This can be seen in figs.7 and 8, which show FFT-derived spectra of the NAD's output while it reproduced a 50Hz tone at 1W and 10W, respectively, into 8 ohms. At the lower power level, an entire series of odd harmonics can be seen that effectively disappear at the higher level. Yes, even at 1W, the third harmonic—the highest in level—lies at -80dB (0.01%), with most of the other harmonics at or below -90dB (0.003%). But this still suggests a somewhat inadequate output-stage bias current.

Fig.7 NAD C370, spectrum of 50Hz sinewave, DC-1kHz, at 1W into 8 ohms (linear frequency scale).

Fig.8 NAD C370, spectrum of 50Hz sinewave, DC-1kHz, at 10W into 8 ohms (linear frequency scale).

Intermodulation distortion, however, is very low—as shown in fig.9, the spectrum of the C370's output with an equal mix of 19kHz and 20kHz tones is just below visible clipping on an oscilloscope. Both the difference product at 1kHz and the second-order tones at 18kHz and 21kHz lie at almost -90dB. Reducing the load to 4 ohms produced a greater number of intermodulation products (fig.10), these surrounded by sidebands at the AC supply frequency, suggesting that the power supply is nearing its limit. However, all these products are still below -80dB.

Fig.9 NAD C370, HF intermodulation spectrum, DC-24kHz, 19+20kHz at 85W into 8 ohms (linear frequency scale).

Fig.10 NAD C370, HF intermodulation spectrum, DC-24kHz, 19+20kHz at 85W into 8 ohms (linear frequency scale).

The NAD C370's power-supply topology allows considerably more output power to be available for the kind of short-duration transients typically found in music (see "Must We Test? Yes We Must!," August 1989). Fig.11, however, shows distortion and noise plotted against the amplifier's continuous power. Into 8 ohms with both channels driven, it still comfortably exceeded its specification, at 172W (22.4dBW) at the usual clipping point of 1% THD. While no more power was available into 4 ohms—again with both channels driven—driving just one channel continuously into 2 ohms gave 305W.

Fig.11 NAD C370, distortion (%) vs continuous output power into (from bottom to top at 1W): 8 ohms, 4 ohms, 2 ohms.

Testing the C370's output power with a low-duty-cycle 1kHz toneburstצ cycles on, 400 cycles off—and just one channel driven gave a much different picture (fig.12). Under these conditions, 279W was available into 8 ohms (24.5dBW, black trace), 512W into 4 ohms (24.1dBW, red), 846W into 2 ohms (23.25dBW, blue), and a whopping 1166W into 1 ohm (21.6dBW, green). The last figure is equivalent to an RMS output current of 34 amperes! Yes, the low-level distortion rises by 6dB or so every time the load impedance halves, but with normal, transient-rich music, the C370 will sound like a much more powerful amplifier than its 120Wpc specification suggests. No more toneburst power is available with the NAD's "soft clipping" engaged, but, as fig.13 reveals, the distortion begins to rise at an earlier level, which is somewhat akin to how a tube amplifier overloads.—John Atkinson

Fig.12 NAD C370, soft clipping off, distortion (%) vs 1kHz burst output power into 8 ohms (black trace), 4 ohms (red), 2 ohms (blue), and 1 ohm (green).

Fig.13 NAD C370, soft clipping on, distortion (%) vs 1kHz burst output power into 8 ohms (black trace), 4 ohms (red), 2 ohms (blue), and 1 ohm (green).