The SDR-4000 Reference CD Processor

The SDR-4000 Reference CD Processor is Spectral’s ultimate compact disc player, completing the Spectral Digital Reference system begun with the SDR-2000 Digital Processor. The development program for the SDR components was initiated over 15 years ago with the introduction of the Spectral SDR-1000 Reference Disc System, the industry’s first truly high-end compact disc playback system. The innovative SDR-1000 was years ahead of its time, pioneering fundamental processing breakthroughs such as balanced decoding and conjugate correction filters. More recently, the ambitious SDR-2000 Professional Processor has stunned the audio industry with its technical advances and resolving power. Sharing its uncompromising circuit architecture with the historic HDCD recording system by Pacific Microsonics, the Spectral SDR-2000 is widely considered to be the finest digital processor available as well as the acknowledged reference for High Definition Compatible Digital recordings.

The arrival of the SDR-4000 Reference CD Processor culminates the SDR program at Spectral and achieves the final integration of our high resolution digital technology. Supporting the challenge are new generation component technologies and state-of-the-art test systems to push forward the envelope of digital audio performance. Like other Spectral SDR components, no detail or expense has been spared in the SDR-4000 to achieve the status of a true reference.


With the recent introduction of digital playback formats for consumers such as DVD-A and SACD it would seem only logical that new digital audio playback components should support these new recordings as well as the established compact disc format. In truth, for general applications, affordable universal disc players may well become the accepted norm for playback of digital audio recordings. In-depth examination of these new high-bit technologies, however, reveals stunning performance limitations when considered for applications of the highest quality. The mass-market origin of the new high-bit digital audio technologies precludes their inclusion in the finest 16 bit CD playback applications due to an array of engineering limitations and performance compromises. Simply put, 24/96 digital audio technology reproduces conventional 16 bit compact disc recordings poorly. In point of fact, reproduction of CD recordings in today’s DVD-A and SACD players is arguably a step backwards, with poorer sonics than previously achieved in CD only players and processors.

We find the technical compromises of the high-bit systems far too serious to ignore when reproducing compact disc recordings. An important musical heritage has been captured in the twenty-two years since the introduction of the compact disc digital format containing some of the finest musical performances of the twentieth century. It is technically clear that the best reproduction of these compact disc recordings will only be accomplished with dedicated CD playback components. Developing a dedicated CD playback system capable of achieving ultimate possible reproduction from compact disc recordings is the goal of the Spectral SDR-4000 Reference CD Processor. Ultimate reproduction from compact disc recordings is assured by the SDR-4000 Reference CD Processor.


One conspicuous area of compromise among today’s high-end audio CD players is the choice of the disc drive itself. Most high-end manufacturers rely on standard mass-market drives developed for DVD, CD ROM or automotive applications. Although available and inexpensive, these drives were never designed for high performance or the durability requirements of true high-end use.
Spectral engineers evaluated premium drives and found virtually all to be compromised by the imperatives of mid-fi cost constraints. A cooperative effort between TEAC and Spectral has resulted in a custom version of their finest hand made Esoteric drive modified exclusively for the SDR-4000.

Unlike most other drives, the Spectral/Esoteric drive is designed exclusively for high-end compact disc applications. Based on the low production Esoteric P3 drive, the Spectral/Esoteric reflects a marriage of state-of-the-art mechanical and opt-electrical performance. Vibration-Free Rigid Disc-Clamping (VRDS) allows the convenience of front disc loading without the performance compromise. The VRDS clamping system offers full vibration damping of the sensitive compact disc while eliminating the mechanical vibration of magnetic pucks or the inconvenience of additional disc weights and stabilizers required with top-loading transports.

The VRDS system is a new generation design built to exacting standards for Spectral. No other drive that we have evaluated offers the data precision and mechanical quality achieved here. Superiority of the Spectral/Esoteric drive starts with its full-size diecast alloy disc turntable and stabilizer clamp which keeps disc and laser pickup in their precise relative positions. The overhead turntable’s heavy design and high-precision clamp greatly reduce undesirable vibration while its massive bearing and 6mm spindle ensure excellent rotational stability and long life. The superstructure of the drive is composite cast, including a massive base unit and damped bridge assembly for the precision motor and bearing of the disc turntable. A floating suspension system with sophisticated resonance damping completes the state-of-the-art mechanicals.


The SDR-4000 has a sophisticated array of five fully discrete power supplies to individually support each section of the player: CD drive and loader, servo circuitry, display and control system, digital system and re-clocking and for analog amplifiers. Extensive design detailing ensures that the elaborate SDR-4000 power supply system isolates the ultra-quiet processor and analog electronics from the vagaries of all types of line-borne distortion and noise as well as eliminating all emissions from the unit which could affect other sensitive system components.


