DIY Analog Synthesizer PCB — Eurorack Module Design

Project Overview

This Eurorack-format synthesizer module combines digital oscillators (STM32 + DAC) with analog filter and amplifier stages. The 4-layer board separates digital control from analog signal path to minimize noise — a critical requirement for audio-quality circuits.

Board Specifications

Parameter	Value
Board Dimensions	100 × 100 mm (3U Eurorack height: 128.5mm panel)
Layer Count	4 layers (Signal / GND / Power / Signal)
Processor	STM32F407VGT6 (ARM Cortex-M4, 168MHz)
DAC	DAC8568 16-bit 8-channel SPI DAC
Analog Section	LM13700 OTA (VCF), SSM2164 (VCA), TL074 op-amps
Power Supply	Eurorack ±12V, onboard 3.3V LDO for digital
Input/Output	CV/Gate 3.5mm jacks × 4, Audio Out 3.5mm, MIDI In (DIN-5)
Copper Weight	1 oz outer / 0.5 oz inner
Surface Finish	ENIG (gold) — better solderability for fine-pitch SMD
Design Tool	KiCad 8.0

Design Highlights

Critical

Analog / Digital Ground Separation

The board uses a split ground plane strategy: digital ground (DGND) and analog ground (AGND) are connected at a single star point near the power input. This prevents high-speed digital switching noise from contaminating the analog signal path — the #1 cause of audio artifacts in hybrid synth designs.

Power

Modular Power Supply Architecture

Eurorack ±12V rail feeds onboard LDO regulators: AMS1117-3.3 for the STM32, and a dedicated low-noise TPS7A47 for the analog op-amps. Bulk capacitance (470µF per rail) plus 100nF bypass caps at every IC power pin.

CV/Gate Inputs — 0-5V buffered through voltage dividers, ADC-sampled by STM32 for quantization and modulation routing
Audio Output Stage — AC-coupled output with adjustable level pot, driven by SSM2164 VCA for voltage-controlled amplitude
SPI Bus Layout — Short, matched-length traces between STM32 and DAC8568, with series termination resistors to prevent ringing
Eurorack Power Header — Shrouded 2×5 pin header with reverse-polarity protection diodes (Eurorack boards are notoriously easy to plug in backwards)

Design Considerations

🔊 Analog / Digital Ground Separation

The single-point ground connection is placed right at the power input header. All analog components reference AGND, all digital components reference DGND. The two planes overlap in the center of the board but are only connected at the star point. This layout achieves <-80dB crosstalk between digital clock noise and audio output.

Ground plane split visible in KiCad PCB viewer — runs diagonally across the board
STM32 decoupling caps (100nF × 4) connect to DGND only
Op-amp decoupling caps connect to AGND only

⚡ Power Filtering and Regulation

Eurorack power rails are notoriously noisy (other modules inject switching noise). The synth PCB includes:

Ferrite beads on ±12V input rails to reject high-frequency noise
LC filter (10µH + 100µF) before analog supply pins
Separate LDO regulators for analog and digital sections
Star-ground topology to prevent ground loops between modules

🎛️ Component Placement Strategy

The board is divided into three distinct zones: digital (top-left, STM32 + DAC + MIDI), analog (bottom-right, VCF + VCA + op-amps), and power (center-left, regulators + connectors). This physical separation reinforces the electrical isolation achieved by the split ground planes.

Building a Synthesizer Module?

Send us your KiCad files for a free DRC review. We specialize in mixed-signal boards where analog/digital isolation matters.

💬 Get Quote on WhatsApp

🎹 DIY Analog Synthesizer PCB