The DMC-30 Reference Preamplifier





Spectral's new concept preamplifier features the proven discrete circuit gain module used in the ultimate performance DMC-20 manual system. Its unsurpassed accuracy combined with exceptionally fast and complete settling is unequaled and essential to preserve inner detail, three dimensional staging, and purity of timbre. The DMC-30 creates an ideal platform for this acclaimed electronic system as its remote control architecture makes possible a tight fine tuned signal routing that is unencumbered by the ergonomic placement of controls and connectors of manual equipment. New sonic aspects are revealed as its direct optimal path construction assures a pristine contamination free amplification and control of audio signals.

Prior to the DMC-30, many convenience features desirable to control and operate sophisticated sound systems inevitable required more electrical parts as well as large convoluted circuit paths within the preamplifier. We know complexity invites sonic loss and as parts and paths increase, designers are tempted to favor integrated circuit amplification to unclutter their products. The final sonic coup de grace for such product development is often the electrically operated variable gain and balance control.

Amongst many practical devices made for this purpose, the integrated circuit DAC is easy to use and it has become the preferred methodology to populate circuit boards for the commodity audio industry. Unfortunately these building block circuits require additional amplifiers in the signal path and their restricted high frequency response is troublesome. Distortion becomes higher and they invariably produce subtle changes of phase and high frequency output from different volume settings. Of course, high-end designers have attempted to improve the situation but even better quality parts built with metal film technology still have the distortion and sweet spot tendencies where one loudness setting sounds better than others.

Clearly this approach is not ideal. Instead, we prefer the moving fader technique, almost universally adapted for automated or robotic control of channel balances in the finest studio consoles. Motor driven variable gain controls are used to adjust mix levels and the best of these systems have evolved to survive critical professional scrutiny. Certainly the most important of today's recordings have had their signals controlled by mechanical attenuator cores used in these devices. The more robust and sonically neutral of these electrically operated components have a common lineage with the uncompromised manual controls used on Spectral preamplifiers.


Moving controls can achieve a substantial performance advantage when there is no exchange of energy between control and amplifier. Only the Spectral gain module and a few rare vacuum tube circuits can provide this virtual open circuit interface along with low noise Class A operation. Almost all IC amplifiers can not. Usually the stressed operation from excessive feedback, consequences of low voltage operation, and reflections from interconnect cables conspire to create large activity bursts within most of these amplifiers. Such distortions sound ugly and electric charge from these can get out. As a result there is little more to be lost from the changeover to solid state controls. Without the inert or infinite impedance connection provided by the Spectral gain module, any energy coupled from the audio signal as well as internal amplification events can become memorialized as microscopic or molecular thermal and voltage stress along moving contacts. Consequences from even the fastest of these events can alter audio transmission later. Sometimes called thermal or charge injection tails, the damage to ongoing signals can be observed and this disturbance is completely eliminated by Spectral's high-speed double cascode circuit and floating shield configurations. Performance accuracy is better than one part per million. Speed is better than one part per million. Speed is independent of settings, and signal events are not memorialized within the level control itself.


Prior to the DMC-30 preamplifier, remote control features created a sonic hindrance to better quality equipment. Only the moving fader recording consoles have maintained their manual performance. Now Spectral has improved preamplifier sonics by combining relay DAC with moving control operations. Microprocessors as well as user presets select optimum gain structures and other operational parameters to provide maximum dynamic range headroom with the lowest amplifier stress. Now the system configuration is best for signal and operational conditions. Unlike manual preamplifiers, the important functional components, such as switches and gain controls are placed in a tight optimum signal path. This arrangement assures full utilization of the high-speed balanced gain module as well as isolation of noise that might propagate from external video and digital equipment in the home audio and theatre environment. This construction and design technique is the method of choice for test instrumentation and recording consoles when compromise of any kind is barred.


The Spectral DMC-30 is a true dual monaural reference preamplifier which incorporates a long list of technical innovations and enhancements to achieve new heights of signal linearity and speed of signal response. The result is stunning musical coherence and transparency. A tour through the DMC-30's outstanding features will server to highlight its remarkable capabilities.


