The Fosi Audio BT20A Pro is a compact 2.0-channel stereo power amplifier designed to drive passive speakers. Its Bluetooth and RCA inputs make it suitable for both wireless sources and conventional analogue audio equipment.

In addition to its speaker outputs, the amplifier features a preamplifier output, allowing flexible integration into a variety of home audio systems. Support for speakers with an impedance of 2–8 ohms and a 24–48 V DC input range allows the system to be matched to the selected speakers and power supply.

The actual output power available from the BT20A Pro is determined by the voltage and current of the connected power supply together with the impedance of the speakers. The 300 W x 2 figure is the amplifier’s maximum power rating into a 4-ohm load, not a continuous output available with every power supply.

With a 32 V / 5 A power supply, the amplifier delivers 100 W rated and 125 W maximum output into 4 ohms, or 50 W rated and 62 W maximum output into 8 ohms. A 48 V / 5 A power supply increases these figures to 220 W rated and 259 W maximum into 4 ohms, or 110 W rated and 129 W maximum into 8 ohms.

At the heart of the BT20A Pro is the Texas Instruments TPA3255 Class D amplifier chip. Its advanced feedback design and proprietary high-speed gate-driver error correction are engineered for accurate signal handling at high output levels.

The chip’s intelligent algorithm optimises the output stage for high efficiency and low distortion. The TPA3255 power stage maintains idle losses below 2.5 W, allowing the design to direct a substantial proportion of the available energy towards driving the speakers.

The BT20A Pro uses two socket-mounted NE5532 operational amplifiers. The removable design allows the op-amps to be replaced, enabling users to tailor the amplifier to their system and sound preferences with compatible alternatives.

Compatible options include the LME49720HA, MUSES02, OPA2134PA, and SS3602. Before replacing the operational amplifiers, always verify the electrical and pin-layout compatibility of the selected components and perform the installation only when the device is switched off and disconnected from power.

The output stage uses inductors with high-precision, stable inductance values. Low DC resistance reduces unnecessary losses and supports efficient power delivery from the amplifier.

Favourable frequency characteristics contribute to consistent output-stage operation across the audible frequency range. The carefully selected components are intended to maintain stable performance and clean sound reproduction.

Bluetooth 5.0 connectivity allows the BT20A Pro to receive music wirelessly from compatible smartphones, tablets, and other Bluetooth sources. Supported Bluetooth transmission formats include SBC and AAC, with a transmission range of up to 15 metres.

Pressing and holding the volume control for five seconds clears the current Bluetooth pairing. Bluetooth memory retains the previously paired device to simplify subsequent connections, while audible alerts indicate the connection status.

The separate bass and treble controls on the front panel allow the sound of the BT20A Pro to be adjusted to suit the speakers, listening room, and individual preferences. The two frequency ranges can be adjusted independently.

When tone correction is not required, both controls can be set to their centre positions. In this position, the amplifier passes the input signal without additional bass or treble emphasis.

The wired RCA input supports connection to CD players, DACs, streamers, and other line-level analogue sources, while the Bluetooth input provides convenient wireless playback. These two input modes allow the BT20A Pro to be used with both conventional and modern audio sources.

Alongside its amplified outputs for passive speakers, the BT20A Pro also provides a preamplifier output. Its compact 127 x 127 x 25.4 mm enclosure takes up very little space, making it easy to accommodate in desktop and smaller home stereo systems.