The Slightly Nasty Model 2231 Asymmetric Slew Limiter is a versatile signal generator and signal processor based on the principle of the Universal Slope Generator originally found on Serge synthesisers. This style of module is often seen as one of the defining elements of the “West Coast” school of synthesiser design, with its emphasis on open-ended versatility and functional abstraction.
Essentially the Model 2231 is a voltage-controlled integrator, with a few additional circuits that allow it to be used for a number of other functions. The basic principle behind the module is that the there are actually quite a few synthesiser circuits that are based internally around some kind of integrator – the Model 2231 takes this central core and presents it in a way that allows it to be used in these different configurations.
There are three operating modes on the Model 2231. “SLEW” takes the incoming signal and slews it according to the rise and fall rates set by the panel controls and CVs. At longer slew times, this can be used as either a traditional slew processor for portamento effects, or to add attack and release times to a gate signal like an AR envelope generator. At higher (audio) rates, the 2231 can operate as a voltage controlled lowpass filter, with the added trick of being able to have different cutoff frequencies for the rising and falling parts of the waveform. This can yield an interesting combination of filtering and waveshaping.
“ENV” turns the 2231 into a voltage-controllable AD envelope generator, triggered by the positive edge of a gate signal. The envelope is auto-completing and retriggerable, and is great for generating percussion envelopes.
“CYCLE” turns the 2231 into an oscillator (or LFO) with a range of around 0-1kHz, with independently controllable rise and fall rates allowing for some interesting waveshape and pitch modulation. Using one subunit of the 2231 as an LFO to modulate the other allows for some interesting rhythmic effects and is an easy way to add movement to a patch.
The slope times have a logarithmic response extending from “As slow as you trim it” to 1 millisecond for a -5v to +5v swing. “ENV” mode outputs a 0-5v signal, and “CYCLE” a +/-5v waveform (10v peak-to-peak). The gate input features a internal comparator that ensures gate signals are always converted to a 0-5v signal before reaching the slew processor, to ensure that sub-5v gate signals still generate a 0-5v output when the module is used as an AR envelope generator.
Updates: The v1.0 Rev B documentation contains important information regarding an error in the v1.0 PCBs that affects builds using certain On-On-On DPDT switches. Additionally there is an optional modification for the LED driver circuit to improve operation of the LED indicator, and some small resistor value tweaks to adjust the voltage range of the CYCLE and ENVELOPE modes. Some typos and mistakes in the circuit descriptions have also been fixed. Many thanks to Guy McCusker for helping troubleshoot everything!
v1.1 PCB – Current Documentation (v1.1 Rev C):
v1.0 PCB – Current Documentation (v1.0 Rev B):