It is Spectral's growing conviction that analog circuits, although conceptualized in two dimensions, should be realized so as to take full advantage of all three physical dimensions for ultimate performance. In the DMC-30 the power and ground conductors, and housekeeping functions such as ferrying control signals from the front panel to the relay system, are carried on a slide-out "mother board." The amplifier modules for line stage amplification and balanced input conversion, are located on their own smaller PC boards which float above the mother board. Power and ground connections are made by "popping up" the relevant conductors at the precise points where they are needed in each circuit.


The gain stage of the DMC-30 employs a cascode front end for improved linearity and complementary power MOSFETs (of a custom-made, proprietary design) as output followers. Localized feedback is used to keep overall feedback at an optimum level, for nearly spontaneous correction of output errors. The DMC-30 exhibits familiar Spectral hallmarks of extremely wide and stable bandwidth and lightning speed as measured by the very fast rise time (70 nanoseconds) and very high and symmetrical slew rate.
The output section of the DMC-30 is virtually a miniature Class A power amplifier, including inboard heat sinks. Its unusually high-current output (up to one ampere per channel) is a result of Spectral's research into the demands made by complex, high quality audio interconnect cables and sophisticated low impedance power amplifiers. This preamplifier's high output current allows it to dominate and control the cable load, making it self-isolating from the sonically disturbing effects of cable energy storage and reflections.
The DMC-30 also produces output voltages more typically associated with tube preamplifiers (80 volts peak-to-peak maximum). As a result, the DMC-30 offers the best of tube-like performance, characterized by a sense of total openness, lack of strain and crystal clear dynamic resolution at both low and high levels.
Today's audio systems are becoming bigger. As complexity increases, the noise propagation through interconnects and power cords must be addressed. Multiple power amplifiers, large screen video equipment, digital players, and computers operate to produce noise at many unsynchronized interference frequencies. Such spread spectrum interference can create ample opportunity to infiltrate and degrade the fragile audio signal path. The DMC-30 is designed to have stealth in this environment. Its tight signal and grounding paths are immune to external influence and its powering has a sophisticated multiple isolation strategy. Inside the DMC-30, the primary balanced gain amplifier module floats on very high impedance power rails. Voltages on those lines are three times higher than the best IC preamplifiers can withstand and this configuration simulates battery powering with far superior regulation.

In the DMC-30 separate power supplies are used for the microprocessor, display, volume control, relays and audio circuits. Each of those functional circuit groups must be carefully isolated from one another and the DMC-30 uses the floating power system to accomplish this. Each system has its own grounding and power paths and only one connection for each is referenced to a main copper ground. Since each path is open ended, electrical currents related to an activity within one group can not propagate to another. This arrangement and the floating regulator can maintain a very low noise, virtual battery like setting or environment for the high-speed DMC-30 amplifier module.

Custom power transformers are internal to the DMC-30 preamplifier and are built with a very large separation between the potentially noisy line input winding or primary coiled and output sections energizing the system. This approach mechanically spaces and isolates line induced noise emanating from subwoofers and large power amplifiers. Nine individual filters remove radio interferences to assure a very quiet battery powered type of environment for audio signals within the DMC-30. Medical equipment has similar construction to prevent circulating noise currents and the overall audio path contamination is often hundreds of times lower than from conventional audio components built with torroidal power transformers.


Most remote controlled audio systems use relays to rout end switch signal paths. Top designers know that sonic and electrical performance can equal wired connections when the electrical contacts are gold and mechanically wipe to expose a new molecular surface each time the relay is energized. Unfortunately as systems get bigger, the electrical signals operating these relays get noisy and cross couple to audio signals. Many examples of reduced staging dimension and uninteresting playback can result from such oversights. To avoid this cross interference problem, all relay coils n the DMC-30 operate at ground potential and individual filters to prevent any interaction through control signals. The main body of the relay then acts as a shield and helps produce a routing path much superior to the wiring and long circuit board traces to panel components of manual preamp systems.


