Spectral Audio DMA-180 High-Current Reference Amplifier

The DMA-180 High Resolution Amplifier

The DMA-180 High Current Reference Amplifier is the first of a new generation of Spectral amplifiers. Like other milestone Spectral products, the DMA-180 is more than an uncompromising audio component of the first rank. It opens a new chapter in the continuing search for an ideal standard of musical reproduction.
For several years, the development paths followed by manufacturers of the most premium grade, solid state power amplifiers diverged. Some makers pursued the goal of higher and higher output current with almost single-minded determination. Their super high current designs grew impressively in size and weight, as well as in the operating temperatures they generated.
Spectral designed and built very powerful amplifiers as well, but with less emphasis on the current component of output power, and more emphasis on Spectral's traditional values: lightning fast responsiveness, ultra-wide stable bandwidth, and uncompromising linearity.
Clearly there are musical issues underlying these two divergent design philosophies.
The trend towards very high current amplifiers coincided with the development of large, complex speaker systems with elaborate networks for crossover, equalization and balancing of the system and drivers. Conflicts between the control of circulating energy and fine tuning were most readily resolved by employing amplifiers with brute force output capability. The objective was to achieve unshakeable control, along with the dynamics and bass power to reward the listener with a room-filling presence. However, component limitations led manufacturers to build monumental amplifiers of often ponderous design and execution. The price of power became the diminished ability to listen long and deeply into the music.

Spectral has always hewed to the line that an audiophile amplifier of uncompromising quality must do many things well. Extraordinary resolution, ultra-wide stable bandwidth, minimal energy storage, the absolute ability to manage load reactions and to minimize interactions with other components in the audio chain: these are factors which translate directly into truthfulness and realism in musical reproduction. As long as it was necessary to compromise them in order to deliver the single goal of very high output current, Spectral was unwilling to make the trade-off.

THE TRADE-OFF TRANCENDED

Spectral's DMA-180 High Current Reference Amplifier finally transcends the historical trade-off. As both a true high resolution Spectral amplifier and a true high output current amplifier, the DMA-180 inaugurates a new generation of high performance power amplifiers without compromise. New insights garnered from a decade and a half of primacy in high-speed audio amplifier design, new technology from the instrumentation and recording industries, and ever more sophisticated manufacturing skills have made this new generation of Spectral amplifiers possible.

Over the years, Spectral's engineering talents have explored every nuance of designing an almost "infinitely fast" amplifier. Very fast slew rates, very short rise and settling times, very broad bandwidth, very swift input-to-output propagation: these are some of the critical factors which help to achieve the most perfect tracking of the music signal and the lowest possible distortion. Speed is particularly associated with the detailed, delicate reproduction of transient musical events, and the realistic, brilliant and timbrally correct presentation of strong impulsive sounds. Examples include voice articulation, and the effects of muted brass and cymbals. These subtleties are often far more central to authentic recreation of the quality of a musical performance than all the industry standard test bench measurements combined.

The fundamental problem has always been how to provide high power at the upper frequencies without introducing artifacts. Electrical energy radiates, and the more one pushes the performance envelope, the harder it is to deal with this radiation. In traditional architectures, it cross-couples into other parts of the circuit, confusing the feedback, and contaminating the incoming program signal with distortion and phantom signals. Reducing this field-interactive distortion to below noise level is an imperative, and is one of the great arts of amplifier design.

The key lies in exquisitely tuned circuit layouts, often interleaved or twisted to eliminate stray electromagnetic field interaction. It also requires the shortest possible signal paths, the ideal choice and placement of a variety of amplifying devices, and boundless attention to a host of other details, including - to name but a few - transformer design, power supply components, printed circuit board materials and grounding systems.

From the time it introduced the world's first high speed, DC-coupled FET power amplifier, the DMA-100, in 1982, Spectral has maintained a reputation for audio components whose circuit design, exotic high technology parts and materials, and manufacturing quality set them apart from all others.

Combining high output current capability with super-responsive, high resolution performance reintroduces the problem of electromagnetic field coupling with a vengeance. Perhaps only a company with Spectral's sophisticated understanding of high speed circuit behavior could have hoped to solve it. The solution required a return to the fundamentals and the invention of a radical new Vertical Dimension Topology and an equally radical new distributed and integrated Focused Power array to square the circle.

