4ms Quad Pingable LFO (2013–2020)

A tap-tempo LFO with four channels of ping-driven modulation that sync, skew, and reset.

Overview

You set the tempo on a 4ms Quad Pingable LFO by feeding it pulses. Plug in a clock, tap the button, or send a burst of pulses, and any rhythmic signal becomes its tempo. It works especially well when you ping it with jagged envelopes or erratic gate patterns. It's not the flashiest module in a Eurorack case, but in a system short on modulation sources or tight on space, it does a lot. That's four independent LFOs in just 12 HP, each able to go from plucky transients to smooth triangle waves and everything in between.

"Pingable" isn't just marketing fluff. The frequency is set by the time between incoming pulses, so it's inherently musical when synced to a sequence or drum pattern. It also handles those pings well. Some LFOs demand clean clock signals. The QPLFO uses comparators on its Ping and Reset inputs, so it accepts noisy waveforms, audio-rate bursts, even feedback from other modules. That helps in chaotic patches, where clock sources might be irregular or mangled on purpose. The tap tempo button is tactile and responsive, and it glows with each press. It runs freely when tapped manually, which is handy for jamming, but it stops when an external clock cuts out, so there's no runaway modulation.

Then there's the skew. It's a full CV-controllable parameter that stretches the waveform from a sharp, plucky attack to a slow ramp-up, with a triangle dead center. At the extremes you get near-exponential curves that act more like envelope generators than traditional LFOs. You can reset each channel externally, or have one channel reset another. Cross-modulating the skew inputs between channels creates shifting rhythm and contour, which suits ambient drones or unpredictable rhythmic textures.

Specifications

Manufacturer	4ms Company
Production Years	2013–2020
Original Price	$295 (built module)
Module Size	12 HP
Depth	1.57" (40 mm)
Power Consumption (+12V)	89mA (external +5V), 154mA (internal +5V)
Power Consumption (-12V)	73mA
Power Consumption (+5V)	65mA (external +5V), 0mA (internal +5V)
Outputs	4 independent LFO outputs
Output Range	0–10V (unipolar) or -5V to +5V (bipolar), jumper-selectable
Frequency Range	0.0002 Hz (71-minute period) to 500 Hz (tracking up to 1 kHz marginally)
Skew Control	Knob and CV input per channel, 0% to 100% (plucky → ramp-down → triangle → ramp-up)
Reset Input	Rising edge triggered, accepts any waveform via comparator
Ping Input	Accepts any complex signal as clock, auto-stops when clock stops
Tap Tempo Button	Free-running when used manually
Mute/On Button	Latching, with optional fire-on-unmute jumper
LED Indicators	Output level (flashing/dimming), tap tempo pulse, on/mute status
Normalization	Ping inputs can normalize to 4ms Quad Clock Distributor via ribbon cable
Special Features	One-shot mode, floating reset mode, comparator threshold trim-pot per channel
Included Accessories	10-to-16 pin power cable, 4 M3 knurled screws

Key Features

Four LFOs That Listen

Most quad LFOs are space-saving conveniences, four copies of the same circuit. The QPLFO is different. Each channel is a full, independent unit with its own tap button, skew control, CV input, reset, and output. You can run one channel synced to a drum clock, another pinging off a sequencer's reset pulse, a third modulating the skew of the fourth, and all of it interacts in real time. Any signal works as a clock, with no need for clean square waves. That allows feedback patches where an envelope triggers a ping, which modulates a filter, which in turn alters the envelope's decay. The QPLFO treats timing as a dynamic parameter rather than a fixed rate.

Skew as Expression

The skew control is where the QPLFO stops being a utility and starts sounding musical. At 0%, you get a fast, plucky rise and a slow exponential fall, good for percussive accents or pseudo-envelope effects. Turn it up, and the wave flattens into a ramp-down, then a triangle, then a ramp-up. Because skew is CV-controllable, you can sweep through these shapes in real time, or let another LFO modulate it. One trick: patch a slow sine wave into the skew CV of a channel running at audio rate, and the waveform morphs from buzzy to smooth and back, a kind of harmonic drift. The skew knob also adds an offset to the CV, so you can use any control voltage range with no need for precise 0-10V scaling.

Sync, Reset, Repeat—On Your Terms

The reset input does more than restart the cycle. Hold a high gate on the reset jack, and the LFO enters "floating reset" mode, where it runs freely without re-syncing to the ping clock. That's useful in generative patches where you want temporary independence from the master tempo. The reset can also trigger a one-shot envelope if the ping clock stops, which makes the LFO act like a quasi-ADSR. With the "fire-on-unmute" jumper (solderable on the PCB), the LFO resets every time you unmute it. That gives consistent timing on activation in live performance.

Historical Context

When the QPLFO launched in 2013, Eurorack was growing fast, but many systems were still space-constrained. Most LFO modules offered one or two channels, and tap tempo was still a luxury feature. 4ms, already known for compact designs like the Dual Looping ADSR and the Spectral Multiband Resonator, saw a gap: a dense, performance-ready LFO that could handle both precise sync and expressive timing. The QPLFO was built around playability. It was about making modulation tactile as well as fitting more in less space. The glowing tap button, the immediate response to pings, and the ability to mute and reset on the fly all cater to live use.

At the time, competitors like Doepfer's LFO and Intellijel's uLFO offered solid performance but lacked the QPLFO's ping-driven flexibility. Using any signal as a clock was still niche, and cross-modulating four channels in such a small footprint was nearly unmatched. Paired with the 4ms Quad Clock Distributor (QCD), it became part of a tightly integrated timing system. That made it a favorite in portable and travel systems, where every HP counts. By 2020, when 4ms discontinued the pre-built version (focusing on kits), the QPLFO had already become a cult favorite.

Collectibility & Value

The 4ms Quad Pingable LFO is no longer in production as a built module, but it remains highly sought after, especially in fully assembled form. Used prices vary widely based on condition and whether it's a kit build or factory module. Factory-built units typically sell between $350 and $450 on the secondary market, while DIY kits (still available from 4ms) go for around $225. Boards and faceplates alone can fetch $100 - $150.

One caveat: the QPLFO is generally reliable, but service technicians observe that the comparator threshold can drift over time, especially in units exposed to voltage spikes or poor power regulation. A trim-pot on the PCB allows adjustment, but it requires opening the module. That's fine for experienced users, less so for buyers who just want to plug and play. The jumpers for +5V source and bipolar/unipolar output matter too. Misconfiguring them can lead to power issues or unexpected signal levels. Collectors note that units with the "fire-on-unmute" jumper installed are slightly more desirable for live performance use.

Common failures are rare but include flickering LEDs (usually a loose connection) or unresponsive tap buttons (wear from heavy use). Forum reports confirm that if the ping rate changes rapidly, the LFO may lag slightly before catching up, especially at extreme frequencies. This isn't a defect. It's a consequence of how the module measures time between pulses, and it expects relatively stable clock sources. For users expecting instant tracking like a PLL, this can be frustrating.

When buying used, check that all four channels respond to pings, that the skew knob sweeps smoothly, and that the mute buttons latch correctly. Verify the jumper settings match your system's needs, especially the +5V selection, since some power supplies don't provide +5V. And if possible, test with both internal tap and external clock to ensure the auto-stop behavior works as expected.