Hi-Rez Disc Player/Transport Reviews

Sidebar 3: Measurements

I examined the Krell Evolution 505's measured behavior using Audio Precision's top-model SYS2722 system (see www.ap.com and "As We See It" in the January 2008 issue), as well as, for some tests, our Audio Precision System One and the Miller Audio Research Jitter Analyzer. The sample measured was S/N 18007010184, the second of the two samples Fred auditioned. As with other Krell components using the company's proprietary CAST current-mode outputs, I could assess the Evolution 505's performance only from the conventional single-ended and balanced jacks. This doesn't affect my assessment of the player's performance in the digital domain, but matters such as the distortion spectra may well be moot when the Evolution 505's CAST output is used.

The maximum output levels (MOLs) at 1kHz from the balanced jacks were 3.95V (SACD) and 3.98V (CD); the single-ended MOLs were exactly half those levels, as expected. The output impedances were 240 ohms balanced and 120 ohms unbalanced, both figures uniform across the audioband. Both outputs preserved absolute polarity; ie, were non-inverting (the XLRs are wired with pin 2 hot). The 505's error correction of CD playback was superb; the player did not mute, or illuminate the Error flag in its S/PDIF output (footnote 1) until the Pierre Verany Test CD's track 34, which has 2mm gaps in its data spiral.

The Krell 505 has four different reconstruction filters for SACD playback, labeled F1, F2, F3, and F4, the first two also available for CD playback. F1 has the widest bandwidth for SACD playback, and the frequency response with this filter is shown as the bottom pair of traces in fig.1. This filter has the slowest rate of ultrasonic rolloff, and the response is –3dB at 50kHz. The next pair of traces in fig.1 was taken with the filter set to F2; the response is +0.56dB at 1kHz, but with a steeper ultrasonic rolloff than with F1. The next two pairs of traces were taken with F3 (+3.3dB of gain at 1kHz, but not quite as steep an ultrasonic rolloff as F2) and F4 (+5.1dB of gain at 1kHz, and the second-slowest rolloff). Because of the different passband gains, direct comparisons of the effects of these filters will not be straightforward.

Fig.1 Krell Evolution 505, frequency response at –3dBFS into 100k ohms, SACD data, with filter set to (from top to bottom): F3, F4, F2, F1 (right channel dashed; 2dB/vertical div.).

The top pairs of traces in fig.2 show the Evolution 505's response for CD playback. F1 has the widest bandwidth; F2 rolls off the top audio octave to reach –3dB at 20kHz. Many listeners prefer CD playback with this kind of filter characteristic, but it should be noted that Fred Kaplan didn't express a preference for one over the other, using (at Krell's suggestion) F1 for all his auditioning. Of greater interest in this graph is the bottom pair of traces, which were taken with preemphasized data. Two peaks and some serious passband ripples appear in the treble, suggesting that the 505's deemphasis equalization has not been optimally designed. Fortunately, preemphasized CDs are very rare these days. Channel separation was superb, at better than 108dB in both directions (not shown).

Fig.2 Krell Evolution 505, frequency response at –12dBFS into 100k ohms without (top) and with (bottom) deemphasis, CD data (right channel dashed, 0.5dB/vertical div.).

For continuity with my tests of digital players over the past 20 years, my initial test to assess a player's resolution is to sweep a 1/3-octave bandpass filter from 20kHz down to 20Hz while the unit plays a dithered 1kHz tone at –90dBFS. The top pair of traces below 8kHz in fig.3 were taken with CD data representing this signal; the middle pair of traces were taken with SACD data. The Krell's conventional outputs are a little noisier than the best players I have tested, SACD giving only a modest increase in resolution compared with CD. Even so, it will still retrieve a tone at –120dBFS from SACD, as shown by the bottom pair of traces in fig.3.

Fig.3 Krell Evolution 505, 1/3-octave spectrum with noise and spuriae of dithered 1kHz tone at –90dBFS with 16-bit CD data (top), SACD data (bottom at 2kHz), and of dithered 1kHz tone at –120dBFS with SACD data (bottom at 1kHz). (Right channel dashed.)

Linearity error with CD (fig.4) was very small to below –105dB. This graph is actually dominated by the recorded dither noise, the Krell's own noise being low enough not to obscure the discrete steps in the waveform of an undithered sinewave at –90.31dBFS from CD (fig.5), or the excellent sinewave shape of a dithered tone at –90dBFS played back from SACD (fig.6).

Fig.4 Krell Evolution 505, linearity error, 16-bit CD data.

Fig.5 Krell Evolution 505, waveform of undithered 1kHz sinewave at –90.31dBFS, CD data (blue left, red right).

Fig.6 Krell Evolution 505, waveform of dithered 1kHz sinewave at –90dBFS, SACD data (blue left, red right).

The Krell's conventional outputs offer very low levels of harmonic distortion into high impedances (fig.7). Though the third harmonic does increase slightly into the punishing 600 ohm load (fig.8), there is no appearance of higher-order harmonics, which can be subjectively more annoying. To assess a player's intermodulation performance, I play back data representing an equal mix of 19kHz and 20kHz tones, the combined waveform peaking at 0dBFS. I don't have this signal on SACD, so I tested the Krell 505 using CD data. The result is shown in fig.9; actual intermodulation is very low, and the granular-looking noise floor is mainly due to the limitations of the 16-bit CD medium. However, the noise floor looks double-humped around the twin fundamentals, suggesting that some modulation noise is present. I noted very similar behavior in the measurements that accompanied Michael Fremer's review of the Krell SACD Standard. As I wrote at that time, low-frequency noise modulation can be audible, but high-frequency effects like this may well be masked both by the music program and by the ear's decreasing sensitivity in this region.

Fig.7 Krell Evolution 505, balanced output, spectrum of 100Hz sinewave at 0dBFS into 200k ohms, SACD data (blue left, red right; linear frequency scale).

Fig.8 Krell Evolution 505, balanced output, spectrum of 100Hz sinewave at 0dBFS into 600 ohms, SACD data (blue left, red right; linear frequency scale).

Fig.9 Krell Evolution 505, balanced output, HF intermodulation spectrum, 19+20kHz at 0dBFS peak into 200k ohms, CD data (blue left, red right; linear frequency scale).

This behavior can also be seen in the narrowband spectrum of the Krell's output while it decoded the Miller Jitter Test signal (fig.10), which comprises an 11.025kHz tone combined with a low-frequency toggling of the LSB. The actual jitter level was very low, however, at 331 picoseconds peak–peak, with data-related components at the residual level.

Fig.10 Krell Evolution 505, high-resolution jitter spectrum of analog output signal, 11.025kHz at –6dBFS, sampled at 44.1kHz with LSB toggled at 229Hz, CD data. Center frequency of trace, 11.025kHz; frequency range, ±3.5kHz (blue left, red right).

Overall, this is excellent measured performance.—John Atkinson

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Footnote 1: The S/PDIF output has to be enabled using the front-panel Menu and arrow buttons.