roughly the same here as with the MM input above. The worst noise peaks are at around -90dBrA at 180Hz (third harmonic of 60Hz) and just below -80dBrA at 60Hz. Of note, is that the 120Hz predominates on the MM input, while on the MC input, it’s the 60Hz peak.

FFT spectrum, 50Hz - MM input

The chart above depicts a fast Fourier transform (FFT) of a 50Hz input sinewave stimulus for the MM input. The X axis is zoomed in from 40Hz to 1kHz, so that peaks from noise artifacts can be directly compared against peaks from the harmonics of the signal. Here we see that the second noise harmonic at 120Hz dominates at -90dBrA (L) and -100dBRa (R). The second signal harmonic (100Hz) is at -100dBrA.

FFT spectrum, 50Hz - MC input

Above is a fast Fourier transform (FFT) of a 50Hz input sinewave stimulus for the MC input. Here we see that the thrid noise harmonic at 180Hz dominates at -80dBrA (L) and -90dBRa (R). The second signal harmonic (100Hz) is barely perceptible above the noise floor at -95dBrA.

Intermodulation distortion FFT (18kHz + 19kHz summed stimulus) - MM input

The chart above represents an FFT of the IMD products for an 18kHz and 19kHz summed sinewave stimulus tone for the MM input. The input rms values are set so that if summed (for a mean frequency of 18.5kHz), would yield 1Vrms (reference or 0dBRa) at the output. Here we find the second order modulation product (i.e., the difference signal of 1kHz) at -95dBrA, which puts them a little below the primary power supply noise products. The third-order modulation products (i.e., 17kHz and 20kHz) are considerably different between the right and the left channels. This is also reflected in our simplified IMD results (which only accounts for the sum of the second and third order modulation products) in our primary measurement table, where the right channel outperformed the left by 15dB on the MM input, and by 5dB on the MC input. The worst-case third-order modulation product peaks are at about -85dBrA for the left channel, but closer to -105dBrA for the right channel.

Intermodulation distortion FFT (18kHz + 19kHz summed stimulus) - MC input

Above is an FFT of the IMD products for an 18kHz and 19kHz summed sinewave stimulus tone for the MC input. Here we find the second-order modulation product at -85dBrA. The third-order modulation products (i.e., 17kHz and 20kHz) are at about -85dBrA for the left channel, but closer to -105dBrA for the right channel.

Diego Estan
Electronics Measurement Specialist