4ms Row Power 40 [RP40]

The quiet workhorse that kept Eurorack systems humming—compact, clean, and built like a tank, until it wasn’t made anymore.

Overview

If your Eurorack case ever slept through the night without a hum, buzz, or brownout, there’s a good chance a 4ms Row Power 40 [RP40] was on duty. This isn’t the flashiest module in the rack—no blinking lights, no knobs, no waveforms—but it’s the one that made everything else possible. Built by 4ms Company, the RP40 is a power distribution module designed to deliver stable, regulated juice to your precious analog circuits. It’s not a power supply on its own; it’s the middleman between your wall adapter and your modules, doing the thankless job of keeping rails clean and current steady. And it did it well—so well that even though it’s discontinued, you still see it tucked into skiffs and desktop rigs from Brooklyn to Berlin.

The RP40 packs 40W of total power across three rails: +12V, -12V, and +5V, with currents rated at 1.5A, 1.25A, and 1.5A respectively. That’s enough to run 150 to 250HP of modules, depending on what you’re running—more if you’re not stacking digital beasts or power-hungry processors. It’s not the biggest in the Row Power family, but it’s the Goldilocks zone for mid-sized systems. At just 4HP wide and only 24mm deep (28mm with the barrel plug attached), it’s skiff-friendly, slipping into tight spaces where deeper supplies would bulge out the back. The height, 27mm with connectors, clears most cases without issue.

It’s made in the United States, which meant something back when it was in production—tight build quality, thoughtful layout, and components that didn’t cut corners. The included 16-to-16 pin power cable and two M3 Knurlie screws mean you’re ready to go out of the box, though serious builders appreciated the extra touches: pads on the PCB for blade terminals, allowing custom power entry without the faceplate, and CV/Gate headers that hinted at expandability beyond just power. This wasn’t just a power strip. It was engineered.

Specifications

Key Features

Separate Regulation, Serious Stability

Each voltage rail—+12V, -12V, and +5V—is independently regulated using high-grade components. That’s not just marketing fluff; it means noise on one rail doesn’t bleed into another. In a system full of sensitive VCOs and low-noise preamps, that separation matters. Owners report it runs cool even under load, which is a quiet win—no fans, no hot spots, just steady delivery. The design prioritizes clean power, and for most users, it delivered.

Daisy-Chaining Done Right

One of the RP40’s smartest features is the dual DC input jacks. You can plug one Row Power module into another, chaining them together while running off a single power brick. This made it easy to scale up—add a second or third unit as your system grew, all fed from one source. And the on/off switch? It cuts power to the module without breaking the daisy chain, so downstream units stay connected and ready. It’s a small thing, but anyone who’s unplugged a chain only to have their whole system reset knows how much it matters.

Built for Builders

The PCB includes pads for blade tabs, letting DIYers or case modders route power directly without using the faceplate-mounted barrel jack. That’s a nod to custom installations—think wall-mounted panels or embedded systems where a protruding connector would get in the way. It’s not a feature you’ll use every day, but when you need it, you’ll be glad it’s there. Same goes for the CV/Gate headers: not part of the power function, but a hint that 4ms was thinking beyond the basics, leaving room for expansion or integration with control systems.

Skiff-Ready, Space-Smart

At 24mm deep, the RP40 slips into shallow cases with room to spare. Even with the power cable attached, it maxes out at 28mm—still within skiff-friendly range. That made it a favorite for portable setups, travel cases, or anyone trying to minimize cable strain in a tight build. The 4HP width doesn’t hog space, and the 27mm height clears standard Eurorack rails without interference. It’s not invisible, but it knows its place.

Historical Context

The Row Power 40 was part of a family that scaled with user needs: the Row Power 25, 30, 35, 40, and 45. The RP40 sat squarely between the 35 and the 45—offering more headroom than the mid-tier models but stopping just short of the top-end 45. It wasn’t the last word in power, but it was the sweet spot for many. Eventually, it was replaced by the Row Power 45. Once that happened, the 40 was quietly discontinued. No fanfare, no limited editions—just a solid run, then silence.

Collectibility & Value

Originally priced at $155.00, the RP40 was never cheap, but it wasn’t outrageous for what it offered. Today, as a discontinued unit, it trades in the secondary market based on condition and completeness. There’s no official failure mode widely reported, but user feedback notes that while the module itself is reliable and “beefy,” the stock power brick that often came with it could introduce noise—something later users solved with third-party adapters. It’s not a rare grail, but it’s respected. If you’re building a vintage 4ms-based system or want proven, quiet power in a compact form, the RP40 still holds value. Just make sure you’ve got a clean DC source to feed it.

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