Accuphase DP-85 SACD player Measurements

Sidebar 3: Measurements

Unless indicated otherwise, I measured the DP-85's performance from its balanced jacks. The maximum output level for CD replay was 2.5V from both balanced and unbalanced jacks—nearly 2dB higher than the CD Standard's 2V RMS—and fractionally lower at 2.47V for SACD playback (assessed using Sony's "tentative" test SACD). The digital volume control offered accurate 1dB attenuation steps down to -60dB.

The source impedance was a low 47 ohms from both sets of jacks over most of the audioband, this increasing inconsequentially to 54 ohms at 20Hz from the RCA outputs, and to 68 ohms from the XLRs. The output polarity was noninverting from the unbalanced RCAs but inverting from the balanced XLRs, these apparently wired with pin 3 rather than pin 2 "hot." Error correction for CD playback was superb, the Accuphase coping with gaps in the data spiral up to 2mm long without missing a beat.

Frequency response for CD playback was flat within the audioband (fig.1, upper pair of traces), and de-emphasis error was negligible (fig.1, bottom traces). With SACDs, the ultrasonic response rolled off to -3dB at 50kHz, -6dB at 60kHz (fig.2), as suggested by the SACD license. With external 96kHz-sampled PCM data—the DP-85's data input successfully locked to sample rates up to 96kHz—the response gently rolled off above 30kHz before dropping like a stone above 45kHz (not shown). Channel separation (also not shown) was superb, at better than 120dB across the audioband and around 130dB at 1kHz.

Fig.1 Accuphase DP-85, CD frequency response at -12dBFS, 44.1kHz sampling, without emphasis (top) and with emphasis (bottom). (Right channel dashed, 0.5dB/vertical div.)

Fig.2 Accuphase DP-85, SACD frequency response at -3dBFS (right channel dashed, 1dB/vertical div.).

Fig.3 shows spectral analyses performed on the DP-85's analog output when its external data input was driven by 44.1kHz PCM data representing a dithered 1kHz tone at -90dBFS with both 16- and 24-bit word lengths. The increase in bit depth drops the noise floor by 12dB or so, suggesting a dynamic range of around 18 bits, which is excellent. Small peaks at 60Hz and 120Hz can be seen in the 24-bit traces (unchanged when I tried floating the AC supply ground), but as these are better than 133dB down from peak level, their presence will be subjectively benign.

Fig.3 Accuphase DP-85, 1/3-octave spectrum of dithered 1kHz tone at -90dBFS, with noise and spuriae (from top to bottom): 16-bit PCM data, 24-bit PCM data (right channel dashed).

Fig.4 shows similar analyses performed with the Accuphase playing back 1kHz tones at -90dBFS and -110dBFS from SACD. Again the player's excellent dynamic range is revealed, at least in the low treble and below. Note, however, the rise in the noise floor above 2kHz, this due to the aggressive noise-shaping used by the SACD's DSD encoding. By comparing fig.4 with fig.3, you can see that SACD has less inherent dynamic range above 10kHz than CD, though this is largely academic, I feel.

Fig.4 Accuphase DP-85, 1/3-octave spectrum of dithered 1kHz tone at -90dBFS (top) and -110dBFS (bottom), with noise and spuriae, SACD data (right channel dashed).

Fig.5 shows a similar spectral analyses, extended up to 200kHz, with the player decoding both DSD and 24-bit 44.1kHz PCM representing a dithered 1kHz tone at -150dBFS; ie, well below the analog noise floor. (I use this test signal rather than "digital black" because so many players and processors mute their analog outputs when they detect the latter signal.) The DP-85's background noise for 24-bit playback (bottom traces) is "white"; ie, it rises slightly with frequency on this logarithmically scaled graph. By contrast, SACD's noise floor rises dramatically above the audioband, peaking at around -53dB between 60kHz and 80kHz. This means that DSD encoding really has only about 10 bits of dynamic range in the octave between 40kHz and 80kHz. However, I don't see this as a real problem, given how little audio information there is above 35kHz.

Fig.5 Accuphase DP-85, 1/3-octave spectrum of dithered 1kHz tone at -150dBFS, with noise and spuriae (from top to bottom): DSD data, 24-bit PCM data (right channel dashed).

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