ALM Busy Circuits Pip Filter (2022–)

A 4HP filter with smooth resonance, pingable envelopes, and a built-in VCA.

Overview

The Pip Filter fits in a 4HP slot and does more than a low-pass. It can shape, trigger, and amplify sound. The filter is a 2-pole (12dB/oct) voltage-controlled design, and the resonance climbs all the way into self-oscillation. It sounds fairly vocal for something this small. It's not a screaming acid monster. Crank the resonance and ping it with a trigger, and it gives a rounded, almost wooden "thwock" that suits plucked basses, marimba-like stabs, or soft FM-style zaps.

The Pip Filter packs a lot into a small panel. There are dual CV inputs for cutoff frequency. One has a manual attenuverter, the other has rough V/Oct tracking, so you can blend modulation sources or feed it pitch data directly from a sequencer. The resonance is manual only, with no CV input. It tracks well enough into oscillation to be musically useful, but not perfectly linearly. Then there's the built-in VCA, normalled to open, which controls the amplitude of the filtered signal on its own. Use it as a full filter+VCA stage with no extra modules, or break the normal and use it as a secondary VCA for ducking, gating, or amplitude modulation.

Despite the size, the Pip Filter doesn't feel like a compromise. The controls are spaced well, the knobs are sturdy, and the module runs cool. It draws a modest 25mA on both rails, so it won't bog down a small power supply, and at 32mm deep it fits even the tightest skiff. There are no flashy LEDs, no experimental topology, and no patch memory. You can use it for filtering, tone shaping, responding to percussive triggers, or as a mini dynamics processor.

Specifications

Manufacturer	ALM Busy Circuits
Model	Pip Filter
Model Number	ALM039
Production Years	2022–
Original Price	£110 GBP / $145 USD
Format	Eurorack
Width	4HP
Depth	32mm
Power Supply	+12V, -12V
+12V Current Draw	25mA
-12V Current Draw	25mA
Filter Type	2-pole (12dB/oct) low-pass
Resonance	Manual control, self-oscillating
Cutoff Control	Knob, dual CV inputs (one with attenuverter)
CV Inputs	2 x Frequency CV (one attenuverted, one V/Oct)
Audio Inputs	1 x Audio In
Trigger Input	1 x Ping (trigger-to-envelope)
VCA	Built-in, normalled open, optional control via rear jumper
Outputs	1 x Filtered Audio Out
Protections	Reverse polarity protection
Warranty	2 years
Country of Origin	England

Key Features

Compact Filter with Full Resonance Range

The 2-pole low-pass core has a warm, rounded roll-off that's more musical than surgical. It won't carve out frequencies with the precision of a 4-pole ladder, but it's designed to be responsive, not clinical. Turn the resonance up and the filter starts to "bloom," adding body and presence before breaking into a smooth, slightly gritty self-oscillation. It's not perfectly stable at high resonance, and that slight wavering suits organic, almost acoustic textures. The lack of CV control over resonance is a limitation, and it forces you to commit to one sonic character and work within it.

Dual CV Inputs with Attenuversion

Two CV inputs for cutoff, one with a built-in attenuverter, is a small luxury. You can patch in an LFO and a sequencer at the same time, then use the attenuverter to set how much of the LFO modulates the cutoff, all without an external mixer or attenuator. The second CV input has rough V/Oct tracking, so you can play the filter melodically from a keyboard or sequencer, though it's not meant as a primary pitch source. That makes the Pip Filter a small modulation hub, with slow drift and fast movement layered together or synced to rhythmic triggers for evolving sweeps.

“Pingable” Trigger Input and Built-in VCA

The trigger input labeled "Ping" briefly opens the filter envelope when triggered. That gives you percussive plucks or resonant "boings" without an external envelope generator. It's not a full ADSR, but it's enough for short, expressive transients, like turning a sine wave into a marimba hit or giving a noise burst a sharp attack. Add the built-in VCA and you have a complete tone-shaping stage in 4HP. The VCA is normalled to open, so the signal passes through unattenuated by default. Break the normal and the VCA input controls amplitude externally. The Pip Filter can be a filter, a filter+VCA, or a standalone VCA if you bypass the filter core entirely (though that's not its intended use).

Historical Context

ALM Busy Circuits has a reputation for modules that bridge classic digital gear and modern modular workflow. Akemie's Castle uses actual Yamaha FM chips, and the MUM M8 recreates the filter of the Akai S950. The Pip Filter doesn't draw from a vintage source in the same way, but it follows ALM's design philosophy of maximum utility in minimal space, with musicality ahead of specs. It was released in the early 2020s, when Eurorack builders were pushing both extremes: ultra-dense utility modules and massive, feature-laden "do-alls." The Pip Filter sits in the former camp. It doesn't emulate a classic synth filter.

Many compact filters at the time were either basic or overly digital-sounding, and the Pip Filter offered something warmer and more responsive. It wasn't the first 4HP filter, but it was one of the first to feel like it belonged in a musical patch rather than just filling space. Competitors like Intellijel's Steiner or Make Noise's Ripples offer more filter types or CV control, but they also take up more room and cost more. The Pip Filter is for the builder who wants a self-contained filter+VCA that doesn't dominate the rack.

Collectibility & Value

The Pip Filter is still in production as of 2026, so it's not a vintage item in the traditional sense. New units sell for around £110 - £130 GBP or $145 - $199 USD depending on the retailer and region. Used prices hover between $100 and $140, with mint-condition units commanding a small premium.

Failures are rare. Service technicians note that the most common issue is power, specifically reverse polarity connections, though the module includes reverse protection, so catastrophic damage is unlikely. The PCB is well laid out, and the components are standard through-hole, so repairs are straightforward if needed. The only user-serviceable part is the rear jumper that controls VCA behavior, which can be reconfigured for different routing options (though most leave it stock). There are no known design flaws or widespread component weaknesses. It's a relatively simple analog circuit with minimal digital content, so it's not prone to firmware issues or clock drift.

When buying used, check that the knobs turn smoothly and that the resonance sweeps cleanly into oscillation without crackling. Test both CV inputs to make sure modulation responds as expected, and verify that the ping function produces a consistent transient. The module ships with a 2-year warranty, and ALM provides full manuals and support documentation online. The Pip Filter isn't a "grail" item for collectors.