Spectral Audio DMA-250 High Resolution Amplifier

The DMA-250 High Resolution Amplifier

The DMA-250 High Resolution Amplifier shares with other Spectral components the creative uncompromising use of edge-of-the-art technology, dedicated precision manufacture, and Spectral's experience with live sound at recording sessions. From extensive listening and design experimentation, Spectral has consistently pioneered component systems having fast settling, high-speed circuit architecture. These have invariably outperformed more traditional often cumbersome high-end design methods to achieve superlative detail, staging and listener involvement. Indeed, test methodology simulating music waveforms and hearing acuity confirm that quickness of response and rapid signal extinction when reproducing complex dynamics are mandatory for accurate music reproduction. Only then can one preserve instantaneous waveform accuracy to prevent cover-up of delicate musical signals by previous events. Construction and performance of the Spectral DMA-250 High Resolution Amplifier for this demanding criterion and resulting sonic resolution are unmatched among contemporary high-end amplifier designs..

BEYOND CONVENTIONAL DESIGN SOLUTIONS

Like the innovative DMA-360 Monaural Amplifier, the DMA-250 utilizes Spectral's unique "Focused Power" output section design with powerful ultra-fast Mega FET devices. Each device, having vacuum tube like operating character, is energized from its own dedicated high energy storage capacitor, rectifier, and individually powered from an isolated ultra-low coupled transformer winding. Individual Teflon bias trimmers are used to calibrate each output device separately. Groups of these individual output device/power supply circuits utilize field folding construction and electronic shielding to further impede propagation of stray interference. The resulting "Focused Array" of individual isolated output sections performs as one with virtually no cross-coupling and energy storage artifacts reflecting between output devices. During extreme program dynamics, this arrangement can launch an instantaneous high current drive of over 70 amps per channel to the most sophisticated loudspeakers with assured precision waveform tracing. Gone are performance damaging magnetic and electrical field propagation problems of conventional multiple device heavy construction. Without stray radiation, critical small signal paths within the DMA-250 and other sound system components can perform with lowest distortion and settle to signal extinction in micro-seconds. Hence, the DMA-250 works with power and speed yet behaves inert to other electronic parts of the system. Reproduction is highly articulate and resolving yet has all the powerful unlimited character of the best very high power amplifier designs, giving little suggestion of its actual 200-watt-per-channel output specification.

THE POWER VAULT

The Focused Power output section of the DMA-250 is only one of the many Spectral innovations designed to create the ultimate in amplifier control and resolution. A unique transformer architecture supports the critical requirements for lowest noise and high power capability of the powered output section array.

In traditional high-end power amplifiers, output transformers are located away from active amplifier circuitry for purposes of noise isolation and construction ease. This virtually universal method extracts a significant sonic price, however, because of the speed and regulation losses which are imposed by the long power supply wiring harness. In addition, the EMI and noise which radiate from the supply harness invariably contaminate sensitive low level circuitry by noise modulating the musical signal. Spectral engineers have long recognized the problems associated with power supply noise contamination and have proposed a unique solution in the DMA-250 High Resolution Amplifier. The Power Vault electromagnetic field containment system virtually eliminates EMI radiation to the amplifier signal path circuitry as well as power and regulation losses through the traditional power supply harness.

The architecture starts with Spectral's elaborate high performance custom power supply transformers. These unique designs are the result of years of research and development to maximize isolation and regulation. Each transformer features individually isolated windings to support each individual output transistor in the amplifier output section, a premium technique shared only with the Spectral DMA-360 Monaural Amplifier. The twin power transformers are mounted between massive machined mounting plates for damping and resonance control. The combined 38 lb. transformer structure is then mounted on a constrained rubber floating suspension system employing multiple compliances similar to a fine turntable suspension. The finished transformer assembly is then suspended inside an EMI, RFI tight welded aluminum containment box located directly beneath the DMA-250 output section. The resulting transformer Power Vault containment system results in an uncompromising solution to the universal power supply noise problem. The high-current power transformers are now intimately sited next to their own high-current output devices with extraordinary noise isolation. The high-speed driver circuitry of the DMA-250 operates in a pristine environment free of the noise radiation and phantom signals which plague other power amplifiers. Dynamic range limitations are removed with a sonic clarity and quietude which is palpable.

PREMIUM DISCRETE CIRCUITRY

Small signal circuitry is based on Spectral's proven discrete circuit multiple cascode double push-pull FET technology. Premium silicon array construction, similar to circuitry in the DMC-30 Reference Preamplifier, is applied to the DMA-250 to achieve enormous internal dynamic range capability. This reserve is linear class A with many times greater dynamic capability than program demands. It allows high current drive for quick controlled response from the large output FET's as well as isolation from their internal electronic activity. Unhindered by interferences and slow cumbersome response of conventional practice, these parts operate with unyielding control and exacting precision. Reproduction is clean, effortless and highly holographic.

Audio amplification paths through the DMA-250 are direct, inherently linear and simple. These minimalist configurations have always sounded and measured best but normally become ponderous and overburdened when protection and support functions are added. The DMA-250 takes a different route to solve the sonic problems of amplifier protection circuitry. To maintain the pristine signal path, a sophisticated analog computing ancillary system is thermal and opto coupled within the amplifier. This "hands-off" operational management system observes device loading and power dissipation as well as speaker damaging out of range signals. It takes control without circuitous cross interfering connections to electronics in the signal path, leaving the musical signal pure and totally untouched.

THE TOPOLOGY

The DMA-250 utilizes FET and high speed bipolar transistors in a dense, fully balanced topology to achieve high-current launch and ultra-fast settling capability. RF (radio frequency) design techniques are employed to achieve a DC coupled power band with intrinsic bandwidth to almost 5 MHz.
Input Stage. The shielded input stage utilizes a balanced, double cascode design using matched differential pairs of J-FET's. The cascode design derives its own ground reference and produces extremely high levels of isolation from the preamplifier and between the stages of the DMA-250 itself.
Gain Stage. The gain stage utilizes super high frequency (RF) bipolar transistors matched 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 custom VMOS-FET output devices. The DC bias setting for each output device is performed individually after burn-in through Teflon trimmers. An individual high-speed switching power supply filter capacitor and full rectifier circuit is sited immediately next to each output device on the circuit board. The total energy storage capacity of this distributed power array is 256 watt seconds per channel. Each output device is also given its own individual set of high isolation transformer windings. The DMA-250's Vertical Dimensional Topology allows the signal traces to enter the output stage along the vertical, or Z axis, and all signal and current carriers are twisted into electromagnetic field-canceling knots.

POWER AND PRECISION IN A UNIQUE MARRIAGE

Improvements in high-current FET technology, high-speed amplifier topologies and new protection systems are combined in the DMA-250 dual monaural amplifier to successfully advance the art of amplifier design. Until now, amplifiers could not offer both powerful speaker control capability and the sonic transparency and life possible only with high-speed, fast settling circuitry. Breakthrough circuits backed by Spectral experience have now brought together the necessary combination of speed, accuracy, control and stealth. The DMA-250 High Resolution Amplifier advances the Spectral tradition for musical resolution and sonic holography. Although of moderate size, the DMA-250 offers remarkable sonic sophistication and drive precision for the most demanding system applications.

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