I usually test D/A linearity by measuring the analog output level with a narrow bandpass filter while a dithered, 16-bit 500Hz tone is swept down from 0dBFS to -120dBFS. The No.30.6 performed almost perfectly on this test, so I raised the bar, driving it with dithered 24-bit data down to -140dBFS. The result is shown in fig.3: the error is less than 1dB down to -130dBFS! Basically, this processor can extract music from the roots of the universe. With its superb linearity and low noise floor, it comes as no surprise that the '30.6's reproduction of an undithered 1kHz tone at -90.31dBFS (fig.4) is essentially perfect. Increasing the word depth to 24 bits gave a good facsimile of a sinewave (fig.5).
Fig.3 Mark Levinson No.30.6, departure from linearity, 24-bit data (2dB/vertical div.).
Fig.4 Mark Levinson No.30.6, waveform of undithered 1kHz sinewave at -90.31dBFS, 16-bit data.
Fig.5 Mark Levinson No.30.6, waveform of undithered 1kHz sinewave at -90.31dBFS, 24-bit data.