Soulnote A-2 Integrated Amplifier

Perfectly balanced, discrete non-NFB channels

SOULNOTE's original perfectly balanced non-NFB channels have been fully optimized and incorporated to put give sound quality the highest priority. Voltage feedback has been completely eliminated in the output stage, the voltage amplification stage, and in the power supply circuits. Because of their impact on sound, DC servo circuits and current mirror circuits have been eliminated, while both the constant current circuits and the bias circuits in the voltage amplification stage employ simple circuitry layouts that only use constant current elements (2SK209) and constant voltage elements (LEDs) carefully selected for utmost sound quality. The power amplification stage is perfectly balanced all the way through the emitter follower output. All this in relentless pursuit of vibrant, expansive, clear, and lush music playback.

Relay-switching balanced attenuator

The volume isn't controlled by electronics or a rotary slider, but instead incorporates SOULNOTE's original relays in a balanced attenuator system that switches high-precision resistors. This dramatically increases clarity, and allows the sound image to emerge in three dimensions. It also significantly improves quality at low volume levels without fear of gang errors.

An ideal construction

The input signal line has been rationalized by concentrating the input terminals, input selectors, volume, and the non-NFB voltage amplification stage onto a single multi-layered board. The board does not use even a single shield or filter that could negatively impact the sound in order to eliminate every factor that reduce sound quality, such as the effects of vibrations from connecting cables, inductance components, radiation noise, etc. This ideal construction has been achieved thanks to the relay-switching balanced attenuator.

4-layer board with 70 micron thick copper foil

Composed of the output stage and the main power supply, the power block features an inner layer stratum of 70 micron thick copper foil as the power supply bus bar and thus minimizes high current loops in the commutating capacitor>>output transistor>>speakers + terminals>>speaker terminals>>commutating capacitor cycle. Eliminating inductance components in high current lines creates outstanding frequency characteristics and stability.

Contactless wiring

Connectors have been eliminated in the power supply lines as well as the signal lines, and cables are instead soldered directly to the boards for contactless connections.

Chimney-style heat sink

The heat sink uses a newly developed chimney shape that provides extremely efficient cooling with less vibration.

Ferocious power supply

The power supply utilizes a 600 VA bifilar wound toroidal transformer, the most powerful in SOULNOTE's history. Commutating current loops have been minimized by placing ± independent commutating circuits from the fast recovery diode close to the transformer winding. The commutating capacitor employs an ELNA product developed specially for SOULNOTE and selected for its sound quality. Connecting multiple small capacity capacitors in parallel helps control and optimize frequency characteristics and mutual capacitance.

4 parallel high-speed output transistors

The final stage is composed of 4 high-speed transistors (2SC2837/2SA1186) with low Cob and high FT connected in parallel. This combines with the strong non-NFB power supply to provide ample speaker driving power.

The output transistors have been selected and paired by using a specially designed device that measures the Vbe current value while avoiding any influence from rising temperatures in the output transistors. Perfectly aligning the 4 parallel current transistors creates abundant speaker driving power and clear, precise sonic reproduction.

4-step Darlington configuration

High frequency transistors create a 4-step Darlington configuration to drive the final transistors completely, thereby optimizing the amount of each current. This produces ample drive power for each speaker.

Two types of monaural modes

In addition to BTL monaural mode, you can also select bi-amp monaural mode that distributes the monaural input to the left and right channels.

Power amplifier mode

The power amp mode interrupts the power supply to the microcomputer and bypasses the selector and volume. This increases the S/N ratio. XLR and RCA input can also be selected in this mode.

Housing design that prioritizes sound quality

Spikes ground the power supply transformers directly, and while combining the spikes with uneven cross-sectional aluminium panels to control resonance has increased rigidity, the aluminum panels are not directly bonded in order to eliminate unwanted resonance. The top cover, which has a particularly large influence on sound quality, features a dual-component construction that joins the base plate and the aluminum panels at three points so that they do not dampen each other. By grounding it at three points without fixing it to the aluminum portions of the housing itself, this design succeeds in pulling down the sound focus while still keeping it expansive.

Single non-NFB differential BTL channels

The conventional BTL monaural principle that uses the left and right channels of the stereo amplifier in reverse phases can produce powerful sound quality, but the sound impression tends to blur with sounds tending to clump together because of the effects from GND lines or matching two amplifiers. The BTL monaural principle in the A-2, however, adopts a new approach that partitions the differential output of one non-NFB differential amplifier each for the left and right final stage. This creates an expansive sound with vigorous, yet delicate BTL driving.

SPECIFICATIONS

Link to the Manual