With these and other innovations, including fully balanced, completely symmetrical circuitry, non-invasive DC servo control, and specially designed high precision power transformers, the DMA-180 achieves impressively high current output while surpassing even Spectral's previous unmatched standards of high resolution performance. It is an example of the art of amplifier design at its finest, and a bright augury of a new generation of Spectral power amplifiers for the years ahead.

VERTICAL DIMENSION TOPOLOGY

Two-dimensional schematic drawings usually provide a fairly good representation of an amplifier's actual, physical layout. This is because most amplifiers utilize an X-Y horizontal layout in which the signal comes in at one end, moves through a single plane, and exits at the other end. Unfortunately, large field generating parts and unstable distributed wiring are not scaled. Once these parts are spread out to accommodate their size, effortless speed becomes impossible. Often the output section will behave like a radio transmitter, literally broadcasting noise across channels and back into earlier stages.

The DMA-180, by contrast, employs a vertical signal path completely new in audio design. The signal literally drops down into the amplifier circuitry along the Z axis, passing through layers of X-Y layout. At the output stage, for extra protection against electromagnetic field interaction, the traces are twisted into field-canceling "knots". This radical new Vertical Dimension Topology is harder to design and harder to implement. But it is one of the keys to making a high-current, high speed amplifier possible.

PERFECT SYMMETRY

Musical realism and transparency in playback require near-perfect waveform tracking and suppression of cross-talk and other noise. The DMA-180 circuits are balanced all the way through, including balanced feedback, for an ideal configuration. The power output stage utilizes push-pull drivers, contributing to the amplifier's extraordinarily fast settling time (settling to -40 dB in 1.5 microseconds). The input and gain sections are configured as back-to-back double cascodes, and the entire circuit is completely symmetrical. This insures that the positive- and negative-going halves of the musical waveform take parallel paths through identical circuitry, resulting in exceptional slew symmetry and overall accuracy.

PROTECTING WITHOUT INTRUDING

The DMA-180 employs a variety of specially developed new nonintrusive protection measures to enhance performance and ensure system safety.

For the first time, output DC offset is controlled by an innovative thermally coupled servo. This strips DC from the amplifier's output, automatically zeroing and protecting the speaker system without electrically intruding on the music signal in any way.

Overload protection is performed by circuitry which looks at the voltage and current product, detecting temperature as well as peak program. It operates on a current limiting principle which leaves the offset bias untouched, thus providing instant onset and virtually instantaneous recovery.

The DMA-180's power-on sequencing is also controlled by current sources. This eliminates the need for relays, which can contaminate the signal as well as pose reliability problems.

FOCUSED POWER

"Power supply" and "output device" are two common and important terms used in describing any amplifier. The discussions of each are usually quite separate because the items themselves are physically quite separate. Not in theDMA-180. The power supply in the DMA-180 is plural - literally a distributed power array in which a separate power supply with its own high-energy filter capacitor and rectifier circuit is physically sited directly adjacent to each output device.

This unique Focused Power geometry is made possible by recent advances in very compact, low ESR filter capacitors used in high speed switching power supplies for computers. Complex wire harnessing and bulky filter banks are rendered unnecessary, eliminating both the power lags and losses and the field radiation effects associated with conventional power supply designs. The result is the DMA-180's remarkable rise time of 100 nanoseconds, a major source of the amplifier's extraordinary articulation and loudspeaker control.

A further dimension of loudspeaker control is the amplifier's ability to handle the electrical energy stored by the loudspeaker and returned to the amplifier. Such load reaction can have very disturbing sonic consequences. With an extremely low internal impedance which is uniform throughout the entire frequency range, the DMA-180 presents a virtual "dead short" to any energy returned by the speaker, sending it to a zero potential point and extinguishing any possible instabilities or distortions which could result from it. This high damping factor capability is a vital prerequisite for dealing with some of the most sophisticated and complex of contemporary speaker systems.

