⚡ Why power and grounding matters in modular systems

Eurorack is an open standard where modules from dozens of manufacturers share a common power bus. Unlike a self-contained synth where one engineer controls the entire signal path, a modular system is a living patchwork of different designs, power draw profiles, and grounding schemes. This flexibility is Eurorack's strength—but it also makes power distribution and grounding the most common source of mysterious noise, hum, and instability.

Poor power design manifests as 60 Hz hum (or 50 Hz in PAL regions), high-frequency whine from switching regulators, random clicks and pops when gates fire, or even module resets and crashes under load. You might patch a beautiful drone only to hear a persistent buzz that gets worse when you touch the panel, or find that adding one more module pushes the system into instability. These aren't usually module defects—they're symptoms of inadequate supply capacity, improper grounding topology, or insufficient decoupling.

This article approaches power and grounding from an engineering perspective: understanding current paths, managing impedance, isolating noise sources, and validating your design with measurements. Whether you're building modules from scratch, assembling a rack, or troubleshooting an existing system, these principles help you achieve clean, reliable operation that lets the music shine through instead of fighting the noise floor.

🔧 Decoupling and local regulation

Bulk electrolytic capacitors (100–470 µF) at power entry points stabilize against transient current spikes, while ceramic capacitors (100 nF–1 µF) placed within 5 mm of IC power pins suppress high-frequency switching noise. The bulk cap handles slow load changes; the ceramic responds to fast edges. Together they form a low-impedance reservoir across a wide frequency range. See Analog Devices' "Staying Well Grounded" article for PCB layout best practices.

Ferrite beads isolate noisy digital sections (microcontrollers, shift registers) from sensitive analog stages. A 100–300 Ω @ 100 MHz bead in series with the digital supply rail blocks hash without significantly dropping DC voltage. For DIY modules with mixed-signal designs, add a small LDO regulator (78L05, LM317) to locally clean the rail for op-amps and voltage references. The few millivolts of extra headroom and localized filtering often eliminate mysterious hum that couples through shared power impedance.

🧲 Grounding topologies: star vs. daisy chain

Ideally, establish a star ground at the power supply entry: all ground returns meet at a single low-impedance point, preventing return currents from one module coupling into another's signal ground. In practice, Eurorack bus boards often daisy-chain grounds along the ribbon cable, which works if the cables are short and currents moderate. Problems arise when high-current digital modules share the same ground trace as sensitive VCOs or preamps.

Avoid ground loops between panels and external gear. If you hear hum only when patching to an audio interface or pedals, suspect a ground loop through the chassis or cable shields. Use balanced (TRS) connections where possible, or lift the shield at one end (typically the modular side) to break the loop. Be cautious—shield lifting can increase susceptibility to RF interference, so test both configurations and choose the quieter one. Understanding ground loop topology is essential for professional audio interconnection.

🧪 Measurement and diagnostics

Probe the +12 V, −12 V, and ground rails with an oscilloscope (AC-coupled, 50 mV/div) while running your heaviest patches. Look for ripple, switching spikes, and low-frequency sag. Ripple above 10–20 mV peak-to-peak suggests inadequate decoupling or an overloaded supply. Spikes coinciding with module clocks or gate changes indicate crosstalk through power rails—add local decoupling or ferrites.

For hum and buzz, use a spectrum analyzer (even a free software FFT on your audio interface output) to identify the frequency. 50/60 Hz and harmonics point to mains coupling or ground loops. Higher frequency hash (kHz to MHz) suggests switching supply EMI or digital crosstalk. Move cables, modules, and power supplies systematically—swap positions, reroute cables away from transformers, and note which changes correlate with noise reduction. Methodical isolation saves hours of guesswork.

🎛️ Practical fixes and preventative design

Twist ribbon cable segments or use twisted-pair wire for power distribution—this reduces the loop area and radiated EMI. Add nylon or fiber washers to jack sockets if they electrically connect panel grounds in ways that create loops. Route high-current modules (large digital sequencers, samplers) physically separated from VCOs, VCAs, and filters to minimize magnetic and capacitive coupling.

When building modules, allocate space for decoupling caps on the PCB layout early—don't treat them as afterthoughts. Run dedicated ground pours or traces for analog and digital sections, tied together at a single point near the power connector. Replace aging power supplies before they become unreliable; electrolytic caps dry out over 5–10 years, increasing ripple and reducing capacity. Proactive replacement is cheaper than debugging intermittent noise or blown modules.