Audio Preamplifier
An audiophile high-fidelity preamplifier, input switcher and digital audio hub featuring an embedded Windows 11 music server, 32-bit AK4399 DAC, Bluetooth LDAC streaming, RIAA Phono stage, and smart ESP32 control via MQTT and IR remote.
After designing and building the Visaton acoustic speakers and Hi-Fi power amplifier, the system needed a versatile, reference-grade preamplifier. This custom unit seamlessly blends analog audiophile sources with high-resolution digital audio streaming in a single cohesive platform.
Key Features and Architecture
- Embedded Mini PC (Music Server): integrated internal mini PC running Windows 11, serving as a dedicated music server. Allows streaming high-res audio through Spotify and lossless audio playback via Foobar2000. Features external USB1, USB2, HDMI, and Gigabit LAN connectors.
- Bluetooth Module with LDAC Support: stream music wirelessly straight from smartphones or tablets with Sony LDAC codec support (up to 990 kbps, 24-bit/96 kHz) for lossless wireless audio transmission.
- Digital-to-Analog Converter (DAC): audiophile 32-bit AK4399 DAC delivers exceptional clarity, wide dynamic range, and ultra-low noise floor.
- Digital Signal Routing & XMOS USB: audio from the embedded PC passes via an asynchronous XMOS USB sound card with direct I2S and S/PDIF output into the hardware I2S selector. Additional digital sources are handled by a low-jitter Wolfson WM8805 receiver (coaxial SPDIF IN, dual optical TOSLINK IN1 & TOSLINK IN2, and SPDIF OUT).
- Integrated RIAA Phono Stage: low-noise shielded phono preamplifier for direct connection of vinyl turntables (PHONO IN).
- Burr-Brown OPA2134 Op-Amps: Burr-Brown / TI OPA2134 SoundPlus™ operational amplifiers are used across the entire analog signal path (input buffers, tone control, and output stages), delivering ultra-low distortion (0.00008% THD+N) and authentic musical warmth.
- Premium ALPS Potentiometers: volume attenuation is powered by a genuine Japanese motorized ALPS potentiometer with servo control. High-grade ALPS potentiometers are also utilized for bass and treble tone controls.
- Source Direct Mode: dedicated direct bypass relay allows bypassing the tone control stage entirely, routing the clean signal directly to the motorized volume stage for maximum transparency.
- Smart ESP32 Microcontroller: centralized system management, relay input switching, motorized ALPS control, and I2S multiplexing are driven by an ESP32 CPU. Controlled via IR remote, front panel rotary encoder with push button, and fully integrated with home automation over MQTT (Wi-Fi).
- Synchronized Mains Control (230V AC OUT): an ESP32-driven 230V power relay automatically switches external power amplifiers on and off with the preamplifier.
Functional Block Diagram
The block diagram below highlights the comprehensive signal architecture: digital stream routing (XMOS, WM8805, Bluetooth, and I2S selector), analog section (Phono stage, input selector, Rec-Loop, OPA2134 buffers, Direct mode, and motorized ALPS volume control), and the ESP32 brain with smart networking.
Chassis, Assembly & Hardware