SPECTRAL IN THE DETAILS

The proprietary "microdetailing" circuit refinement techniques developed at Spectral are able to isolate fleeting low level noises and distortions as much as -100 dB below musical program material. Spectral's unique and innovative tone cluster test, developed by Director of Engineering Keith Johnson, accurately simulates the upper harmonics of musical instruments. This permits the circuit designer to pinpoint and eliminate audible subharmonic intermodulation distortions created by inaudible high frequency transients. The results of this test correlate very strongly to subjective impressions of resolution, focus and air, and a lack of muddiness, or subtle confusion, when listening to music.

Spectral's characteristically painstaking attention to detail carries through to every particular of the componentry and construction of the DMA-180. The unusual demands of the DMA-180's output section required a new kind of transformer design. The result is an innovative audio grade stack laminate power transformer precision-made for ultra-stiff regulation and dense field structure. Superior to toroidal designs in reducing high frequency coupling from primary to secondary, this new stacked design is particularly suited to dealing with today's noisy power lines which often serve as carriers of high frequency digital pikes generated by CD players, home computers and the like.

Spectral's philosophy has always been to use "the right device in the right place", matching the specific performance characteristics of each type of amplifying device to the specific requirements of each place in the circuit. In the DMA-180 this philosophy has been carried to new levels of refinement, yielding an optimal combination of carefully selected and matched vertical and lateral MOS-FETs, junction FETs, monolithic arrays, and super high frequency bipolars. The bias setting for each output device is performed independently after extensive burn-in and maturing, producing lifetime calibration individually tailored to each unit. For the highest possible linearity and stability, we believe there is no substitute for the individual bias adjustment and matching of each output device, as has been traditional with the finest tube amplifiers.

Following the precedent set by Spectral's state-of-the-art DMC-20 Reference Preamplifier, the DMA-180 employs exotic, phase compensated internal cabling designed and produced exclusively for Spectral by MIT (Music Interface Technologies).

CIRCUIT DESCRIPTION

Input Stage. The shielded input stage utilizes a balanced, double cascode design employing a matched differential pair of J-FETs. The cascode design derives its own ground and produces an extremely high level of isolation from the preamplifier and between the stages of the DMA-180 circuit itself.

Gain Stage. The gain stage utilizes super high frequency (RF) bipolar transistors in a cascode configuration to bring the circuit into the power device realm.

Output Stage. The power output stage utilizes push-pull drivers for the power MOS-FET output devices. The DC bias setting for each output device is performed independently after burn-in. An individual high speed switching power supply filter capacitor and rectifier circuit is sited immediately beneath each output device on the printed circuit card. The total energy storage capacity of the distributed power supply array is 256 watt-seconds per channel. The DMA-180's Vertical Dimension Topology allows the signal carrying traces to enter the output stage along the vertical, or Z axis, and all signal and current carriers to be twisted into electromagnetic field-canceling knots.

RADICAL IN DESIGN AND QUALITY

This discussion has focused on the radical innovations to be found in the DMA-180's electronic design. While this is the "news" about the first high current, high speed audio amplifier, it should not obscure the fact that the DMA-180 also continues the Spectral tradition of radical quality in every detail of materials and craftsmanship.

This obsession with quality is reflected by the exotic high frequency printed circuit board materials from Tektronix, the military-grade hand silver soldering, the precision metal film resistors, heavy gold-plated connectors and the heli-arc welded, Class A Polane painted aluminum case. It is also reflected by the most stringent quality control standards in the audio industry, including hundreds of hours of continuously cycled burn-in, painstaking individual calibration and a final personal listening test for every unit performed by the company's founder. Spectral's tradition of modular design and construction greatly facilitates the infrequent servicing needs and the periodic optional upgrading of its products. The DMA-180's new Vertical Dimension Topology and Focused Power concepts actually enhance the modularity of the amplifier's design.

In the end, it is always and only the musical listening experience that counts. With the fastest output launch and the most linear signal response in the industry, the DMA-180 High Current Reference Amplifier offers a whole new perspective on fine recorded music. Subtle colorations and phase distortions are banished, and the complex fabric of transients, dynamics and timbres is resolved with grace and utter realism. Never before has a Spectral amplifier been able to dominate and control the most complex, reactively demanding speaker loads with such ease. Never before has the discriminating music listener been offered such a wholly satisfying choice in a fine audio power amplifier.

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