FiiO CLASS A brings fully discrete Class-A amplification to a 4W balanced desktop amplifier

The FiiO CLASS A is a fully discrete Class-A desktop headphone amplifier and preamplifier based on a fully balanced differential architecture. Its analog stage uses Toshiba twin matched-pair and complementary audio transistors alongside Sanken output transistors, with manually calibrated quiescent current supporting continuous Class-A operation. Balanced headphone output reaches 4,000mW + 4,000mW into 32Ω, while five gain levels, independent bass and treble adjustment, balanced and single-ended I/O, and a 60W linear power supply give the amplifier enough flexibility to handle headphones with substantially different impedance and sensitivity requirements.

Fully discrete Class-A amplification

Rather than relying on an integrated power-amplifier stage, the CLASS A uses individually selected transistors, resistors, and capacitors throughout its amplification circuit. The design incorporates 16 Toshiba twin matched-pair audio transistors and eight Sanken Electric audio transistors, with the quiescent current of the amplification stage manually calibrated during production.

Class-A operation keeps the output devices conducting continuously instead of switching them on and off around the signal waveform. That operating method requires substantially more standing current and thermal capacity than lower-bias amplifier classes, which shapes the CLASS A’s power-supply, cooling, and protection requirements. A DC servo circuit continuously monitors output offset and applies correction to keep DC voltage at the headphone output under control.

Balanced architecture with up to 4W per channel

The amplifier uses separate positive and negative signal paths for each channel in its fully balanced differential circuit. Through the 4.4mm and 4-pin XLR balanced outputs, maximum rated power reaches 2,000mW + 2,000mW into 16Ω, 4,000mW + 4,000mW into 32Ω, and 1,360mW + 1,360mW into 300Ω.

The 6.35mm single-ended output delivers up to 2,000mW + 2,000mW into 16Ω, 2,850mW + 2,850mW into 32Ω, and 350mW + 350mW into 300Ω. Five selectable gain positions—Low, Medium, High, Super High, and Ultra High—allow the amplifier’s gain structure to be adjusted around headphones with different voltage and sensitivity requirements rather than relying on a single amplification range.

60W linear power supply

A custom 60W toroidal transformer supplies the CLASS A’s split regulated power rails. The transformer uses a continuous silicon-steel strip core, copper windings, magnetic shielding, and a resettable thermal switch, reflecting the higher continuous-current demand of the Class-A output stage.

A 20,000μF capacitor reservoir, including Nichicon audio capacitors, provides local energy storage while reducing residual power-supply ripple reaching the amplification circuit. The amplifier accepts AC 100–120V or 220–240V at 50/60Hz, depending on the configured mains version.

Measured noise and distortion performance

FiiO specifies a signal-to-noise ratio of at least 126dB A-weighted and an A-weighted noise floor below 3.7μV. The low residual noise level is particularly relevant when the amplifier is paired with more sensitive headphones, where circuit noise can become audible at comparatively low listening levels.

THD+N is specified below 0.00046% at the XLR output with a 1kHz signal at -2dBV into a 10kΩ load. Frequency response extends from 20Hz to 95kHz with less than 3dB attenuation, giving the analog stage bandwidth well beyond the conventional audible range.

Five gain levels and analog tone adjustment

The CLASS A provides five gain settings to accommodate a wider range of headphone loads without requiring the volume control to operate within the same range for every headphone. Lower gain positions are suited to more sensitive loads, while the higher settings provide additional voltage gain for headphones that require greater drive.

Independent bass and treble controls provide ±10dB of adjustment. Both controls operate within the amplifier itself, allowing low- and high-frequency balance to be changed without replacing the source or inserting an external equalizer into the signal chain.

Balanced and single-ended system connectivity

Front-panel headphone connections include 6.35mm single-ended, 4.4mm balanced, and 4-pin XLR balanced outputs. Analog inputs cover 3.5mm single-ended, RCA, 4.4mm balanced, and XLR balanced sources. Input impedance is specified at 5kΩ for the RCA and 3.5mm inputs and 8kΩ for the XLR and 4.4mm balanced inputs.

RCA and XLR line outputs also allow the CLASS A to function as a preamplifier for an external power amplifier or active speakers. This makes the unit usable as the analog control point of a desktop system rather than exclusively as a headphone amplifier.

Desktop control and protection

12V trigger input and output provide coordinated power control with compatible upstream and downstream components. A USB-C connection is included for firmware updates, with the control system managed by an ESP32-S3 controller. Front-panel indicators display output mode and mute status, while the illuminated volume control also handles muting, output switching, and selectable indicator-light behavior.

Protection circuits monitor DC output, power-input overcurrent, amplifier temperature, overvoltage, output overload, and transformer temperature. These systems address the sustained current draw and heat generation associated with the amplifier’s Class-A operating point while also providing protection for connected headphones and downstream equipment.

Availability

The FiiO CLASS A headphone amplifier and preamplifier is available from Apos. View the FiiO CLASS A product page.