“I ended up creating something new, an FM drum circuit that combines the punchy character of the 909 with the metallic textures of FM synthesis,” explains Moritz Klein after hours spent “staring at this absolute maze of components” in Roland’s classic drum‑machine schematics. What starts as a repair job on a misbehaving vintage unit quickly turns into a full‑blown rebuild, giving producers a step‑by‑step tour of the circuits that make the 909’s kick so enduring.

Watch video Recreating the 909 Kick Drum (with an FM twist)

From triangle core to cleaner sine

Klein isolates the 909’s oscillator first, noting that Roland “traded efficiency and lower manufacturing costs for a more complex but also more flexible subtractive synthesis approach” when they swapped the 808’s bridged‑T for a proper VCO. On the LABOR breadboard he demonstrates a trimmed‑down integrator and Schmitt‑trigger loop, then soft‑clips the resulting triangle into a “pretty decent sine wave” with a handful of resistors and diodes.​​

A four‑part VCA in classic Roland style

To carve percussive amplitude‑decay without bulky parts, Klein replicates Roland’s famously lean VCA: “just four components, one op‑amp, one NPN transistor and two resistors." A minimal diode‑capacitor envelope feeds this VCA and turns the static sine into a tight transient, all verified on LABOR’s scope before he adds gate‑to‑trigger conversion for modern sequencers.​​

Pitch envelope and impact tricks for authentic punch

The hallmark 909 thump arrives when a pitch envelope “quickly rises and then drops back down,” mixed passively with the base tuning.​​ Klein also borrows Roland’s transient enhancers: impact distortion that temporarily alters the waveshaper curve and a subtle trigger injection that “adds some much‑needed complexity to the sound’s transient."

FM modulation opens new timbres

With the vanilla 909 sound secured, Klein clones the VCO, slows it with a larger cap and feeds its output through a switchable 100 kΩ resistor into the carrier’s CV node. Flicking the switch yields metallic growls and alien kicks, though a protective diode is needed to stop the carrier “crashing into either supply rail” on deep bass settings.

XOR cymbals and high‑pass shaping

Square outputs from both oscillators feed a diode‑transistor XOR gate, faking ring‑mod‑style clang for hats and cymbals. An added Sallen and Key high‑pass at 7.5 kHz thins the result into crisp noise bursts, routable through the same VCA via an SPDT switch for multi‑mode percussion.

Extra CV inputs broaden performance scope

Simple PNP hacks give the circuit decay‑length CV and a shared pitch CV, letting producers “play some simple melodies and bass lines."A small diode‑drop compensation brings back sub‑bass range when external CV is at zero.

DIY drum system on the horizon

The video ends with a look ahead: Klein and Erica Synths are folding the FM drum into an eight‑module DIY drum system, alongside kick, snare, hi‑hat, compressor, BBD delay, four‑channel mixer and sequencer, all prototyped on LABOR, the joint circuit‑design platform the two launched last year.​​ For builders and sound designers alike, the collaboration promises a complete playground where vintage inspiration meets modern modular experimentation.

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