4ms Row Power 40 [RP40]
A compact, clean power distribution module that kept Eurorack systems quiet, until it was discontinued.
Overview
If a Eurorack case ran all night without a hum, buzz, or brownout, there's a good chance a 4ms Row Power 40 [RP40] was powering it. It has no blinking lights, no knobs, no waveforms. Built by 4ms Company, the RP40 is a power distribution module that delivers stable, regulated power to your analog circuits. It's not a power supply on its own. It sits between your wall adapter and your modules and keeps the rails clean and the current steady. It did that well enough that, even discontinued, you still see it in 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 processors or power-hungry modules. It's not the biggest in the Row Power family, but it suits mid-sized systems. It's just 4HP wide and only 24mm deep (28mm with the barrel plug attached), so it fits skiffs and 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, with tight build quality, a 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. Serious builders also got pads on the PCB for blade terminals, which allow custom power entry without the faceplate, and CV/Gate headers that hinted at expandability beyond power.
Specifications
| Manufacturer | 4ms Company |
| Model | Row Power 40 [RP40] |
| Total Power | 40W |
| Output | +12V @ 1.5A, -12V @ 1.25A, +5V @ 1.5A |
| Power Input | 2.1mm barrel jack, positive center, 15V-20V DC |
| Width | 4HP |
| Depth | 24mm (0.96") |
| Depth with power cable | up to 28mm (1.1") |
| Height | 27mm (1.1") with power connectors installed |
| Weight | 0.15 kg |
| Item Number/MPN | RP40 |
| Made In | United States |
| Included accessories | 16-to-16 pin power cable, 2 M3 Knurlie screws |
| Output Connectors | 10-pin and 16-pin Eurorack power headers |
Key Features
Separate Regulation, Serious Stability
Each voltage rail - +12V, -12V, and +5V - is independently regulated using high-grade components. That means noise on one rail doesn't bleed into another, which matters in a system full of sensitive VCOs and low-noise preamps. Owners report it runs cool even under load. No fans, no hot spots. 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 and chain them together off a single power brick. That made it easy to scale up: add a second or third unit as the system grew, all fed from one source. The on/off switch cuts power to the module without breaking the daisy chain, so downstream units stay connected and powered. Anyone who's unplugged a chain and watched the whole system reset knows why that matters.
Built for Builders
The PCB includes pads for blade tabs, so DIYers and case modders can route power directly without the faceplate-mounted barrel jack. That suits custom installations like wall-mounted panels or embedded systems, where a protruding connector would get in the way. The CV/Gate headers are not part of the power function. They 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 fits 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, and 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.
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 between the 35 and the 45, with more headroom than the mid-tier models but stopping short of the top-end 45. Eventually the Row Power 45 replaced it, and the 40 was quietly discontinued. No fanfare, no limited editions.
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 on the secondary market based on condition and completeness. No official failure mode is widely reported. User feedback notes that the module itself is reliable and "beefy," but the stock power brick that often came with it could introduce noise, which later users solved with third-party adapters. It's not a rare grail, but it's respected. For a vintage 4ms-based system, or anyone who wants proven, quiet power in a compact form, it still holds value. It needs a clean DC source to feed it.
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