I’ve finished building the Slightly Nasty Model 2331 Asymmetric Slew Limiter. Here I’ll describe the build and show scope photos of the various operating modes.

Features

The ASL is another take on the venerable Serge Dual Universal Slope Generator.

• Separate control over rise and fall times
• Individual CV inputs with attenuverters for rise and fall times
• Three modes: Slew, AD Envelope, and Cycling
• Bi-color LED indicating the output

Modes

SLEW mode is the typical VC rate of change function. Can be used as a portamento glide, AR envelope generator following the Input, and even a type of low pass filter at faster slew rates. The default curve is linear, but by patching the output to one or both of the CV inputs, exponential and logarithmic curves can be obtained. The input can be presented to either the Input or the Gate in SLEW mode. The Gate input has a internal comparator that ensures gate signals are always converted to a 0-5v signal before reaching the slew processor to ensure a 0-5V output when the module is used as an AR envelope generator.

ENVELOPE mode is a voltage-controllable AD envelope generator, triggered by the positive edge of a gate signal patched to the Gate input. The envelope is auto-completing and re-triggerable.

CYCLE mode turns the ASL into an LFO or oscillator with a range of 0Hz to 4 kHz, with independently controllable rise and fall rates. The maximum frequency without added CV lies around 90 Hz, the way I trimmed it. But adding 10V to both rate CV inputs can produce a triangle wave up to 4 kHz.

Since the modes are selected by a switch, instead of being patch programmable like the DUSG, the end pulse output is not available on the panel, due to panel space limits.

The Build

Each of the two slew limiters is built as a completely separate module on two PCBs, the rear one for most of the circuit, and the front one for the panel parts.

As usual, I made a few changes to the front panel look. I used my BugBrand jack color theme, where inputs are Yellow, outputs are Green, and gate/trigger inputs are Black. I used Blue Davies 1900 clone knobs for the CV attenuators and the Bold, Small Skirted Davies Plastic Knobs from Thonk.

See my previous post about the Slightly Nasty Discrete Oscillator for details of the headers and standoff hardware mounting parts. The most important part to know about for this ASL is the DPDT On-On-On miniature toggle switch. No specific part was given in the BOM. The one I found at Mouser fits the board almost perfectly. (It uses the S2 configuration in the assembly manual.)

The switch is Mouser #611-7211-002. It is quite expensive, but you’ll know it fits.

Scope shots of operation

Cycle Mode

Below is the ASL in Cycle mode with both rate knobs about in the middle. It’s about 3.1 Hz, going from +4.9V to -4.6V. I can’t explain the flat part at the bottom. I’m going to ask the designer about it. There’s no global offset trimmer.

The photo below shows the LFO with the fall time minimized.

By adding voltage to the CV inputs to minimize both rise and fall times, a maximum of around 4 kHz is shown on the scope. Notice the amplitude remains about the same as with low frequencies.

SLEW Mode

Below you see a square wave being slewed into a trapezoid. Blue trace is the input; yellow is the output. The input is a unipolar square wave from 0-5V.

Envelope Mode

Photo below shows input Gate (blue) and the output AD envelope. The envelope begins falling as soon as it reaches the peak. It completes because the envelope time is shorter than the trigger cycle time.

The following shows re-triggering, which happens if a new trigger arrives at the gate input before the envelope is completed. The envelope does not reach zero volts, but starts the rise segment when the next trigger arrives.