Variatic Erumption#

Dual-burst generator with built-in LFO


Variatic Erumption is a dual-burst generator. When triggered, it produces a controllable sequence of gates synchronized to a LFO. A 1 V per octave LFO is included. Any external LFO/clock may optionally be used. Bursts can be terminated early with the choke. Burst length can be controlled as well as the pattern used for the burst.

  • Type: Burst Generator
  • Size: 10HP Eurorack
  • Depth: 0.8 inch
  • Power: 2x5 Eurorack
  • +12 V: 50 mA
  • -12 V: 5 mA

Interface (LFO)#

An illustration of Variatic Erumption's interface

This switch selects the burst clock source between the internal LFO or an external clock.
In internal LFO mode this jack is a 1 V per octave CV for the internal LFO and has 8 octaves of range. In external mode, it is a clock input. Burst gate length follows both rising and falling edges so clock duty cycle is maintained between LFO, Out and End.
In internal LFO mode, this knob controls the LFO tempo. It allows 4 octaves of control. In external mode, it is a controllable clock divider with 1 to 32 divisions.
This switch controls the base tempo of the internal LFO. It is a three-position switch. Each position is half tempo of the position above. The lowest tempos in each range are: 1/2 Hz (30 BPM), 2 Hz (120 BPM) and 8 Hz (480 BPM).
This jack outputs the LFO used for burst generation.

Interface (Burst)#

An illustration of Variatic Erumption's interface

Hit inputs
This gate input will start the burst on a rising edge. If using internal clock mode, it will also reset the phase of the internal LFO to match the Hit phase.
Hit switches
This tactile switch will start the burst when depressed. If using internal clock mode it will also reset the phase of the internal LFO to match the HIT phase.
Chk inputs
This gate input will terminate the current burst on a rising edge.
Chk switches
This tactile switch will terminate the current burst when depressed.
Cnt inputs
This jack allows to control the burst count with a 0 V to 8 V control voltage.
Cnt knobs
This knob allows the control of the burst count. It acts as a scaler when using an external CV.
This gate output sends a burst based on the clock when triggered with a Hit. There is an orange LED to indicate the current output state.
This gate output sends a pulse based on the clock at the next clock following the end of a burst.
Pat input
This jack allows 0 V to 8 V CV control over the pattern.
Pat knob
This knob controls the current pattern used for the burst. Built-in patterns are documented in detail in the patterns section. When rotated fully clockwise this knob will double the maximal length of the pattern to 64 gates. When in double length mode the bust pattern is the same as when fully counterclockwise (every clock outputs a gate).

Patch tutorial#

  1. Put the Int/Ext switch into the Int position. Put Fst/Slw switch into the Slw position.
  2. Set Pat knob fully counterclockwise. Set left Cnt fully clockwise.
  3. Connect the left Out and End outputs to two gateable modules such as an envelope or Basimilus Iteritas.
  4. Hit the left Hit switch. A long burst will be generated which will blink the orange LED and trigger the attached device. At the end of the burst the red LED will blink and any device attached to the End output will trigger.
  5. Adjusting the Tmp knob while hitting Hit will change the burst speed. The tempo can be controlled by CV (Clk). Send an envelope or LFO to modulate the burst rate. The tempo range can be changed with the Fst/Slow switch. It has three positions each is half tempo of the position above. The Cnt knob and jack input the number of gates in the burst. The Pat knob and input control the pattern of the burst.
  6. Connect an external trigger source to the Hit input to trigger the burst externally.
  7. Hitting the Chk switch or sending a trigger to the Chk input will terminate the burst.
  8. To use an external clock flip the clock switch to Ext and send a clock into the LFO's In. Everything will work the same except it will use the external clock for output timing. In this mode the Div knob acts as an adjustable divider to the clock input. To match the external clock without division put Div in the fully counterclockwise position.


Illustration showing the grid of patterns

Variatic Erumption's patterns are based off of the bit-reversed concatenation of two 16-bit prime numbers. Prime numbers are always odd so in binary the lowest bit is always set. Reversing these will always generate a pattern who's first beat is set. Note the first two patterns are not prime as they are all bits set since this is the most simple burst pattern.

Design notes#

Photo of the first prototype

Variatic Erumption started in Cyrus Makarechian's loft when Aaron Funk asked me why there was no burst generator in Eurorack. I responded: "What is a burst generator?". Cyrus sent me some links to existing products and a few days later I had a rudimentary prototype designed and ordered. Many features were suggested by the testers of that first prototype which led to the second prototype. The second prototype is nearly identical to the final module you hold in your hand.

I had very little to do with the overall formulation of the module. It was very much a community product based on the ideas and feedback of friends.

Special thanks#

  • Kris Kaiser
  • Aaron Funk
  • Cyrus Makarechian
  • Richard Devine
  • Rodent516
  • Baseck
  • Shawn Jimmerson
  • William Mathewson