Traditionally, protection circuits have intruded sonically on the program signal, either because of the contacts between the circuits or the sensing techniques used.

With the DMC-30, Spectral becomes the first audio manufacturer to utilize optical coupling to eliminate this problem. The protection circuits (which protect against DC offset in the inputs, short circuits, and so on) in effect "look at" the music signal without "touching" it. They have their own independent power supply connection and ground returns, as well as a defeat switch which allows the protection circuitry to be bypassed for diagnosis and service.

The result is a still quieter, more noise-free signal bus, conveying maximum ambiance and low level detail through the music system to the listener's ears.

Remote control operations in DMC-30 include signal routing choices, gain and balance adjustments, and channel function modes. These and many internal operations for startup and protect conditions are microprocessor controlled. Normally, remote control systems of this functional complexity stay digitally active and ready to respond to commands and the results are evidenced by the strobing or flickering displays as well as noise added to audio signals. To maintain a silent environment for audio signals, all digital activity within the DMC-30 is shut down once an operation change is complete. Even the display is latched to fixed states during the microprocessor sleep interval. Clock cross talk becomes nonexistent and can not propagate to add jitter to digital playback.


Just as control processors pollute the sensitive signal path of remote control preamplifiers, front panel displays and LEDs and their digital interface introduce switching and digital noise. Display clock interaction occurs in today's most ambitious preamplifiers with resulting losses of resolution and transparency. To conquer this distortion Spectral introduces clockless displays on the DMC-30 preamplifier using analog indicator arrays. The LED bar displays and annunciators feature current-mode operation with separate power supplies and are transient and flicker-free. With this display system, interaction with digital clocks and signal path interference is virtually eliminated.


In addition to directing the internal gain settings and balance requirements of the DMC-30, microprocessor systems also control various features and housekeeping functions. Software control of the motorized volume fader makes possible a progressive ramp-up function for the mute control. Available rate selection allows fader control speed o be adjusted by the user. A remote operated reset function allows all preamplifier modes and functions to be bypassed and reset to start-up positions with one button. Even protection functions are under the eye of the DMC-30 processors, with programmed start-up and protection programs to monitor all aspects of circuit operation and system status. Unlike embedded processors in conventional preamplifiers, DMC-30 programming software is fully upgradable. Removable memory allows easy feature revision as future software developments occur.


While no high-end audio component can be expected t be absolutely "future proof", the DMC-30 preamplifier anticipates state-of-the-art advances in design with its fully modular circuit architecture and its upgradable control software. The fully balance output section and balanced input section (optional) are built on easy exchange modules. For service and upgrade, even the main signal buss and power supplies are incorporated on the removable Systemboard assembly. Front panel control commands and future options are also updated easily in the field via the removable DMC-30 software module.


Experienced enthusiast know that preamplifier quality holds and essential key to sonic performance in today's high-end music systems. Spectral's engineering focus on preamplifier development for over two decades reflects a keen awareness of the major impact this crucial component makes. The DMC-30 reference preamplifier reflects our effort to combine or renowned high-resolution signal amplification technology with innovative control systems for a new level of musical refinement and user convenience. In keeping with its role as the reference heart of the most ambitious music systems, every DMC-30 represents the absolute summit of materials and workmanship. The exhaustive testing and burn-in standards employed by Spectral in its production are unparalleled in the audio industry.

Like all Spectral components, the DMC-30 incorporates totally modular design for fast and easy diagnosis and service, and to make any DMC-30 purchased today readily upgradable to future Spectral standards.

Is the DMC-30 Reference Preamplifier the "ultimate"? Ultimate status can only be conferred over time by the world of serious and discriminating music enthusiasts. We know that the DMC-30 sets new standards for musical realism and transparency in a full feature preamplifier with remote control convenience. It combines the flexibility to accommodate the demands of evolving music systems with a new benchmark in listening satisfaction. We are pleased to offer the DMC-30 as the epitome of Spectral's long experience in preamplifier engineering and design and confident that it will bring fine recordings to life in the systems of music enthusiasts for years to come.

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