Pro-Ject Stereo Box S3 BT black

Pro-Ject Stereo Box S3 BT: the amp that fits in your hand

Ten centimeters wide, 450 grams on the scale, and yet 2 x 40 watts into 4 ohms. The Stereo Box S3 BT takes the concept of its predecessor, the S2 BT, and adds Bluetooth 5.0 with aptX HD codec and a clearly improved signal-to-noise ratio (96 dB versus 76 dB on the S2). Pro-Ject delivers here a class D integrated stereo amplifier designed and hand-assembled in Europe, in a format that fits on a bookshelf.

An aluminum chassis 103 mm wide

The Stereo Box S3 BT measures 103 x 37 x 115 mm, connectors excluded. With plugs connected, allow 138 mm in depth. The chassis is entirely made of machined aluminum, available in black or silver. This material choice is not just about looks: the rigid metal enclosure protects the circuit from vibrations and electromagnetic interference, a sensitive point on a class D amplifier where the switching frequency can generate unwanted noise.

On the front panel, a single rotary knob controls the volume. It is a motorized analog potentiometer, which remains uncommon in this size category: the adjustment is mechanical (no digital conversion of the audio signal in the volume path), and the motor allows the level to be controlled from the supplied infrared remote. To the left of the knob, a push button controls power on and standby. To the right, three LEDs indicate the active input, and a second push button lets you switch between sources.

Three inputs, including one wireless in aptX HD

On the connectivity side, the rear panel brings together two pairs of gold-plated line-level RCA inputs and a Bluetooth 5.0 input. Supported codecs are aptX and aptX HD (A2DP profile), allowing wireless transmission up to 24 bits / 48 kHz. To take full advantage of it, the source device (phone, tablet, computer) must also support aptX HD.

An external Bluetooth antenna screws onto the rear of the chassis. Bluetooth is active only when input 3 is selected, and the device can store up to eight paired devices. When no Bluetooth device is connected, the amplifier automatically switches to energy-saving mode (the LED turns off).

On the output side, there are speaker terminals compatible with 4 mm banana plugs, spades, or bare wire, as well as a variable line output on a 3.5 mm jack. This variable output allows connection to an active subwoofer, a pair of active speakers, or a second power amplifier for a multiroom installation.

Power and measurements: the benefit of moving to the S3

The class D amplifier delivers 2 x 21 W into 8 ohms and 2 x 40 W into 4 ohms (at 1% THD). Frequency response covers 20 Hz to 20 kHz with less than 1.5 dB attenuation into 4 ohms.

The clearest improvement over the S2 BT lies in the signal-to-noise ratio: 96 dB (100 dB A-weighted) versus 76 dB for the previous generation. That is a 20 dB difference, which results in much lower background noise, noticeable when listening at low volume or with high-sensitivity speakers. Channel separation reaches -67 dB at 10 kHz, and gain is 32.5 dB. Input impedance, set at 25 kΩ, is sufficient for most line-level sources.

Total harmonic distortion is specified at less than 0.12% at 5 W into 4 ohms. The S2 BT was rated at less than 0.05% at 10 W: the measurement conditions differ (power and load), which makes direct comparison difficult. Power into 4 ohms still improves, however (40 W versus 37 W), and the SNR makes a significant leap.

On a desk, in a TV cabinet, or in a second room

With its 21 W into 8 ohms, the Stereo Box S3 BT does not claim to fill a concert hall. But it powers bookshelf speakers of متوسط to high sensitivity (86 dB/W/m and above) without difficulty in a 15 to 25 m² room, which covers most offices, bedrooms, and small living rooms.

Pairing it with a Pro-Ject turntable requires an external phono preamp (the Phono Box S2 from the same range, for example), since the Stereo Box S3 BT does not include a phono input. Likewise, there is no digital-to-analog converter or headphone output. For these functions, Pro-Ject offers other enclosures in the same format in the Box Design series (Pre Box, Head Box, DAC Box), stackable to create a modular system.

The 3.5 mm variable jack output adds flexibility: connect a subwoofer to reinforce the low end, or use it to send the signal to a more powerful amplifier if your speakers are in a neighboring room.

Technical Specifications

Key Features

• Ultra-compact integrated amplifier
• High power up to 2 x 40 W at 4 ohms
• Bluetooth 5.0 with aptX HD (24-bit / 48 kHz)
• 2 RCA inputs for analog sources
• Variable output for subwoofer or secondary amplifier
• Infrared remote control included
• Rigid interference-resistant aluminum chassis
• European manufacturing

Connectivity

• Inputs
  • 2 x RCA (line)
  • Bluetooth 5.0 (aptX, aptX HD, A2DP)
• Outputs
  • Speaker terminals compatible with banana plugs, spades, or bare wire
  • Variable line output (3.5 mm jack)

Audio Performance

• Output power
  • 2 x 21 W at 8 ohms
  • 2 x 40 W at 4 ohms
• Frequency response: 20 Hz – 20 kHz (< -1.5 dB at 4 ohms)
• Signal-to-noise ratio: 96 dB (100 dB A-weighted)
• Harmonic distortion (THD): < 0.12% (5 W at 4 ohms)
• Channel separation: -67 dB at 10 kHz
• Gain: 32.5 dB
• Input impedance: 25 kΩ

Bluetooth

• Version: 5.0
• Codecs: aptX, aptX HD, A2DP
• Quality: up to 24-bit / 48 kHz

Power Supply and Consumption

• Power supply: 20 V / 3 A (100–240 V)
• Standby power consumption: < 0.5 W

Dimensions and Weight

• Dimensions (W x H x D): 103 x 37 x 115 mm (138 mm with connectors)
• Weight: 450 g (excluding power supply)

Design and Manufacturing

• Robust vibration-resistant aluminum chassis
• Available finishes: black, silver
• Designed and manufactured in Europe

Ideal Use

• Compact Hi-Fi systems
• Desk or small living room
• High-quality Bluetooth streaming
• Pairing with bookshelf speakers

Key Benefits

• Mini format, powerful sound
• Audiophile Bluetooth (aptX HD)
• Versatile (analog and wireless)
• Elegant and durable design