Wadia 850 CD player Measurements

Sidebar 3: Measurements

As supplied, the Wadia 850's maximum output level was 4.33V RMS, this measured at both the balanced and unbalanced jacks. Its output impedance from the balanced outputs was 16 ohms from below 20Hz to 20kHz; from the unbalanced outputs I measured 0.5 ohm! The output signal was in the correct (noninverted) polarity from both sets of outputs. The volume control operated in accurate 0.5dB steps, with superb channel matching. With an indicated setting of "99" equivalent to full level, "87" was -6dB and "75" -12dB. A setting of "5" was equivalent to -47.15dB, with "0" being a full mute. The Wadia's channel separation was better than 100dB below 1kHz, which is excellent. Capacitive coupling between channels reduced this to 84dB at 20kHz, which is still excellent.

Wadia's DigiMaster reconstruction filter sacrifices both initial stop-band attenuation and upper passband response to achieve its goal of optimal time-domain behavior. The latter can be seen in the 850's frequency-response graph (fig.1, top trace); the output is fully 3dB down at 20kHz. As my hearing cuts off above 16kHz and most classical musical material has very little top-octave energy, this probably explains why I only very occasionally felt that the Wadia's highs sounded dull. The bottom trace in fig.1 shows the 850's response with pre-emphasized data, offset by -1dB for clarity. It maps the top trace exactly, indicating an accurate de-emphasis curve.

Fig.1 Wadia 850, frequency response at 0dBFS (top) and de-emphasis response (bottom) (right channel dashed, 0.5dB/vertical div.).

Fig.2 shows spectra of the Wadia's analog output while it decodes a dithered 1kHz tone at -90dBFS from CD (top) or from the Audio Precision System One Dual Domain set to a 24-bit word length (bottom). The increase in word length gives just a 6dB reduction in the player's noise floor, which implies that while the DigiMaster software may operate with 24-bit precision, the player's ultimate resolution is limited by the noise present, which is either analog in origin or is more likely due to Wadia's use of dither in their DSP engine. However, this noise is very low in level. Note that with the longer data words, a slight amount of 120Hz hum is unmasked in the right channel; at -124dBFS, however, this will never be heard. Similar behavior can be seen in fig.3, which extends the analysis bandwidth to 200kHz while the player is being driven with digital silence. Again, the top trace is from CD, the bottom trace from the System One set to 24-bit words.

Fig.2 Wadia 850, spectrum of dithered 1kHz tone at -90.31dBFS, with noise and spuriae (16-bit data top, 20-bit data bottom, 1/3-octave analysis, right channel dashed).

Fig.3 Wadia 850, spectrum of digital silence, with noise and spuriae (16-bit data top, 20-bit data bottom, 1/3-octave analysis, right channel dashed).

Unless realized with long internal word lengths and optimally dithered, a digital-domain volume control has the potential of reducing resolution with increasing attenuation. If you think about it, reducing the level by 6dB is approximately equivalent to sliding all the bits in a 16-bit digital word one bit to the right and replacing the Most Significant Bit with a "0." What was the Least Significant Bit now is the 17th bit—but only as long as the word length has been increased to more than 16 bits. Otherwise, it just "falls off the end" of a 16-bit word, a process described in engineering terms as "truncation."

Wadia's white paper on its volume control implies that it has been correctly implemented with respect to preserving absolute resolution. I examined its operation by feeding the 850 24-bit data representing a dithered 1kHz tone at -90dBFS and performing a spectral analysis with the volume control set at 0dB, -6dB, -12dB, -18dB, and -24dB. The result, with the reference level normalized for the -90dBFS tone with each successive reduction in level, is shown in fig.4. By comparing this graph with fig.2, it can be seen that the noise floor is at or below the 16-bit level as long as the volume-control operation is between 0dB ("99") and -6dB ("87").

Fig.4 Wadia 850, spectrum of dithered 1kHz tone at -90.31dBFS, with noise and spuriae, with volume control set to 0dB, -6dB, -12dB, -18dB, and -24dB (24-bit data, 1/3-octave analysis, right channel dashed).

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