BONASUS

BONASUS

INTEGRATED TUBE AMPLIFIER

Discontinued

For many years, we have been selling top-of-the-line audio equipment. We combine our work with passion, which has allowed us to develop our own definition of great sound. We are fully aware that a system’s sound is the result of the quality of its individual components. We strive not to favor any single component, focusing instead on maintaining the proper balance across the entire signal path. In our search for new solutions in the field of audio system configuration, we decided to design our own amplifier that embodies our vision of a device intended for use in a high-end home audio system. Among the many factors influencing how well an amplifier matches the other components, we selected a few that, in our opinion, determine its suitability in terms of both sound quality and usability:

• Integrated amplifier – this design is optimal in many respects for home use. For many users, the compact design is the primary criterion for selection. The need to purchase additional cabling required for separate-component systems significantly increases the cost of the entire system, which has a considerable impact on the choice of a specific solution.
• Dual-mono configuration – despite being housed in a single enclosure, our amplifier essentially consists of two circuits, each containing independent power supply, preamplifier, and power amplifier sections. The two channels are connected only via the circuit’s ground. There are many advantages to this solution, such as the full integration of all circuits within a single enclosure, which prevents interference, particularly from the power supply. On the other hand, each channel is electrically isolated, which significantly improves stereo imaging and increases signal resolution.
• Output power—a parameter to which less experienced users attribute an almost magical significance—the more “watts,” the better the equipment—but which, in a home setting, is usually more of a nuisance. Our experience shows that the actual power used by owners of home audio systems ranges from 0.1 to 6 watts. Most amplifier owners likely struggle with what’s known as the “11 o’clock alarm clock syndrome.” This means that once the volume control is turned past the “11 o’clock” mark, the system stops playing music and simply starts making noise. In our design, we eliminated this problem by building an amplifier with an output power of “only” 16 watts. Naturally, we opted for a tube-based design, as only tubes are capable of operating at low gain levels while maintaining the full sound spectrum. Semiconductors have many shortcomings that fundamentally complicate their use in low-output-power audio circuits. Thanks to its carefully designed circuitry, our amplifier ensures very high gain linearity across its entire adjustment range. This means that regardless of the gain knob setting, the user enjoys uniform performance across the entire frequency response. The configuration of the output stage itself—operating in an ultra-linear push-pull Class A/B circuit—is also significant. We’re often asked why we limit the output to just 16 watts, when this circuit can easily produce 25 watts or even more. The answer is quite simple—the output stage operates at its optimal operating point, which we selected based on listening tests and experience, and its power output is precisely at that level. Of course, this output power makes our amplifier best suited for use with speakers that have at least average sensitivity—that is, at least 88 dB.

• Lack of negative feedback. A circuit designed to remedy the shortcomings of high-gain systems is not necessary in our application. Setting aside its well-known negative impact on sound quality, we focused on another factor of fundamental importance to our device. Feedback reduces the circuit’s gain, which forces designers to increase power and produce “empty watts” that negatively affect the amplifier’s impulse response. This situation can be mitigated to some extent by expanding the power supply circuits, but this entails much greater design complexity and does not always yield the desired results—something we sought to avoid.
• Power Supply – a traditional circuit with a tube rectifier and a filter consisting of capacitors and an inductor. A polypropylene capacitor is used in the first stage, followed by an inductor, and then electrolytic capacitors with a total capacitance of 1,680 uF per channel. After the choke, the filtering is split into two circuits: one for the input and control circuits, and the other for the output stage. The input circuit also includes polypropylene capacitors designed to decouple interference generated directly in that stage. All other amplifier circuits—logic, inputs, displays—are powered by a separate transformer.
• Operation Control and Monitoring – We have equipped our amplifier with a “soft-start” circuit that ensures the tubes are properly warmed up before the anode voltage is applied. This is of paramount importance for extending the tubes’ service life. BONASUS also features a circuit that monitors the condition of the output tubes, allowing the user to easily check whether they need to be replaced.
• Mechanics—this element, which has a huge impact on the amplifier’s performance and sound, was treated with the utmost seriousness in our design. The extremely rigid steel chassis has a high mass, allowing it to perfectly absorb any vibrations from the components inside. All transformers and chokes are housed in shielded enclosures and are filled with a special semi-elastic damping compound. This is designed to eliminate vibrations generated during their operation.
• Components and circuit design—the key to an amplifier’s good sound lies in the proper selection of its components. After numerous trials and experiments, we achieved results that met our expectations by using carefully selected components from manufacturers such as RIKEN, KIWAME, ELNA, MILLS, AUDYN, and SOLEN. The components in the signal path are mounted directly onto the board using the “point-to-point” method. The tubes used in our amplifier are 5AR4 rectifiers, 12BH7 dual triodes, and 6CA7 tetrodes.

As you can see, BONASUS is a carefully designed amplifier that guarantees high-quality sound combined with ease of use. In addition to the features mentioned above, elements such as the direct coupling of the input stage to the phase inverter and the use of AC current for tube heating—a factor that is also of immense importance for extending the tubes’ lifespan—played a very significant role in shaping the amplifier’s sound. The positive impact of AC heating on sound quality is undeniable; however, its use typically results in increased hum from the speakers. Our solution is very simple: a top-quality power transformer and proper routing of the power cables allow you to enjoy the benefits of AC filament heating without any issues. BONASUS is an example of a device that combines traditional, proven circuit designs with modern technology. The result is an amplifier that is the perfect solution for discerning listeners.

Technical Specifications