As digital audio design has matured, cookbook design compromises and cost cutting have become entrenched in products at all price levels. Circuit compromises believed necessary for design practicality are ubiquitous in even today’s most soverign high-end digital designs. The endless engineering debate concerning what constitutes superior circuit performance is ironic considering the relative ease with which superior music reproduction is identified by the human ear. We believe the solution to the dilemma of digital audio quality lies in the design fundamentals, examining every design convention to identify a superior solution.
In approaching the engineering of the analog sections of a quality digital audio component you would, of course, expect unanimous agreement concerning the use of fine discrete amplification circuits for all analog stages, just as those implemented in any quality high-end preamp or amplifier. The surprising reality is that virtually all of today’s digital audio products, processors and CD players utilize integrated circuit amplifiers (IC op-amps) for most, if not all, of their analog stages. The use of low voltage, low bias integrated circuit amplifiers is the de-facto standard in the digital audio today.
Audiophiles and traditionalists may be shocked at the use of IC chips in high-end digital components due to the poor performance of monolithic circuits. However, in reality it is almost impossible to design practical fully discrete amplifiers to replace inexpensive ICs designed as I/V, equalizer and output amplifiers in digital audio components. For those reasons virtually all digital audio products use IC chips in the analog signal path.
Just how good can digital audio be? In reality, it is rather difficult to say since circuit quality compromises are virtually universal. Digital audio is always heard through the sonic filter of mediocre integrated circuit amplification and buffers. Is there any other option than the compromise of IC amplification? Yes, Spectral engineers have developed all-discrete, high performance analog circuitry for the SDR-4000 CD processor. All analog sections from DAC amplifier and equalizer to high-level output stage are discrete and uncompromised by IC monolithic circuits. Now analog sonics and performance are equal that of the finest high-end preamplifiers for the first time.

Spectral's two decades of state-of-the-art digital audio design experience are essential when it comes to extracting maximum performance in the critical DAC stage. Digital to analog conversion within SDR-4000 is performed at an extraordinarily high level of accuracy and features a Spectral-developed, double balanced implementation of the Analog Devices AD1853 multi-bit Sigma/Delta DAC. This unusual multi-bit double push-pull architecture is a proprietary Spectral innovation achieving DAC resolution and accuracy unrivaled by any other system. Directly inspired by the uncompromising discrete DAC topology of the Spectral SDR-2000 Professional Processor, the SDR-4000 DAC utilizes a differential current (I/V) conversion stage with Spectral’s own dedicated DAC amplifiers.


As the first analog signal stage following the DAC converter, the DAC amplifier or I/V stage is arguably the most critical analog signal stage in the system. The DAC amplifier is where ‘the rubber meets the road’ so to speak and where circuit linearity, speed and resolution are absolutely essential in determining ultimate system sonics. Unfortunately, today’s DAC and I/V amplifiers in digital audio components are universally implemented with low bias, low voltage IC op-amp circuits due to their ease of design, stability and low cost. Research at Spectral under live recording conditions indicates that the use of integrated circuit DAC amplifiers in today’s high-end digital products results in severe performance compromise.
Although designing a stable, discrete circuit DAC amplifier is a challenging task, solving the dilemma of IC compromise is a priority at Spectral. Our solution is the new SHHA I/V module (Spectral High-Speed Hybrid Amplifier) for use after the digital decoder. This discrete high performance amplifier is fully balanced and employs eight amplifier sections with superlative risetime and bandwidth characteristics. Only matched discrete through-hole fet and high-speed bipolar transistors are capable of the degree of precision required. The SHHA DAC amplifier module in the SDR-4000 solves the classic IC compromise in this critical analog stage for the very first time.


The output equalizer section of current digital audio components is another critical analog circuit plagued by performance compromises. Today’s output equalizers are fully active designs utilizing easy to implement IC filter circuits. These circuits are very attractive in their size and cost, but subject the sensitive signal to all the various distortions inherent in low voltage monolithic amplification. Spectral engineers have side-stepped the performance problems of integrated circuit filters entirely with an ingenious passive equalizer topology. The SDR-4000 passive equalizer is developed around a transient perfect circuit utilizing the finest available passive components such as handmade silver litz wire chokes and low noise bulk metal precision resistors. Eliminating the distortions inherent in IC based equalization results in a new level of signal purity in the SDR-4000.


The innovative, high performance output section of the SDR-4000 is a classic example of the difference uncompromising Spectral engineering makes. Although routinely neglected in digital audio design, the line level output stage is as critical to ultimate performance as it is in high-end preamplifier. For this reason, Spectral engineers have banished typical integrated circuit compromises for state-of-the-art discrete analog amplifiers proven in our finest high-end preamplifiers. Our entire DMC-15 line level stage and power supplies have been adapted for the output section of the SDR-4000. This architecture utilizes the spectacular Spectral SHHA high-speed amplifier modules which have revolutionized Spectral reference preamps. This uncompromising single-ended line section completes the distinguished analog stages of the SDR-4000, rivaling the performance and sonic sophistication of the world’s finest high-end preamplifier designs.

Engineering refinement in the SDR-4000 CD Processor extends to the quality of the user interface as well. The SDR-4000 front panel is a model of understated elegance and usability, in the Spectral tradition, with a hand polished satin front panel and machined acrylic construction. Large alphanumeric displays monitor the CD drive and playback functions, allowing visual confirmation of remote operations from a distance.
Refinement is also reflected in the convenience and precision of the SDR-4000 remote control. The deluxe remote duplicates SDR-4000 front panel controls and offers full input keypad and repeat functions. All metal construction of fully machined aluminum alloy components and premium powder coat finishing ensures rugged durability. The instrumentation grade momentary switching used on the SDR-4000 front panel is also used on the SDR-4000 remote for positive actuation and long operation. Two infrared transmitter diodes maximize signal power for extended operating range.


The SDR-4000 Reference CD Processor is designed to achieve the finest reproduction ever from compact disc recordingss. As the prevailing medium of high-fidelity music recordings currently available, state-of-the-art reproduction from compact disc is a Spectral engineering priority. Many performance compromises long indigenous to even the finest digital audio design have been eliminated for the first time. Utilizing a sweeping array of technological advances, the Spectral SDR-4000 CD Processor extracts new musical coherence and life from fine compact disc recording, proving clear insight into important musical performances of our time.

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