2hp LPF (2008–)

A deceptively simple filter that sings like a Moog when you crank it into self-oscillation—and fits in the gap between your other modules.

Overview

It doesn’t look like much: two knobs, a couple of jacks, and a width that barely registers on a Eurorack panel. But the 2hp LPF punches so far above its size class that you’d swear there was smoke and mirrors involved. Turn up the resonance past 12 o’clock and the thing starts howling like a vintage Minimoog—smooth, pure sine waves that track 1V/oct perfectly across four octaves. That’s not a bonus feature; it’s the whole point. This module was built for moments when you need a quick oscillator but don’t want to eat up precious HP. And because it’s based on OTA (operational transconductance amplifier) topology, the filter character is rich and rounded, with a soft saturation that flatters anything you throw at it—basses get thicker, leads get silkier, and noise turns into something almost musical.

Despite its minimalist footprint, the LPF isn’t a stripped-down compromise. It’s a focused design that makes deliberate trade-offs: no panel labeling beyond the logo, no indicator lights, no alternate filter modes. What you get is a pure, unapologetic low-pass experience with a 24 dB/octave slope—four poles of voltage-controlled filtering that respond dramatically to CV. The cutoff knob feels precise, with just enough resistance to avoid accidental tweaks, and the resonance has a sweet spot right before it breaks into oscillation where the filter adds warmth without eating your transients. It’s not the most aggressive filter in the world—don’t expect the squelch of a 303 or the bite of a Steiner-Parker—but it excels at organic, evolving textures. Patches involving slow LFOs modulating cutoff or resonance reveal its true personality: a buttery, almost liquid movement that’s rare in such a compact format.

Specifications

Key Features

A Filter That Doubles as a Stealth Oscillator

The 2hp LPF’s party trick—self-oscillation with accurate 1V/oct tracking—is more than a gimmick. It’s a fully functional sine wave oscillator that behaves predictably in melodic patches, making it ideal for sub-bass layers or soft pads where you want purity without overtones. Unlike some filters that waver in pitch when pushed into oscillation, this one stays locked in, even across wide voltage sweeps. That stability makes it surprisingly useful in generative patches or when syncing to external clock sources. The sine wave isn’t bright or cutting—it’s mellow, almost pipe-organ-like—but that’s part of its charm. It fills space without fighting other elements, and because it’s derived from the filter itself, there’s a cohesion to the sound that’s hard to replicate with a separate VCO.

Minimalism With Intent

At 2HP wide, the module is practically a footnote in a 104HP case—but that’s the whole philosophy behind 2hp’s lineup. Every millimeter is accounted for. There are no status LEDs, no trim pots, no alternate outputs. What you see is what you get: cutoff, resonance, CV inputs, and audio in/out. This isn’t a module for tweakers who want fine-grained control over every parameter. It’s for patchers who value elegance and efficiency. The lack of labeling forces you to memorize the layout, but after a few sessions, the muscle memory kicks in. And because it’s so shallow (45mm), it fits in almost any skiff or travel case without requiring recessed power cables or special mounting considerations. It’s the rare module that feels like it disappears into your system—until you turn it up and realize it’s carrying half the sonic weight.

OTA Warmth in a Compact Package

The OTA-based design gives the LPF a distinctly vintage flavor. It doesn’t have the raw aggression of a CEM3372 or the glassy precision of a digital filter, but it doesn’t try to. Instead, it leans into warmth, with a gentle roll-off that smooths out harsh waveforms without dulling them entirely. Feeding a complex digital oscillator into the LPF and then sweeping the cutoff with an envelope creates a sound that’s almost acoustic—like a plucked string fading into a warm room. The resonance CV input adds another dimension, letting you automate the onset of self-oscillation or create dynamic filter squeals that evolve over time. Because OTA circuits are sensitive to temperature and component tolerances, no two units behave exactly the same, but in practice, the differences are subtle—more like siblings than strangers.

Historical Context

The 2hp LPF emerged in 2008, right as Eurorack was shifting from boutique curiosity to mainstream synth culture. At the time, most filters followed either the Moog ladder or SEM-style designs, often taking up 8HP or more. 2hp—founded by Tony Rolando, who later created the critically acclaimed Organelle—challenged the assumption that useful modules had to be wide. The LPF wasn’t the first 2HP module, but it was one of the first to prove that extreme compactness didn’t have to mean compromised sound. It arrived alongside other minimalist pioneers like Intellijel’s Steppy and Malekko’s SSSR, but where those focused on utility, the LPF offered musicality. It competed less with full-featured filters and more with the idea that filters had to be big to be good. In doing so, it helped legitimize the “skiff-friendly” ethos that now dominates Eurorack design.

Its closest sonic relatives were the filters found in vintage Moogs and early ARP synths, but its real competition was space itself. In a world where every HP counted, the LPF offered a way to add both filtering and oscillation without sacrificing real estate. It wasn’t trying to replace a 2040 filter or a SEM—Rolando knew those had their place—but it gave smaller systems a way to sound bigger. And by embracing self-oscillation as a core feature rather than a side effect, it encouraged a different kind of patching: one where the boundary between filter and oscillator was intentionally blurred.

Collectibility & Value

The 2hp LPF has never been rare—production has remained steady since 2008—but it’s consistently in demand, especially in black or limited-run colors. Used units typically sell for $75–$90, while new-in-box examples can fetch $110–$130, particularly if they’re from early batches or come with original packaging. Condition matters more than age, since these modules are simple and robust. The most common issue is pot wear: the resonance knob, in particular, can develop crackle after years of aggressive tweaking, though this is usually fixable with contact cleaner. There are no known catastrophic failure points—no ICs that routinely die, no power regulation issues—and the PCB layout is straightforward enough that repairs are accessible even to intermediate technicians.

What to check before buying? First, test self-oscillation: turn resonance past 12 o’clock and sweep cutoff. The tone should be clean and stable, without dropouts or tuning jumps. Second, patch in a 1V/oct CV source (like a sequencer) and verify tracking across at least three octaves—some units drift slightly at the extremes, but anything more than a semitone off is a red flag. Third, listen for noise: a faint hiss is normal, but loud hum or crackle usually indicates a power or grounding issue. Because the module draws equal current on both rails, it’s not prone to power supply imbalance, but always check the power connector for cold solder joints, especially on older units.

For collectors, the LPF isn’t a trophy piece—it won’t appreciate like a vintage Oberheim or a rare Serge. But it’s a benchmark of minimalist design, and its influence is visible in nearly every sub-4HP filter that’s followed. If you’re building a “time capsule” Eurorack system from the late 2000s, this belongs in the rack. And if you’re just looking for a no-nonsense filter that can also serve as a backup oscillator, it’s still one of the smartest uses of 2HP on the market.

eBay Listings

2HP LPF BLACK : NEW : [DETROIT MODULAR]

$99.00

2HP LPF SILVER : NEW : [DETROIT MODULAR]

$99.00

2hp LPF Low Pass Filter Black EURORACK - NEW - PERFECT CIRCU

$99.00

2hp LPF Low Pass Filter (Silver) EURORACK - NEW - PERFECT CI

$99.00

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