Spectral Audio DMC-20

The DMC-20 Reference Preamplifier

The Spectral DMC-20 Reference Preamplifier is a response to opportunity and challenge. The opportunity is given by new, ever-advancing technology and design insights. The challenge is presented by home audio systems of greatly increased complexity and sophistication. Once every decade or so such a combination of circumstances gives rise to a new, benchmark product which sets an uncompromising standard of performance for years to come. Spectral offers the DMC-20 for your consideration as the preamplifier by which all others may be measured for the 1990's and beyond.
Spectral was founded in the belief that the world needed a better preamplifier, and that we knew how to make it. Our first product, the Spectral MS-ONE, introduced in 1976, was a radical departure from earlier premium preamplifier design and construction. Its true dual monaural topology featuring extremely fast and wideband all-FET circuitry represented an extraordinary rethinking of conventional ideas. Innovations like its direct-coupled circuits, moving-coil-ready phono stage and independent power supplies with AC line conditioning were made possible by the use of then-esoteric devices and techniques pioneered by advanced high technology disciplines such as instrumentation and microwave. The MS-ONE launched Spectral as "the audio maverick from Silicon Valley," a characterization which is still apt, although undergirded in the intervening years by numerous highly successful product introductions and the satisfaction of music enthusiasts all over the world.
Starting in 1980, the DMC-10 became Spectral's flagship preamplifier, bringing MS-ONE design philosophy and performance within the range of a much larger audience. In production for over a decade, the DMC-10 is esteemed by many as the longest-lived of classic preamplifier designs.
In the mid-1980's, Spectral determined that the time had come to take a completely fresh look at preamplifiers in response to, and anticipation of, new developments in audio technology and user preference. We embarked on the creation of an all new, top-of-the-line preamplifier based on cleansheet engineering design. The DMC-20 is the result of this long and searching process.
Above all, the DMC-20 had to be a lasting design. Spectral's lengthy product development programs and small scale, handcrafted production methods can only be justified by non-obsolescent, almost ageless designs. Designs characterized by state-of-the-art performance, durability and the capacity to evolve.

Such a design is the DMC-20.

DESIGN FEATURES

The spectral DMC-20 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-20's outstanding features will serve to highlight its remarkable capabilities.

THREE-TIERED ARCHITECTURE

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-20 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 optional phono stage 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. And fully 90% of the signal path is carried on specially designed, phase-aligned, individually terminated MIT/Spectral cabling which acts as the preamplifier's signal bus. With the DMC-20, Spectral for the first time carries the audiophile cable concept which it pioneered with MIT into the active heart of the amplifier circuit.

This three-tiered architecture offers the most innovative and uncompromising response to the classic preamplifier challenge to reduce the contamination of sensitive, low level music signals by extraneous noise to an absolute minimum. Power, ground and signal conductors no longer snake around one another on two dimensional printed circuit boards with their inherently imperfect dielectrics.

Spectral's polysulfone (Teflon derived) printed circuit boards, custom made for us by Tektronix, have no rival in the audio field. But the low level music signal's extreme sensitivity to radiant energy fields makes it imperative, we believe, to remove it from the crowded domain of the PC board at every opportunity.

The signal bus is routed in bundles to prevent interference from radiated fields, and interconnections are made at a 90 degree angle to eliminate crosstalk between channels. Intricate and costly, this approach is a challenge to engineers and craftspeople alike. But we think the resulting sonic purity and impeccable transparency make any other approach unthinkable for a state-of-the-art instrument like the DMC-20.

LINE LEVEL GAIN STAGE

The final gain stage of the DMC-20 employs a cascade 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 signal errors. The DMC-20 exhibits the 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-20 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 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-20, powered by the massive DMS-20 Power Supply, also produces output voltages more typically associated with tube preamplifiers (80 volts peak-to-peak maximum). As a result, the DMC-20 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.

UNIVERSAL PHONO MODULE

the Model 202 Phono Module is direct-coupled, employing a carefully tuned DC servo to avoid the necessity for any capacitor in the signal path. Since all capacitors, however finely made, add some sonic coloration, the benefit of this approach is clear.

DC servos are a relatively unsung area of audio design in which many potential problems lurk. Spectral's Keith Johnson has specialized in the design of sonically transparent servos which avoid such difficulties as imperfectly calculated reaction times, causing the servo control and the music signal to interact. The DC servo employed in the Model 202 Universal Phono Module is the product of a year of development effort.

