2hp LPF (2008–)

A simple filter that self-oscillates like a Moog when you crank the resonance, 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.

Small footprint, but it isn't a stripped-down compromise. The trade-offs are deliberate: no panel labeling beyond the logo, no indicator lights, no alternate filter modes. What you get is a low-pass with a 24 dB/octave slope. That is four poles of voltage-controlled filtering, and they respond dramatically to CV. The cutoff knob has just enough resistance to avoid accidental tweaks. The resonance has a sweet spot right before it breaks into oscillation, where the filter adds warmth without eating your transients. It isn't the most aggressive filter around. Don't expect the squelch of a 303 or the bite of a Steiner-Parker. It does well at organic, evolving textures, though. Slow LFOs modulating cutoff or resonance give a buttery, almost liquid movement that is 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 a footnote in a 104HP case, and that's the philosophy behind the 2hp lineup. Every millimeter is accounted for. There are no status LEDs, no trim pots, no alternate outputs. You get cutoff, resonance, CV inputs, and audio in/out. It isn't for tweakers who want fine control over every parameter. The lack of labeling means you have to memorize the layout, but after a few sessions the muscle memory kicks in. It is only 45mm deep, so it fits in almost any skiff or travel case without recessed power cables or special mounting.

OTA Warmth in a Compact Package

The OTA-based design gives the LPF a vintage flavor. It doesn't have the raw aggression of a CEM3372 or the glassy precision of a digital filter. It leans into warmth, with a gentle roll-off that smooths out harsh waveforms without dulling them entirely. Feed a complex digital oscillator into it, sweep the cutoff with an envelope, and the result sounds almost acoustic, like a plucked string fading into a warm room. The resonance CV input lets you automate the onset of self-oscillation or create filter squeals that evolve over time. OTA circuits are sensitive to temperature and component tolerances, so no two units behave exactly the same. In practice the differences are subtle.

Historical Context

The 2hp LPF emerged in 2008, as Eurorack was shifting from boutique curiosity to mainstream synth culture. Most filters then 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 show that extreme compactness didn't have to mean compromised sound. It arrived alongside other minimalist modules like Intellijel's Steppy and Malekko's SSSR, but where those focused on utility, the LPF offered musicality. It helped legitimize the "skiff-friendly" ethos that now dominates Eurorack design.

Its closest sonic relatives were the filters in vintage Moogs and early ARP synths, but its real competition was space. The LPF added both filtering and oscillation without costing much real estate. It wasn't trying to replace a 2040 filter or a SEM, and Rolando knew those had their place. It gave smaller systems a way to sound bigger. Self-oscillation was a core feature rather than a side effect, which encouraged a patching style where the boundary between filter and oscillator was blurred on purpose.

Collectibility & Value

The 2hp LPF has never been rare. Production has stayed steady since 2008, but it is 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 sturdy. The most common issue is pot wear. The resonance knob can develop crackle after years of aggressive tweaking, though contact cleaner usually fixes it. 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 within reach of 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. The module draws equal current on both rails, so it isn't prone to power supply imbalance, but 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 Low Pass Filter (Silver) EURORACK - NEW - PERFECT CI

$93.00

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