Catalyst Audio - Mig VCO

The MIG VCO from Catalyst Audio and Electroserf is an analog oscillator with an unusual heart: a dual tube core where two matched 1J24b pentodes reset each other at twice the output frequency, each tube drawing one half of the final waveform. The Balance and Offset controls then slide those two halves against each other, and that's how the module arrives at its range of shapes — much easier to grasp from the oscilloscope drawings in the manual than from any description. A Resonance control feeds part of the output back into the tubes to bend the waveforms further, with a switch for two flavours of that feedback. It's a waveshaping trick that, as the makers note, isn't really doable with conventional oscillator cores.

From there the signal fans out across four simultaneous outputs. Raw is the oscillator core itself. Pulse runs that through a comparator whose threshold you set with the PWM knob — modulate Balance and Offset for proper PWM-style movement. Smooth sends the Raw signal through a third tube that can be pushed from clean into full saturation, rounding the edges and eventually settling into a soft square, with the Shape knob tilting its tone brighter or darker by re-biasing that tube. Fold takes the smoothed wave through a four-stage discrete transistor wavefolder. For control there's 1V/oct, linear FM, and an adjustable sync that resets one or both tubes depending on the incoming waveform.

On tuning, Catalyst are refreshingly straight: the VCO tracks roughly ±10 cents across four octaves and is temperature compensated, which they liken to a 1970s analog synth. If you need pinpoint accuracy across eight octaves, this isn't the oscillator for you. What you get instead is a voice that hums and sizzles and always sounds a little on the edge — raw electrical aggression that's just barely under control. This makes for a great main sound source in patches where character matters more than precision.

Features:

• Analog vacuum tube oscillator with a dual-tube core (three 1J24b tubes in total)
• Two matched tubes each generate half the waveform; Balance and Offset shape the result
• Resonance feedback path with two flavours, alters the waveshapes dynamically
• Four simultaneous outputs: Raw, Pulse, Smooth, Fold
• Pulse output with PWM threshold control
• Smooth stage: third tube from clean to full saturation; Shape re-biases its tone
• Fold output via a four-stage discrete transistor wavefolder
• 1V/oct tracking (~±10 cents over 4 octaves, temperature compensated)
• Linear FM input
• Adjustable sync that can reset one or both tubes
• CV inputs with attenuverters for Balance, Offset and Smooth/Fold
• NOS (New Old Stock) Cold War-era Soviet pentode tubes
• 100% analog