ALM Busy Circuits - FMco

The ALM Busy Circuits FMco is a compact FM synthesis voice for Eurorack. Two operators, eight waveforms, a built-in decay envelope, and stereo outputs. If you know ALM's Akemie's Castle, which used new-old-stock Yamaha chips to bring four-operator FM into the modular world, think of the FMco as a spiritual descendant. This time around, ALM have written their own DSP engine from scratch, tuned for a more "modern" sound and a faster workflow.

The controls are few but well-chosen. A large encoder handles tuning, while knobs and CV inputs cover Index, Ratio, and Wave selection across eight operator waveforms. The Ratio knob has a Lock switch that toggles between free-spinning mode (for inharmonic, metallic sounds) and snapped intervals (for the cleaner, more tonal side of FM). A trigger input fires the onboard decay envelope, which simultaneously shapes both the FM depth and the output level — send it a gate and you've got it working on its own without needing external envelopes or VCAs. Dial the decay short for percussive plonks and bells, stretch it out for longer tones.

Next up, chords. The FMco can stack up to four voices with CV-controllable chord selection and inversion, giving you anything from simple octaves to lush clusters — again, all from a single module. The stereo-widened left and right outputs can spread everything out nicely across the field.

This is a small, quick and capable FM source that delivers a surprising amount of range from a very simple set of controls. Really great stuff!

Features:

• 2-operator FM synthesis voice with ALM's own DSP engine
• 8 operator waveforms with direct and CV control (with attenuator)
• FM Index with manual offset and CV control (with attenuverter)
• Ratio control with Lock switch for free or snapped intervals
• Built-in decay envelope routed to FM depth and output VCA
• Trigger input for envelope activation
• Encoder-based tuning for quick, accurate pitch setting
• CV-controllable four-voice chord mode with inversion
• Stereo-widened left and right outputs
• USB-C for firmware updates
• Skiff-friendly (32mm deep)
• Reverse power protection