HP—short for horizontal pitch—is the standard measurement unit of panel width in a Eurorack system. One HP equals 0.2" (5.08 mm), and a standard 19"- wide studio rack typically offers 84HP of panel space.
When assembling a modular system, it’s not uncommon to end up with a small gap in a row of panels due to the variety of products available. Doepfer and others offer blank panels to fill these gaps—from 1 to 42 HP—and DIYers often wire up a passive mult to fill the empty space. A 2HP panel is about as narrow as you can go and still have usable 3.5mm jacks.
The appropriately named 2hp (twohp.com) goes a step further by offering a line of gap-filling modules that provide basic synth functions. Priced individually at $150, the first six (with character-reduced names to fit the narrow panels) include an analog oscillator (OSC), an analog filter (FILT), a digital LFO, an envelope generator (ENV), a digital noise generator (NSE), and a random voltage/gate generator (RND). I tested OSC, FILT, and LFO, mounting them close together in my case: The panel layouts are well thought out, with the switches and thin knobs positioned above the jacks and kept reasonably away from patch cables. Nonetheless, it can be a challenge to turn these knobs in a busy patch when other feature-rich modules are mounted nearby.
OSC has Coarse and Fine controls; separate outputs for sine, upward saw (despite the front panel marking), triangle, and square waves; and CV inputs for 1V/oct, FM, and hard sync. This analog oscillator has a robust sound, overall, and its wide frequency range—from LFO-range 2.7Hz to 20kHz—is especially useful to have.
The digitally derived Morphing LFO offers two sets of four unipolar waveforms, with the Wave knob interpolating between each group of four. In NRM mode you morph between sine, triangle, downward ramp, and square; ALT mode goes from a phase-modulated sine, through a stepped triangle, upward ramp wave, and a more extreme phase-modulated wave that resembles a moustache. The module also includes a Gate output.
At slower rates, the LFO’s wave shapes are stepped rather than smooth, which I was surprised by at first, but found to be handy for creating sequence-like modulation when controlling the rate with a CV or using the Reset input. You’ll also hear stepping while slowly turning the Wave knob when the LFO rate is slow. At faster LFO speeds, the wave shapes, and the interpolation between them, sounds smoother.
FILT is an exceptional sounding resonant, analog filter that provides individual highpass, lowpass, bandpass, and notch outputs, as well as inputs for audio and frequency cutoff and controls for cutoff and resonance. Crank up the resonance with no audio input, and the filter immediately sings; here’s another sine oscillator with CV tracking in your system. And FILT can get squawky and gritty with an audio input and the resonance on fully, especially with some modulation added.
One can easily imagine having a case full of such narrow yet powerful modules, though, with a power cable required for each, you’d quickly use up power-bus connectors before filling the rack. (It would be feasible only if, like Circuit Abbey’s thin line, you could connect adjacent modules together behind the panel and use a single power cable for the group.)
But while other manufacturers offer much simpler modules at this width, 2hp is the only one packing so much power into each panel, while getting a rich sound that belies the diminutive size.