Oct 8, 2020

Slope

The Slope module is a voltage controlled slew limiter and modulation source. The core idea behind this module comes form the classic Serge Dual Universal Slope Generator, which has in turn inspired many popular modules including the Makenoise Maths and and the Befaco Rampage.


This is version 2 of my adaptation of the module; the original version is available here as circuit diagram. This version comes as a proper PCB and front panel set, featuring a single slope generator in an 8 hp eurorack module. At the modules' heart is a voltage controlled slew limiter, whose output is a copy of the input voltage except with any sharp steps replaced by smooth ramps, with a rate set by the front panel Rise and Fall controls as well as the corresponding control voltages. When used on pitch CV, the slew limiter creates a glide effect.



Additional logic circuitry turns the Slope into a versatile modulation source. By feeding a signal to the Gate, the module becomes a voltage controlled envelope generator, with the attack time set by Rise, an optional Sustain phase, and the decay time by Fall.

About the circuit

The circuit is described in more detail in the original slope generator article. This module retains popular features of the original design, such as individual shape controls for rise and fall, and sustain and loop controls.

Internally several improvements were made to the circuit. The configuration of the slew limiter core has been changed from inverting to a non-inverting unity-gain amplifier. This ensures very accurate tracking (0.01% gain error) between the input and output voltage, so pitch control voltage can be processed by the unit without tuning errors. Accurate gain is further preserved by a high 1M input impedance to avoid loading down the voltage source, and a low-impedance output to drive the subsequent stage accurately.

The logic circuit has been improved to provide well-defined end-of rise (EOR) and end-of-cycle (EOC) gate outputs, which open up gate delay and pulse timing functions, as well as complex cycling when multiple modules are used together. The gate outputs are driven by push-pull comparators, so they can both source and sink current and will trigger nearly any module properly.

Finally all outputs have status LEDs to show what is going on visually; the main output LED is bicolor.

Typical applications

Like it's predecessors, the Slope module can be used in numerous applications, especially if patched together in pairs or with other modules. However this module was designed with the following appications in mind, that don't need any special patching.

Voltage controlled slew limiter.


This is the most basic function of the slope. Fed with a voltage to the In jack, the Out signal will be a copy of the input but with any fast changes limited by the Rise and Fall slew rates. Rise and fall rates are independently controllable, and each have a CV input with attenuverter.

Used on pitch CV, this gives a glide effect. The CV inputs can be used to change the glide times, or for example fed with a gate to turn the glide effectively on or off (the response polarity and amount can be set with the attenuverters).

The slew limiter can also process audio signal, in which case it is a non-linear low-pass filter.


ADSR envelope



The Slope has an internal set-reset latch, turning it into an envelope generator. The envelope is started by a rising edge on the Gate input, and the attack time is set by the Rise control. When reaching about 8V, the output will go to the decay/release phase, at a rate set by the Fall control. The Sustain control feeds part of the buffered gate signal back to the input, making the Slope a complete 4-stage voltage controlled envelope generator. The only limitation is that the decay and release rates are set by the same control, but their effect can be balanced by adjusting the Shape controls (see below).


The EOR and EOC outputs generate gate signals at different envelope stages, and can be used to trigger other modules at, for example triggering a second envelope to create a faux-delay effect. Even more gate timings can be found by combining EOC, EOR and the original gate input with an external logic module. 

Low-frequency oscillator



The Loop switch internally connects the EOC output to the Gate input, which will cause the Slope output to loop forever. By adjusting Rise and Fall rates, the speed of the LFO can be set, and the shape can be varied between rising ramp, triangle and falling ramp. In loop mode the LFO can go up to several kHz in frequency, and down to cycles lasting 10s of seconds. Even slower LFO rates can be achieved by feeding a fixed voltage to the CV inputs, and adjusting the CV controls counter-clockwise.


The shape can be further adjusted with the Shape controls, independently for the rising and falling part of the cycle. These controls internally feed part of the output back to the input, adjusting the shape between exponential decay and exponential growth, as shown in the figure above. Linear ramps are achieved with the shape controls at the center position. The shape controls similarly affect the shape in ADSR and Slew limiter usage.

Gate delay



A special use of the ADSR envelope is the gate delay, where only the EOR output is used. It will output a gate delayed by an amount controlled by the Rise control. The length of the gate can be adjusted by the Fall time. Such a delayed gate can be used to create off-beat elements in a sequence, or if comined with the original gate on an OR module create a double trigger delay effect on a synth or drum voice.

When the Sustain control is fully CCW, the EOR output length is only determined by the Fall time. However when Sustain is added, the EOR will last at least as long as the original Gate, plus some time which depends on both Sustain and Fall.

PCBs and front panels

PCBs and front panels are for sale in the kassutronics store. To build your own, please read the build documentation, which includes the full schematics, BOM and specifications. 



15 comments:

  1. Woohoo, ordered 3. Very happy it's through hole.

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  2. This will be a nice addition to the other Kassutronics modules I already have. Just wondering if I get one or two right away.

    Since the DUSG, rampage, maths, all come in pairs. What magic are we missing if we only get one?

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    1. I think them coming in pairs is tradition based on the DUSG. There are some cross-modulation possiblities, but more commonly you'll just want one more in your patch. No need to restrict to only two! :)

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  3. Hi! I have just built this using a custom home-etched board, could it be possible that the shape controls also affect the rise or fall time?

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  4. I accidentally got the TLC393 (open collector) rather than the TLC3702 (push-pull). Can I make the circuit work using a pullup?

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    1. finally got the right IC (I was so stupid and order it wrongly the second time again; so finally I ended up paying 6€ for 2 TLC3702, postage included)

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  5. Hi! I've build 2 versions of the slope using the pcb's, and I'm wondering if they are operating properly. When the sustain is all the way down, the envelope will not reach 7.5 volts, more like less than 1 volt, it is basically completely closed and the VCA it is controlling barely opens. In order for it to open fully I need to turn the sustain up basically past half way. So I can't get a simple AD envelope without sustain. Reading the guide, I would think it should still open fully even with the sustain completely closed. Am I correct in assuming this? If so, do you have any suggestions on what could be going on? Thank you!

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    1. Just to add to this, I haven't installed the toggle switch yet as I'm still waiting for them to arrive, so have just connected EOC to gate if I want it to loop. I assume this should not impact the function though?

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    2. If anyone else has this same problem in the future: it turned out to be a bad TL074. Great that you found the problem!

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  6. Hey! Please forgive my inexperience here - does anyone have a Mouser or Tayda cart for the BOM on the Slope? I'm still bad with figuring out the right things to order...

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    1. I sourced the parts for my Slope from those two vendors and can either compile a BOM now, or, what would be better, wait until the Tayda order arrives (should be two days from now) and I've built & confirmed everything is working (this weekend at the latest) and we'll know the parts I chose are correct. Happy to help either way.

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    2. That would be wonderful, thank you! Totally happy to wait, I'm in no rush - I'm just falling at the first hurdle, confused by the range of capactitors available :)

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  7. Are the LEDs a functional part of the circuit or just indicators? I have to wait on bicolor 2 lead LEDs but have the rest built and would like to use it. Also could a 2 pin SPST switch work or should I just bridge it on or off? Waiting on SPDT as well

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  8. What a well written and compassionate article. I found your ideas and wisdom encouraging and helpful. https://www.magicare.com/collections/hand-sanitizer

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