Mark Levinson No.383 integrated amplifier:
Measurements part 2
Small-signal THD was very low over most of the audioband, rising slightly in the top audio octave and into lower impedances (fig.4). The actual harmonic content was buried in noise at reasonable power levels; fig.5 shows that it is fundamentally third harmonic. At high powers (fig.6), the third harmonic rose to a still low 0.05% (-66dB) and was joined by the fifth at 0.01% (-80dB). Intermodulation levels were also respectably low, even just below visible clipping into a 4 ohm load (fig.7).

Fig.4 Mark Levinson No.383, THD+noise (%) vs frequency at (from top to bottom at 4kHz): 4W into 2 ohms; 2.83V into simulated loudspeaker load; 2W into 4 ohms; and 1W into 8 ohms (right channel dashed).

Fig.5 Mark Levinson No.383, 1kHz waveform at 12.5W into 8 ohms (top), distortion and noise waveform with fundamental notched out (bottom, not to scale).

Fig.6 Mark Levinson No.383, spectrum of 50Hz sinewave, DC-1kHz, at 145W into 4 ohms (linear frequency scale).

Fig.7 Mark Levinson No.383, HF intermodulation spectrum, DC-22kHz, 19+20kHz at 120W into 4 ohms (linear frequency scale).