Ieaskul F. Mobenthey


Pictured are six modules, in four flavors, from Peter Blasser.  He designated his line of Eurorack modules as Ieaskul F. Mobenthey (IFM).  I have four of the five IFM modules.

Why the Blue Knobs?

IFM modules have knobs with all yellow pointers.  It can be confusing as to which are the basis knobs and which are the attenuverters.  Well, I happened to have lying about some knobs from Blacet modules that I had converted to 5U MOTM format.  They fit the pots perfectly and are only slightly smaller in diameter than the ones that were replaced.  Now all the pots with a bow-tie symbol are blue.


The Denum module is essentially a bounds/bounce oscillator. It has the “triangle core” circuitry used in many oscillators, but it adds a correlated bounds modulation.  Here, bounds and bounce are each given equal treatment, with separate linear and exponential controls and inputs. In addition, this module has a fully featured bi-polar VCA with complimentary inputs, to get you quickly going sending “bb-objets” out the left and right channels.

Bounds Modulation

In the simplest terms, ‘bounds’ are the upper and lower voltage boundaries that the output reaches, before reversing direction.  In most modular oscillators the bounds are fixed, typically at +5 and -5 volts.  Modulating the frequency does not change the bounds; the amplitude remains constant.  Bounds modulation means controlling the upper and lower bounds, and hence the amplitude of the output.  But, changing the bounds also changes the frequency, since smaller bound take less time to reach.  Thus bounds modulation is simultaneous frequency and amplitude modulation.

Denum is a wide-range VCO, with three switch-selected ranges going from very slow LFO to virtually ultrasonic.  Without any bounds modulation, the output swings 5 volts peak-to-peak around zero.  The pulse output is a huge +/-10 volts.  Patching the triangle output back into its own Bounce CV input results in different, but still symmetrical waveforms.

The VCA in Denum

The VCA is unusual in a number of ways.  There are two outputs, left and right, each having its own input, with both inputs normally connected to the triangle output of the oscillator.  There are two CV inputs, but they behave in an interesting way.

If the left (CV) input is greater than right, sound goes out the “left out”. If right is greater than left, sound goes out the “right out”. The voltage amount of difference maps to loudness in these channels.

You can patch into the VCA left and right inputs, disconnecting them from the Denum oscillator. My notes made while checking out the VCA:

  • It’s a low-pass filter that rounds out the edges of the triangle wave, resulting in a pseudo sine wave.
  • It appears to be AC-coupled, so it doesn’t pass very low frequencies.
  • Unity gain in linear mode is at about 5 volts of CV, but in exponential mode, 5 volts results in only about 0.25.


The Fourses module consists of four bounds/bounce oscillators, stacked on top of each other so that their bounds are mutual. Imagine four bouncy balls in a greased perspex tube only wide enough to permit them to travel along it. They bounce off of each other in the tube, and generate intricate but inter-related chaotic outputs. Each bouncy ball has the requisite basis and attenuverter knobs, and bounds inserts, control input, and triangular output.

The bounds of any operator are dictated by the ones immediately above and below, except for the very top and bottom operators; the top upper bound is set to eight volts, and the bottom lower bound to negative eight volts.

Fourses can be thought of as a single complex oscillator cluster.  Changes made to one affect them all.  This is an interesting beast with a learning curve.  My notes from checking it out:

  • Both the Basis and Bounce CV attenuverter inputs operate in a bipolar way:  When centered the frequency is highest, because turning right or left increases the rise or fall time of the wave, thus decreasing the frequency.
  • Patching a +5 or 0V into a Bounds input seems to have a stabilizing effect.
  • The top and bottom bounds act differently (as expected from the above quote).
  • Try audio rate Bounds modulation.


Peter says,

The Swoop module is essentially a bound/bounce control voltage processor, in the lineage of such multi-purpose tools as the “Serge Dual Universal Slope Generator”.  It differs, however from the DUSG, in that it truly is a free-running oscillator, that will bounce between any given boundaries; bounds take a more correlative role in deciding the frequency. In fact, it is already running when given zero input, bouncing infinitesimally at a very high resultant frequency.

It is well to note that Swoop is always bouncing!  The output is barely heard with no inputs, but the pulse output is detecting the zero-crossings and putting out a pulse wave.  To make it cycle, it needs a defined upper or lower bound, or both.  Patch either of the trigger outputs to one of the bounds input, and it oscillates.  More of my check-out notes:

  • With +5 volts patched to a bounds input, the output is from 0V to +5V.
  • Pulse output travels +/- 10 volts.
  • Trigger outputs travel 0 to +5 volts.
  • If you also patch a -5 volts to the other bounds input, now the output travels between -5V and +5V.
  • The trigger outputs do not change when cycling.
  • Due to it always oscillating, when triggered there is a bit of oscillation riding along with the DC of the generated envelope.  This is a little weird!
  • The attenuverters have a fine tuning effect, if no CV patched into them.
  • Patch the main output into one or both of the Bounce CV inputs to obtain more ramp wave shapes.


Peter says,

The Sprott module is a model of chaotic jerk system with standardized voltage control of all parameters. Thus, the chaotic attractor can be shrank down to distill the module into a resonant filter that is the dynamical sub- circuit underlying most jerk systems. It is named after J.C. Sprott who has published numerous papers, articles and books on chaos and chaotic circuits, from a rigorous physical view-point. This module is of course for musical purposes; although the voltage control feature has limited import in physics experiments, it is crucial to aesthetic purposes.

Sprott is a sort of filter.  It’s definitely non-intuitive!  My notes:

  • The Q pot (second down) is minimized at the fully clockwise position.
  • The Q is very sensitive and will cause the module to self-oscillate into clipping easily.
  • If the VCA level (top pot) is not turned up high enough, it seems to contribute to self-oscillation.
  • Chaos modulation is cool!  Chaos CV impacts all outputs, not just the Chaos pass output.

As Ieaskul says, “It’s experimental!”

Here is demo of the Sprott filtering a Denum VCO, with two different outputs going to two stereo channels.  I twiddled the Denum frequency.




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