My recent build of a Befaco Rampage prompted me to revisit four variations of the Serge Dual Universal Slope Generator that I have in Eurorack: Befaco Rampage, Random*Source DUSG, Steady State Fate Mini Slew and After Later Audio Tilt. I will compare them for both features and behavior. I learned a few things, too. But first let’s briefly look at the original Serge. I built a CGS Serge DUSG board some years ago, in a banana jack format. It’s very much like the graphic above, with the addition of a Bipolar output.

There’s beauty in its simplicity. Patch from End to Trigger to make it cycle. Patch from Out to CV in for different curves. The Eurorack versions I’m looking at here all have added features to this basic design. And they are:

Random*Source Serge DUSG

The R*S DUSG is the closest in design to the original. It is also the largest, at 26hp width. I built mine from a kit.

Befaco Rampage

The Rampage implements the functionality with a completely different technical design. It’s 18hp wide. I built mine from a full kit. The Rampage circuit is open source, with schematic supplied.

Steady State Fate Mini Slew

The Mini Slew is a single unit; you need two for the equivalent of the DUSG, in 16hp total. I wrote a comparison of Mini Slew with the Make Noise Function.

After Later Audio Tilt

Tilt also comes as a single unit, so you need two for the Dual Slope functionality. But it’s only 6hp in width!

Feature Comparison

This is a simple list of features. We’ll get into functionality later.

All have the following common features.

• Input for slewing
• Main output
• Output indicator
• Rise pot
• Fall pot
• Rise CV input
• Fall CV input
• 1V/Octave FM input
• Trigger input
• Shape control
• Cycle switch

R*S DUSG adds:

• One square wave output
• One bipolar output (inverted)
• End pulse output
• Both CV input (summed to Rise and Fall CVs)
• Rise CV attenuverter
• Rise Exponential switch
• Fall CV attenuverter
• Fall Exponential switch

Rampage adds:

• Cycle gate input
• Trigger button
• Range switch
• Rising gate output (10V)
• Falling gate output (10V)
• End pulse output (10V)
• A>B gate output (10V)
• Min output (trough)
• Max output (peak)
• Balance pot between A and B to comparator input

Mini Slew adds:

• Cycle gate input
• Variable level output (+/- 9.5V)
• Variable level CV input, i.e. 4-quadrant multiplication
• CV Sum
• Time Compensation
• Shape CV input
• End of Rise pulse output
• End of Cycle pulse output

Tilt adds:

• Unipolar/bipolar cycling switch
• Variable level output (+/- 9.5V)
• Bipolar output (+/- 4.5V)
• Rising gate output (9V)
• Falling gate output (9V)
• Separate shape knobs for rise and fall
• Gated ASR with manual sustain level

Generating Envelopes

Envelope peak voltage

The R*S DUSG main output peaks at +5V when generating an envelope or cycling. Mini Slew main output is about 8.5V and the variable output can go up to +/- 9.5V. Rampage outputs peak at +10V. Tilt outputs up to +/- 9.5V unipolar, or +/- 4.5V bipolar. Nothing standard there!

A common use for slope generators is to make an Attack-Decay envelope. Attack is started by a rising edge on the Trigger input. The output voltage rises to maximum at the rate set by the mix of Rise pot, Rise CV input, and FM input. The curve of the rise can be linear, logarithmic (log) or exponential (exp). Upon reaching the max voltage, the decay phase starts heading back to zero at a rate set by all the fall controls and with a linear, log, or exp shape. Of special interest is what happens if a new trigger is received before the cycle is completed. For all of these modules, a new trigger (positive transition) arriving during Rise is ignored (contrary to what the Rampage manual states, that it will be a hard sync, i.e. a reset to zero). But the behaviors differ when a new trigger is received during Fall.

When a trigger is received during Fall

The R*S DUSG just ignores triggers throughout the whole cycle. This makes it ideal for use as a pulse frequency divider. The End pulse will be a division of the number of trigger inputs that happen during the cycle.

Mini Slew, touted also as a pulse divider, exhibits this behavior as well, but very oddly only if the trigger pulse height is around +5V. It was very confusing when triggering Mini Slew from a Rampage 10V pulse output. With a 10V trigger Mini Slew will re-trigger during Fall! I view this as a design flaw, since the level of a trigger should never make a difference to anything.

Rampage behaves just as stated in the manual and re-triggers upon receiving a new trigger during Fall. Re-triggering means that the voltage begins rising again.

Tilt also re-triggers when getting a trigger during Fall.

Envelope shapes

Each of these modules has a different idea about shape. The traditional curves for an envelope uses a logarithmic rise and an exponential fall, or a linear curve for both.

