Unison Research Unico 90

Unison Research Unico 90 Hybrid Integrated Amplifier

The Unico 90 is a hybrid integrated amplifier developed according to specific sonic and scientific criteria set by the Unison Research design team. These criteria have resulted in a product distinct from anything else Unison Research has ever released, making it different from every other amplifier, whether tube or solid-state, on the market.

Design Philosophy

The Unico 90 is a model of structural simplicity. Modern amplification devices inherently introduce distortion as amplification stages are added. The more stages present in a design, the more complex the distortion structure becomes. Therefore, structural simplicity is a highly sought-after goal in amplifier design.

Unison Research's development process led to two circuit performance standards essential for achieving high performance: a three-stage circuit and a two-stage circuit, both designed with zero global feedback. The three-stage circuit is used in the Unico 150, which is better suited for higher-power integrated amplifiers, while the two-stage circuit used in the Unico 90 is optimized for medium-power amplifiers.

Both approaches enable Unison Research to achieve exceptional linearity, a standard difficult to reach with conventional technologies. The linearity of these designs closely matches that of high-end solid-state amplification.

Circuit Design of the Unico 90

At the core of the Unico 90 lies its hybrid circuit structure. All gain, and ultimately the amplifier’s sonic character, is shaped by a single tube stage using ECC83 and ECC81 tubes in a multiple-parallel totem pole configuration. Each channel is equipped with six triodes, arranged to provide limited gain while ensuring the lowest possible output impedance.

The choice of triodes was based on extensive testing and research, considering factors such as:

• Triode linearity, ensuring mutual and automatic compensation of nonlinearity.
• High linear signal excursion and current capacity, necessary to drive the complementary HEXFET pairs in the output stage.
• A dedicated gain-stage triode with partial negative feedback, maintaining a low output impedance while preserving the integrity of the sound signature.

Distortion Characteristics

Both transistors and tubes introduce distortion, but high-order harmonics in tubes remain invariant across most of the audible frequency range. Many audiophiles describe tube sound as smooth, which is evident on an oscilloscope, where the residual distortion waveform remains relatively stable. Unlike transistor-based circuits, tube circuits, especially in parallel configurations, exhibit almost flat distortion trends across the frequency range.

Power Delivery

One of the key goals in designing the Unico 90 was to ensure consistent performance across different power levels. This consistency is maintained almost entirely, with only minor deviations at maximum output levels.

This is achieved through:

• Linearity in the first two tube stages.
• Direct coupling between the tube stages and the MOSFET output stage.

The innovative design eliminates the need for a thermal compensation stage, which is present in most amplifiers and can be a source of distortion. The Unico 90 achieves nominal linearity within just a few tenths of a second and reaches optimal linearity within 10 minutes of operation.

Preamplifier Section

The preamp section of the Unico 90 incorporates several innovative solutions, including:

• A fully passive preamp design, reducing circuit complexity.
• A high-precision ALPS potentiometer, configured in four sections and connected in parallel to minimize noise and maintain accurate channel balance.
• Mechanical miniature relays for switching inputs, ensuring low signal interference.
• Truly balanced transmission using transformers, which provide galvanic isolation and significantly reduce interference.

Although transformer-based solutions may introduce minor performance compromises at low frequencies, Unison Research's extensive listening tests confirmed the effectiveness of this design, leading to its final implementation despite the higher production cost.

Construction and Power Supply

The Unico 90 features a dual-mono design powered by a single transformer capable of delivering 600 watts continuously. Each channel’s power MOSFETs can handle up to 900 watts, with a stationary current capacity of ±40 amperes.

The circuit design prioritizes safety and reliability, with real-time protection mechanisms that can instantly disconnect the load in case of faults. The Unico 90 maintains 70 percent of its maximum power rating to provide an adequate safety margin without compromising drive capability.

Speaker Compatibility

The Unico 90 can drive challenging loudspeaker loads, though it is not recommended for speakers with a minimum impedance below two ohms. Increasing the number of MOSFETs could accommodate lower-impedance loads, but doing so would compromise frequency consistency and linearity.

Capacitors and Components

Key design features include:

• High-frequency, low-loss capacitors throughout the circuit.
• Extensive use of polypropylene capacitors to enhance signal speed and accuracy.
• Three Mundorf capacitors per channel, providing a final energy reserve and reducing parasitic inductance.
• Custom-made Itelcond electrolytic capacitors, optimized for low stray inductance.

Conclusion

The Unison Research Unico 90 is the result of extensive research, development, and listening tests. Every detail has been meticulously engineered to deliver exceptional performance while maintaining structural simplicity.

Unlike many amplifiers, where measurements dictate the design, the Unico 90 was developed with sound quality as the primary objective, ensuring an engaging and natural listening experience.

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