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The Classé Omega SACD 2 uses newly developed technology for Super Audio CD and standard Compact Disc to give outstanding musical performance with both formats. It was designed for two-channel music systems where extracting the best sonic performance from a wide variety of SACDs and CDs is the highest priority.

Two Players in One Box
The SACD 2 provides separate signal paths for both the PCM (standard CD) and DSD (Super Audio CD) data streams. Treating the signal paths separately means that no compromises needed to be made for either data stream. The separate signal paths begin with two laser pick-ups in the transport mechanism: one for SACD and another for standard CD. The two signals are then processed separately until the last stage of digital-to-analog conversion. The best available digital filters and sample-rate conversion were selected for the PCM signal path, while the DSD signal bypasses all processing and routes directly to a state-of-the-art delta-sigma digital-to-analog converter. The result is excellent performance for both formats.

As with all Classé components, the Omega SACD 2 was developed using measurements in combination with careful listening tests to produce the most realistic and compelling musical performance possible. Through years of experience, the Classé design team has learned which measurements correlate with sonic performance and which do not. Several aspects of the SACD 2 design could have been changed to yield improved performance on some measured tests. However, these changes would degrade performance on other tests and, more importantly, degrade sonic performance.

The transport mechanism selected for the SACD2 is the Sony 555ES. To get the best possible performance from both formats, this mechanism uses dual laser pickups—one for the reflective CD layer and the other for the semi-transmissive SACD layer.

Continuing the theme of separation and optimization, the SACD 2 has dedicated digital filters for each type of signal. This allows standard CD, HDCD, and external PCM sources to be processed using the optimum filter type. In addition, the user can choose different sample rates by switching the sample-rate converter from the remote or front panel controls.

Sample rate conversion produces a high-sample rate signal, up to 192 kHz, from a standard CD data stream. In addition, this asynchronous converter discards the incoming clock signal and generates a new clock. Any jitter on the incoming clock does not affect the new clock signal. Jitter is not only reduced overall, but also the spectrum of any residual jitter on the outgoing clock is not correlated with the incoming clock. The combination of higher data rate and reduced jitter results in a significant improvement in sound quality.

The filter used to decode standard CD (PCM) signals in the SACD 2 is designed to work specifically with the digital-to-analog converters. It performs up to 8-times interpolation with moderately slow roll-off characteristics. The result is a finely tuned balance between frequency response and time domain response, giving optimum sonic performance.
A separate processor is used to decode HDCD encoded discs. Although it can also perform 8-times interpolated filtering, we route HDCD decoded signals to filter/DAC combination for optimum performance.

One of the main advantages of the Sony SACD system is that the DSD signal does not require any interpolation filtering before being converted to analog. The SACD2 takes full advantage of this by routing the DSD signal clear of all digital processing and connecting directly to the delta-sigma digital-to-analog converters.

Digital-to-analog conversion is provided by an array of three Crystal CS4397 stereo delta-sigma DACs. In other words, the two-channel balanced and single-ended outputs of the SACD 2 are derived from six D-to-A converters. These state of the art converters from Cirrus Logic (one of the top manufacturers of delta-sigma DACs) offer performance near the theoretical limits for dynamic range and linearity with both PCM (standard CD) and DSD (SACD) signals.

Each channel of the SACD2 uses one stereo DAC to generate the balanced output, with one of the internal converters for the positive signal and the other for negative signal. The third stereo DAC is used by both channels to convert the single-ended signals. The outputs from these DACs are routed to dedicated balanced and single-ended analog circuitry respectively. This way both balanced and single-ended outputs are not compromised and do not interfere in any way with each other.

The role of the analog circuitry is to convert the DAC output, a stream of high-frequency pulses, into the analog output that drives the output cables and preamplifier input. The DAC output is a stream of pulses of equal amplitude at a frequency of 2.8 MHz. This digital stream is pulse-density modulated; that is, the average value of the stream of pulses is the desired analog signal. The analog filter provides a low-pass (or averaging) function to remove the high-frequency pulses and leave the analog signal remaining. The noise that remains after this process is essentially random noise, but has been noise shaped so that the amplitude is highest at higher frequencies.

The filter designed for the SACD 2 is a 2^nd-order passive filter. At 12 dB per octave roll-off, it does not offer the out-of-band rejection of a more complex, steep slope filter. However, with a corner frequency at 35 kHz and 96 dB of attenuation at 2 MHz, this filter provides more than sufficient noise suppression to prevent both audible artifacts and intermodulation distortion. In addition, this filter has excellent time domain performance with virtually zero group delay from 100 Hz to 10 MHz – a level of performance that cannot be attained with steep slope filters. Because the filter is passive, composed of high quality capacitors and resistors, it is sonically pure, with no added distortion or noise.

THD measurements and the Analog Filter
The choice of filter has an effect on the THD measurement. Distortion analyzers used to measure harmonic distortion cannot distinguish between harmonic distortion and non-harmonic noise. For this reason, distortion specifications are typically expressed as THD + Noise. Residual noise from the delta-sigma conversion process can cause an increase in the THD + Noise figure. This noise, and therefore the THD + Noise specification, could be reduced by using a more complex, active, steep slope filter. While this would improve the THD + Noise performance, it would come at the cost of overall sonic performance. While other designs offer lower THD + Noise specifications, they cannot match the sonic performance of the Omega SACD 2.

The voltage obtained from the second stage of analog filtering is sufficient to drive the output, but current buffering (gain) is still required to drive the interconnect cables and pre-amplifier input circuitry. This is provided by a zero-feedback output driver, where the lack of feedback has a particular advantage. The output driver must prevent external noise, such as RF noise picked up by the audio cables, from entering the unit. An output driver with feedback feeds the output signal back to the input, offering an opportunity for such noise to enter the circuitry.

The entire output circuit of the SACD 2 requires no voltage gain and no feedback. This elegant simplicity is representative of the approach taken on every aspect of the SACD 2 design. It is a design philosophy that results in an extremely natural, relaxed, and yet highly detailed portrayal of music that will satisfy and captivate any music enthusiast.