R*S DUSG has the most flexible shape control of the four modules. Rise and Fall shapes can be set independently, making any combination of lin/log/exp Rise and lin/log/exp Fall possible. It’s accomplished by a reversible attenuator (attenuverter) plus an Expo switch. It works by feeding back some of the output into the Rise or Fall CV input. Thus, it mixes with whatever CVs are patched into the Rise, Fall, and Both CV inputs. And there are two positions of the Expo switch, which differ regarding the target voltage of the curves.

Mini Slew has a Shape knob that goes from log through linear to exp. There’s also a Shape CV input which sums with the Shape knob setting. Given that changing the shape impacts rate, using this CV input might have unexpected results. With Mini Slew both Rise and Fall are the same shape, log, lin, or exponential. Exponential attacks are musically odd, but if the rate is fast it sounds OK.

Rampage has yet another way. The shape knob allows to choose between log attack and exp decay, or linear, or exp attack and log decay (weird).

Tilt features separate pots for rise and fall shapes, which facilitates the traditional log attack and exponential decay, if desired.

Here’s a nice instructional video about envelope shapes by someone else.

Relation of shape change to frequency

Altering shape always has some impact on the rate (frequency). Each module behaves differently.

R*S DUSG sees the rate slowed by increasing log shape and made faster by an exponential shape.

Rampage sees the rate slowed by turning the shape knob either direction, away from center (linear).

Mini Slew has two different behaviors. With T-Comp off it’s like the DUSG: log shapes make it slower and exp shapes faster. With T-Comp on it behaves like Rampage, with rate slowed by by either log or exp shapes. The T-Comp feature doesn’t work very well to keep the rate the same as the shape changes.

Tilt acts still differently. In bipolar mode, the rate is slowed by deviation from linear shape, like Rampage. But in unipolar mode, log shapes make it slower and exp shapes faster.

Let’s get cycling!

On three of the modules, patching from End to Trigger will cause cycling. But each module has a switch to turn on cycling. R*S DUSG has a toggle switch; Mini Slew has a button (with status LED) that turns cycling on and off (remembered across power off and on), plus a Cycle gate input that toggles the state set by the button. Rampage has a toggle switch plus a Cycle gate input, but when toggling into cycling it’s also necessary to press the trigger button to get it started. (Patching to both Trigg in and Cycle gate will turn on cycling and start it.)

Note that the End pulse of the Rampage is always a very short 10V spike. The width of EOC on Mini Slew and End on DSUG depends on the setting of the Rise time.

Tilt has no End pulse, so to get cycling one must toggle the three-position switch to either bipolar or unipolar. (Center is for gating envelopes or following an input.)

Frequency control

On three modules, the 1V/Octave, FM, or Exp CV input (same function, just differently named) causes an exponential increase in the frequency with a positive voltage and a decrease with a negative voltage. (Some digital modules do not accept a negative voltage on frequency CV inputs; all of these do.)

On Tilt, the Both CV input acts exponentially, at a measured 1.1V/octave. Patching to both the Rise and Fall CV inputs gets the same response as patching to the Both input.

On the R*S DUSG turning knobs clockwise and adding positive CV to the Rise, Fall, or Both CV inputs will increase the frequency.

On the Mini Slew, turning knobs clockwise or adding positive CV to the Rise and Fall CV inputs will decrease the frequency.

On the Rampage, moving sliders Up decreases the frequency. Adding positive voltage to the Rise or Fall CV inputs also decreases the frequency.

On Tilt, turning knobs clockwise decreases the frequency, but adding positive CV increases the frequency.

Audio Oscillating

The R*S DUSG is the best audio oscillator among the three. It has a 1V/Octave input, as well as trim pots to tweak this response. It also has the highest claimed maximum frequency, at 10 KHz. I tested all four with a linear triangle wave shape and found the DUSG to top out at 1.5 KHz.

Mini Slew tops out at 300 Hz with both knobs full CCW, going up to 560Hz with +5V into the FM CV input.

Rampage top in Lo, Mid, and Hi range is respectively 2.5 Hz, 24 Hz, and 500 Hz, going to 1.2 KHz with +5V on the Exp CV input. These top frequencies could be increased by using exponential shapes.

Tilt max frequency with linear shapes is 1.2 Khz (both knobs fully CCW), and will go to 2.3 KHz with +5V on the Both CV input.

Conclusions

I am happy to have all of these (and my collection isn’t complete yet). Some of the notable features:

• R*S DUSG is best for audio with its range, stability and tracking.
• Rampage and Minislew both have control over cycling on/off.
• Rampage and Tilt both feature gate outputs, active during Rise and Fall.
• Minislew and Tilt both feature variable and invertible outputs.