Subtle variations in timbre and rhythm are a major part of what makes music, well, musical. Much of the allure of analog circuits, tube-based products, or a real drummer is in the subtle nonlinearities they produce.
In electronic music, Don Buchla addressed this with his Model 266 Source of Uncertainly, which produced a variety of ever changing voltages for modulation purposes. Although a number of companies have sought to duplicate the characteristics of the 266, none have improved on the design to the degree that Frap Tools has with the Sapèl, a Eurorack module that provides more than twice as many voltage sources while adding features that significantly increase their potential.
The Sapèl is divided into two identical blocks (yellow and green), each of which is controlled by an independent clock and delivers four kinds of random voltages. The clocking features (positioned in the middle of the panel) include a clock generator with external control input, a CV/Gate input that can modulate either the clock frequency or the manual sample-and-hold (S&H), a button to trigger the S&H, an unprocessed clock output, and a random clock output with a switch for determining the kind of signal that passes through and to what degree it is influenced by other clocks. The switch next to each frequency knob allows you to integrate the other clock’s output into its own sample-and-hold circuit.
Above and below are the sections that provide the primary voltage sources. Starting with the outputs beside the yellow arrows (upper right of the panel), the random-voltage options are (top to bottom) quantized 2n (exponential, useful for semitones), quantized n+1 (linear, for octaves), a fluctuating random voltage, and a non-quantized S&H. To the left of the arrows are switches for introducing a user selectable amount of probability distribution to each voltage output. The magnitude of probability is globally set for each block using the small knob below the switches.
The quantized and fluctuating random voltage sources can be adjusted manually and externally. For the quantized voltages (2n and n+1), the knobs set the “n” in each equation, from 1 to 6, while the input adds CV control. The fluctuating voltage has a knob and CV input for determining its rate of change. (The green section on the lower half of the panel has the exact same features as those in yellow, but in reverse order starting from the bottom of the panel.)
Lastly, the Sapèl has four noise outputs. In addition to white noise, there are three others with a distinctive spectral tilt—blue (+3db/octave), pink (-3db/octave), and red (+6db/octave).
This seemingly complicated description might lead you to believe the Sapèl is difficult to use, but it’s not. Each voltage source has an audibly distinct character when used for modulation, and the colorful markings on the panel provide visual clues to what everything does. Surprisingly, the mirror image positioning of the top and bottom sections makes it easy to keep track of what you’re doing in a patch.
All of this makes the Sapèl a rich resource for modulation that is well suited to both self-generative patches and more traditional uses. This is the module you’ll go to when you want your drum patterns and sequences to have an organic quality.
And although at $449, it’s priced in the upper tier of Eurorack gear, the Sapèl’s solid build and extensive functionality easily justify the cost.