Naim NAC 552 preamplifier Measurements
Measured using its RCA input and output jacks, the Naim preamp offered a maximum gain of 21.1dB, requiring just 88mV at 1kHz to give an output of 1V. (This is very slightly higher than the specified sensitivity.) The unity-gain position on its volume control was 11 o'clock. The NAC 552 didn't invert absolute polarity, and the input impedance was a more or less constant 38.5k ohms across most of the audioband, this dropping inconsequentially to 34k ohms at 20kHz.
The output impedance was a very low 3 ohms at middle and high frequencies, this figure including the series resistance of the interconnect used. It rose in the bass, to 418 ohms at 20Hz, presumably from its using a finite-sized output coupling capacitor. While this is still a low source impedance in absolute terms, it does result in curtailed bass response into a 600 ohm load (fig.1, lower pair of traces below 300Hz). This suggests the partnering power amplifier should have an input impedance of at least 8k ohms.
Fig.1 Naim NAC 552, frequency response at 1V into 100k ohms (top) and 600 ohms (bottom). (0.5dB/vertical div., right channel dashed.)
Into a more reasonable load (fig.1, top traces), the NAC 552 offers good low-frequency extension, though it still rolls off slightly earlier at infrasonic frequencies than do typical American designs. As MF notes in his review, Naim has also arranged for the ultrasonic response to be rolled off earlier than is usual in US designs. This results in 0.6dB of rolloff at 20kHz, though this will not be subjectively significant, I feel.
Channel separation was excellent in the midrange (fig.2), though it did decrease at higher frequencies due to capacitive coupling, probably at the volume control. However, at 65dB in both directions at 20kHz, this, too, will not be an issue for listeners. Due to the capacitive coupling of the output, DC offset was negligible. The NAC 552's unusual wiring topology, in which the audio output is first taken to the power supply, then to the power amplifier, is, to the best of my knowledge, done to place the preamp's ground reference at the point where the impedance to true ground is at its lowest. Certainly, the unweighted, wideband S/N ratio, taken with the input shorted but the volume control set to its maximum, was good at 75.9dB, this improving to 88.3dB when A-weighted. There were no AC power-supply-related spuriae present in the audio output signal.
Fig.2 Naim NAC 552, channel separation (10dB/vertical div., R-L dashed).
Fig.3 plots the THD+noise present in the output against the output voltage. The increase in the measured percentage as the level decreases shows that the figure is dominated by noise, not distortion per se. The latter starts to increase slightly above 750mV or so—the NAC 552's nominal output is specified as 775mV—but the true clipping point (1% THD) is not reached until 8V into 100k ohms, or a still high 6.2V into 600 ohms.
Fig.3 Naim NAC 552, distortion (%) vs output voltage at 1kHz into 100k ohms (bottom) and 600 ohms (top).
The manner in which the THD+noise percentage changes with frequency (fig.4) was plotted at 1V output, which means that, as shown by fig.3, the graph does show the true level of distortion. Into 100k ohms (bottom trace above 200Hz), the THD is constant with frequency at 0.025%, but something weird happens into 600 ohms: there appears to be a notch in the measured distortion at 60Hz, with a rising percentage (associated, I assume, with the rising source impedance) at low frequencies. Such a notch usually means that some 60Hz AC hum is leaking into the signal somewhere, giving rise to phase cancellation in the measured level at the same frequency. But a frequency of 60Hz usually implies magnetic coupling from the AC transformer, not electrical leakage, and nothing I did with regard to the grounding of the NAC 552 or my Audio Precision System One changed this behavior. A mystery, but it does again suggest that the Naim not be used to drive low impedances.
Fig.4 Naim NAC 552, THD+N (%) vs frequency (from bottom to top above 200Hz): 1V into 100k and 600 ohms (right channel dashed).
The distortion that is present is primarily a low level of the innocuous second harmonic (fig.5), though the third harmonic rises when the preamp drives low impedances. Intermodulation distortion is also low (fig.6), the 1kHz difference component resulting from an equal mix of high-level 19kHz and 20kHz tones lying at -77dB (0.014%).
Fig.5 Naim NAC 552, spectrum of 1kHz sinewave, DC-10kHz, at 1V into 8k ohms (linear frequency scale).
Fig.6 Naim NAC 552, HF intermodulation spectrum, DC-24kHz, 19+20kHz at 1V into 8k ohms (linear frequency scale).
As long as it not used to drive unrealistically low impedances—which it won't be in an all-Naim susyem—the NAC 552 gets a clean bill of health for its measured performance.—John Atkinson