NAD 218 THX power amplifier Measurements
A complete set of measurements of the NAD Model 218 was made in the unbalanced configuration, with selected readings taken in balanced mode. Unless otherwise noted, the results here refer to unbalanced operation with the amplifier's soft-clipping circuit engaged.
Following its one-hour pre-conditioning test, the NAD's heatsinks were quite hot, but could still be touched without discomfort. The amplifier's voltage gain measured 29dB unbalanced and virtually the same (28.9dB) balanced. The input impedance measured 67k ohms unbalanced and 123k ohms balanced. DC offset was a very low 0.4mV left, 0.0mV (ie, unmeasurable) right. The 218 is noninverting from its unbalanced input; pin 2 of the balanced input is wired as positive.
The 218's unweighted S/N ratios at 1W into 8 ohms, left channel only measured, were 89.8dB (92.1dB balanced) over a 22Hz-22kHz bandwidth, 84.7dB (86.1dB balanced) from 10Hz to 500kHz, and 92.5dB (94.8dB balanced) A-weighted. Output impedance varied from 0.04 to 0.09 ohms, depending on the load impedance and frequency. (The highest value occurred at 20kHz.) These figures should make the amplifier's frequency response a constant with any real-world loudspeaker load.
The 218's unbalanced frequency response (fig.1) needs no comment, with the balanced-mode (not shown) identically flat. Fig.2 shows the 10kHz squarewave response of the amplifier. The slight rounding of the leading edge is common to most amps with this frequency ultrasonic response; otherwise, the result is excellent, as is the near-ideal 1kHz squarewave (not shown). The channel separation (fig.3) is excellent, the only oddity being the long plateau in the left-to-right balanced crosstalk. But none of the crosstalk measurements is of any practical concern.
Fig.1 NAD 218 THX, frequency response at (from top to bottom): 1W into 8 ohms, 2W into 4 ohms, and 2.828V into simulated loudspeaker load (0.5dB/vertical div., right channel dashed).
Fig.2 NAD 218 THX, small-signal 10kHz squarewave into 8 ohms.
Fig.3 NAD 218 THX, channel separation (from top to bottom at 20kHz): R-L, L-R, unbalanced; R-L, L-R, balanced (10dB/vertical div.)
Fig.4 plots the 218's THD+noise percentage against frequency, again typical of a solidly designed amplifier. The higher result into 2 ohms is not unusual (the amplifier is not rated for this load), and the balanced THD+noise performance into 8 and 4 ohms (not shown) is comparable. The 1kHz THD+noise waveform at an output of 60W into 4 ohms is shown in fig.5. At lower power output, the 218's low-level noise obscures the waveform. The result is a combination of second and third harmonics, plus noise.
Fig.4 NAD 218 THX, THD+noise (%) vs frequency at (from top to bottom at 1kHz): 4W into 2 ohms, 2W into 4 ohms, 1W into 8 ohms, and 2.83V into simulated loudspeaker load (right channel dashed).
Fig.5 NAD 218 THX, 1kHz waveform at 2W into 4 ohms (top), distortion and noise waveform with fundamental notched out (bottom, not to scale).
The distortion spectrum resulting from a 50Hz input at 228W into 4 ohms is shown in fig.6. All of the artifacts are lower than -90dB (0.003%), an excellent result, with the second harmonic highest in level. The 19+20kHz IM spectrum at 179W into 4 ohms (visible signs of clipping start to appear at higher levels with this signal) is plotted in fig.7. The 1kHz intermodulation artifact is at -75.2dB (0.015%), the 18kHz artifact at -66.3dB (0.05%), both solid results. The 19+20kHz spectrum at 100W into 8 ohms (not shown) is very similar to the result of fig.7.
Fig.6 NAD 218 THX, spectrum of 50Hz sinewave, DC-1kHz, at 228W into 4 ohms (linear frequency scale).
Fig.7 NAD 218 THX, HF intermodulation spectrum, DC-22kHz, 19+20kHz at 179W into 4 ohms (linear frequency scale).
The Model 218's THD+noise vs level curves are shown in figs.8 and 9, the latter with the soft-clipping circuit disengaged. Though the amplifier is not recommended for use with loads under 2 ohms, I did run the single sweep shown in fig.8 to test its 2 ohm performance (but did not perform discrete clipping measurements on the amplifier with this load). Note that the 218's power output is higher with its soft clipping disengaged. Also note that the abruptness of the clipping (indicated by the abrupt upturn in the curve as it passes the "knee") is not noticeably more gradual with soft clipping engaged. NAD was the first company to include soft clipping in their amplifier designs. While this feature is useful with low-powered amplifiers, it's hard to imagine that it will have any audible benefits with anything as powerful as the 218, especially when it's used in its bridged mode: 800W at clipping into 8 ohms (1kHz, 1% THD+noise)! The 218's discrete clipping levels are shown in Table 1.
Fig.8 NAD 218 THX with soft clipping, distortion (%) vs continuous output power into (from bottom to top): 8 ohms, 4 ohms.
Fig.9 NAD 218 THX without soft clipping, distortion (%) vs continuous output power into (from bottom to top): 8 ohms, 4 ohms.
John Atkinson also measured the NAD's output power on tonebursts, using the Miller Audio Research Amplifier Profiler. Fig.10 show the results with the soft clipping engaged: the increase in THD as the amplifier starts to overload is indeed more gentle than without soft clipping, and massive amounts of power were available with the 1kHz toneburst signal, which is closer to a musical signal than a continuous tone: 247W into 8 ohms (black), which almost doubles to 484W into 4 ohms (red), 950W into 2 ohms (blue), and an astonishing 1555W (green) into a punishing 1 ohm load! Slightly more power was available without soft clipping—305W into 8 ohms, 1575W onto 1 ohm—but again, this is academic.
Fig.10 NAD 218 THX with soft clipping, distortion (%) vs burst output power into 8 ohms (black trace), 4 ohms (red), 2 ohms (blue), and 1 ohm (green).
Altogether, this is a fine set of measurements for one very powerful amplifier.—Thomas J. Norton