NAD C370 integrated amplifier Measurements
The chassis of the NAD C370 was hot after the usual one-hour preconditioning period at 1/3 power into 8 ohms, but, at around 55 degrees C, well within the bounds of safety. (The internal heatsinks were much hotter, of course, but these cannot be reached by inquisitive fingers.) The amplifier didn't invert signal polarity, and the maximum voltage gain (into 8 ohms) was a high 39.4dB. Input impedance at 1kHz was 300k ohms and the output impedance across most of the audioband a moderate 0.16 ohm, this rising slightly to 0.19 ohm at 20kHz. This is equivalent to a damping factor of 50, lower than specified.
With the tone controls defeated, the change in small-signal frequency response with changing load impedance can be seen (in fig.1) to be small, though a channel imbalance of 0.6dB is also evident. (The volume control was set to 12:00 for this measurement; the imbalance decreased at higher settings, but got worse at lower ones.) The response itself was wide and flat, with a -3dB point at around 200kHz. The shape of a 10kHz squarewave was correspondingly square (fig.2). With the tone controls in-circuit but set to their detented flat positions, the response was not quite flat (fig.3, central pair of traces), with a slight lack of energy in the lower midrange apparent. The bass and treble controls give a sensible ±6dB of boost or cut.
Fig.1 NAD C370, frequency response with tone controls defeated at (from top to bottom at 2kHz): 2.83V into dummy loudspeaker load, 1W into 8 ohms, 2W into 4 ohms, 4W into 2 ohms (right channel dashed, 0.5dB/vertical div.).
Fig.2 NAD C370, small-signal 10kHz squarewave into 8 ohms.
Fig.3 NAD C370, frequency response with tone controls set to maximum and minimum settings (right channel dashed, 0.5dB/vertical div.).
The C370's midrange channel separation (fig.4) was slightly disappointing at 73dB, though this is still good enough in absolute terms. The crosstalk starts to increase in the high treble, due to the usual capacitive interchannel coupling, and is worse in the L-R direction. Interestingly, the crosstalk was also 180 degrees out of phase. The A-weighted signal/noise ratio (ref. 1W/8 ohms) was better than specified at 98dB, but worsened to 72dB (unweighted wideband), apparently due mainly to some very-high-frequency noise.
Fig.4 NAD C370, channel separation (10dB/vertical div., R-L dashed).
The NAD's total distortion was low in level and didn't change appreciably with load or frequency (fig.5), other than at high frequencies above the audioband. There appeared to be a drastic increase in THD in the mid-treble with the amplifier driving Stereophile's simulated loudspeaker load, but at this 8.5V output level, I suspect that an inductor core in the load was saturating. Fig.5 sums both the true distortion harmonics and the residual noise. The lower trace in fig.6—taken at 71W into 8 ohms, where the harmonics poke up above that noise—shows that, in addition to some third harmonic, some distortion spikes are apparent at the signal zero-crossing points. Bear in mind, however, that the absolute THD level was just 0.0047%, which will mitigate the audibility of this higher-harmonic content. At higher output currents (not shown), the distortion content became more pure low-order harmonic.
Fig.5 NAD C370, THD+noise (%) vs frequency at (from top to bottom at 10kHz): 8.5V into simulated loudspeaker load, 40W into 2 ohms, 10W into 8 ohms, 20W into 4 ohms (right channel dashed).
Fig.6 NAD C370, 1kHz waveform at 71W into 8 ohms (top), distortion and noise waveform with fundamental notched out (bottom, not to scale).