2HP Filter (2013–2018)

A small module with a resonance knob that goes all the way to self-oscillation.

Overview

Turn the resonance past noon on the 2HP Filter and it self-oscillates into a sine wave. That makes it a working VCO, with 1V/oct tracking across four octaves. The module takes up only two horizontal pitch units in a Eurorack case. 2HP always went for density. The Filter, later renamed MMF to avoid confusion with other designs, was their answer to getting an expressive analog filter into the smallest possible footprint. It uses an OTA-based, 12dB-per-octave design. That slope is gentler than the steeper 24dB designs, and it trades raw aggression for fluidity and versatility.

It's not a Moog-style ladder and doesn't try to be. With the cutoff wide open it's nearly transparent, which suits subtle tonal shaping. Wind it down with a touch of resonance and it gets creamy, with the kind of low-pass warmth that makes basslines throb and pads breathe. Push resonance harder and it snarls, and it can do acid squelch when fed a sharp transient or a well-timed envelope. The big feature is the simultaneous outputs: low-pass, high-pass, band-pass, and notch. Route all four to separate VCAs, modulate them independently, and you have a lot of spectral room in 2HP of space.

The controls are spaced well, the knobs are sturdy, and the panel comes in both black and silver. It has the minimalist 2HP look, more lab instrument than guitar pedal. It runs on standard Eurorack power, draws modest current, and at 45mm depth it fits skiff builds. It's deeper than its size suggests. For a metallic bell tone, feed noise into the band-pass output and ping it with a short gate. For a rhythmic sweep across a drone, modulate cutoff with an LFO and send the high-pass output to a reverb. The notch output is often overlooked. It makes hollow, phasey textures that sit in a mix without muddying the low end.

Specifications

Manufacturer	2HP
Production Years	2013–2018
Original Price	$99 USD
Module Type	Analog multimode voltage-controlled filter
Filter Slope	12 dB/octave
Filter Types	Low-pass, high-pass, band-pass, notch (simultaneous outputs)
Topology	OTA-based
Self-Oscillation	Yes, with 1V/oct tracking across four octaves
CV Input	Cutoff frequency CV, resonance CV
Audio Input	1 x normalized input
Audio Outputs	4 x simultaneous: LP, HP, BP, Notch
Power Requirements	+12V, -12V
Current Draw	+12V: 30 mA, -12V: 30 mA
Width	2 HP
Depth	45 mm
Panel Options	Black, Silver

Key Features

Parallel Filter Outputs Unlock Sonic Multiplicity

Most multimode filters make you pick one at a time - low-pass or band-pass, high-pass or notch. The 2HP Filter gives you all four at once. You can split a single oscillator into four timbral paths without daisy-chaining or switching. Send the low-pass output to a distortion, the band-pass to a delay, the high-pass to a granular processor, and the notch to a spring reverb, all from one source in real time. It works well in generative patches, where independent modulation of each output creates evolving textures. Even in simple setups, the high-pass output lets you pull crisp, clicky transients from a plucked waveform without losing the body in the main low-pass signal.

Resonance as Instrument, Not Just Effect

Past a certain point the resonance control turns the filter into a sound source. It oscillates with a clean, stable sine wave that tracks accurately across four octaves with 1V/oct input. That's rare in such a compact module. Most self-oscillating filters in the 2HP form factor either drift out of tune or need constant tweaking. This one stays locked, so it can stand in for a dedicated VCO in a pinch. Short of self-oscillation, the resonance is not harsh or brittle. It has a rounded peak that enhances without piercing. CV control over resonance opens up dynamic timbral shifts, like a snare hit that starts bright and collapses into a subby thud, or a pad that swells from silence into a resonant peak before fading. The resonance CV input accepts both unipolar and bipolar signals, so it can be modulated with envelopes, LFOs, or audio-rate sources for FM-like effects.

OTA Topology with a 12dB Personality

The 12dB/octave OTA (Operational Transconductance Amplifier) design, rather than the more common 24dB transistor-ladder approach, sets the Filter's sound. It's less aggressive and less "vintage synth." Where a 24dB filter might carve deep notches or dramatic sweeps, the 12dB slope is smoother and more gradual, better for subtle shaping than radical change. In dense patches, a gentler filter can sit in the mix without pulling focus. It's also more forgiving with feedback loops and complex modulation, and less likely to go unstable or distort unexpectedly. The OTA core gives a slightly organic edge, with a softness in the highs and a warmth in the mids that digital filters often struggle to replicate.

Historical Context

2HP launched the Filter in 2013, when Eurorack cases were getting bigger, modules were getting wider, and "more is more" ruled. The brand was founded by engineer Tony Rolando, known for his work on the Buchla 200e and later the Make Noise 0-Coast. The idea behind it was that minimalism could be maximal in expression. The Filter showed that a two-pitch module could do the work of four. Multimode filters at the time were typically 8HP or wider. Competitors like Doepfer and Intellijel offered more features, such as morphing filters, drive circuits, and multimode envelopes, but they cost more space. The 2HP Filter didn't try to beat them at that. It appealed to skiff builders, travel synth enthusiasts, and minimalists who valued elegance over excess.

The Filter also arrived as the DIY Eurorack scene was exploding. Builders wanted compact, reliable modules they could fit into custom cases. 2HP delivered with precision-machined panels, clean layouts, and straightforward functionality. It wasn't the first 2HP module - Rolando had already released the Dual Attenuator and Dual Switch - but it was the first to show that such a narrow form factor could host a full-featured analog circuit. Its success paved the way for later designs like the MMF (a refined version), the VCA, and the Dual Oscillator. The original Filter was eventually rebranded as the MMF to distinguish it from the later Comb and LPF models.

Collectibility & Value

The original 2HP Filter is no longer in production, having been succeeded by the MMF, but collectors and minimalists still look for it. On the used market, prices typically range from $120 to $180, depending on condition, color, and whether it includes the original packaging. Silver panels tend to fetch slightly more than black, though both are common. The module is all-analog and built with high-quality components, so failure rates are low, but not zero. The most common reported issues are intermittent connections at the power header or jack sockets, usually from mechanical stress from frequent patching. These are repairable with basic soldering skills, but buyers should test all outputs and CV inputs before purchasing. The module has no user-serviceable parts inside, so opening the case voids any remaining warranty and risks damaging the PCB.

One thing to watch for: counterfeit or clone modules. Due to the popularity of 2HP’s designs, third-party manufacturers have released unlicensed versions that mimic the layout and functionality but use inferior components. These often lack the precise 1V/oct tracking and exhibit instability at high resonance. Authentic units have “2HP” laser-etched on the panel and a clean, tight fit between the PCB and front panel. The original manual is available online, and service technicians recommend checking for consistent tracking across octaves when testing self-oscillation. For those building a period-correct 2010s Eurorack system, the Filter is a genuine artifact of a pivotal moment in modular history—when size stopped being a limitation and started being a design philosophy.