Accurate tracking of the RIAA equalization curve is a more difficult task than is generally appreciated, and improvements in this area yield quite appreciable sonic results. By using extremely high quality components (custom made film foil construction Teflon capacitors and better than 1% tolerance Vishay resistors) in a passive-active network, the model 202 Phono Module sets new standards of phono equalization accuracy.

At the ultra-low signal level of moving coil cartridges, it is mandatory to maintain signal-to-noise ratios of astronomical levels. Here is where the benefits of the DMC-20's three-tiered design with its exceptional ability to banish noise, are most in evidence. At this stage, every consideration - even thermoelectric effects between connectors and solder - must be examined and controlled. Knowing that any nuance of the music signal which is lost at this stage of the audio system can never be reconstructed, we individually select the fastest, lowest noise devices (J-FETs) for the front end of the phono module. Large area gold switch contact posts rather than conventional tin-plated contacts are employed for the same reason.

RELAY SYSTEM

The relay system allows choices made by the user at the front panel of the DMC-20 - selection of signal source, tape monitoring, absolute phase, stereo/mono and muting - to be executed at the optimal points in the circuitry without requiring that the all-important music signal be shunted to the front panel and passed through a switch mechanism for each of these functions. This maintains the shortest, most straight-line signal path, a vital consideration for preamplifier performance.

OPTICAL PROTECTION CIRCUITRY

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-20, 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 return, 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.

BALANCED INPUT MODULE

Spectal took an unhurried approach to introducing all-balanced systems because of our conviction that properly designed and executed single-ended systems, with careful grounding and shielding, could not easily be surpassed in musical performance. Many consumer audio manufacturers jumped into balanced systems in emulation of Pro-sound practices, where, because of extremely long cable lengths and a multiplicity of devices at different potentials, problems of noise contamination and ground currents require balancing for recording systems to function at all. These conditions do not prevail in home listening environments.

Nevertheless, the trend is toward all-balanced operation, with some of the finest source components now offering a balanced output signal. We believe that the DMC-20 sets the standard for "balanced done right" and is the first truly differential preamplifier for use with premium balanced amplifiers.

The DMC-20's optional balanced input module employs only discrete circuitry, like all Spectral products. It is perfectly matched in terms of gain, frequency and phase response, offering fully differential operation, with uncompromised common mode rejection, over an extraordinarily wide bandwidth, from DC to 15 megaHertz. Very high frequency differential operation to perfectionist standards demands such careful engineering that most audio devices are not even measured (or their measurements are not published) in this region. But we believe that it is in the very high frequency range that common mode rejection is most needed (or the noise "seepage" will find its way back into the audible range). This is another instance of Spectral's importing the more stringent standards of other high technology disciplines for the first time into a home audio product.

DMS-20 POWER SUPPLY

One look at the DMS-20 confirms the boldness and meticulous attention to detail with which Spectral engineered this ground-breaking preamplifier power supply. Using cascaded power supplies, individually regulated for each rail of each channel, the DMS-20 delivers flawlessly clean and well-regulated DC power to the DMC-20, completely isolated from ground currents and decoupled from high frequency interference. With a total output of 80 volts, rare for a solid state device, the DMS-20 has the reserves of a 100-watt-per-channel power amplifier.

PRINCIPAL CONTROLS

Volume. The DMC-20's volume control is a custom made, conductive plastic, precision stepped attenuator built to Spectral's design. Our extensive comparative testing confirms that this device, with its satisfying torque feel, offers by far the most sonically transparent operation, precise tracking (channels within .10 decibel) and extended life of any volume attenuator made today.

Balance. Spectral takes the most uncompromising approach to balance control, utilizing a precision stepped attenuator with 14 settings employing metal film resistors. The DMC-20's balance control features a center position bypass - in this position the control is out of the circuit altogether.

Input selector. The input selector is not in the signal path, but controls the relay logic system which switches between source inputs at optimal positions in the circuit. This sealed, mil-spec rotary switch features highest-grade heavy plated gold over coin silver contacts for extended durability and reliability.

IN THE SPECTRAL TRADITION

In keeping with its role as the preeminent reference preamplifier of its time, every DMC-20 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 products, the DMC-20 incorporates totally modular design for fast and easy diagnosis and service, and to make any DMC-20 purchased today readily upgradable to future Spectral standards.

Is the DMC-20 Reference Preamplifier a "classic"? Classic status can only be conferred over time by the world of serious and discriminating music listeners. We know that the DMC-20 sets new standards for musical realism and transparency. 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-20 as the epitome of Spectral's long experience in preamplifier engineering and design, and confident that it will represent the Spectral name well for many years to come.

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